Xue-hui Ge,Liangji Mo,Anhe Yu,Chenzi Tian,Xiaoda Wang,Chen Yang,Ting Qiu

Engineering Research Center of Reactive Distillation,Fujian Province University,College of Chemical Engineering,Fuzhou University,Fuzhou 350108,China Qingyuan Innovation Laboratory,Quanzhou 362801,China

Keywords:Stimuli-responsive Emulsions Switchable surfactants Interface

ABSTRACT Responsive emulsions are the emulsions that can be reversibly switched on-demand between “stable” and “unstable” by environmental stimulus or trigger,which allows a simple and effective adjustment approach to achieve emulsification and demulsification.In recent years,stimuli-responsive emulsions acting as smart soft material are received considerable attention with the advantages of simple manipulation,good reversibility,low cost,easy treatment,and little effect on the system.In this paper,the recent research progress of emulsions that can respond to external stimuli,including pH,light,magnetic field,CO2/N2 and dual responsive are reviewed.Also,the potential applications based on responsive emulsion are discussed,such as catalytic reactions,heavy oil recovery,polymer particles synthesis and optical sensor,aiming to summarize the latest achievements and put forward the possible development trends of responsive emulsions.

1.Introduction

Emulsion,composed of a liquid dispersed in another immiscible liquid in the form of micro-droplets,plays an important role in the field of food,material,chemical,and biological medicine,etc.The emulsion is a thermodynamically unstable multiphase dispersion system.It spontaneously tends to decrease free energy,that is,small droplets collide with each other and coalesce into large droplets until oil–water separation.To prevent the emulsion from coalescing,researchers have developed surfactants [1],nanoparticles[2],and amphiphilic polymers [3] to stabilize the emulsion and keep long term stability.However,in some practical applications,it sometimes needs to demulsify the emulsion to efficiently recover and purify products.The traditional demulsification methods,such as centrifuge,heat and the addition of demulsifier,consume large amounts of energy and would have irreversible effects on the system [4],which need additional operation cost and expenses in industrial application.Based on this,it is very valuable to develop a method to conveniently,quickly and accurately control the emulsion stability on purpose,that is,to make the emulsion more “smart”.The development of external stimuli-responsive stabilizers has made it possible.The approach is to use external stimuli to activate changes in molecular structures of responsive surfactants,hence it is possible to affect surface activity,aggregation,solubility and lead to change in emulsions stability.Similarly,this approach can be applied to soft materials based on stimulusresponsive surfactants such as foams [5,6],gels [7,8] and micelles[9,10] and can control their stability,viscoelasticity and selfassembly behavior on demand,which have been successfully used in mineral flotation[11],drug delivery[12,13],template synthesis[10] and other applications.

In the last two decades,emulsions with stimuli-responsive property have been investigated deeply owing to its ability to be reversibly switched on-demand between “emulsification” and “demulsification” by environmental stimulus or trigger with the advantages of simple manipulation,reversibility,low cost,facile treatment,and little effect on the system.The stimulation could be mainly divided into external physical stimulation such as light,magnetic and temperature and invasive chemical stimulation such as pH,redox,enzyme and so on.Both of stimulation can affect the physical and chemical properties of the stabilizer adsorbed at the oil–water interface and change the adsorption behavior,leading to oil–water separation in the emulsion.Different stimulus systems with different response mechanisms have their corresponding advantages and disadvantages.Therefore,appropriate design ideas are adopted to meet the needs of applications at specific situations.At the beginning,the responsive emulsions are mainly designed for smart and facile emulsification and demulsification of certain systems.However,this purpose is no longer sufficient to meet the need for the expansion of emulsion applications into more sophisticated and intelligent fields.

Thus multiphase emulsions with more complex structures and elaborate adjustment are researched.The structure of multiphase emulsion mainly driven by interface tension at equilibrium state has been concluded to four typical geometries:shell core,antishell core,snowball-Janus,and perfect-Janus [14].Crucially,the properties and functions of double emulsions are related to their geometry and composition,so it is critical to develop fast,simple,and reversible methods to precisely control the topology of a multiphase emulsion.Swegar and co-workers [15] used stimulationresponsive surfactants for the first time to realize a dynamically reconfigurable three-phase emulsion that could respond to pH and light,taking the application of responsive emulsions to a new level.By introducing various stimuli or triggers,they provide access to dynamically and precisely control of the multiphase emulsions configuration,and simultaneously broadening the emulsion application space.After that,stimuli-responsive double emulsions have been received much attention and employed as tunable micro-droplets in many application such as Janus particles synthesis [16,17] and biosensors [18,19].In addition,microemulsions with micron size (0.1–50 μm) have the advantages of thermodynamic stability,high solubility and transparency,better dispersion than ordinary emulsions,and are applied in the field of enhancing oil recovery,catalytic reaction and drug delivery[20].Furthermore,the microemulsions have three structure types,namely O/W,W/O and bicontinuous structure,which means that the responsive microemulsions have richer phase transition behaviours.Therefore,the development of responsive microemulsions is of great practical significance to expand the application range and performance of emulsions.In addition to surfactant molecules,the nanoparticles also have been confirmed to stabilize emulsions[21].The preparation of a Pickering emulsion involves the dispersion of solid particles into the continuous phase,and based on their partial wettability in each of the two immiscible phases,they adsorb at the oil/water interface to form an effective steric and electrostatic protective shield for the emulsified droplets.In recent years,the stimuli-responsive Pickering emulsions which stabilized by nanoparticle have also attracted huge attention due to its high stability,low toxicity,environmental friendliness and easy recycling.Therefore,this review focus on the recent advance of various the stimuli-responsive emulsions in term of the design idea,response process and mechanism as well as problems to be solved.At the beginning of each section,we provide simple schematic diagrams to illuminate each classical response process and the role of stabilizers (nanoparticles and surfactants) in regulating emulsion stability.Moreover,the potential applications based on responsive emulsions including biphase catalytic reactions,heavy oil recovery and transport,polymer particles synthesis and optical sensors are discussed.We anticipate this review will provide some guidance for the design of responsive emulsions and put forward the possible development trends.

2.Stimuli-Responsive Emulsions

At present,stimuli-responsive emulsions can be divided into physical or chemical stimulation.Triggers such as pH and CO2,which work by directly contacting and reacting with emulsion,are classified as chemical stimulation.Such stimulation often changes the characteristics of sensitive molecules(surface activity,polarity or solubility) by triggering minor chemical reactions and thus influence the macroscopic performance of material [20].Other triggers such as light,magnetic field and temperature,which act without directly invading emulsion components,are classified as physical stimulation.Moreover,the development of dual stimulation has received great attention recently due to its advantages of high flexibility,wide application range and rich regulatory direction.In this section,a review of physical,chemical and dual stimuli-responsive emulsion systems recently reported in the literatures is presented.

2.1.pH-responsive emulsions

Although acids and bases may have a destructive effect on the substances in the system and cause salt accumulation during multiple cycles,the pH stimulus–response method is still popular and widely used owing to its facile operation and quick response [10].The forming of a pH-responsive system is mainly through introducing a pH-responsive group (carboxy,amine,phenolic groups)or moieties.As shown in Fig.1,the responsive groups of pHresponsive surfactants can be protonated or deprotonation via adjusting pH value which affects the stabilizers’ interface adsorption capacity to control on demand the stability of emulsion.Recently,Cui et al.[22] designed a new pH-responsive surfactant by introducing a tertiary amine group to the end of the alkyl chain to form cationic surfactant N+-(n)-N,which can stabilize conventional O/W emulsions alone or emulsions coupled with charged inorganic nanoparticles,respectively.When the pH of aqueous solution decreases from 7.1 to 5.4,the N+-(n)-N cationic surfactant at the oil–water interface is transformed to Bola form N+-(n)-NH+due to protonation of the tertiary amine group,which become an inferior emulsifier whether used alone or together with charged nanoparticles,resulting in demulsification of the emulsions.When the pH increases back,the N+-(n)-NH+turns to N+-(n)-N to stabilize the emulsions again.Thus,these emulsions can be switched between stable and unstable state many times triggered by pH.This method provides an effective way to design a responsive system to reversibly control emulsion stability with the advantages of reversible recovery of surfactants for multiple cycles.However,this kind of method above is unrealistic for large-scale applications due to cumbersome synthesis and difficulty in purification of the pHresponsive surfactants.

Fig.1.Schematic diagram of the switchable mechanism of the O/W emulsion stabilized by(a)nanoparticles and pH-responsive surfactant or(b) pH-responsive surfactants alone.

To avoid this problem,Jia et al.[23] recently developed a pHswitchable system without synthesizing complex surfactants but by introducing a pH-switchable noncovalent interaction between 4,4′-oxydianiline (ODA) and conventional anionic surfactant sodium dodecyl benzene sulfonate (SDBS).With the decreasing of pH,the protonation of amino group induces ODA into the ODA+cations in solution (Fig.2),which significantly reduces the W/O interfacial tension through electrostatic attraction and π-π stacking interaction between ODA+and SDBS.Furthermore,when the molar ratio of ODA/HCl reaches 1:4,ODA+further protonates to form ODA2+which attracts two anionic surfactants SDBS molecules via the electrostatic attraction and π-π stacking interaction to form a Gemini-like surfactant with higher surface activity.Thus,by adding ODA or HCl,the emulsion system could be switched between O/W and W/O type (Fig.2).

The introduction of dynamic covalent bonds to establish pHresponsive systems is also served as one of the important methods[24–26].Among all of the dynamic covalent bonds,the dynamic imine bond is very popular for developing a pH-responsive system due to its simple synthesis and rapid pH-responsiveness.The imine bond is stable under alkaline conditions and dissociates under acidic conditions.Sun et al.[26] designed a pH-responsive surfactantviathe formation of a dynamic covalent imine bond between the non-amphiphilic monomer polyethyleneimine (PEI) and benzaldehyde (B).As shown in Fig.3(a),the formation of stable emulsion is ascribed to the PEI-B which possesses excellent surface activity and adsorbs at the oil–water interface.When the pH decreased from 7.8 to 3.5,the demulsification occurred due to the dissociation of dynamic covalent bond between PEI (hydrophilic)and B(amphiphilic)as shown in Fig.3(b).After adjusting the pH to 7.8,the rebuilding of the covalent bond enables PEI-B to stabilize emulsion again.

Fig.2.(a) The different protonated states of ODA by adding HCl or NaOH;(b) Photograph of the pH-switchable process for the emulsion stability [23].

Fig.3.Schematic diagram illustrates the response mechanism of pH-induced demulsification.(a) At pH 7.8,the formation of PEI-B contributes to stabilizing emulsion droplet (grey);(b) After demulsification by adding HCl,the PEI-B breaks into super-hydrophilic polyethyleneimine (PEI) and amphiphilic benzaldehyde leading to demulsification [26].

Pickering emulsion can also be designed to respond to pH trigger,thus achieving precise control of emulsion stability and effective recycling of the emulsifier.pH-responsive Pickering emulsions are stabilized by the pH-responsive solid particles,which can be classified into three types:(1) amphiphilic molecules or polymers attached to the surface of the solid particles by chemical modification[27,28];(2)direct synthesis of microgels or inorganic composites composed of pH-sensitive polymers [29,30];and (3)hydrophilic particles modified by electrostatic attraction [31,32].Recently,Ju et al.[31]developed a novel pH-responsive emulsifier,benzyl-polyethyleneimine modified cellulose nanocrystals (Ben-PEI-CNCs).As shown in Fig.4,the Pickering emulsions stabilized by Ben-PEI-CNC2shows different stability under different pH conditions.At pH 7,stable emulsions could be prepared due to the suitable hydrophilic-hydrophobic balance and electrostatic repulsion between the positively charged Ben-PEI-CNCs which effectively adsorbed at the oil–water interface.As the pH approached 10,Ben-PEI-CNC2particles change to electrically neutral to get aggregation,resulting in the coalescence of droplets and the formation of a poorly-dispersed emulsion layer.At pH 3,Ben-PEI-CNC2had a strong hydrophilic ability to be well dispersed in water,and could not stabilize oil droplets,resulting in the separation of the oil phase and the water phase.Although the Pickering emulsions stabilized by the above-modified nano-particles have achieved some success [27,32,33],these systems still have drawbacks such as the requisition of extra purification procedure due to the addition of particles,and the difficulties in the large-scale production because of the complex particle modification process.Thus,it is still challenging to find a simple way to prepare the pHresponsive Pickering emulsion with different types of oil phases by using readily available hydrophilic particles at a low concentration under mild conditions [34].

Fig.4.(a) Photographs shows the stability of hexadecane-in-water emulsions stabilized by Ben-PEI-CNC2 at pH 3 (left),pH 7 (middle) and pH 10 (right);(b)Schematic diagram illustrating the response mechanism of the pH-responsive Pickering emulsion [31].

2.2.CO2/N2 responsive emulsions

CO2/N2responsive emulsion is also popular owing to its nontoxic and non-accumulating system with commercially available materials.As shown in Fig.5,the stability of emulsion can be reversibly controlled by alternate addition and removal of CO2,depending on the protonation/deprotonation degree of CO2responsive surfactants.Similar to the response type introduced above,the key to develop this responsive emulsion is how to design and construct a sensitive functional unit that can respond to CO2,and use it as a CO2-responsive switch to control the stability of the emulsion.The groups that respond to CO2can be mainly summarized into four categories,including primary amine [1],guanidine/guanidine[35,36],tertiary amine[37,38]and nitrogen-containing azole heterocyclic system[39].Lu et al.[40]reported a series of CO2responsive surfactants containing hydrophobic tertiary amines with varying alkane carbon numbers and coupled with SDBS as emulsifiers to stabilize dodecane-in-water emulsions.By bubbling CO2for 20 s,the hydrophobic tertiary amines with a long alkane carbon could be transformed into bicarbonate salts,allowing to form ion pairs with SDBS via electrostatic interaction,and thus disrupt the stability of the emulsions.The stable emulsions were obtained again via bubbling N2at 60 °C for 5 hours,aiming for removing CO2from the system.By varying the number of tertiary amine groups and the position of hydroxyl groups,this research group also studied the effect of tertiary amines with different molecular structures on the CO2responsiveness of O/W emulsion [38].According to the research,more tertiary amine groups bring more stable emulsions and higher demulsification speed by bubbling CO2.Moreover,the appropriate position of the hydroxyl group could also strengthen the stability of the emulsions but show few benefits on demulsification due to their good water solubility.

Fig.5.Schematic diagram of the switchable mechanism of the O/W emulsion stabilized by(a)nanoparticles and CO2 responsive surfactant or(b)CO2 responsive surfactants alone.

Although the tertiary amine based-CO2responsive emulsions possess many advantages,such as commercial availability and high demulsification efficiency,the process of removal of CO2to recover emulsifiers required more time with injecting N2at 60°C.Also,the synthesis of conventional small amphiphiles with CO2responsiveness is complicated and time-consuming.To solve the problem,the supramolecular amphiphiles (i.e.superamphiphiles) with simpler design and construction process are developed [24].Supreamphiphiles or supramolecular amphiphiles refer to amphiphiles that are fabricated by noncovalent interactions or dynamic covalent bonds,which can be reversibly controlled by a trigger.Recently,many researchers have interests in utilizing the superamphiphiles molecule to control the stability of emulsions[23,26,41–43].Most of these superamphiphiles were assembled based on fatty acids and organic amines.For example,Sun et al.[44] developed CO2responsive superamphiphiles (D-OA) as emulsifiers by facilely mixing Jeffamine D230 and oleic acid (HOA) at a stoichiometric ratio of 1:2,leading to the formation of stable O/W emulsion for over 14 days.After bubbling CO2within 20 s,part of D-OA molecules was converted into neutral HOA and charged D230(D2+)with inefficient interfacial activity,leading to complete phase separation.And then stable emulsions were obtained again due to the re-construction of D-OA as efficient stabilizers at the oil/water interface after bubbling N2at 60 °C.Xu et al.[41] reported the CO2switchable microemulsions stabilized by the CO2responsive superamphiphile and co-surfactant 1-butanol.The stable heptane-in-water microemulsions were obtained via electrostatic interactions between anionic oleic acid and cationic Jeffamine D-230 (1:1) to form superamphiphile as emulsifier.As shown in Fig.6,with bubbling CO2for 20 s,the demulsification occurred because the carbonate substituted oleic acid bonded to D-230 to form a compound with low interfacial activity.After bubbling N2at 60 °C for 10 min,the stable and transparent microemulsions were obtained due to the in-situ formation of the amphiphile at the oil–water interface with high interfacial activity,demonstrating the high efficiency of the reversible switching characteristics.Xu et al.[43] designed a novel CO2responsive amphiphile BTOA by facilely mixing an anionic HOA and a newly cationic amine(1,3-Bis (aminopropyl) tetramethyldisiloxane) at a 1:1 molecular ratio,exhibiting excellent interfacial activity and reducing the dynamic interfacial tension of n-decane in water from 45 mN·m-1to 5 mN·m-1at equilibrium.More importantly,less dosage of the novel surfactant is required for stabilizing the emulsion compared with previous studies.

Fig.6.Schematic illustration of proposed emulsification and demulsification mechanism of O/W emulsions prepared using CO2 responsive superamphiphile D-OA [41].

The nanoparticles that served as a stabilizer to develop CO2responsive emulsion are also attractive because of their long term stability of emulsions with less surfactant.Zhang et al.[37,45,46]utilized the tertiary amine or amine oxide-based surfactants together with silica particles through electrostatic interaction to prepare Pickering emulsions with similar CO2responsive behaviors.Rao et al.[47] designed a novel biosurfactant maleopimaric acid glycidyl methacrylate ester 3-(dimethylamino)-propylamine imide (MPAGN) to stabilize CO2responsive Pickering emulsions with silica nanoparticles.All of these surfactants above are with positive charge group,which could adsorb on the surface of nanoparticles with opposite charge(e.g.silica particles)via electrostatic attraction.Besides ionic surfactants,the CO2responsive Pickering emulsions could also be stabilized by nonionic surfactants with silica nanoparticles.For example,Zhang et al.[48] utilized hydrogen bonding to adsorb conventional polyoxyethylene nonionic surfactant (C12E9) on the surface of silica nanoparticles to make the particles hydrophobic in situ,and thus stabilize the Pickering emulsions.The addition of CO2is to simply decrease the number of silanols on the particle surface and the adsorbing sites for hydrogen bonding with oxyethylene groups,which make the silica particles become ineffective emulsifiers.These Pickering emulsions could be effectively switched “on”(stable) and “off”(unstable) by the addition and removal of CO2.In recent years,a large number of studies have applied CO2/N2-responsive Pickering emulsion as droplet microreactor in heterogeneous catalysis [49–51]which would be detailed in the following section on the application of heterogeneous catalytic reactions.

2.3.Photoresponsive emulsions

The light is attractive for being regarded as a “green” stimulus to control the stability of the emulsion.It could achieve reversible switching between the “on” and “off” state in the metastable systems,such as foams [6,11,52,53],emulsions [54,55] and multiphase emulsions [15,56].Remarkably,various photochemical reactions are depending on the different chromophores (azobenzene,fluorenyl,stilbene,spiropyran,etc.) [57].By incorporating appropriate moieties,the photoresponsive systems can be adjusted with high spatial and temporal precision through modification of a broad range of parameters (wavelength,intensity,duration,etc.)[58].To the best of our knowledge,it is popular to utilize the amphiphilic molecule containing azobenzene as a photoresponsive emulsifier owing to its high photosensitiveness and rapid responsiveness.As shown in Fig.7,azobenzene surfactants have two configurations.Commonly,the surfactnats are mainly trans structures with effective surface activity,which will be isomerized and transformed into curved cis structures with ineffective surface activity by uv irradiation.In 2014,Takahashi et al.[55] firstly achieved photoinduced demulsification by utilizing a photoresponsive Gemini surfactant (C7-azo-C7) with an azobenzene group.However,complete demulsification on a macroscopic scale required six hours due to the poor transmittance of light in the dispersion system.In 2016,Takahashi et al.[54] employed the mixture of photoresponsive surfactant 2-(4-(4-butylphenyl)diazenylphenoxy)ethyltrimethylammonium bromide(C4AzoTAB)and anionic surfactant Sodium dodecyl sulfate (SDS) to stabilize octane/aqueous emulsion and then achieved demulsification with UV irradiation for 90 min.As shown in Fig.8 (a),the surfactant containing azobenzene photo-converts reversibly from a trans to a cis state by UV irradiation.This change from a trans to a cis state in conformation led to the increase of the polarity of azobenzene surfactant molecules,thus,increasing by up to 20 mN·m-1of the interfacial tension between octane and surfactant mixtures (SDS/C4AzoTAB,1/9,mol·mol-1) after UV irradiation (Fig.8 (b)).This makes the emulsion more likely to coalesce and eventually separate into oil and water.To avoid the disadvantage of poor light transmittance of emulsions,they also used the microreactor to achieve photoinduced demulsification within several minutes.

Fig.7.Schematic diagram of the switchable mechanism of the O/W emulsion stabilized by (a) nanoparticles and photoresponsive surfactant or (b) photoresponsive surfactants alone.

Fig.8.(a) Trans-cis photoisomerization of C4AzoTAB upon UV/Vis light;(b) the interfacial tension between n-octane and SDS/C4AzoTAB mixed surfactants with various concentrations,the hollow represents trans-C4AzoTAB,solid represents cis-C4AzoTAB [54].

Such powerful ability of azobenzene photosensitive surfactants in controlling the interfacial activity has attracted application in the reconfigurable structure of double emulsions.In 2015,Swager et al.[15]first used C4AzoTAB combined with Zonyl as emulsifiers to stabilize photoresponsive double emulsions.The dynamic transformation between core–shell structure and Janus structure could be achieved via tunable interfacial tension upon alternatively UV and blue irradiation.When exposed to the ultraviolet light at 365 nm,part of the trans-C4AzoTAB at the oil–water interface changed to cis-C4AzoTAB with interfacial inactivity,resulting in the transformation of fluorocarbon/hydrocarbon/water(F/H/W)double emulsion into Janus structure.And after further UV irradiation,the double emulsion completely reversed to hydrocarbon/fl uorocarbon/water (H/F/W) double emulsion,while blue light can reverse the process above.To investigate the dynamic reconfigurable process and interfacial adsorption behavior of photoresponsive double emulsion,Jia and co-workers[56]measured interfacial tension among three phases under UV/Blue light,as shown in Fig.9(a),which found that only the hexane-water interfacial tension γHwas significantly affected by light with a rise under UV irradiation and a decrease under blue light.As shown in Fig.9 (b),they believed that C4AzoTAB only adsorbed on hexane-water interface due to polarity,and Zonyl tends to adsorb on the perfluorohexane-water interface.They proposed that the dramatic change in dynamic interfacial tension drove the Marangoni flow,resulting in the inversion of the double emulsion.Such lightinduced transformation is appealing and possesses great potential application on drug delivery and optical material.

Fig.9.(a)The dynamic mechanism of three-phase emulsions hexane(H)/fluorohexane(F)/water(W)structure inversion,(b)dynamic interfacial tensions for hexane–water(γH),perfluorooctane–water (γF),and hexane–perfluorooctane (γHF) in an aqueous solution of Zonyl FS-300 and AzoPS under UV/blue light irradiation [56].

The photoinduced transformation systems mentioned above are all based on the cis–trans isomerization of azobenzene groups of surfactants.Different from the cis–trans isomerization of a single group,Pickering emulsions provide new design strategies via modification of nanoparticles or the self-assembled aggregation of microstructures.Recently,Li et al.[59]developed the photoresponsive Pickering emulsion using β-cyclodextrin-grafted alginate(Ugi-Alg-CD,host molecule)and azobenzene derivative supramolecular(Azo-PEG,guest molecule) self-assemblies as particles emulsifier.Before UV irradiation,Ugi-Alg-CD and Azo-PEG self-assembled to form aggregates through host–guest interactions and acted as emulsifiers to stabilize the emulsions.Most of Azo-PEG/Ugi-Alg-CD host–guest aggregates disintegrated after UV irradiating for 5 min,causing oil–water separation,while the stable emulsions were obtained again due to the phototransformation of Azo moiety after irradiation of visible light.The stability of responsive Pickering emulsions depends on the microstructure of Ugi-Alg-CD/Azo-PEG self-assemblies at the oil/water interface in response to UV/Vis light.Bai et al.[60] chemically grafted trichlorododecylsilane on the surface of TiO2nanoparticles to improve their hydrophobicity.Thus the silane-grafted TiO2(S-TiO2) nanoparticles have suitable surface hydrophobic property,which can be used to stabilize W/O emulsion.After light irradiation,the surface of S-TiO2nanoparticles becomes hydrophilic due to the adsorption of the hydroxyl group,leading to droplet coalescence and emulsion demulsification.

2.4.Magnetic responsive emulsions

Similar to the light,the magnetic field could act as a “physical’’trigger without the introduction of any chemical additives.It has attracted huge attention due to its irreversible adsorption,low toxicity,and low cost.The magnetic responsive emulsions can be prepared using either magnetic emulsifiers or amphiphilic molecules or nanoparticles adsorbed at the bi-phase interface and magnetically responsive solution as the dispersion phase.The rapid demulsification of the emulsions can be achieved by only introducing an external magnetic field,and the magnetic nanoparticles can be recycled after being cleaned by chloroform or ethanol and dried.As shown in Fig.10 (a),Fe3O4nanoparticles are widely used as responsive particles in magnetic responsive Pickering emulsions because of their excellent superparamagnetism [61–64].Fig.10(b)shows that formulate magnetically responsive emulsions stabilized by a new class of magnetic surfactant stabilizers.

Yang et al.[63] prepared magnetic responsive Pickering emulsions stabilized by 3-aminopropyltriethoxy silane coated magnetic(Fe3O4@SiO2–NH2) nanoparticles with excellent stable emulsion performance and rapid demulsification ability.As shown in Fig.11 (a),the stable O/W emulsion could be obtained and kept for 52 days (0.4% (mass) particles concentration),attributed to the large remaining surface area of the hydrophilic particles in the aqueous phase to prevent droplet coalescence.When applied the external magnetic field to emulsions,oil–water separation was observed within a few minutes due to the aggregation of the magnetic nanoparticles triggered by magnetic forces on nanoparticles (Fig.11(a)).Peng et al.[64] synthesized magnetically responsive particles as emulsifiers to stabilized the ionic liquid (IL) -based emulsions.The magnetic nanoparticle emulsifier (MNCHOL) was synthesized based on Fe3O4by utilizing the surfaceinitiated ATRP method with a specially designed cholesteryl derivative for further modification.The stable IL(C4mim[PF6])/water Pickering emulsions were obtained by MN-CHOL without cosurfactants.Moreover,the droplet sizes of Pickering emulsion depended on the concentration of MN-CHOL(negative correlation)and NaCl (positive correlation).Remarkably,the Pickering emulsions showed magnetically responsive movement property controlled by magnetic field as shown in Fig.11 (b).It is found that droplets of IL are not destructed and demulsified during manipulation by a magnet,showing incredible stability and controllable movement.This kind of magnetically responsive emulsions provides an attractive application prospect in term of wastewater purification,extraction and targeted drug delivery.

Fig.10.Schematic diagram of the switchable mechanism of the O/W emulsion stabilized by (a) Fe3O4@nanoparticles or (b) magnetive-responsive surfactants.

Fig.11.(a) Photographs of emulsions stabilized by Fe3O4@SiO2–NH2 nanoparticles after preparation [63];(b) Photographs of utilizing a hand-magnet to control the movement of magnetically responsive C4mim[PF6]-based Pickering emulsion droplets against gravity [64].

Traditionally,the preparation of magnetically responsive emulsions has been limited to Pickering emulsions[65].However,there are many limitations in the magnetic particles for the scale-up application,including complicated synthesis,poor biocompatibility and severe aggregation.In recent years,nano-particle free magnetic emulsions have also been achieved by utilizing magnetically ionic liquids(MIL),providing an alternative to magnetic responsive Pickering emulsions as shown in Fig.10(b).However,recent studies can only control the movement of emulsion droplets by applying magnetic fields.On the one hand,MIL only needs simpler and fewer synthesis step than magnetic particles.On the other hand,MIL as molecular liquids is better than the traditional ferrofluids which usually need a flammable organic carrier fluid.The magnetic properties of MIL are contributed from F-zone metal ions that chelate to the surfactant head group or exist as counter-ions [66].Brown et al.[67] firstly developed the magnetic responsive emulsions stabilized by MIL with simple synthesis and purification.MIL,mixing the halides of the F-zone metal (Gd3+,Fe3+) with the imidazole salt,can stabilize the emulsion with magnetic property,allowing for easy control of the emulsion droplets movement under a magnetic field.In addition,MIL,as the polar or surfactant component,respectively,can establish magnetic responsive microemulsions with excellent magnetic responsiveness and magnetic susceptibility.Recently,Dai et al.[62]developed a novel MIL by utilizing the trimethylammonium bromide surfactants with different length of carbon chains as cations and [FeCl3Br]-as anions to stabilize hexane-in-water microemulsion.MILs as emulsifier and 1-butanol as co-surfactant,showing high magnetic susceptibility and low viscosity.

2.5.Temperature-responsive emulsions

Temperature is another commonly investigated physical stimulus used to control emulsion stability.Most surfactants are temperature sensitive,for example,ionic surfactants have a Krafft point,that is,when the temperature of the ambient solution reaches a critical value,the solubility of the surfactant molecules will increase significantly,becoming more hydrophilic;In contrast,non-ionic surfactants are more hydrophobic due to dehydration of hydrophilic groups.Heating will not only affect the hydrophilicity of emulsifiers but also reduce their adsorption capacity on the interface and weaken the protective film.Furthermore,heating can reduce the viscosity of the continuous phase,which is conducive to increasing the chance of droplet collision and demulsification.For this reason,the temperature control range of the thermal responsive emulsion should be as close to room temperature as possible.At present,temperature-responsive stabilizer is commonly achieved by surface grafting with polymers that exhibit thermo-responsive properties,such as,poly(Nisopropylacrylamide)(PNIPAM)[68,69],and poly[2-(dimethylamino)ethyl methacrylate] (PDMAEMA)[70].Both of the thermo-responsive polymers have a lower critical solution temperature (LCST).As shown in Fig.12,when the temperature is higher than LCST,the solubility of the polymer in solution decreases.Taking advantage of this property,Li et al.[69] synthesized a novel temperature-responsive Janus silica nanoparticle with one silanol side and one PNIPAM side.Compared with symmetrical structures,the Janus particle exhibit more excellent emulsification and thermo-responsive performance in both tetracaine/water and crude oil/water emulsion systems.As shown in Fig.13,it was found that the emulsion remained stable for more than 24 h,which was due to the increase of electrostatic repulsion of nanoparticles at the oil–water interface,and the agglomeration of nanoparticles did not occur below LCST.However,when the temperature of the emulsion exceeds LCST,the insolubility of the Janus nanoparticles in water increases,which leads to the rapid instability of the emulsion.

Fig.12.Schematic diagram of the switchable mechanism of the O/W emulsion stabilized by temperature-sensitive polymer with (a) hydrophobic or (b) hydrophilic corrsponding to the cooling or heating stage.

Fig.13.Schematic diagram illustrating the different state of the Janus stabilizers at different ambient temperatures and use for controlling the stability of emulsions via heating and cooling [69].

Most of the thermal responsive emulsions are stable at low temperature and unstable at high temperature.However,applications for emulsion reactions usually require the emulsion to be stable at heating conditions and then become demulsification when the reaction is complete.Recently,Ranka et al.[72] developed a temperature-responsive emulsion that is stable at high temperature and unstable at low temperature.A co-block polymer composed of polysulfobetaine methyl acrylamide (PSBMA) and temperature-responsive polymer PNIPMAM was grafted onto silica nanoparticles via conjugated siloxane and used as a stabilizer to prepare temperature-responsive Pickering emulsions.This approach is likely to be broadly adaptable to other diblock copolymers and find significant utility in applications such as enhanced oil recovery and heterogeneous catalysis.

2.6.Dual responsive emulsions

Over the past decade,much attention has been devoted to emulsion systems that respond to more than one stimulus.The combination of multiple stimuli is particularly advantageous because it can improve the controlling ability either by broadening the controllable range or increasing the adjustable precision of a given system.Jiang et al.[73] fabricated a dual responsive Pickering emulsion system stabilized by mixing negatively charged SiO2nanoparticles and the dual responsive surfactant (4-butyl-4-azobenzene bicarbonate,AZO-B4) which was responded to both light and CO2.The dual responsive surfactant AZO-B4 contains an amine group and an azophenyl group which is sensitive to CO2/N2and photoactive.Bubbling CO2into emulsions could convert uncharged tertiary amine of AZO-B4(surface-inactive)into amphiphilic ammonium bicarbonate (surface-active).And the stable Pickering emulsions were obtained as a result of the adsorption of positive AZO-B4 on SiO2at the oil–water interface and the decrease of the interfacial tension.In addition,azophenyl groups can be photoisomerized between cis (surface-active) and trans(less surface-active) states under ultraviolet or blue irradiation.This photoinduced change can make the surface activity of the emulsifier switch between active and inactive state to further control the droplet size of the emulsions.Wang et al.[71] developed synergetic CO2and Near-infrared light(NIR)dual responsive polymer nanoparticles through grafting CO2-and temperaturedependent poly[2-(diethylamino)ethyl methacrylate] (PDEAEMA)onto NIR responsive polydopamine (PDA) surface.Initially,the W/O emulsions stabilized by PDA-PDEAEMA could be converted into O/W emulsions via bubbling CO2within 1 min due to the protonation of PDEAEMA.Then,upon NIR irradiation for 1 min,the temperature of the O/W emulsion raised to 58 °C due to the photothermal conversion of PDA,leading to the conversion of the PDEAEMA chain from an expansion state (hydrophilic) into a contraction state (hydrophobic) and the change of emulsion type to W/O,as shown in Fig.14.

Fig.14.(a) Fluorescence microscopic images of the inversion of Pickering emulsions stabilized by PDA–PDEAEMA particles under CO2 and NIR irradiation.(b)Schematic illustration shows the change in charge and structure of the PDEAEMA chain on the PDA surface under CO2 and NIR irradiation [71].

The above process can be reversed by cooling (O/W) and then removing CO2by bubbling N2to change the emulsion type back to the initial state(W/O).This synergistic stimulus–response is attractive,which can control the stability of the emulsions more accurately and widely and have negligible effects on the system.Ren et al.[75] prepared the temperature and CO2dual responsive hydrophobic CNCs-M2005 through the electrostatic interaction between hydrophilic cellulose nanocrystals (CNC) and thermosensitive amino-terminated poly(EO6-st-PO29) (M2005).The Pickering emulsions were stable at 20 °C and became unstable once heating to 60 °C.The temperature-induced demulsification was ascribed to the dehydration of polyethylene oxide and polypropylene oxide leading to the aggregation of CNCs-M2005 particles.In addition,CO2can also cause the emulsion to demulsify,which was mainly due to the desorption of CNCs-M2005 on the surface of the droplet,which cannot form a mechanical barrier at the oil–water interface to prevent droplet coalescence.Lu et al.[74]developed a redox and pH dual switchable emulsifier FA-DMDAOx composed of ferrocenecarboxylic acid (FA) and N,Ndimethyldodecylamine (DMDA).The Pickering emulsions were stabilized by the electrostatic interaction between negatively charged silica nanoparticles and FA-DMDA-Ox.Commonly,the stability of Pickering emulsions could be easily controlled by alternatively adding Na2SO3and H2O2,exhibiting the ability of redox responsiveness.It is noted that both acidic and alkaline ambient solutions could cause demulsification due to the disaggregation of FA-DMDA-Ox,exhibiting the unique pH-switch behaviour as shown in Fig.15.

Fig.15.Photographs (a)and micrographs (b) of the Pickering emulsion stabilized by 0.5%(mass) silica nanoparticles and 0.075% (mass) FA-DMDA-Ox after adding HCl and NaOH alternately [74].

Apart from forming Pickering emulsions proposed above,utilizing dual responsive surfactants as the emulsifier is also served as an effective pathway to design dual responsive emulsions.For example,the morphological control of a heptane/perfluorohexane/water (H/F/W) double emulsion could be achieved by only using a pH and light dual-responsive surfactant.Dong et al.[76]employed the dual-responsive surfactant 1-[2-(4-decylazoben zene-phenoxy)-ethyl]-1-diethylenetriamine (C10ANOC2N3) to control the topological structure of the double emulsion.First,the surfactant C10ANOC2N3with an amine group undergoes protonation when adding HCl,causing the double emulsions to transform from Janus droplets to F/H/W core–shell double emulsions.Then,after UV irradiation,C10ANOC2N3containing azobenzene group will change from trans state to cis state,and the oil–water interfacial tension will therefore increase,leading to the conversion of H/F/W core–shell double emulsions firstly to Janus emulsion,and finally to F/H/W double emulsion.When irradiated with blue light,the double emulsions reverted to the H/F/W type.What’s more,emulsions reconfiguration could even be obtained without surfactants.Recently,Zhang et al.[31] reported a series of CO2and temperature dual responsive surfactant-free microemulsions composed of hydroxyethylamine (2-(dimethyl amino)ethanol DMEA,N-methyldiethanolamine MDEA and triethanolamine TEA)solution as water phase and octanol as oil phase without conventional surfactants.It is found that the microemulsions using DMEA exhibited the most sensitivity to both CO2and temperature stimuli.Bubbling and removing CO2from emulsions could switch the stability of microemulsions between “on” and “off”.Moreover,the microemulsions could convert from W/O to bicontinuous phase (BC) and then to O/W upon cooling.The former response mechanism is the protonation of DMEA under the CO2atmosphere,while the latter is the influence of temperature on hydrogen bond interaction.To summarize,the construction of such a dualresponsive emulsion system could not only widen the range of adjustment with higher precision but also enrich the phase behavior of emulsions to adapt to specific environmental conditions.

3.Recent Application of Responsive Emulsions

3.1.Responsive emulsions as droplet microreactor for biphase catalytic reactions

Heterogeneous catalytic reaction refers to a reaction in which the system is composed of more than two phases due to the incompatibility of reactants and catalysts in media.These reactions usually occur at the phase interface,so the area of the phase interface has a significant impact on the reaction rate.The emulsion,particularly microemulsion,is a highly dispersed system with large phase contact areas,which is conducive to heat and mass transfer of the reaction.Considering product separation and emulsion recycling,an emulsion that can be “switched off” by a trigger is preferred for chemical reactions.For this reason,recent studies have focused on utilizing responsive emulsions as droplet microreactor for heterogeneous catalytic reactions.

CO2has attracted increasing attention owing to its green,pollution-free,easily available and nontoxic abilities [49–51,77].Compared with pH-responsive stimuli,introducing or removing CO2usually makes less effect on reactants and catalysis in the emulsion system.Therefore,the CO2responsive emulsion can be recycled many times by separating the resulting product,which is an ideal reversible medium for biphasic reactions.For example,Wang et al.[51] reported a CO2/N2switchable ionic liquid-based microemulsion as a droplet microreactor for the Knoevenagel condensation reaction.The droplet microreactor is composed of n-pentanol as a reaction medium with [C12DMEA][Im] solution as the alkaline environment to promote the reaction.The Knoevenagel condensation reaction in emulsion droplets only took one hour,showing excellent catalytic activity (Fig.16).Additionally,the microemulsion was separated into two phases by introducing CO2and the product was suspended in the aqueous phase in the form of precipitation.Then,the resulting products could be separated from the reaction mixture simply by filtration,and the microemulsion system could be readily recycled at least three times upon N2bubbling.Similarly,this group designed a CO2responsive zeolitic imidazolate framework(ZIF)to stabilize Pickering emulsion for Konevenagel condensation reaction in the recent year [49].The complete separation of the Pickering emulsion consumes only 1 minute,showing the efficiency ability of demulsification.But a similar reaction required 2 hours due to the larger droplet size of the Pickering emulsion than the microemulsion.In addition,the uses of CO2responsive droplet microreactors for other biphase catalysis reactions are also reported,such as alcohol oxidation [50],the hydrolysis of olive oil and the esterification of octanol with oleic acid[77],proving the universality and practicability of this platform in the catalytic reaction.

Fig.16.(a) Reaction between benzaldehyde and malononitrile in the n-pentanol/[C12DMEA][Im]/H2O microemulsions;(b) the coupling of the chemical reaction,product separation and recycling of the microemulsions [51].

Fig.17.Schematic diagram of two major stages in a heavy oil recovery process [78].

Besides CO2,the other stimulus to adapt specific condition for different reactions are also needed.For example,Wang et al.[79]developed a novel photoresponsive Pickering emulsion as a microreactor for the highly efficient catalytic hydrogenation of several unsaturated hydrocarbons in mild condition.The emulsion is stabilized by both silica microspheres functionalized with surfaceloaded metal Pd (Pd@SM) and water-soluble azobenzene-based ionic liquid surfactant.During the reaction,the silica microspheres also act as catalysts and the ionic liquid is the key to photoinduced emulsification.As the reaction finishes,the stable Pickering emulsions underwent switching from emulsions to complete oil–water separation upon UV irradiation with stirring for 30 min and the resulting product dissolves in the oil phase.The reformation of emulsions requires visible light with stirring for 60 min.The pH,which is rapid and easy to manipulate,is also an attractive method for designing a recyclable Pickering emulsion catalyst system.In recent year,Luo et al.[33]designed novel pH-responsive polymeric nanoaggregates to stabilize the toluene-in-water emulsion.The pH-responsive emulsion provides an efficient platform for the biphasic interfacial catalytic hydrogenation of olefins.The large oil–water interface brings the reactants in full contact with the solid catalyst resulting that the yields of the reaction reach more than 99% within 3 hours.Moreover,the separation of products and recovery of catalyst is realized by adding alkaline substances to the emulsion leading to the separation of oil from water.More importantly,this process can be repeated over eight cycles with negligible effect on catalytic activity.Recently,Wu et al.[80]developed a CO2,temperature-and pH-responsive block copolymer to prepare multi-responsive emulsion and employed it for sequential multienzyme cascades reaction.The three-step cascade enzymecatalyzed reaction included hydrolysis of acetic acid benzyl ester,oxidation of benzyl alcohol,and benzoin condensation and every step requires specific enzyme and optimal pH condition,respectively.Thus,in this emulsion system,the appropriate pH is provided for the different stages of the reaction to increase the activity of specific enzymes.And after every step,the multiresponsive emulsions are disrupted on demand by heating or bubbling CO2to recycle product and enzyme catalyst.To sum up,responsive emulsions provide a promising platform for cascade reactions by not only enhancing catalytic activity and facilitating the recovery of resulting products and catalysts,but also eliminating intermediate operations and purification steps,which greatly simplify the reaction process and reduce reaction costs.

3.2.Responsive emulsions for enhancing heavy oil recovery and transport

Emulsions have great potential in the petroleum industry.Firstly,emulsions could be used as oil displacement agents that can reduce the interfacial tension between oil and water and the wettability between oil and rock to enhance oil recovery.Also,the formation of emulsions helps greatly reduces the viscosity by dispersing heavy oil in water which benefits the pipeline transport of heavy crude oil.However,when the processes of oil recovery and transport are completed,the emulsions need to be separated to recycle oil product.Therefore,developing the responsive emulsion with controllable stability is beneficial for effectively recycling the oil product to enhance oil recovery and transport.

CO2responsive emulsions are widely used for heavy oil recovery due to their low cost,environmentally friendliness and renewability.For example,Lu et al.[78]applied a supramolecular approach to construct a CO2responsive surfactant system composed of oleic acid and monoethanolamine for promoting oil–solid and oil–water separation in heavy oil recovery.The main process is shown in Fig.17 as follow,the flooding water containing switchable surfactant promotes the heavy oil-solid separation during heavy oil liberation.This is because the addition of surfactant reduces the interfacial tension between solid and liquid,making it easier for oil to desorb from the surface of sand and gravel.To recycle the oil product,the emulsion needs to be broken by bubbling CO2in the system due to the combination of organic amines with carbonate to form hydrophilic compounds.Finally,the water solution can be reused as an oil displacement agent by bubbling inert gas to remove CO2and adding new oleic acid.Liu et al.[81] developed heavy oil-in-water emulsions with naphthenic acids and DMCHA as an emulsifier.The demulsification effects of CO2and organic acids were compared.For emulsions with low crude oil content (5%,mass),the demulsification efficiency of CO2exceeds 90%,achieving almost complete separation of heavy oil and water.For emulsions with high crude oil content (70%,mass),due to the high viscosity of the emulsion,the CO2release pressure increases,and the CO2demulsification efficiency is only 56%,which cannot be completely demulsified.Using organic acids,especially citric acid and oxalic acid instead of CO2,the demulsification efficiency can be increased by more than 90%.The main reason for this difference is that organic acids are more acidic than carbonic acid.Additionally,Saliu et al.[82] investigated several nitrogen-containing organic bases which can interact with the naphthenic acids that existed in the petroleum crude oil to form a CO2responsive surfactant.Both the charged naphthenic acids and the organic salts can adsorb at the water–oil interface,playing an important role in the decrease of the IFT and emulsification process.Within a few minutes of CO2bubbling,the emulsions could be “switched off” completely.The stable emulsion could be formed with a relatively low concentration(0.05%,mass)of PEG bearing terminal amino moieties and could keep long-term stability.

Fig.18.Schematic illustration of continuous morphological control of polymer particles induced by electricity via double emulsions template [16].

In addition to CO2,the pH-responsive emulsion is also a popular method for oil recovery and transport applications.Hirasaki et al.[83] developed the pH-responsive polymer-coated nanoparticles to emulsify bitumen with higher density and viscosity than other oils,which makes it more difficult to recover and transport.The pHresponsive particles were obtained by grafting Poly-(2 (Dimethylamino) ethyl methacrylate) (DMAEMA) covalently onto the surface of silica nanoparticles.The stable heavy oil in water emulsion can be stabilized by DMAEMA nanoparticles at low concentration (0.1%,mass) and neutral pH condition without any additives.To verify the practicability of the system,the solution containing 0.1% (mass)DMAEMA nanoparticles are used as flooding water in the sand pack column.It is found that the polymer nanoparticle solution could recover 10% (mass) bitumen from the sand pack column which is much better than the DMAEMA linear polymer (about 1% (mass)).After recovery,the emulsion can be destabilized by adding HCl to reduce pH from 7 to 2.During oil recovery,considering that the particles in the emulsion tend to agglomerate and block the rock pores,the design of pH-responsive and the solid-free emulsion is more desirable for oil recovery.In recent year,Onaizi group[84,85]developed the pH-responsive heavy oil/water nanoemulsion (＜200 nm)stabilized by 0.1% (mass) rhamnolipid biosurfactants and investigated the pH-responsive phase behavior.In general,there is no doubt that responsive emulsions make heavy oil recovery and transportation more intelligent,convenient and efficient.And they are similarly suitable for sewage sludge treatment and other applications.However,the influence of various stabilizers on the formation damage,pore blockage and effective permeability reduction,have to be further investigated in future researches.

3.3.Responsive emulsions for synthesizing polymer particles

More intelligent properties can be obtained by using responsive emulsions as templates for synthesizing particles.Latex is a colloidal emulsion system of polymer particles dispersed in water,which is widely used in surface coatings and adhesives.During the preparation of latex via emulsion polymerization,it is necessary to add surfactants into the system to prevent the polymer particles from coalescing.After finishing latex synthesis,the system contains a large amount of water (30%-50%,mass),which is not needed for some further processes and greatly increases transportation costs.Therefore,developing such a latex that can be aggregated and re-dispersed on demand is desirable.Kondo et al.[86] developed the photoresponsive polymer latexes that can be reversibly aggregated and re-dispersed via UV/Vis irradiation.The photoresponsive latexes are composed of mixed surfactants (C4-AzoTAB and SDS),polystyrene monomer and AIBN as initiator using microemulsion polymerization.The destabilized/redispersed mechanism is based on the light-induced cis-trans isomerization reaction of C4AzoTAB.To further improve the light-induced aggregation efficiency,a continuous-flow microfluidic device is used to achieve the aggregation of nanoparticles in 90 s.Shieh et al.[87]utilized the pH and CO2dual responsive polymer poly(2-dimethylamino-ethyl methacrylate) as surfactant and PMMA as polymer monomer to prepare latexes via emulsion polymerization.These newly developed nanoparticles not only possess efficient,reversible CO2-/N2-switchable aggregation/redispersion ability but also can remain in stable colloidal dispersion at low pH and promote rapid precipitation of nanoparticles at high pH of 9.

Fig.19.(a)-(c) Two dimensional finite difference time domain simulation of emulsion droplets with different curvature radii on the incident light of 500 nm wavelength.(d) Local areas of the emulsion droplets based microlenses stabilized by C4AzoTAB exposed to UV irradiation show a smiley face pattern [91].

In addition to controlling the dispersity of the particles,utilizing responsive emulsions to produce Janus particles also allows the formation of precise morphologies manipulated by external stimuli.Current synthesis methods of Janus particles with responsive emulsions include solvent evaporation[17] and phase separation[88].Recently,the multiphase emulsions with rich morphological characteristics,are widely used as templates to prepare multifarious Janus particles by introducing appropriate monomers and polymerization [89,90].Jia and co-workers [16] firstly achieved the fabrication of anisotropic particles with configurable morphology via tunable interfacial tension using responsive double emulsions.The electricity responsive double emulsions are fabricated in an immiscible three phase system consisting of two oils:photo-induced polymerization monomer hexanediol diacrylate(H) and immiscible fluorocarbon oil (F) after homogenizing.The mixed surfactants ferrocenyl cationic surfactant (11-ferrocenylun decyl)-trimethylammonium bromide (FTMA) and fluorosurfactant are employed as the continuous phase.FTMA surfactant under electric field stimulation leads to protonation of the ferrocene group,which affects its adsorption capacity at the oil–water interface.These changes lead to the morphologic transformation of the double emulsion driven by the Marangoni flow.Meanwhile,hollow,hemispherical,mushroom-like,and spherical type particles were obtained continuously after photopolymerization at different times of electrical action as shown in Fig.18.This strategy makes it possible to achieve large scale,precise,and facilitative manipulation of particle morphology in a single synthesis system.

3.4.Responsive double emulsions for optical sensor

An optical lens is a device that focuses or scatters light.Micronscale droplets have inherent optical lensing behavior due to their ability to create curved interfaces according to the lowest Gibbs free energy.Swager group [31] reported a pioneering work that utilizing double emulsion droplets to prepare responsive microscale compound lenses with dynamic refractive ability.The immiscible hydrocarbons and fluorocarbons are used as the inner phase of droplet microlenses because that the contrast of refractive index between the two oil phase is beneficial to enhance the focusing power.As shown in Fig.19,it is found that there have different effects on light transmission depending on the configuration of emulsion with different internal interface curvature.To be specific,the double emulsion with a high refractive index of n-heptane as the core has a strong focusing effect on light,while the Janus emulsion has no obvious effect on light transmission,and the double emulsion with fluorocarbon as the core has a strong scattering effect on light.To explore the feasibility of droplet-based microlenses application in imaging display,the photoresponsive surfactant C4AzoTAB is used to reversibly photoinduced variations in double emulsion morphology,and thus control the focal length of microlenses.As shown in Fig.19(d),The local areas of microlens are exposed to UV light by a photomask with a smiley face,which results in the conversion of the exposed double emulsion from the transparent Janus configuration(light zone)to the strong light scattering core–shell configuration (dark zone).Such responsive double emulsion droplets can be dynamically reconfigured between core–shell and Janus configuration via external stimuli,which makes these droplets very promising as an optical sensor.

Based on the above findings,Swegar group [19] further developed an optical sensor based on tunable droplet microlenses to quantitatively measure enzyme activity.The sensor mechanism of the enzyme activity is described in Fig.20(a).As the action of the targeted enzyme,the enzyme responsive surfactant with ineffective surface activity gradually converts to the effective surfactant,corresponding to the change of the emulsion structure from core-shell type (H/F/W) to anti-core–shell (F/H/W) type through Janus type driven by interface tension.During the transformation of emulsion type,the curvature of the internal interface changes constantly,so that the transmission of light changes accordingly.From a macro perspective,it can be observed that the clarity of the image below the emulsions changes between opaque and translucent as shown in Fig.20(b).More importantly,such alterations in the optical transmission of the micro-lenses are sensitive enough for quantification of α-amylase,lipase,and sulfatase activity.And then utilizing common electronic products as light intensity detectors to collect the transmission data.Similarly,they [92]also developed a pathway that employing dynamic tunable double emulsion as an optical sensor to rapid detect Salmonella enterica.Moreover,they [18,93] reported a more sensitive transduction mechanism that detected analytes by utilizing the aggregation of Janus droplets instead of the change of interface tension.The configuration of Janus droplets lenses is maintained during the binding process and the analyte could be directly observed through the tilted Janus droplets.Recently,Swegar et al.[94] developed a unique coupling of an optical resonator and double emulsion,providing a chemical sensing platform.Full-liquid double emulsions are adjustable droplets that can undergo dynamic and reversible morphological changes according to changes in the chemical environment.This chemical-induced change of emulsion greatly enhances the effective refractive index,making the tunable droplets act as a chemical sensor and signal amplifier and resulting in an obvious variation in the resonance wavelength measured by the optical resonator.In general,the dynamically reconfigurable emulsion can be used as an optical sensor to transform chemical and biological signals into visual signals,providing a convenient,low-cost,portable and rapid detection platform.However,since the optical sensor depending on droplet morphology is easily affected by other factors in the real solution environment,the application research remains in the lab stage.

4.Conclusions and Outlooks

Emulsion plays an important role in the chemical industry and daily life.However,considering the separation and extraction of products and recycle,it is necessary to demulsify emulsion.Stimuli-responsive emulsions can achieve efficient and convenient oil-water separation on demand with negligible effect on the system.Different stimulus systems have their advantages and disadvantages because of their response mechanisms.Therefore,different design ideas are adopted to meet the needs of applications at specific situations.What is more,with the expansion of emulsion applications into more sophisticated and intelligent fields,the demulsification of single emulsions through stimulation is no longer sufficient to meet the need.The three-phase emulsions with a rich topological structure introduced in this field accordingly.Therefore,the research on responsive three-phase emulsion has been reported successively,which further broadens the application range of responsive emulsion.

Fig.20.Reconfigurable droplets act as tunable lenses and the optical transmission of an emulsion film depends on the droplet morphology.(a)Schematic ray diagrams of the complex droplets composed of hydrocarbon and fluorocarbon within a continuous phase of aqueous solution containing the enzyme and substrate.(b) Below the droplet schematic is a photograph of the corresponding polydisperse emulsion,placed on a smiley face in a petri dish to show changes in light transmission.Below are photographs and microscopic images of the three different configurations of the emulsion droplets [19].

In this review,we focus on the recent advance involved two and three-phase emulsions with stimuli responsive property ranging from pH,light,magnetic field,CO2and dual stimuli.The key to design a responsive emulsion is how to select stabilizers with an appropriate group or interaction with the ability to respond to external stimuli,such as,the azobenze group with cis-trans isomerization under UV light,the tertiary amine group that can be protonated in the presence of CO2,and organic amines and oleic acid group which the change of pH would affect their electrostatic interaction and so on.Additionally,we also elaborate on the great application potentials for the stimuli responsive emulsions in fields such as biphase catalytic reactions,heavy oil recovery and transport,polymer particles synthesis and optical sensors.These applications based on responsive emulsions take full advantage of emulsions as microreactors,oil dispersants,emulsion templates,and sensors coupling with external stimuli-responsive property to achieve efficient recovery of catalysts and products,enhanced oil recovery and transport,functional particles synthesis,and portable detection of biomolecules.

From the perspective of the direction of the stabilizer,the future development trend of the stabilizer may be more involved in multi-response.However,most of the current responsive stabilizers require complex synthesis steps or expensive raw materials,which limits the large-scale application of responsive emulsions.Utilizing the noncovalent interaction or covalent bonds to build a responsive system is served as an important way to effectively avoid complex synthesis problems.However,this way is generally only applicable to chemical stimuli-responsive types.The development of surfactant-free emulsion systems also has a bright future.For example,adding reactive solutions such as ionic liquids,polymer to replace constituents in the emulsion system may significantly reduce the amount of surfactant.Meanwhile,if responsive emulsion systems are to be adopted industrially,then more precise and green control over responsiveness will be necessary.Coupling microfluidic technology with responsive emulsions can greatly improve demulsification efficiency and provide better control in droplet morphology,but this must take into account the equipment cost of microchannel systems.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21908026),the Fujian Province science and technology guidance project (2021Y0007) and Key Program of Qingyuan Innovation Laboratory (00221004).