Stimuli-Responsive Bubbles and Foams Stabilized with Solid Particles

Invited Feature Article pubs.acs.org/Langmuir

Stimuli-Responsive Bubbles and Foams Stabilized with Solid Particles Syuji Fujii* and Yoshinobu Nakamura Department of Applied Chemistry, Faculty of Engineering, Osaka Institute of Technology, 5-16-1 Omiya, Asahi-ku, Osaka 535-8585, Japan ABSTRACT: Particle-stabilized bubbles and foams have been observed and used in a wide range of industrial sectors and have been exploited as a technology platform for the production of advanced functional materials. The stability, structure, shape, and movement of these bubbles and foams can be controlled by external stimuli such as the pH, temperature, magnetic fields, ultrasonication, mechanical stress, surfactants, and organic solvents. Stimuli-responsive modes can be categorized into three classes: (i) bubbles/foams whose stability can be controlled by the adsorption/desorption/ dissolution of solid particles to/from/at gas−liquid interfaces, (ii) bubbles/foams that can move, and (iii) bubbles/foams that can change their shapes and structures. The stimuli-responsive characteristics of bubbles and foams offer potential applications in the areas of controlled encapsulation, delivery, and release.

1. INTRODUCTION A foam can be defined as a dispersed system of many gas bubbles in a liquid or solid.1,2 The liquids may be water (or aqueous solutions), oil (or organic solutions), ionic liquids, or liquid-state metals. To date, gas-in-aqueous media bubbles/ foams have been systematically studied and are the focus of this feature article. In general, macromolecules and small molecules having surface activity adsorb at the gas−liquid interface of a bubble and lower the surface free energy; therefore, they are employed as bubble/foam stabilizing agents. In addition to these molecular-level bubble/foam stabilizers, it has been known for more than a century that solid particles can also act as bubble/foam stabilizers,3 and several books and review articles have been published on this topic.4−8 The ability of particles to stabilize bubbles/foams by adsorption at the gas− liquid interface is not surprising considering the known ability of particles to stabilize emulsions through adsorption at their water−oil interfaces9−11 and the recognition of gases as fluids resembling an oil of extremely low polarity. Particle-stabilized foams are sometimes called Pickering foams by analogy to particle-stabilized emulsions consisting of oil and water, which are frequently called Pickering emulsions9−11 in honor of the researcher9 who described them in his paper published in 1907. However, if these foams are to be named after an individual, then they should actually be called Ramsden foams because Ramsden had already reported in 1903 that bubbles/foams can be stabilized by particles.3 In this feature article, we simply refer to these foams as particlestabilized bubbles/foams or an equivalent expression. Particlestabilized bubbles/foams have been widely observed and used in ore flotation (also known as froth flotation), cleaning, food products, water purification, radioactive material processing, crude oil refining, and many other industrial fields.12,13 © XXXX American Chemical Society

Unwanted bubbles/foams are often generated when particles are present in unit operations involving the mixture of gases and liquids by pumps or agitators; this is a problem in papermaking, where undesirable bubble/foam production is induced by fibers, tars, clays, or other particles and may lead to pipe and filter clogging. Additionally, in the petroleum industry, bubbles/foams stabilized by asphaltene particles tend to form in the distillation processes, thereby posing problems. In the areas of ore flotation and ink removal, however, foams have been used beneficially in the recovery of target materials through their adsorption at gas−liquid interfaces. In all of these fields, the particles involved are generally nonuniform in shape and surface chemistry and polydisperse in size distribution. These qualities present particular difficulties for the accurate experimental characterization and reproducibility of results regarding bubble/foam formation, stability, and structure. Therefore, in recent years, scientific studies using particles of well-defined size, size distribution, and surface chemistry as bubble/foam stabilizers have been conducted. Particle synthesis and characterization technologies have continued to develop in the 21st century. These developments highlight the importance of examining particles that adsorb at gas−liquid interfaces and studying these bubbles/foams using model particles with strictly controlled surface structure and surface chemistry based on interfacial and colloidal science. Intelligent materials, which can respond to external stimuli, represent one of the most exciting and newly emerging areas of scientific interest with a wide range of potential applications.14,15 The introduction of stimuli-responsive properties Received: March 25, 2017 Revised: May 6, 2017 Published: May 6, 2017 A

DOI: 10.1021/acs.langmuir.7b01024 Langmuir XXXX, XXX, XXX−XXX

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Invited Feature Article

Figure 1. Conceptual illustrations of stimuli-responsive bubbles and foams. Bubbles and foams (i) break, (ii) move, (iii) deform, and (iv) change their structure in response to stimuli such as pH, temperature, surfactants, magnetic fields, ultrasound, mechanical stress, and organic solvents.

is adsorbed at the interface with a high adsorption energy, then it will strongly resist desorption. For this reason, the stability of particle-stabilized bubbles/foams is thought to be higher than that of foams stabilized by typical molecular-level bubble/foam stabilizers. It has been experimentally confirmed that particles having contact angles of 63° to 66° stabilize foam, imparting a high level of stability.20,21 It is possible that bubble/foam stabilization may be caused by the adsorption of particle aggregates at the gas−liquid interface rather than by the adsorption of individual particles. In this case, it is necessary to determine the wettability of the uneven aggregate surfaces. In future studies examining foam stability, it will be necessary to measure the contact angles of the individual particles and the particle aggregates at the gas−liquid interface and to clarify their relationship to foam stability.22,23 The stability of the thin film of the continuous liquid phase formed between the bubbles is also important for controlling the stability of the bubbles/foams.11 The stability of the thin film is influenced by the capillary pressure preventing film thinning and rupture and the interfacial rheological properties affecting film drainage.

into bubble/foam systems is of significant interest in various research fields, including physical chemistry, materials chemistry, colloid science, nanotechnology, and biochemistry.16 This feature article highlights the scientific progress in the area of recently developed stimuli-responsive bubbles and foams that are stabilized with particles (Figure 1). We first discuss the physicochemistry of particles adsorbed at the gas−liquid interface and the mechanism of film stabilization. Then, the methods for producing particle-stabilized bubbles and foams are described, followed by discussions of stimuli-responsive bubbles and foams stabilized with solid particles whose structure, stability, and motion can be controlled by external stimuli.

2. PHYSICOCHEMISTRY OF PARTICLES ADSORBED AT THE GAS−LIQUID INTERFACE AND MECHANISM OF FILM STABILIZATION Understanding the wettability of particles at the gas−liquid interface and their adsorption energy is crucial for designing/ fabricating particle-stabilized bubbles and foams and for controlling their structure and stability. Considering the curvature effect of the particle-adsorbed gas−liquid interface, particles with relatively hydrophilic surfaces (exhibiting a contact angle θ of less than 90°, as measured through the liquid phase) are appropriate for the stabilization of bubbles and foams in liquid media. Conversely, when particles with relatively hydrophobic surfaces (exhibiting a contact angle larger than 90°) are used, it is expected that water drops dispersed in air (liquid marbles and dry liquids) are preferably stabilized. In this regard, the phase transition between a dispersion of bubbles in a liquid (foam) and a dispersion of liquid drops in a gas (dry liquid) has been demonstrated by controlling the hydrophilic−hydrophobic balance of the surfaces of nanosized silica particles.17 Note that particle wettability is analogous to the hydrophilic−lipophilic balance of surfactant molecules.18 The adsorption energy of a spherical particle adsorbed at a gas−liquid interface from the liquid phase (ΔG) can be described using eq 119 ΔG = − γglπa 2(1 − cos θ )2

3. PARTICLE-STABILIZED BUBBLES AND FOAMS 3.1. Methods for Producing Bubbles and Foams Stabilized by Solid Particles. Particle-stabilized bubbles and foams can be produced by methods similar to those used to produce bubbles and foams stabilized by molecular-level surfactants. Bubbles and foams can be generated by mixing the particle, liquid, and gas components by manual shaking, blender agitation, or the injection of fine bubbles into the particle dispersion.24−26 Other methods include first dissolving the gas in the liquid phase by applying pressure to the particle dispersion and then sharply reducing the pressure to produce bubbles in the liquid and by generating bubbles using a foaming agent. Vanderhoff and Shaffer discovered serendipitously that nitrogen bubbles were stabilized in an aqueous medium by 7.9 μm polystyrene (PS) particles adsorbed at the gas−liquid interface during seeded emulsion polymerization under zero gravity on the Challenger space shuttle (STS 7, June 1983).27 Nitrogen bubbles were generated when the 2,2′-azobis(isobutyronitrile) azoinitiator was decomposed to generate free radicals.

(1)

where γgl is the surface tension of the liquid, a is the particle radius, and θ is the contact angle. As shown by this equation, the adsorption energy increases with the particle radius, liquid surface tension, and contact angle proximity to 90°. If a particle B


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Figure 2. pH-responsive bubbles and foams: Schematic representation of the pH-induced disruption of bubbles and foams stabilized with a pHresponsive particulate foam stabilizer. The bubble/foam exhibits long-term stability when the pH values of the aqueous medium promote hydrophobicity in the particles adsorbed onto the bubble surface. By contrast, the addition of an acid/base leads to immediate bubble/foam disruption and the spontaneous dispersal of the particles because of the hydrophilization of the particle surface by protonation/deprotonation. Reproduced with permission from ref 34. Copyright 2011, American Chemical Society.

the nanometer scale, the surface chemistry of the particulate bubble/foam stabilizers is important to understanding the wetting behavior of the particles at a gas−liquid interface. At the submicrometer and micrometer scales, the adsorption/ desorption of the particles to/from the gas−liquid interface is an important event. Chemical and physical changes in the liquid and gas phases at larger scales (>micrometer) can affect the properties of both the particulate stabilizers and the liquid− gas interfaces; consequently, these changes can alter the bubble/foam structure, stability, and motion. The fabrication of bubbles/foams whose structure, stability, and motion can be controlled in a well-defined manner by external stimuli is also accomplished by the insertion of stimuli-responsive materials into the bubble/foam systems. In this section, bubbles and foams that are capable of responding to various external stimuli (pH, temperature, magnetic fields, ultrasonication, mechanical stress, molecular surfactants, and organic solvents) are reviewed (Figure 1). 3.2.1. pH-Responsive Particle-Stabilized Foams. Studies have been performed on particle-stabilized bubbles/foams whose stability can be controlled by the addition of an acid or a base (Figure 2).32−39 In these systems, the hydrophilic− hydrophobic balance of the particle surfaces and their wettability at the gas−liquid interface are controlled by exposure to a pH stimulus, thereby changing the interfacial absorption energy of the particle to the interface. This process effectively controls the adsorption and desorption of the particles, thus providing a means of obtaining foam stabilization and defoaming on demand. Foams that break down upon exposure to a base have been produced using particulate bubble/foam stabilizers carrying acid groups on their surfaces. The particles in acidic aqueous media have hydrophobic surfaces with protonated acid groups, and they can stabilize aqueous foams by adsorption at the gas− liquid interface. Conversely, for particles that have hydrophilic surfaces with negatively charged deprotonated acid groups, nonadsorption and desorption of the particles to/from gas− liquid interfaces occur. It was demonstrated that PS particles carrying a poly(acrylic acid) (PAA) colloidal stabilizer, which is

It has been reported that particle-stabilized highly monodisperse bubbles can be produced in microchannels using microfluidic devices. In addition to the simple production of highly monodisperse bubbles, the microchannel method enables research on the adsorption rate of particles at the gas−liquid interface. Using a three-channel system (one for gas and the other two for aqueous media), Subramaniam et al. finely controlled the size and stability of particle-stabilized bubbles.28 In the three-channel system, particle-covered bubbles could be formed with good reproducibility only at a high flow speed (10 cm/s), which indicated that particle adsorption kinetics was important for the formation of stable bubbles. Related to particle delivery to the gas−liquid interface, Buchcic claimed that the application of ultrasound could supply energy to ensure particle adsorption to the gas−liquid interface against energy barriers for the interfacial adsorption of particles.29 Park et al.30 produced highly monodisperse carbon dioxide bubbles (5% or less polydispersity) at a rate of 3000 bubbles per second using microchannels. A unique method of preparing nonspherical particlestabilized bubbles, named the air pocket trapping technique, was proposed by the Stone group.31 Water is quickly added to a thin, porous powder layer consisting of solid particles to ensure the entrapment of air between the particles. The water penetrates the porous layer until the capillary pressure of the advancing water interface is balanced by the pressure of the trapped air. This process results in the formation of a monolayer of particles trapped at the air−water interface, and the nonadsorbed particles can be removed to obtain bubbles. Using this method, it is possible to fabricate nonspherical bubbles with sizes greater than 10 μm. 3.2. Stimuli-Responsive Particle-Stabilized Bubbles and Foams. Various studies have been performed on the fabrication of stimuli-responsive bubble/foam systems in which the bubble/foam structure, stability, and motion can be controlled by responses to specific external stimuli. Particlestabilized bubbles and foams are multiscale systems, and their properties, including stability and responsiveness to external stimuli, arise from the complex combination of these scales. At C


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Figure 3. Thermoresponsive bubbles and foams: (a) Schematic representation of temperature- and pH-induced disruption and structural change in bubbles and foams stabilized with polystyrene (PS) particles carrying poly[2-(dimethylamino)ethyl methacrylate] (PDMA) hair as a pH and temperature doubly responsive particulate foam stabilizer. Reproduced with permission from ref 36. Copyright 2015, Royal Society. (b) Evolution of the foam volume as a function of both temperature and time for foam produced with fatty acid tubes, showing the thermoresponse of the foam. At 20 °C, foams are ultrastable because of the presence of fatty acid tubes inside the foam liquid channels. At 60 °C, the tubes reassemble into spherical micelles, leading to a drastic decrease in the foam volume. By decreasing the temperature to 20 °C, tubes are reformed in the foam liquid channels, and the foam volume remains constant again with time. Reproduced with permission from ref 45. Copyright 2011, Wiley-VCH. (c) Optical micrograph of a single bubble covered by adsorbed monolaurin solid particles dispersed in the squalane medium. Reproduced with permission from ref 48. Copyright 2006, American Chemical Society.

a pH-responsive polyacid, can function as an effective pHresponsive foam stabilizer.33 The isoelectric point of the PS particles is pH 3.5, and the particle size is approximately 800 nm at and above pH 5.0, whereas the size is substantially larger at and below pH 4.5. These results indicate that the particle surface becomes highly hydrophilic in a high pH environment owing to deprotonation of the carboxyl groups in the PAA colloidal stabilizer, and the particles are colloidally stable via both electrostatic and steric stabilization mechanisms. Conversely, when the carboxyl groups are protonated in a low-pH environment, the electrostatic and steric stabilization mechanisms no longer function, resulting in particle aggregation. It has been demonstrated that more than 90% of the PAA at the particle surface is protonated at and below pH 3.5 and that the particle surface is rendered relatively hydrophobic under these conditions, thus adsorbing at the gas−liquid interface to form foams that are stable for over 1 month. By contrast, at pH levels of 6 and above more than 90% of the carboxylic acids of PAA are deprotonated, and the particle surface becomes negatively charged and highly hydrophilic; therefore, the particles simply disperse in the continuous aqueous medium rather than adsorbing at the gas−liquid interface. Because bases can induce the disruption of bubbles and foams in the system described above, it is possible to produce foams exhibiting behavior complementary to those that are disrupted by the addition of acids. To attain an acid-induced

disruption, particles carrying base groups on their surfaces have been utilized. The bubbles and foams stabilized by these particles are stable when dispersed in basic aqueous media because the particles have hydrophobic surfaces with nonprotonated base groups and remain adsorbed at the gas−liquid interfaces. By contrast, the bubbles and foams rapidly disintegrate upon the addition of an acid because the particles will contain positively charged water-soluble protonated base groups with hydrophilic characteristics. PS particles carrying poly[2-(diethylamino)ethyl methacrylate] (PDEA) hair (PDEA−PS particles)40,41 can be utilized to produce particle-stabilized foams that break down upon acid addition.34,35 PDEA is a pH-responsive polybase with a pKa value of 7.3. In basic media, the amino groups of PDEA are nonprotonated, thus exhibiting hydrophobic characteristics; in addition, PDEA is insoluble in water under these conditions. Conversely, in acidic media, the amino groups of PDEA are protonated, and positively charged PDEA becomes a watersoluble polymer. The PDEA−PS particles closely reflect this property.42 At pH values greater than 8.0, the particle surfaces are hydrophobic, and they can adsorb at the gas−liquid interface, thereby stabilizing the bubbles and foams, which are stable for at least 1 month. Optical microscopy and scanning electron microscopy (SEM) studies indicated that aggregated PDEA−PS particles were adsorbed at the air−water interface and stabilized the aqueous foams. At pH 6.1 and 7.1, relatively D


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temperature (Figure 3a). It was demonstrated that the PDMA− PS particles can function as temperature-responsive stabilizers.36 PDMA has a low critical solution temperature (LCST) of 32 °C and a pKa of 7.1, and its hydrophilic−hydrophobic balance can be controlled by both temperature and pH. At and above pH 6.0, where the PDMA hairs were either nonprotonated or partially protonated, particle-stabilized foams were formed at both 23 and 55 °C. The foam prepared at 55 °C was the more stable of the two, lasting for at least 24 h, whereas the foam prepared at 23 °C destabilized within 24 h. SEM studies on dried foams confirmed that the PDMA−PS particles adsorbed at the air−water interface of the bubble as monolayers at 23 °C and as multilayers at 55 °C. At and below pH 5, where the hairs were positively charged, hydrophilic, and watersoluble, no foam was formed, irrespective of temperature. Ondemand defoaming could be rapidly induced by lowering the solution pH at both temperatures because of the in situ protonation of the PDMA hairs, thus prompting the PDMA− PS particles to desorb from the air−water interface and to disperse in continuous aqueous media. Recently, PDEA was found to be responsive to temperature and pH,43 although it had been known to display a response only to pH: PDEA shows an LCST at near neutral pH, and its hydrophilic−hydrophobic balance could be tuned by varying the temperature. (The LCST of PDEA at pH 6.86 was 41 °C.) This fact indicates that PDEA−PS particles can act as temperature-sensitive stabilizers for aqueous foams.44 Particlestabilized foams were formed, which coalesced with time, and the size of the bubble increased at 25 °C, where the PDEA is hydrated and water-soluble. At 40 and 45 °C, where the PDEA hairs were partially nonhydrated, foams were formed, and the coalescence and increase in bubble size were slower than those observed at 25 °C. At and above 50 °C, where the PDEA hairs were nonhydrated, the more stable creamlike foams were formed. SEM studies confirmed that the particles mainly adsorbed at the air−water interface as monolayers at 25 °C and as multilayers at and above 40 °C. A temperature-responsive morphological change in particles is another trigger that induces the destabilization of particlestabilized bubbles and foams (Figure 3b). Fatty acids with an aliphatic alkyl tail and a polar carboxylic acid group are wellknown to self-assemble to form materials with various shapes (e.g., spheres, rods, and tubes) in aqueous media, and their morphology can be controlled by a change in temperature. For example, 12-hydroxystearic acid and hexanolamine selfassemble to form micrometer-sized tubes in aqueous media at 20 °C, and the tubes change to nanometer-sized spherical micelles at 60 °C. At 20 °C, the fatty acid tubes adsorb at the air−water interface and form a gel-like network within the foam lamellas and in the plateau borders, resulting in the formation of ultrastable foams. Heating above 60 °C causes the tubes to convert to spherical micelles in aqueous media and desorb from the air−water interface, thus destroying the gel-like network, which results in a destabilization of the foam. Interestingly, when the temperature returned to 60 °C, the fatty acid tubes were reformed, and the foam became ultrastable again.45 Reversible foaming and defoaming cycles can be attained by other stimuli via the incorporation of internal heat sources in the fatty acid tube-stabilized foam system. Foams stabilized with a mixture of fatty acids and carbon black with photothermal properties are stable in the dark but are readily disrupted upon UV or solar light irradiation. Note that the incorporation of carbonyl iron particles into fatty acid-stabilized foam leads to

stable foams, which remain stable for at least 24 h, can be formed. SEM studies confirmed that the PDEA−PS particles were adsorbed at the air−water interface as a monolayer at pH 6.1. At pH 5.1 and 3.1, the particles have cationic water-soluble PDEA hairs with hydrophilic characteristics, which provide the particles with electrostatic and steric stabilizing effects; thus, the PDEA−PS particles do not adsorb at the gas−liquid interface to form foams but instead lead to macrophase separation of the air−aqueous dispersion of the particles. It was also shown that the foam stabilized by PDEA−PS particles at pH 9.0 was disrupted when the pH was lowered to 4.0 by acid addition, thus confirming their pH-responsive property. The foaming− defoaming cycle was repeatable. In a similar manner to that of the PDEA−PS particles, an acid-induced defoaming of a particle-stabilized foam system can be attained using particles with pH-responsive surfaces, namely, PS particles carrying poly[2-(dimethylamino)ethyl methacrylate] (PDMA) hair (PDMA−PS particles)36,37 and silica particles surface-treated with triamine and octyl organosilanes.38 In the above-described studies, solid particles with a non-pHresponsive core and a pH-responsive outer layer were used as a particulate stabilizer. By contrast, it is also possible to use particles with a pH-responsive core and a non-pH-responsive outer layer to produce pH-responsive bubbles and foams. In this context, Dupin et al.39 demonstrated that pH-responsive poly(2-vinylpyridine) (P2VP) particles with poly(ethylene glycol) (PEG) hairs functioned as an effective pH-responsive foam stabilizer. The surface of the P2VP particles displayed non-pH-responsive PEG and pH-responsive P2VP dual characteristics; hence, the particles exhibited pH-controllable wettability. The pH-responsive characteristics can be finely tailored by making excellent use of surface and polymer chemistries, and precise control of the stable/disrupted transition by a narrow pH transition range can be attained. The introduction of additional functionalities into the core components is also possible. Although pH-responsive foam systems are diverse and simple, a disadvantage exists regarding the use of acids/bases to tune the pH. After pH cyclings, the undesirable accumulation of salts produced via neutralization reactions could limit the application of these foam systems. In this context, carbon dioxide is a unique stimulus that is relatively inexpensive, biocompatible, benign, and easily applied. Carbon dioxide can dissolve in water to form carbonic acid and can be easily removed by bubbling with a different inert gas, such as nitrogen. Acidification−neutralization cycling can be conducted without any significant increase in the ionic strength, and this process is nonaccumulative. 3.2.2. Temperature-Responsive Particle-Stabilized Foams. Temperature can be used as an external stimulus to control the stability and structure of bubbles and foams (Figure 3). The advantage of a temperature stimulus is that its control is easily exercised without directly changing the chemical composition of the system, in contrast to a pH stimulus, which requires the addition of an acid/base. Thus, temperature is an attractive stimulus for the stability control of systems that are sensitive to pH or ionic strength. The introduction of thermoresponsive characteristics into bubble/foam systems can be achieved using thermoresponsive particles. Foams whose stability and structure can be tuned by temperature can be produced using particulate foam stabilizers whose hydrophilic−hydrophobic balance can be controlled by E


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Figure 4. Surfactant-responsive bubbles and foams: (a) Possible approaches to attaching colloidal particles at gas−liquid interfaces by tuning their surface-wetting properties. (i) Schematic illustration of the stabilization of gas bubbles with colloidal particles (the particle size is exaggerated for clarity). (ii) Adsorption of partially hydrophobic particles at the gas−liquid interface. (iii) Approaches used to tune the wetting properties of originally hydrophilic particles to illustrate the universality of the developed foaming method. The same principles can be easily extended to other types of particles using different surface modifiers as well as liquid and gaseous phases. Reproduced with permission from ref 51. Copyright 2006, Wiley-VCH. (b) Phase diagram of the armored bubble response to surfactant. C(1)critical signifies the critical surfactant concentration above which the armored bubbles are no longer stable to dissolution, and C(2)critical denotes the boundary of the uncrowding transition where the particles on the shell resume Brownian motion and the bubble reverts to a sphere. The lifetime of the bubble decreases as the surfactant concentration increases. Reproduced with permission from ref 61. Copyright 2006, American Chemical Society.

hydrophilic−hydrophobic balance of the interfaces. By adding surfactant to the particles, the surfactant adsorbs onto the particle surfaces, thus controlling the contact angle of the particles at the air−water interface. The adsorption of surfactant on the particles can make the particle surface hydrophilic or hydrophobic. If the surfactant makes a particle surface hydrophobic, then foamability and foam stability are improved (Figure 4a).51−56 The Studart and Gonzenbach group is one of the pioneers in this research area. This group demonstrated that particles with various surface chemistries can be partially hydrophobized with tailored surfactants to produce aqueous foams showing remarkable stability. Bubbles and foams were stabilized by such partially hydrophobized particles adsorbed at the air− water interface. Hydrophobization of the particle surface is

the production of foams that are responsive to temperature, light, and magnetic fields.46,47 Morphological change-induced defoaming can be applied to nonaqueous foam systems (Figure 3c).48−50 Shrestha et al. achieved foam stabilization in nonaqueous media with a monoglycerol fatty acid ester.48 When monolaurin and squalane are used as the foam stabilizer and the liquid phase, respectively, the monolaurin self-assembles to form needleshaped particles at room temperature that can adsorb at the gas−liquid interface, resulting in the formation of foam that can remain stable for 12 h or more. If the temperature increases, the monolaurin starts to dissolve into the medium, thus disrupting the foam. 3.2.3. Surfactant-Responsive Particle-Stabilized Bubbles and Foams. Surfactants adsorb at interfaces and change the F


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Figure 5. Magnetic field-responsive bubbles and foams: (a) Image sequence showing the clockwise rotation of a particle-stabilized bubble with the direction of the magnetic field gradient at 6 T/m. Reproduced with permission from ref 66. Copyright 2011, Elsevier. (b) Translation of magnetic bubbles of various sizes in a capillary tube as a function of magnetic field gradient. (i) No magnet, (ii) magnet at 14 mm away and (iii) magnet at 4 mm away from the bubbles. Reproduced with permission from ref 66. Copyright 2011, Elsevier. (c) (i) Wet foam prior to exposure to a magnetic field. The optical microscope image below shows that the foam bubbles are surrounded by iron particles. (ii) Collection of fresh foam in the direction of the magnet upon exposure to the field. The microscope picture shows the withdrawal and alignment of the magnetic particles. The bubbles are squeezed out of the particle mass and will undergo coalescence and popping. (iii) Breakdown of dry foam in the direction of the magnet upon exposure to the field. The micrograph shows the collection of the cellulose and iron particle mass as a whole. Reproduced with permission from ref 68. Copyright 2011, American Chemical Society.

and nonionic surfactant Triton X-100 (critical micelle concentration (cmc), 0.22−0.5 mM) as a bubble stabilizer and surfactant, respectively.61 The particle-stabilized nonspherical bubbles were prepared in a water medium, and then, an aqueous solution of surfactant was added. Triton X100 concentrations of 0.085 mM and higher caused the nonspherical bubble to quickly become spherical by ejecting the particles from the gas−liquid interface. Then, the spherical bubble proceeded to dissolve continuously until it disappeared completely. Interestingly, a high-aspect-ratio spherocylindrical bubble destabilizes at the axial ends and grows spherical caps, which eventually collapse to form a sphere31 because the hoop stress is twice as large as the axial stress. At Triton X-100 concentrations of between 0.066 and 0.0043 mM (below the cmc), the bubble maintains its nonspherical shape but starts ejecting particles while losing volume. A Triton X-100 concentration of 0.17 μM and lower results in no destabilization. Note that the stabilization−destabilization of particlestabilized foams can be cycled using both anionic and cationic surfactants.62 Negatively charged hydrophilic silica nanoparticles in water are initially hydrophobized in situ with a conventional cationic surfactant, rendering their surface hydrophobic, which leads to the formation of stable aqueous foams by shaking. The addition of an equal number of moles of an anionic surfactant to the aqueous foam leads to defoaming. Zhu et al. claimed that the trigger toward defoaming is the electrostatic interaction between the oppositely charged ionic surfactants rather than that between the cationic surfactant and the negatively charged particles. The added anionic surfactant forms ion pairs with the cationic surfactant, resulting in the desorption of the cationic surfactant from the particle surfaces

achieved through the adsorption of surfactants carrying a functional group that efficiently anchors on the particle surface and a hydrophobic part that remains in contact with the aqueous phase. Short-chain carboxylic acids, alkyl sulfosuccinates, alkyl sulfates, alkyl gallates, alkylamines, quaternary ammonium gemini surfactants, and nonionic surfactants can be utilized as surface modifiers that can in situ hydrophobize the particle surfaces. It is noteworthy that polymer nanoparticles can also work as a hydrophobizing agent along with the abovementioned molecular-level surfactants. Pelton and co-workers demonstrated that hydrophobic polymer nanoparticles with positive surface charges can adsorb onto much larger hydrophilic glass particles with negative surface charges, and the resulting composite particles can work as a stabilizer for aqueous bubbles and foams.57−59 If the surfactant causes the particle surface to become hydrophilic, then bubble/foam destabilization can be expected (Figure 4b). The Velev group demonstrated that the stability of a foam stabilized by rod-shaped particles can be significantly reduced by the addition of an anionic surfactant, sodium dodecyl sulfate (SDS).60 The SDS adsorbed in an oriented manner onto the particle surface (with the hydrophobic tail oriented toward and the hydrophilic head oriented away from the particle surface) and changed the particle surface properties from hydrophobic to hydrophilic in situ. Hence, the particles’ affinity for the interface decreased, consequently disrupting the foam. Note that SDS, which is typically used as an effective foaming agent, acts as a defoamer in this particle-stabilized foam system. The Stone group systematically investigated the effect of surfactant concentration on the structure and stability of individual particle-stabilized bubbles using 4.0 μm PS particles G


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Figure 6. Ultrasound-responsive bubbles: (a) Volume oscillations of particle-stabilized bubbles and ultrafast monolayer compression, buckling, and particle expulsion. The bubble is stabilized by 3 μm particles undergoing compression−expansion in ultrasound at 50 kHz. Reproduced with permission from ref 70. Copyright 2015, National Academy of Sciences. (b) Shape oscillations of particle-coated bubbles and directional particle expulsion. Examples of shape oscillations of particle-coated bubbles with different modes n observed in an experiment. R0 is the resting radius of the bubble. Scale bar = 80 μm. Reproduced with permission from ref 71. Copyright 2017, Royal Society of Chemistry. (c) Patterned particle desorption for nonspherical bubble oscillations. Shape oscillations of a bubble coated with 500 nm particles, with a dominant 4-fold mode developing over time and directing particle expulsion. Reproduced with permission from ref 70. Copyright 2015, National Academy of Sciences.

and dehydrophobization of the particles. Further addition of another trace amount of cationic surfactant leads to rehydrophobization of the particles in situ, thus restabilizing the foams. Along with the destabilization of particle-stabilized bubbles and foams by the addition of a surfactant, Subramaniam et al. showed that it is possible to produce colloidosomes filled both internally and externally with water by adding double-chain quaternary ammonium surfactant at twice the cmc to aqueous bubbles stabilized by micrometer-sized PS particles.63 During the colloidosome formation process, the particle assembled structure was maintained, allowing water to flow into the bubble. The study of this phenomenon has been limited to its observation, but it is hoped that further studies will reveal the mechanism by which water flows into the interior and the reason for the maintenance of the capsule structure by the particles. 3.2.4. Magnetic-Field-Responsive Particle-Stabilized Foams. Magnetic fields are of special interest as external stimuli because of their potential for noncontact stimulus transmission. Magnetic stimuli can remotely apply long-range and large forces in a controlled manner. These stimuli exhibit very low interactions with nonmagnetic materials and can penetrate most materials, including biological materials. The use of magnetic field-responsive particles as bubble and foam stabilizers allows the motions, shapes, and stabilities of the particles to be changed through the application of an external magnetic field (Figure 5). These magnetic particles have dual

functions as both the force mediator and the bubble/foam stabilizer. Soetanto and Watarai produced 10 μm magnetic-fieldresponsive bubbles by the adsorption of submicrometer-sized magnetite particles. The bubbles in water could move toward a magnet with a magnetic force of 2 kG. The bubbles also showed an excellent ultrasonic scattering property. The authors claimed that the magnetic microbubbles are a good candidate as an ultrasonic contrast agent for drug delivery systems.64 Campbell et al. demonstrated that magnetic microrods consisting of shellac or an ethyl cellulose matrix and a dispersion of magnetite nanoparticles could adsorb at the air−water interface to stabilize aqueous bubbles. It was found that the stabilized bubbles could be easily manipulated using a neodymium magnet and that 3 wt % magnetic nanoparticles in the microrods was sufficient for the manipulation.65 Rodrigues et al. demonstrated that magnetic nanoparticles with an iron oxide core and a silica shell morphology, whose surfaces are hydrophobized by treatment with octyltrimethoxysilane, could function as an effective stabilizer for an air-in-aqueous solution of ethanol bubbles/foams by adsorbing to the gas−liquid interface.66 The addition of ethanol (5−20 vol %) to the water medium is required for obtaining a suitable contact angle of the particles to the air−liquid interface (i.e., between 80° and 130°) to generate stable foams. Optical microscopy studies indicated that aggregates of magnetic nanoparticles are adsorbed at the air−liquid interface, and the bubbles display a lumpy surface morphology. The bubbles are stable against disruption for 1 year in the presence of a magnetic field (65 T/m) because the H


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Figure 7. Organic-solvent-responsive bubbles and foams: (a) Scheme for the fabrication of polymer hollow microspheres by the solvent treatment of aqueous bubbles stabilized with polystyrene (PS) particles. (b, e) Optical microscopy and (c, d, f, g) SEM images of PS-particle-stabilized bubbles (b−d) before and (e−g) after solvent treatment. (d, g) Magnified images of bubble surfaces. Reproduced with permission from ref 73. Copyright 2015, The Chemical Society of Japan.

adsorption energy of the particles at the interface is sufficiently high for the particles to remain at the interface. The bubbles can rotate by simply changing the direction of the magnetic field gradient, and they can move from top to bottom in a liquid prepared in a capillary tube by moving the magnet close to the bottom of the capillary. Furthermore, it was demonstrated that the magnet-particle-stabilized bubbles can be used as localized and remotely controllable heat sources by exposing them to an oscillating field. Zhao et al. investigated the motions of magnetic-field-responsive particle-stabilized bubbles and modeled these motions using Newton’s second law by balancing the magnetic, buoyancy, drag, and added mass forces.67

In the above-mentioned studies, the manipulation of bubbles was realized; however, the destruction of the bubbles was not achieved, possibly because the magnetic field did not overcome the adsorption energy of the particles at the gas−liquid interface because of the low magnetic characteristics of the particulate stabilizer. The first on-demand destruction of foams by a magnetic field was demonstrated by Lam et al. (Figure 5c).68,69 Foams were produced by aerating an aqueous mixture of oleic acid-coated iron particles (4.5 to 5.2 μm in size) and hypromellose phthalate particles, and it was confirmed that both the hypromellose phthalate particles and the iron particles adsorbed at the air−water surface to form foams that remained stable for several weeks. The creamed foam retained a large water content during the first 5 h after preparation; over a I


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Figure 8. Mechanical-stress-responsive bubbles: (a) Bubbles are covered with polystyrene particles. (i) Two initially spherical particle-stabilized bubbles. (ii) The bubbles are compressed between two glass plates, which exposes naked interfaces that spontaneously coalesce. (iii) The gas bubble maintains a stable ellipsoidal shape even after the side plates are removed. Scale bar = 100 μm. Reproduced with permission from ref 74. Copyright 2005, Nature Publishing Group. (b) Extremely high aspect ratio gas bubbles such as these millimeter-length spherocylinders formed from successive coalescence events. Scale bar = 400 μm. Reproduced with permission from ref 31. Copyright 2006, American Chemical Society.

in the interfacial area in the compression phase. This process results in a sufficiently large surface pressure within the particle monolayer to overcome the desorption energy. When the bubble experiences shape oscillations, particle desorption occurs at the antinodes of the deformed bubble. The antinodes are the points where the radial acceleration of the interface, the interface curvature, and the rate of change in area have their maximum values. By tuning the conditions, it is possible to remove the particles from the interface completely within a millisecond. Ultrasound, which causes mechanically forced desorption of the particles from the interface, offers the advantage of a system that is manipulated remotely and at precisely programmed times. The applications of this ultrasound-induced particle detachment phenomenon can range from controlled release to interfacial catalysis. 3.2.6. Organic-Solvent-Responsive Particle-Stabilized Bubbles. The rapid disruption of aqueous bubbles and foams can be induced by the addition of organic solvents with a lower surface tension than that of water; these solvents (partially) dissolve in aqueous media, thus decreasing the surface tension. The particle surface gains mediaphilic characteristics owing to the addition of organic solvents, and the particles desorb from the gas−liquid interface. When the organic solvents cause the particles to swell, the film formation of the particles occurred at the air bubble surfaces, leading to a reduced pore size among the particles and the formation of a rigid shell on the air bubbles after the removal of the solvent. In this context, polymer latex particles are attractive materials with film-forming ability that have been widely employed in the adhesive, paint, and paper industries.72 It has been demonstrated that PS particle-stabilized bubbles in an aqueous medium could be transformed into polymer hollow microspheres by the addition of dichloromethane (DCM) to an aqueous medium (Figure

period of 5 to 10 min, the iron particles alone were slowly drawn toward a magnet placed nearby, and defoaming was observed. However, 11 days after foam formation, the water content greatly decreased, and both the iron particles and the hypromellose phthalate particles were quickly drawn toward the magnet when it was placed nearby, resulting in defoaming within 1 to 3 s. This phenomenon may be useful in applications for the development of particle-stabilized foams characterized by noncontact defoaming. As described above, magnetic-particle-stabilized bubbles can be manipulated using a magnetic bar. The bubbles floating at the gas−liquid interface can be moved horizontally and can be rotated by moving the magnetic bar. The speed of the bubbles can be arbitrarily controlled by the speed of the magnet. Additionally, the bubbles can be dragged downward in the liquid phase by the placement of the magnetic bar underneath the liquid phase. When the magnet is removed, the bubbles float up to the planar gas−liquid interface because of buoyancy. Furthermore, the disruption of bubbles and the release of inner gas can be achieved under a high magnetic field. These features enable the bubbles to be moved from one spot to another without a guide and on demand by the release of gas. 3.2.5. Ultrasound-Responsive Particle-Stabilized Bubbles. Ultrasound is an attractive stimulus because of its noninvasiveness, cost effectiveness, and easily controlled intensity by tuning the frequency. Ultrasound can be utilized to change the size, shape, and stability of particle-stabilized bubbles (Figure 6). The Garbin group discovered that particle-stabilized bubbles experience volumetric oscillations and shape oscillations depending on the acoustic pressure.70,71 When the bubble maintains its spherical shape during volumetric oscillations (compression−expansion cycles), particles are expelled from the gas−liquid interface because of the decrease J


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7).73 The bubbles before DCM treatment were nearly spherical in a dispersed state in water, but greater deformation was observed after water evaporation. Capillary forces working among the particle-stabilized bubbles during drying should cause the bubbles to deviate from their nearly spherical shape. The PS particle layer that formed at the air−water interface of the bubble was soft and deformable, and it could not prevent bubble deformation during drying. However, the bubbles after DCM treatment retained their nearly spherical shapes during/ after drying. The PS particles at the bubble surface were swollen with DCM, and the PS molecules interdiffused among the particles to form a polymer film. The amount of added DCM is an important factor to control for the fabrication of polymer hollow microspheres: too much DCM results in a disruption of the particle-stabilized bubbles. 3.2.7. Mechanical-Stress-Responsive Particle-Stabilized Bubbles. Buoyancy influences the shape of particle-stabilized bubbles. When the particle-stabilized bubble is placed in a liquid medium, it rises up to the planar gas−liquid interface. When the particle layer on the bubble surface contacts the bulk gas phase, the particle layer deforms to produce a distinct flat facet. In this configuration, the particles form a bridge between the gas phase in the bubble and the bulk gas phase, with only a thin layer of liquid in between. Through the thin water film in the bridge, gas can diffuse out of the bubble directly into the bulk gas phase. It was confirmed that the bubble progressively deformed to a nonspherical shape by losing some inner gas. The buckling of the bubble surface and the stable nonspherical shapes indicated that the interface of the particle-stabilized bubble behaved in a solidlike manner because nonspherical shapes with isotropic surface tension in the equilibrated state were thermodynamically prohibited. This solidlike property of the bubble arises from the crowding of the adsorbed particles at the bubble interface.28,74,75 The bubbles cannot relax into a spherical shape, which is thermodynamically stable, because of the crowding of the particles or because of the rigid particle network (i.e., the solid particles were strongly adsorbed at the gas−liquid interface and could not be desorbed under ambient conditions). This unique behavior of particles highlights their difference from molecular surfactants, which are not crowded at the interface owing to a dynamic exchange between the adsorbed molecules and freely diffusing species into the continuous phase.76 The solidlike behavior of the particleadsorbed interface was also observed in particle-stabilized emulsions77,78 and liquid marble79 systems. Owing to the solidlike property of the particle-adsorbed gas−liquid interface, stable ellipsoidal- and spherocylinder-shaped bubbles with various aspect ratios can be fabricated by the fusion of multiple bubbles via squeezing them between two glass plates (Figure 8).31,74

regarding the design, fabrication, and engineering of stimuliresponsive materials. The development of a broad range of functional solid particles that can act as effective stimuli-responsive foam/ bubble stabilizers is particularly important. Edible particles, including fat crystals, proteins, and solid particles derived from vegetables, fruits, plants, animals, and insects, can serve as important stabilizers. Biodegradable/bioabsorbable particles fabricated from synthetic polymers and from grass and wood may also function as interesting foam/bubble stabilizers. For biorelated purposes, it is crucial to ensure the nontoxicity and biocompatibility of these stabilizers. Natural particles with a characteristic morphology, such as pollens and spores, are of interest as bubble/foam stabilizers. (Nano)composites and element-block polymer-based particles with unique magnetic and optical properties are expected to become more widely applied.80 Molecularly imprinted particles may be interesting foam stabilizers, and they can selectively absorb/adsorb and recover larger amounts of target materials from continuous liquid media compared to molecular-level surfactants. This function may be extended to applications in water purification and ion flotation technologies. Particles with a proper wettability with respect to the gas−liquid interface can function as a foam/bubble stabilizer; therefore, it is important to synthesize stimuli-responsive solid particles with a surface that has a suitable hydrophilic−hydrophobic balance and to develop surface modification technologies for these particles. In this context, synthetic polymer latex particles are especially attractive because their surface chemistries and hydrophilic− hydrophobic balance can be designed by utilizing various functional monomers and via post polymer reactions (e.g., esterification and hydrolysis) on demand. The development of foams/bubbles responsive to other stimuli, such as biorelated molecules/materials including proteins, viruses, bacteria, and cells, is also interesting. Sunlight-responsive particles with photothermal properties should be promising stabilizers for the fabrication of light-responsive foams/bubbles because the sun provides free, inexhaustible, and clean energy. Elucidating the correlations among the size, size distribution, and shape of the particles and the foamability as well as the properties of the foams/bubbles, including the size, size distribution, shape, stability, and responsiveness, will also be crucial. Recent years have witnessed the emergence of vigorous and growing research efforts into the development of microparticles and nanoparticles with different shapes,81,82 including fibers, rods, disks, bowls, snowmen, octopuses, and worms. The proliferation of foam-stabilizing particle types and properties is expected to introduce an expanding range of uses for particlestabilized foams utilizing their respective unique properties. The selections of inner gas and outer liquid are important factors requiring the development of stimuli-responsive bubbles/foams. It is possible to introduce stimuli-responsive properties into the outer liquid phase, which would in turn affect the stability and movement of bubbles/foams. For example, it will be possible to control the stability and movement (velocity and direction) of the bubbles by utilizing liquids whose viscosity can be tuned by an external stimulus (e.g., magnetorheological and electrorheological fluids). The introduction of dry liquids, which can also be stimulusresponsive, into the inner gas phase is possible. Furthermore, it is also possible to coat particle-stabilized bubbles with stimuliresponsive materials by chemical reactions in the liquid phase.

4. CONCLUDING REMARKS AND FUTURE RESEARCH DIRECTIONS There is a long history of ore flotation studies in which illdefined solid particles (nonuniform in shape and surface chemistry and polydisperse in size distribution) were used as foam/bubble stabilizers; currently, stimulus-responsive foams/ bubbles stabilized with well-defined particles are developed. Foams/bubbles that can respond to external stimuli are one of the most exciting and newly emerging scientific research areas and have many unexplored potential commercial applications. This evolving research field faces many exciting challenges K


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and to inspire their design philosophies toward reliable intelligent bubble/foam-based materials. The development of novel and functional multiple gas− liquid dispersed systems, such as liquid-in-gas-in-liquid (antibubbles), gas-in-liquid-in-gas, liquid-in-liquid-in-gas-inliquid, and gas-in-liquid-in-gas-in-liquid systems, will be possible by utilizing particle-stabilized bubbles/foams. Some of these systems can be fabricated only by using particlestabilized bubbles but not molecular surfactants. The introduction of stimuli-responsive properties to these dispersed systems will enable the stepwise release of materials. Particle-stabilized bubbles/foams share many similarities with particle-stabilized emulsions formed from oils and water, as they relate to the ability to achieve high stability and stable nonspherical shapes. Therefore, the principles established in stimuli-responsive particle-stabilized foam/bubble systems should also be applicable to other particle-stabilized soft dispersed systems, including Pickering-type emulsions,86,87 liquid marbles,88 and dry liquids.89,90 It is technically feasible to fabricate stimuli-responsive materials, including porous materials and microreactors, from foams and bubbles. The utilization of gas−water−solid contact lines as a chemical reaction platform can enable the fabrication of stimuli-responsive multiple catalysis reaction systems.38 These breakthroughs should be an important technological milestone that bodes well for the commercial exploitation of particle-stabilized foams/bubbles. Stimuli-responsive particlestabilized foams/bubbles are expected to function as a platform for smart soft materials, and a wide range of academic and industrial applications will be proposed. Interdisciplinary research is crucial to continue developing foam/bubble science and engineering.

External stimuli-induced chemical reactions at bubble surfaces will be a critical research topic. One of the important extensions of the research on stimuliresponsive foams/bubbles is the development of stimuliresponsive modes. Opening and closing of the particleadsorbed surface of the bubble are interesting modes that have been pursued in liquid marble systems.83 These modes can also be investigated using magnetic particulate bubble stabilizers. Self-oscillation of bubbles induced by external stimuli is another interesting mode, which can be achieved using self-oscillating materials.84 Another important extension is the development of bubbles/foams that are responsive to multiple stimuli. Creating synthetic bubble/foam systems capable of responding to multiple stimuli and resulting in one or more responses in a controllable and predictable manner still represents a significant challenge. Bubbles/foams that can be disrupted and release their inner gas materials on demand by selected external stimuli are examples of intelligent capsule materials. In this context, it is important to obtain a precise response and a wide range of stimulus-response pairs. It is also crucial to control the release kinetics of inner gaseous materials (i.e., as either a digital on/off signal or a slow release). Swelling and shrinking behaviors of stimuli-responsive polymeric materials at particle-stabilized bubble surfaces can control the permeability of a gas to the liquid phase and vice versa. Film formation on the bubble surfaces via annealing and coating of the particle-adsorbed surfaces, which can be used to control the pore size in the shell, is expected to lead to a slow release of inner gaseous materials and the slow diffusion of a liquid into the gas phase. The development of methods to apply the stimuli to bubbles/foams will be useful. If the size of a bubble ranges from the micrometer to centimeter scale, then it is possible to apply a stimulus to a small area (comparable to or smaller than the bubble size) of the bubble on site. By achieving these conditions, the simultaneous application of multiple stimuli to different positions of a single bubble will be possible, which should lead to the fabrication of lab-on/in-a-bubble systems. Furthermore, the development of multi-stimuliresponsive bubbles/foams that can be actuated and disrupted on demand by an independent external stimulus is also desirable and would enable advanced material delivery and release systems. The development of methods for characterizing particlestabilized bubbles/foams is also crucial. Imaging of the bubbles/ foams using the powerful techniques of stereomicroscopy, optical microscopy, laser microscopy, confocal laser scanning microscopy, SEM, and X-ray computed tomography is important for detailed characterization. In particular, laser microscopy, confocal laser scanning microscopy, SEM, and Xray computed tomography are useful methods to characterize the surface morphologies of bubbles with sizes ranging from nanometers to millimeters. The combination of microscopebased imaging techniques with high-speed cameras will allow an investigation of the dynamics of deformation, disruption, coalescence, and motion of the bubbles induced by external stimuli at the millisecond time scale.85 Cryo-electron microscopy studies on ultramicrotomed foam/bubble samples will be useful for characterizing foam/bubble nanomorphology in the future. Our understanding of the mechanism of formation, disruption, and deformation of bubbles/foams is substantially incomplete. Advances in characterization methods are expected to elucidate the relationships among the fabrication, structure, and properties of the foams/bubbles

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Syuji Fujii: 0000-0003-2562-9502 Notes

The authors declare no competing financial interest. Biographies

Syuji Fujii graduated from Kobe University (Ph.D. 2003). His postdoctoral studies were carried out at the University of Sussex (U.K.) from 2003 to 2004 and at the University of Sheffield (U.K.) from 2004 to 2006. He joined Osaka Institute of Technology as a lecturer in 2006 and was promoted to associate professor in 2013 and professor in 2017. His major research interests focus on synthetic L


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(11) Binks, B. P., Horozov, T. S., Eds.; Colloidal Particles at Liquid Interfaces; Cambridge University Press: Cambridge, U.K., 2006. (12) Garrett, P. R., Ed.;Defoaming: Theory and Industrial Applications; Surfactant Science Series 45; Marcel Dekker: New York, 1993. (13) Prud’homme, R. K., Khan, S. A., Eds.; Foams; Theory, Measurement and Applications; Surfactant Science Series 57; Marcel Dekker: New York, 1997. (14) Stuart, M. A. C.; Huck, W. T. S.; Genzer, J.; Muller, M.; Ober, C.; Stamm, M.; Sukhorukov, G. B.; Szleifer, I.; Tsukruk, V. V.; Urban, M.; Winnik, F.; Zauscher, S.; Luzinov, I.; Minko, S. Emerging Applications of Stimuli-Responsive Polymer Materials. Nat. Mater. 2010, 9, 101−113. (15) Yoshida, M.; Lahann, J. Smart Nanomaterials. ACS Nano 2008, 2, 1101−1107. (16) Fameau, A.-L.; Carl, A.; Saint-Jalmes, A.; von Klitzing, R. Responsive Aqueous Foams. ChemPhysChem 2015, 16, 66−75. (17) Binks, B. P.; Murakami, R. Phase Inversion of Particle-Stabilized Materials from Foams to Dry Water. Nat. Mater. 2006, 5, 865−869. (18) Davies, J. T.; Rideal, E. K. Interfacial Phenomena, 2nd ed.; Academic Press: New York, 1963. (19) Levine, S.; Bowen, B.; Partridge, S. J. Stabilization of Emulsions by Fine Particles I. Partitioning of Particles between Continuous Phase and Oil/Water Interface. Colloids Surf. 1989, 38, 325−343. (20) Schwarz, S.; Grano, S. Effect of Particle Hydrophobicity on Particle and Water Transport across a Floatation Froth. Colloids Surf., A 2005, 256, 157−164. (21) Ata, S.; Ahmed, N.; Jameson, G. J. The Effect of Hydrophobicity on the Drainage of Gangue Minerals in Flotation Froths. Miner. Eng. 2004, 17, 897−901. (22) Paunov, V. N.; Al-Shehri, H.; Horozov, T. S. Attachment of Composite Porous Supra-Particles to Air−Aater and Oil−Water Interfaces: Theory and Experiment. Phys. Chem. Chem. Phys. 2016, 18, 26495−26508. (23) Al-Shehri, H.; Horozov, T. S.; Paunov, V. N. Preparation and Attachment of Liquid-Infused Porous Supra-Particles to Liquid Interfaces. Soft Matter 2016, 12, 8375−8387. (24) Fujii, S.; Ryan, A. J.; Armes, S. P. Long-Range Structural Order, Moiré Pattern and Iridescence in Latex-Stabilized Foam. J. Am. Chem. Soc. 2006, 128, 7882−7886. (25) Fujii, S.; Iddon, P. D.; Ryan, A. J.; Armes, S. P. Highly Stable Aqueous Foams Stabilized Solely with Polymer Latex Particles. Langmuir 2006, 22, 7512−7520. (26) Fukuoka, K.; Tomikawa, A.; Nakamura, Y.; Fujii, S. Aqueous Foams Stabilized with Several Tens of Micrometer-Sized Polymer Particles: Effects of Surface Hydrophilic-Hydrophobic Balance on Foamability and Foam Stability. Chem. Lett. 2016, 45, 667−669. (27) Urban, D., Takamura, K., Eds.; Polymer Dispersions and Their Industrial Applications; Wiley-VCH: Weinheim, 2002. (28) Subramaniam, A. B.; Abkarian, M.; Stone, H. A. Controlled Assembly of Jammed Colloidal Shells on Fluid Droplets. Nat. Mater. 2005, 4, 553−556. (29) Buchcic, C.; Tromp, R. H.; Meinders, M. B. J.; Cohen Stuart, M. A. Assembly of Jammed Colloidal Shells onto Micron-Sized Bubbles by Ultrasound. Soft Matter 2015, 11, 1326−1334. (30) Park, J. I.; Nie, Z.; Kumachev, A.; Abdelrahman, A. I.; Binks, B. P.; Stone, H. A.; Kumacheva, E. A Microfluidic Approach to Chemically Driven Assembly of Colloidal Particles at Gas-Liquid Interfaces. Angew. Chem., Int. Ed. 2009, 48, 5300−5304. (31) Subramaniam, A. B.; Abkarian, M.; Mahadevan, L.; Stone, H. A. Mechanics of Interfacial Composite Materials. Langmuir 2006, 22, 10204−10208. (32) Kettlewell, S. L.; Schmid, A.; Fujii, S.; Dupin, D.; Armes, S. P. Is Latex Surface Charge an Important Parameter for Foam Stabilization? Langmuir 2007, 23, 11381−11386. (33) Binks, B. P.; Murakami, R.; Fujii, S.; Schmid, A.; Armes, S. P. pH-Responsive Aqueous Foams Stabilized by Ionizable Latex Particles. Langmuir 2007, 23, 8691−8694.

polymer chemistry, the design and characterization of polymer-based particles, biomimetics, and particle-stabilized soft dispersed systems including emulsions, foams, liquid marbles, and dry liquids.

Yoshinobu Nakamura graduated from Kobe University (Ph.D. 1983). He worked for Nitto Denko Co. from 1980 to 1997 and then joined Osaka Institute of Technology as an associate professor in 1997. He was promoted to professor in 2002. His major research interests focus on polymer-based composite materials and pressure-sensitive adhesives.

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ACKNOWLEDGMENTS This work was supported by JSPS-DAAD (Germany) and JSPS-OP (Australia) Bilateral Joint Research Projects and by a Grant-in-Aid for Scientific Research (B) (JSPS KAKENHI grant number JP16H04207) and Scientific Research on Innovative Areas “Engineering Neo-Biomimetics (No. 4402)” (JSPS KAKENHI grant numbers JP15H01602 and JP25120511), “New Polymeric Materials Based on ElementBlocks (no. 2401)” (JSPS KAKENHI Grant Numbers JP15H00767 and JP25102542), and “Molecular Soft Interface Science (no. 2005)” (JSPS KAKENHI grant number 23106720).

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REFERENCES

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Stimuli-Responsive Bubbles and Foams Stabilized with Solid Particles

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