Direct observation of giant Pickering emulsion and colloidosome droplet interaction and stability

Adsorption of sterically-stabilized diblock copolymer nanoparticles at the oil-water interface: effect of charged end-groups on interfacial rheology

The RAFT aqueous emulsion polymerization of either methyl methacrylate (MMA) or benzyl methacrylate (BzMA) is conducted at 70 °C using poly(glycerol monomethacrylate) (PGMA) as a water-soluble precursor to produce sterically-stabilized diblock copolymer nanoparticles of approximately 30 nm diameter. Carboxylic acid- or morpholine-functional RAFT agents are employed to confer anionic or cationic functionality at the ends of the PGMA stabilizer chains, with a neutral RAFT agent being used as a control. Thus the electrophoretic footprint of such minimally-charged model nanoparticles can be adjusted simply by varying the solution pH. Giant (mm-sized) aqueous droplets containing such nanoparticles are then grown within a continuous phase of n-dodecane and a series of interfacial rheology measurements are conducted. The interfacial tension between the aqueous phase and n-dodecane is strongly dependent on the charge of the terminal group on the stabilizer chains. More specifically, neutral nanoparticles produce a significantly lower interfacial tension than either cationic or anionic nanoparticles. Moreover, adsorption of neutral nanoparticles at the n-dodecane-water interface produces higher interfacial elastic moduli than that observed for charged nanoparticles. This is because neutral nanoparticles can adsorb at much higher surface packing densities owing to the absence of electrostatic repulsive forces in this case.

Buckling and Drying Kinetics of Particle-Stabilized Water Droplets Fully or Partially Immersed in an Oil Layer

We have investigated the effect of buckling of particle-stabilized water droplets on the drying kinetics. Particle-stabilized water droplets in an oil phase were prepared and the shrinking modes of the droplets during drying were controlled by the wettability of the particles. We obtained water droplets with and without buckling and used them in drying experiments. The drying times were comparable when the droplets were fully immersed in a thick oil layer. However, when the thickness of the oil layer was smaller than the droplet diameter, the buckled droplets showed faster drying. Observation of the reflection images around the droplets suggested that the buckled droplets preferentially shrank in the height direction, while the droplets without buckling isotropically shrank. Mathematical models that assumed diffusion of dissolved water molecules in the oil layer showed good agreement with the experimental data. The effective water-oil interfacial area was constant in the buckled droplets, whereas it shrank in the droplets without buckling. This would be a reason for the faster drying of the partially immersed buckled droplets. Particulate shells on liquid droplets could be used to enhance droplet drying.

Pickering Emulsifiers Based on Block Copolymer Nanoparticles Prepared by Polymerization-Induced Self-Assembly

Block copolymer nanoparticles prepared via polymerization-induced self-assembly (PISA) represent an emerging class of organic Pickering emulsifiers. Such nanoparticles are readily prepared by chain-extending a soluble homopolymer precursor using a carefully selected second monomer that forms an insoluble block in the chosen solvent. As the second block grows, it undergoes phase separation that drives in situ self-assembly to form sterically stabilized nanoparticles. Conducting such PISA syntheses in aqueous solution leads to hydrophilic nanoparticles that enable the formation of oil-in-water emulsions. Alternatively, hydrophobic nanoparticles can be prepared in non-polar media (e.g., n-alkanes), which enables water-in-oil emulsions to be produced. In this review, the specific advantages of using PISA to prepare such bespoke Pickering emulsifiers are highlighted, which include fine control over particle size, copolymer morphology, and surface wettability. This has enabled various fundamental scientific questions regarding Pickering emulsions to be addressed. Moreover, block copolymer nanoparticles can be used to prepare Pickering emulsions over various length scales, with mean droplet diameters ranging from millimeters to less than 200 nm.

Unique Oil-in-Brine Pickering Emulsion Using Responsive Antipolyelectrolyte Functionalized Latex: A Versatile Emulsion Stabilizer

A simple and straightforward approach to synthesize oil-in-water (O/W) emulsions under high salinity and temperature using zwitterion-functionalized latexes are presented in this work. First, well-defined functionalized latexes were synthesized by emulsifier-free emulsion copolymerization in the presence of precursor sulfobetaine comonomer using brine as a continuous phase. The surface-functionalized latex particles were then characterized by DLS, SEM, TEM, XPS, and TGA. The functionalized latex exhibited antipolyelectrolyte behavior in high salinity brine and at high temperatures. The effects of salinity, temperature, and pH on the long-term stability of the particles were investigated. Further, to evaluate the potential in high salinity brine and high temperature, the saltphilic functionalized latexes were utilized to stabilize the oil/brine (O/W) interface without any other additives. The latex enabled the formation of a stable Pickering emulsion system with low solid content (<0.02% w/w) in the presence of 50% v/v n-decane. The functionalized latexes were self-assembled at the O/W interface as a spherical colloidosome in high salinity brine through hydrophobic interactions and irreversible adsorption. The supraparticles were imaged with SEM, providing an insight that the exterior of the emulsion droplets is stabilized by the saltphilic latex particles, forming a protective layer at the oil-water interface through electrostatic repulsion. The antipolyelectrolyte latex can be utilized as a novel emulsion stabilizer, which can provide a versatile alternative for applications in a complex environment such as high salinity, temperature, and low or high pH.

Stabilization of Pickering Emulsions by Hairy Nanoparticles Bearing Polyanions

Pickering emulsions are increasingly applied in drug delivery, oil-water separation, composite materials preparation, and other fields. However, systematic studies on the stabilization of Pickering emulsions to satisfy the growing application demands in multiple fields with long-term conservation are rare. Compared to conventional solid nanoparticles, polyanion-modified hairy nanoparticles are more stable in practical environments and are investigated in this study. Poly (sodium p-styrenesulfonate) was grafted to a polystyrene (PS) core via a photoemulsion polymerization. A hairy nanoparticle bearing polyanions called poly (sodium p-styrenesulfonate) brush (PS@PSS) was synthesized. The size and uniformity of the Pickering emulsions stabilized by PS@PSS were investigated via a polarizing microscope. The stability of Pickering emulsions were optimized by adjusting critical factors like ultrasonic power and time, standing time, oil phases, salt concentration, and water:oil ratio. Results indicated that the Pickering emulsions could be stabilized by PS@PSS nanoparticles, which showed remarkable and adjustable partial wetting properties. It was found that the optimized conditions were ultrasonic power of 150 W, ultrasonic time of 3 min, salt concentration of 0.1 mM, oil phase of hexadecane, and water:oil ratio of 1:1. The formation and stability of Pickering emulsion are closely related to the hairy poly (sodium p-styrenesulfonate) brush layer on the nanoparticle surface.

Giant Pickering Droplets: Effect of Nanoparticle Size and Morphology on Stability

The interaction between a pair of millimeter-sized nanoparticle-stabilized n-dodecane droplets was analyzed using a high-speed video camera. The droplets were grown in the presence of either poly(glycerol monomethacrylate)-poly(benzyl methacrylate) (PGMA-PBzMA) diblock copolymer spheres or poly(glycerol monomethacrylate)-poly(2-hydroxypropyl methacrylate)-poly(benzyl methacrylate) (PGMA-PHPMA-PBzMA) triblock copolymer worms prepared by polymerization-induced self-assembly. The effect of nanoparticle morphology on droplet coalescence was analyzed by comparing 22 nm spheres to highly anisotropic worms with a mean worm width of 26 nm and comparable particle contact angle. Both morphologies lowered the interfacial tension, providing direct evidence for nanoparticle adsorption at the oil-water interface. At 0.03 w/v % copolymer, an aging time of at least 90 s was required to stabilize the n-dodecane droplets in the presence of the worms, whereas no aging was required to produce stable droplets when using the spheres, suggesting faster diffusion of the latter to the surface of the oil droplets. The enhanced stability of the sphere-coated droplets is consistent with the higher capillary pressure in this system as the planar interfaces approach. However, the more strongly adsorbing worms ultimately also confer stability. At lower copolymer concentrations (≤0.01 w/v %), worm adsorption promoted droplet stability, whereas the spheres were unable to stabilize droplets even after longer aging times. The effect of mean sphere diameter on droplet stability was also assessed while maintaining an approximately constant particle contact angle. Small spheres of either 22 or 41 nm stabilized n-dodecane droplets, whereas larger spheres of either 60 or 91 nm were unable to prevent coalescence when the two droplets were brought into contact. These observations are consistent with the greater capillary pressure stabilizing the oil-water interfaces coated with the smaller spheres. Addition of an oil-soluble polymeric diisocyanate cross-linker to either the 60 or the 91 nm spheres produced highly stable colloidosomes, thus confirming adsorption of these nanoparticles.

Preparation and attachment of liquid-infused porous supra-particles to liquid interfaces

We prepared model porous composite supra-particles and investigated the effect of the initial infused fluid phase on their attachment at the liquid-fluid interface. We used a simple method for fabrication of millimetre-sized spherical porous supra-particles from much smaller monodisperse latex microparticles as building blocks by evaporation of a polystyrene sulphate latex suspension on a hot super-hydrophobic surface. We annealed the dried supra-particles at the polymer's glass transition temperature to fuse partially their latex particle building blocks. Spherical porous supra-particles were produced above 40 wt% initial concentration of the latex particles in the suspension, which had a rough surface, with a porous and amorphous structure. We controlled the supra-particle size by varying the initial volume of the latex suspension drop, the latex particle concentration and the drop evaporation temperature. This preparation technique allowed limited control over the porosity of the supra-particles by varying the initial concentration of the latex particle suspension, the rate of evaporation and the annealing temperature. We characterised the surface morphology and the inner structure of supra-particles by SEM imaging. We report for the first time results of an MRI study of supra-particles attached to an air-water or an oil-water interface, which indicated that only the surface layer of the building block particles attaches to the liquid interface while the pore fluid was not displaced by the outer fluid. We observed that supra-particles infused with water had different wettability and attachment positions at the oil-water interface compared with the same particles infused with oil. Similarly, the infusion of the porous supra-particles with water led to a different attachment at the air-water interface compared to the attachment of the same supra-particle when dry. The fundamental importance of this result is that the porous particles (or colloid particle agglomerates) may give an oil-in-water or water-in-oil Pickering emulsion depending on whether they are initially impregnated with oil or water. The results of this study are relevant for particle-stabilised emulsions and foams in a range of pharmaceutical, food and cosmetic formulations as well as ore flotation.

Controlling Pickering Emulsion Destabilisation: A Route to Fabricating New Materials by Phase Inversion

The aim of this paper is to review the key findings about how particle-stabilised (or Pickering) emulsions respond to stress and break down. Over the last ten years, new insights have been gained into how particles attached to droplet (and bubble) surfaces alter the destabilisation mechanisms in emulsions. The conditions under which chemical demulsifiers displace, or detach, particles from the interface were established. Mass transfer between drops and the continuous phase was shown to disrupt the layers of particles attached to drop surfaces. The criteria for causing coalescence by applying physical stress (shear or compression) to Pickering emulsions were characterised. These findings are being used to design the structures of materials formed by breaking Pickering emulsions.

Behavior of Bubble Interfaces Stabilized by Particles of Different Densities

Stability of bubbles laden with particles of different densities was investigated. Capillary-held bubbles were produced and coated with particles across the density range of 1.2-3.6 g·cm(-3). The materials used were poly(methyl methacrylate) (PMMA), glass, and anatase. The interaction of the bubbles, once brought into contact, was monitored using high-speed video recording. Visual inspection indicated that denser particles were more easily displaced during the contact of the bubbles and therefore the PMMA particles provided a particle barrier more resistant to coalescence. The coalescence events yielded information on the surface properties of the bubble and the detachment of particles. The attached particles commonly dampen the oscillation of the coalesced bubbles through viscous drag and change in the surface properties (e.g., area-exclusion principle). The dampening of the oscillation generally leads to a reduced mass of particles detaching from the bubble surface. It was found that the different materials investigated did not offer clear evidence of the effect of particle detachment on the bubble surface properties in the present systems. On the other hand, the detachment of different particle materials seemed to be consistent with one another when comparing the attachment and detachment forces exerted on the particles based on their density, size, and hydrophobicity. It was concluded that particles of lower density are more effective in stabilizing interfaces, and thus particle density is an important parameter in the selection of materials for the handling of dispersions.

Giant pH-responsive microgel colloidosomes: preparation, interaction dynamics and stability

The interactions of two oil droplets grown in the presence of swollen, lightly cross-linked cationic poly(tert-butylamino)ethyl methacrylate (PTBAEMA) microgels was monitored using a high-speed video camera. Three oils (n-dodecane, isopropyl myristate and sunflower oil) were investigated, each in the absence and presence of an oil-soluble cross-linker [tolylene 2,4-diisocyanate-terminated poly(propylene glycol), PPG-TDI]. Adsorption of the swollen microgel particles was confirmed by interfacial tension, interfacial elasticity and dilational viscosity measurements on single pendant oil droplets, and assessment of the oscillatory dynamics for coalescing droplet pairs. Like the analogous bulk emulsions, particle adsorption alone did not prevent coalescence of pairs of giant Pickering emulsion droplets. However, prior addition of surface-active PPG-TDI cross-linker to the oil phase results in the formation of highly stable microgel colloidosomes via reaction with the secondary amine groups on the PTBAEMA chains. Colloidosome stability depended on the age of the oil-water interface. This reflects a balance between the adsorption kinetics of the PPG-TDI cross-linker and the microgel particles, each of which must be present at the interface to form a stable colloidosome. Colloidosome formation was virtually instantaneous in n-dodecane, but took up to 120 s in the case of isopropyl myristate. The impact of an acid-induced latex-to-microgel transition on the interaction of giant colloidosomes (originally prepared at pH 10 using isopropyl myristate) was also studied. This acid challenge did not result in coalescence, which is consistent with a closely-related study (A. J. Morse et al., Langmuir, 2014, 30(42), 12509-12519). No evidence was observed for inter-colloidosome cross-linking, which was attributed to retention of an aqueous film between the adjacent pair of colloidosomes.

The roles of particles in multiphase processes: Particles on bubble surfaces

Particle-stabilised foams (or froths) form the fundamental framework of industrial processes like froth flotation. This review provides an overview of the effects of particles on bubble surfaces. The characteristics of the particles have a profound effect on the stability of the bubbles although the stabilisation mechanisms may differ. It is well known that layers of particles may provide a steric barrier between two interfaces, which prevents the coalescence of bubbles. Although perhaps considered of lesser importance, it is interesting to note that particles may affect the bubble surface and momentarily suppress coalescence despite being absent from the film separating two bubbles. Foams are at best metastable and coalescence occurs to achieve a state of minimum energy. Despite this, particles have been reported to stabilise bubbles for significant periods of time. Bubble coalescence is accompanied by a release of energy triggered by the sudden change in surface area. This produces a distinctive oscillation of the bubble surface, which may be influenced by the presence of incompressible particles yielding unique surface properties. A survey of the literature shows that the properties of these composite materials are greatly affected by the physicochemical characteristics of the particles such as hydrophobicity and size. The intense energy released during the coalescence of bubbles may be sufficient to expel particles from the bubble surface. It is noted that the detachment of particles may preferentially occur from specific locations on the bubble surface. Examination of the research accounts again reveals that the properties of the particles may affect their detachment upon the oscillation of the bubble surface. However, it is believed that most parameters affecting the detachment of particles are in fact modifying the dynamics of the three-phase line of contact. Both the oscillation of a coalescing bubble and the resulting detachment of particles are highly dynamic processes. They would greatly benefit from computer simulation studies.

Liquid marble and water droplet interactions and stability

The interactions between two individual water droplets were investigated in air using a combination of coalescence rig and high speed video camera. This combination allows the visualization of droplet coalescence dynamics with millisecond resolution which provides information on droplet stability. Bare water droplets coalesced rapidly upon contact, while droplet stability was achieved by coating the droplets with polystyrene particles carrying pH-responsive poly[2-(diethylamino)ethyl methacrylate] hairs (PDEA-PS particles) to form liquid marbles. The asymmetric interaction of a water droplet (pH 3 or 10) armoured with the PDEA-PS particles (liquid marble) with a bare droplet at pH 3 exhibited intermediate stability with coalescence observed following an induction time. The induction time was longer for the pH 10 liquid marble, where the PDEA-PS particles have a hydrophobic surface, than in the case of a pH 3 liquid marble, where the PDEA-PS particles have a hydrophilic surface. Furthermore, film formation of PDEA-PS particles on the liquid marble surface with toluene vapour confirmed capsule formation which prevented coalescence with the neighbouring water droplet instead wetting the capsule upon contact within 3 milliseconds. This study illuminates the stability of individual particle-stabilized droplets and has potential impact on processes and formulations which involve their interaction.

Arrested coalescence behaviour of giant Pickering droplets and colloidosomes stabilised by poly(tert-butylaminoethyl methacrylate) latexes

The coalescence of two oil droplets grown at pH 10 in the presence of lightly cross-linked 260 nm diameter charge-stabilised poly(tert-butylamino)ethyl methacrylate (PTBAEMA) latexes was monitored using a high-speed video camera. Three model oils (n-dodecane, isopropyl myristate and sunflower oil) were investigated, each in the absence and presence of an oil-soluble cross-linker [tolylene 2,4-diisocyanate-terminated poly(propylene glycol), PPG-TDI]. In the absence of PPG-TDI, rapid coalescence was observed for giant PTBAEMA-stabilised Pickering oil droplets, which exhibited faster coalescence times compared to bare oil droplets. However, an increase in the damping coefficients for coalescing Pickering droplets (compared to those of bare oil droplets) indicated PTBAEMA latex particle adsorption. Addition of PPG-TDI cross-linker to oil droplets in the absence of latex particles led to a reduction in the interfacial tension confirming its surface-active nature. The oil-soluble PPG-TDI reacts with the secondary amine groups on the PTBAEMA latex, producing giant colloidosomes that remain stable to coalescence when brought into contact. This stability to coalescence was not observed for bare oil droplets in the presence of PPG-TDI, confirming that the cross-linked latex particles at the interface provide the additional stability. Finally, interactions between asymmetric n-dodecane droplets were examined. Adding oil-soluble cross-linker to only one droplet resulted in "arrested coalescence" behaviour in the presence of PTBAEMA latex particles. In this context, the droplet ageing time was found to be critical and is attributed to the relatively slow particle adsorption kinetics. Ageing times of less than 60 s led to catastrophic droplet coalescence, whereas ageing times longer than 60 s indicated cross-linker diffusion from one droplet to the other, which produced inter-cross-linked colloidosomes. Arrested coalescence was only observed for ageing times of approximately 60 s.

Problematic stabilizing films in petroleum emulsions: shear rheological response of viscoelastic asphaltene films and the effect on drop coalescence

Adsorption of asphaltenes at the water-oil interface contributes to the stability of petroleum emulsions by forming a networked film that can hinder drop-drop coalescence. The interfacial microstructure can either be liquid-like or solid-like, depending on (i) initial bulk concentration of asphaltenes, (ii) interfacial aging time, and (iii) solvent aromaticity. Two techniques--interfacial shear rheology and integrated thin film drainage apparatus--provided equivalent interface aging conditions, enabling direct correlation of the interfacial rheology and droplet stability. The shear rheological properties of the asphaltene film were found to be critical to the stability of contacting drops. With a viscous dominant interfacial microstructure, the coalescence time for two drops in intimate contact was rapid, on the order of seconds. However, as the elastic contribution develops and the film microstructure begins to be dominated by elasticity, the two drops in contact do not coalescence. Such step-change transition in coalescence is thought to be related to the high shear yield stress (~10(4) Pa), which is a function of the film shear yield point and the film thickness (as measured by quartz crystal microbalance), and the increased elastic stiffness of the film that prevents mobility and rupture of the asphaltene film, which when in a solid-like state provides an energy barrier against drop coalescence.

Hollow colloidosomes prepared using accelerated solvent evaporation

We demonstrate a new, scalable, simple, and generally applicable two-step method to prepare hollow colloidosomes. First, a high volume fraction oil-in-water emulsion was prepared. The oil phase consisted of CH2Cl2 containing a hydrophobic structural polymer, such as polycaprolactone (PCL) or polystyrene (PS), which was fed into the water phase. The water phase contained poly(vinylalcohol), poly(N-isopropylacrylamide), or a range of cationic graft copolymer surfactants. The emulsion was rotary evaporated to rapidly remove CH2Cl2. This caused precipitation of PCL or PS particles which became kinetically trapped at the periphery of the droplets and formed the shell of the hollow colloidosomes. Interestingly, the PCL colloidosomes were birefringent. The colloidosome yield increased and the polydispersity decreased when the preparation scale was increased. One example colloidosome system consisted of hollow PCL colloidosomes stabilized by PVA. This system should have potential biomaterial applications due to the known biocompatibility of PCL and PVA.

An investigation of bubble coalescence and post-rupture oscillation in non-ionic surfactant solutions using high-speed cinematography

Most processes involving bubbling in a liquid require small bubbles to maximise mass/energy transfer. A common method to prevent bubbles from coalescing is by the addition of surfactants. In order to get an insight into the coalescence process, capillary bubbles were observed using a high speed cinematography. Experiments were performed in solutions of 1-pentanol, 4-methyl-2-pentanol, tri(propylene glycol) methyl ether, and poly(propylene glycol) for which information such as the coalescence time and the deformation of the resultant bubble upon coalescence was extracted. It is shown in this study that the coalescence time increases with surfactant concentration until the appearance of a plateau. The increase in coalescence time with surfactant concentration could not be attributed only to surface elasticity. The oscillation of the resultant bubble was characterised by the damping of the oscillation. The results suggested that a minimum elasticity is required to achieve an increased damping and considerable diffusion has a detrimental effect on the dynamic response of the bubble, thereby reducing the damping.

Direct observation of individual particle armored bubble interaction, stability, and coalescence dynamics

The interactions between two individual particle-stabilized bubbles were investigated, in the absence of surfactant, using a combination of coalescence rig and high-speed video camera. This combination allows the visualization of bubble coalescence dynamics which provide information on bubble stability. Experimental data suggested that bubble stability is enhanced by both the adsorption of particles at the interface as indicated by the long induction time and the increase in damping coefficient at high surface coverage. The interaction between an armored bubble and a bare bubble (asymmetric interaction) can be destabilized through the addition of a small amount of salt, which suggested that electrostatic interactions play a significant role in bubble stability. Interestingly, the DLVO theory cannot be used to describe the bubble stability in the case of a symmetric interaction as coalescence was inhibited at 0.1 M KCl in both the absence and presence of particles at the interfaces. Furthermore, bubbles can also be destabilized by increasing the particle hydrophobicity. This behavior is due to thinner liquid films between bubbles and an increase in film drainage rate. The fraction of particles detached from the bubble surface after film rupture was found to be very similar within the range of solution ionic strength, surface coverage, and particle hydrophobicity studied. This lack of dependence implies that the kinetic energy generated by the coalescing bubbles is larger than the attachment energy of the particles and dominates the detachment process. This study illuminates the stability behavior of individual particle-stabilized bubbles and has potential impact on processes which involve their interaction.