Mechanics of interfacial composite materials

Model aggregated 2D suspensions in shear and compression: From a fluid layer to an auxetic interface?

HYPOTHESIS

Particle-laden interfaces play a crucial role in engineering stability of multiphase systems. However, a full understanding of the mechanical properties in shear and compression, especially in relation to the underlying microstructural changes, is as yet lacking. In this study, we investigate the interfacial rheological moduli in heterogeneous networks of aggregated 2D suspensions using different deformation modes and relate these moduli to changes in the microstructure.

EXPERIMENTS

Interfacial rheological experiments were conducted at different surface coverages and clean kinematic conditions, namely in (i) simple shear flow in a modified double wall-ring geometry and (ii) isotropic compression in a custom-built radial trough, while monitoring the evolution of the microstructure.

FINDINGS

The compressive moduli increase non-monotonically with decreasing void fraction, reflecting the combined effect of aggregate densification and reduction of void structures, with rotation of rigid clusters playing a significant role in closing voids. However, the shear moduli increase monotonically, which correlates with the increase in fractal dimension of the aggregates making up the backbone network. We also observe that these interfaces act as 2D auxetic materials at intermediate coverages, which is surprising given their amorphous structure. This finding has potential implications for the resilience of particle-coated bubbles or droplets subjected to time-varying compression-expansion deformations.

Rheology of a 2D granular film

We study experimentally the rheology of a macroscopic particle-laden soap film, designated as a "Granular Film", in the simple shear configuration. Macroscopic particles are dispersed in a soap film, while being large enough that they bridge both fluid interfaces. We simultaneously perform macroscopic rheological measurements with a classical rheometer and investigate interactions at the particle scale with a camera underneath the film. The determination of the velocity field of the grains reveals the presence of an inhomogeneous shear within the granular film. Trying to correlate both measurements unveils the non-locality of the rheology of the granular film: similar to what has been observed in a dry granular material, we find an highly-sheared zone close to the moving wall contrasting with a large quasistatic area. This behavior can be accounted for through extended kinetic theory and correlated with a transition in the dominant component of the stress.

Synthesis of Wrinkle-Free Metallic Thin Films in Polymer by Interfacial Instability Suppression with Nanoparticles

Synthesis of a smooth conductive film over an elastomer is vital to the development of flexible optics and wearable electronics, but applications are hindered by wrinkles and cracks in the film. To date, a large-scale wrinkle-free film in an elastomer has yet to be achieved. We present a robust method to fabricate wrinkle-free, stress-free, and optically smooth thin film in elastomer. Targeting underlying mechanisms, we applied nanoparticles between the film and elastomer to jam the interface and subsequently suppress interfacial instabilities to prevent the formation of wrinkles. Using polydimethylsiloxane (PDMS) and parylene-C as a model system, we have synthesized large-scale (>10 cm) wrinkle-free Al film over/in PDMS and demonstrated the principle of interface jamming by nanoparticles. We varied the jammer layer thickness to show that, as the layer exceeds a critical thickness (e.g., 150 nm), wrinkles are successfully suppressed. Nano-indentation experiments revealed that the interface becomes more elastic and less viscoelastic with respect to the jammer thickness, which further supports our assertion of the wrinkle suppression mechanism. Since the film was embedded in a polymer matrix, the resultant film was highly deformable, elastic, and optically smooth with applications for deformable optical sensors and actuators.

Influence of particle wettability on foam formation in honey

The rising level of obesity is often attributed to high sugar and/or fat consumption. Therefore, the food industry is constantly searching for ways to reduce or eliminate sugar or fat in food products. Therefore, honey foam, which contains little sugar and no fat, can be used as cake, cracker or bread spread instead of butter or margarine which contains a substantial amount of fat or jam that contains a substantial amount of sugar. Small solid particles (nanometers to micrometers) of suitable wettability are now considered outstanding foam-stabilizing agents. However, while the degree of particle wettability necessary to obtain very stable aqueous and nonaqueous foams is well-known, that needed to obtain very stable honey foam is unknown. In this study, the influence of the degree of wettability of fumed silica particles, indicated by their % SiOH (14-100), was investigated in honey in relation to foam formation and foam stability. The honephilic particles (61%-100% SiOH) formed particle dispersion in honey, while foams were obtained with the honephobic particles (14%-50% SiOH). The thread-off between particle dispersion and foam formation occurs at 50% SiOH, meaning foam formation in honey is possible when the particles are at least 50% honephobic. At relatively low particle concentration <1 wt.%, foam volume decreases with increasing honephobicity, but increases with honephobicity at relatively high concentration >1 wt.%. Also, as particle concentration increases, the shape of the air bubbles in the foam changes from spherical to non-spherical. After a little drainage, the foams remain stable to drainage and did not coalesce substantially for more than six months. These findings will guide the formulation of edible Pickering honey foams.

Generating Lifetime-Enhanced Microbubbles by Decorating Shells with Silicon Quantum Nano-Dots Using a 3-Series T-Junction Microfluidic Device

Long-term stability of microbubbles is crucial to their effectiveness. Using a new microfluidic device connecting three T-junction channels of 100 μm in series, stable monodisperse SiQD-loaded bovine serum albumin (BSA) protein microbubbles down to 22.8 ± 1.4 μm in diameter were generated. Fluorescence microscopy confirmed the integration of SiQD on the microbubble surface, which retained the same morphology as those without SiQD. The microbubble diameter and stability in air were manipulated through appropriate selection of T-junction numbers, capillary diameter, liquid flow rate, and BSA and SiQD concentrations. A predictive computational model was developed from the experimental data, and the number of T-junctions was incorporated into this model as one of the variables. It was illustrated that the diameter of the monodisperse microbubbles generated can be tailored by combining up to three T-junctions in series, while the operating parameters were kept constant. Computational modeling of microbubble diameter and stability agreed with experimental data. The lifetime of microbubbles increased with increasing T-junction number and higher concentrations of BSA and SiQD. The present research sheds light on a potential new route employing SiQD and triple T-junctions to form stable, monodisperse, multi-layered, and well-characterized protein and quantum dot-loaded protein microbubbles with enhanced stability for the first time.

Stable liquid foams from a new polyfluorinated surfactant

Liquid foams exhibiting long-term stability are a key-challenge in material design. Based on this perspective, new pyridinium polyfluorinated surfactants were synthesized from simple building blocks enabling unusually stable liquid foams. While the batch-generated foams were used for qualitative foaming evaluation, microfluidics allowed a quantitative insight into the aging effects of monodisperse foams.

Shear-induced deformation and interfacial jamming of solid-stabilized droplets

We use rheo-microscopy to directly investigate the dynamics of solid-stabilized droplets subjected to shear flow of a surrounding bulk fluid. In our system, the stabilizing particles are weakly attractive through the continuous fluid phase and along the droplet interface. Under shear, droplets stabilized by these particles at near-complete surface coverage exhibit a number of previously unforeseen phenomena, including negative-then-positive deviations from the predictions of Taylor and the behavior of bare droplets, evolution toward spherocylindrical shapes, and an earlier onset of rupture than their bare counterparts, which we explain in light of the weak attractive interparticle interactions along the droplet interface. We also demonstrate the formation of long-lived anisotropic particle-coated droplets by flow cessation, and provide evidence that this is due to the formation of a jammed, disordered particle network along the interface at surface coverage lower than the starting conditions. Importantly, these newly observed phenomena are shown to be sensitive to the droplets' initial surface coverage. Our findings provide new technologically-relevant insights into the physics of particle-coated droplets under fluidic or other external stresses, and introduce avenues for future research to better understand the roles of interparticle interactions and surface coverage in mediating the interfacial rheology of particle-laden interfaces and solid-stabilized emulsions.

Microbubbles as Heterogeneous Nucleation Sites for Crystallization in Continuous Microfluidic Devices

Injecting a stream of microbubbles and thereby introducing a heterogeneous interface is proposed for enhancing nucleation and controlling particle formation in continuous microfluidic devices. Different gas and liquid flow rates were investigated to establish the two-phase flow regime map and to identify the optimum characteristics for microbubble flow. Subsequently, the effect of microbubbles was studied for the cooling crystallization of paracetamol. An enhanced nucleation rate compared to that in the operation without bubbles was observed and the presence of microbubbles results in the formation of more crystals, which indicates that nucleation is faster than that in operation without bubbles, i.e., the metastable zone width is reduced. Determining the crystal yield confirmed that a larger mass of crystals is obtained in a two-phase flow with microbubbles. Furthermore, results showed that the presence of microbubbles allows crystallizing continuously without clogging of the microreactor.

Nanoparticle Assembly at Liquid-Liquid Interfaces: From the Nanoscale to Mesoscale

In the past few decades, novel syntheses of a wide range of nanoparticles (NPs) with well-defined chemical composition and structure have opened tremendous opportunities in areas ranging from optical and electronic devices to biomedical markers. Controlling the assembly of such well-defined NPs is important to effectively harness their unique properties. The assembly of NPs at liquid-liquid interfaces is becoming a central topic both in surface and colloid science. Hierarchical structures, including 2D films, 3D capsules, and structured liquids, have been generating significant interest and are showing promise for physical, chemical, and biological applications. Here, a brief overview of the development of the self-assembly of NPs at liquid-liquid interfaces is provided, from theory to experiment, from synthetic NPs to bio-nanoparticles, from water-oil to water-water, and from "liquid-like" to "solid-like" assemblies.

Effects of Contact Angle and Flocculation of Particles of Oligomer of Tetrafluoroethylene on Oil Foaming

Oil foams have been stabilized by using particles of oligomer of tetrafluoroethylene (OTFE). OTFE particles were dispersed in oil mixtures prior to aeration, to exclude the oil-repellency nature of the particles due to the formation of the metastable Cassie-Baxter state and properly evaluate the effects of contact angle on the foaming behavior. The particle contact angle (θ^Y) against air/oil surfaces were controlled by changing a composition of two oils with different surface tension (n-heptane and methyl salicylate). The θ^Y value increases with increasing a mole fraction of methyl salicylate, from 42° (for pure n-heptane) to 89° (for pure methyl salicylate). The air volume incorporated in the oils after aerating OTFE dispersions in the oil mixtures shows a maximum when θ^Y = 55°. The flocculation of OTFE particles in bulk oils is responsible for the unexpected behavior of foaming observed when θ^Y is relatively high. The increase in the degree of the flocculation reduces the effective concentration of OTFE particles in bulk oil, leading to the inefficient bubble stabilization. These findings suggest the efficient oil foaming using particles as a stabilizer is achieved by optimizing both the particle contact angle and the degree of flocculation in oils.

A Study of the Stability Mechanism of the Dispersed Particle Gel Three-Phase Foam Using the Interfacial Dilational Rheology Method

The dispersed particle gel (DPG) three-phase foam is a novel profile control and flooding system. The stability mechanism of the DPG three-phase foam was studied using an interfacial dilational rheology method. The results show that the elastic modulus of the DPG three-phase foam is up to 14 mN/m, which is much higher than the traditional foam. The increase in interface elasticity produces significantly positive effects on foam stability. Emphasis is given to the influences of frequency, temperature, pressure, and concentration on the viscoelasticity and interfacial adsorption of DPG particles, which change the modules of the foam interface and have a significant effect on foam stability. In addition, the microstructure of the DPG three-phase foam was observed. A viscoelastic shell is formed by the aggregation of the DPG particles on the interface. The irreversible adsorption gives the interface high elasticity and mechanical strength. The electrostatic repulsion between particles increases the spacing between bubbles. The combined effects of these factors give the interface higher mechanical strength, slow down the film drainage, effectively prevent gas permeation, and significantly improve the foam stability.

Capture of colloidal particles by a moving microfluidic bubble

Foams can be stabilized for long periods by the adsorption of solid particles on the liquid-gas interfaces. Although such long-term observations are common, mechanistic descriptions of the particle adsorption process are scarce, especially in confined flows, in part due to the difficulty of observing the particles in the complex gas-liquid dispersion of a foam. Here, we characterise the adsorption of micron-scale particles onto the interface of a bubble flowing in a colloidal aqueous suspension within a microfluidic channel. Three parameters are systematically varied: the particle size, their concentration, and the mean velocity of the colloidal suspension. The bubble coverage is found to increase linearly with position in the channel for all conditions but with a slope that depends on all three parameters. The optimal coverage is found for 1 μm particles at low flow rates and high concentrations. In this regime the particles pass the bubbles through the gutters between the interface and the channel corners, where the complex 3D flow leads them onto the interface. The largest particles cannot enter into the gutters and therefore provide very poor coverage. In contrast, particle aggregates can sediment onto the microchannel floor ahead of the bubble and get swept up by the advancing interface, thus improving the coverage for both large and medium particle sizes. These observations provide new insight on the influence of boundaries for particle adsorption at an air-liquid interface.

CO2-Triggered microreactions in liquid marbles

CO₂-Triggered microreactions in liquid marbles were developed by using CO₂ to coalesce contacting patchy liquid marbles containing separate reagents.

Elasticity and failure of liquid marbles: influence of particle coating and marble volume

When coated with microscale hydrophobic particles, macroscopic liquid droplets can become non-wetting liquid marbles that exhibit an array of fascinating solid-like properties. Specifically, the force required to uniaxially compress liquid marbles depends on their volume, but it is unclear if the particle coating plays a role. In contrast, the failure of marbles upon compression does depend on the particle coating, but the conditions for failure do not appear to change with marble volume. Here, we experimentally study the elastic deformation and failure of liquid marbles and, by applying a doubly truncated oblate spheroid model to quantify their surface area, explore the role of marble volume and particle composition. First, we find that the work required to compress liquid marbles agrees with the product of the core fluid surface tension and the change in the marble surface area, validating that the elastic mechanics of liquid marbles is independent of the particle coating. Next, we study marble failure by measuring their ductility as quantified by the maximum fractional increase in marble surface area prior to rupture. Not only does marble ductility depend on the particle coating, but it also depends on marble volume with smaller marbles being more ductile. This size effect is attributed to an interaction between marble curvature and particle rafts held together by interparticle forces. These results illuminate new avenues to tailor the rupture of liquid marbles for applications spanning smart fluid handling and pollution mitigation.

Stimuli-Responsive Bubbles and Foams Stabilized with Solid Particles

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.

Mechanical properties in crumple-formed paper derived materials subjected to compression

The crumpling of precursor materials to form dense three dimensional geometries offers an attractive route towards the utilisation of minor-value waste materials. Crumple-forming results in a mesostructured system in which mechanical properties of the material are governed by complex cross-scale deformation mechanisms. Here we investigate the physical and mechanical properties of dense compacted structures fabricated by the confined uniaxial compression of a cellulose tissue to yield crumpled mesostructuring. A total of 25 specimens of various densities were tested under compression. Crumple formed specimens exhibited densities in the range 0.8–1.3 g cm⁻³, and showed high strength to weight characteristics, achieving ultimate compressive strength values of up to 200 MPa under both quasi-static and high strain rate loading conditions and deformation energy that compares well to engineering materials of similar density. The materials fabricated in this work and their mechanical attributes demonstrate the potential of crumple-forming approaches in the fabrication of novel energy-absorbing materials from low-cost precursors such as recycled paper. Stiffness and toughness of the materials exhibit density dependence suggesting this forming technique further allows controllable impact energy dissipation rates in dynamic applications.

Mechanics of a granular skin

Magic sand, a hydrophobic toy granular material, is widely used in popular science instructions because of its nonintuitive mechanical properties. A detailed study of the failure of an underwater column of magic sand shows that these properties can be traced to a single phenomenon: the system self-generates a cohesive skin that encapsulates the material inside. The skin, consisting of pinned air-water-grain interfaces, shows multiscale mechanical properties: they range from contact-line dynamics in the intragrain roughness scale, to plastic flow at the grain scale, all the way to sample-scale mechanical responses. With decreasing rigidity of the skin, the failure mode transforms from brittle to ductile (both of which are collective in nature) to a complete disintegration at the single-grain scale.

Mechanics of Pickering Drops Probed by Electric Field-Induced Stress

Fluid drops coated with particles, so-called Pickering drops, play an important role in emulsion and capsule applications. In this context, knowledge of mechanical properties and stability of Pickering drops are essential. Here we prepare Pickering drops via electric field-driven self-assembly. We use direct current (DC) electric fields to induce mechanical stress on these drops, as a possible alternative to the use of, for example, fluid flow fields. Drop deformation is monitored as a function of the applied electric field strength. The deformation of pure silicone oil drops is enhanced when covered by insulating polyethylene (PE) particles, whereas drops covered by conductive clay particles can also change shape from oblate to prolate. We attribute these results to changes in the electric conductivity of the drop interface after adding particles, and have developed a fluid shell description to estimate the conductivity of Pickering particle layers that are assumed to be non-jammed and fluid-like. Retraction experiments in the absence of electric fields are also performed. Particle-covered drops retract slower than particle-free drops, caused by increased viscous dissipation due to the presence of the Pickering particle layer.

Sinking a Granular Raft

We report experiments that yield new insights on the behavior of granular rafts at an oil-water interface. We show that these particle aggregates can float or sink depending on dimensionless parameters taking into account the particle densities and size and the densities of the two fluids. We characterize the raft shape and stability and propose a model to predict its shape and maximum length to remain afloat. Finally we find that wrinkles and folds appear along the raft due to compression by its own weight, which can trigger destabilization. These features are characteristics of an elastic instability, which we discuss, including the limitations of our model.

Shape oscillations of particle-coated bubbles and directional particle expulsion

Bubbles stabilised by colloidal particles can find applications in advanced materials, catalysis and drug delivery. For applications in controlled release, it is desirable to remove the particles from the interface in a programmable fashion. We have previously shown that ultrasound waves excite volumetric oscillations of particle-coated bubbles, resulting in precisely timed particle expulsion due to interface compression on a ultrafast timescale [Poulichet et al., Proc. Natl. Acad. Sci. U. S. A., 2015, 112, 5932]. We also observed shape oscillations, which were found to drive directional particle expulsion from the antinodes of the non-spherical deformation. In this paper we investigate the mechanisms leading to directional particle expulsion during shape oscillations of particle-coated bubbles driven by ultrasound at 40 kHz. We perform high-speed visualisation of the interface shape and of the particle distribution during ultrafast deformation at a rate of up to 10⁴ s^-1. The mode of shape oscillations is found to not depend on the bubble size, in contrast with what has been reported for uncoated bubbles. A decomposition of the non-spherical shape in spatial Fourier modes reveals that the interplay of different modes determines the locations of particle expulsion. The n-fold symmetry of the dominant mode does not always lead to desorption from all 2n antinodes, but only those where there is favourable alignment with the sub-dominant modes. Desorption from the antinodes of the shape oscillations is due to different, concurrent mechanisms. The radial acceleration of the interface at the antinodes can be up to 10⁵-10⁶ ms^-2, hence there is a contribution from the inertia of the particles localised at the antinodes. In addition, we found that particles migrate to the antinodes of the shape oscillation, thereby enhancing the contribution from the surface pressure in the monolayer.

Pickering emulsions stabilized by coloured organic pigment particles

The possibility of stabilizing emulsions of water and non-polar alkane with pure, coloured organic pigment particles is explored. Seven pigment types each possessing a primary colour of the rainbow were selected. Their solubility in water or heptane was determined using a spectrophotometric method and their surface energies were derived from the contact angles of probe liquids on compressed disks of the particles. As expected, most of the pigments are relatively hydrophobic but pigment orange is quite hydrophilic. At equal volumes of oil and water, preferred emulsions were water-in-oil (w/o) for six pigment types and oil-in-water (o/w) for pigment orange. The emulsion type is in line with calculated contact angles of the particles at the oil-water interface being either side of 90°. Their stability to coalescence increases with particle concentration. Emulsions are shown to undergo limited coalescence from which the coverage of drop interfaces by particles has been determined. In a few cases, close-packed primary particles are visible around emulsion droplets. At constant particle concentration, the influence of the volume fraction of water (φ_w) on emulsions was also studied. For the most hydrophilic pigment orange, emulsions are o/w at all φ_w, whereas they are w/o for the most hydrophobic pigments (red, yellow, green and blue). For pigments of intermediate hydrophobicity however (indigo and violet), catastrophic phase inversion becomes possible with emulsions inverting from w/o to o/w upon increasing φ_w. For the first time, we link the pigment surface energy to the propensity of emulsions to phase invert transitionally or catastrophically.

Direct calculation of anisotropic surface stresses during deformation of a particle-covered drop

The modification of the surface tension and the surface shear elasticity by particles in particle-covered drops can be attributed to a particle-induced surface stress. This stress represents at the macroscopic, continuum level the microscopic effect of lateral particle-particle interactions. Understanding the link between the isotropic and anisotropic components of the surface stress and the particle microstructure, and how these components change when structured interfaces deform, is a crucial problem in the field of particle-laden interfaces. In this paper, we analyse static and transient three-dimensional simulations of a pendant drop whose surface is covered by colloidal particles displaying purely repulsive particle-particle interactions. We compute the isotropic and anisotropic surface stress from the inter-particle forces using a version of the Kirkwood-Irving formula suitable for interfacial suspensions; we validate the approach by comparing against surface tension values obtained using Fordham's method (Proc. R. Soc. London, Ser. A, 1948, 194). In the parameter range simulated, the combination of parameters for which the drop does not pinch off (stable drop) gives rise to a homogeneous and isotropic surface stress; we argue that in the absence of attractive interactions the drop becomes unstable before anisotropic effects can manifest themselves. For unstable drops, stress non-uniformity and anisotropy are significant when the drop deformation and the solid area fraction are sufficiently large. Our results have implications for the dynamic deformation of structured interfaces with geometrically complex and time dependent morphologies.

Particles at Oil-Air Surfaces: Powdered Oil, Liquid Oil Marbles, and Oil Foam

The type of material stabilized by four kinds of fluorinated particles (sericite and bentonite platelet clays and spherical zinc oxide) in air-oil mixtures has been investigated. It depends on the particle wettability and the degree of shear. Upon vigorous agitation, oil dispersions are formed in all the oils containing relatively large bentonite particles and in oils of relatively low surface tension (γla < 26 mN m(-1)) like dodecane, 20 cS silicone, and cyclomethicone containing the other fluorinated particles. Particle-stabilized oil foams were obtained in oils having γla > 26 mN m(-1) where the advancing air-oil-solid contact angle θ lies between ca. 90° and 120°. Gentle shaking, however, gives oil-in-air liquid marbles with all the oil-particle systems except for cases where θ is <60°. For oils of tension >24 mN m(-1) with omniphobic zinc oxide and sericite particles for which advancing θ ≥ 90°, dry oil powders consisting of oil drops in air which do not leak oil could be made upon gentle agitation up to a critical oil:particle ratio (COPR). Above the COPR, catastrophic phase inversion of the dry oil powders to air-in-oil foams was observed. When sheared on a substrate, the dry oil powders containing at least 60 wt % of oil release the encapsulated oil, making these materials attractive formulations in the cosmetic and food industries.

Dispersion behavior and aqueous foams in mixtures of a vesicle-forming surfactant and edible nanoparticles

In an attempt to prepare ultrastable aqueous foams composed entirely of food-grade ingredients, we describe the foamability and foam stability of aqueous phases containing either calcium carbonate particles (CaCO3), sodium stearoyl lactylate surfactant (SSL), or their mixtures. Techniques including zeta potential measurements, adsorption isotherm determination, contact angles and optical and cryo-scanning electron microscopy are used to probe the interaction between particles and surfactant molecules. Aqueous dispersions of inherently hydrophilic cationic CaCO3 nanoparticles do not foam to any great extent. By contrast, aqueous dispersions of anionic SSL, which forms a lamellar phase/vesicles, foam progressively on increasing the concentration. Despite their foamability being low compared to that of micelle-forming surfactant sodium dodecyl sulfate, they are much more stable to collapse with half-lives (of up to 40 days) of around 2 orders of magnitude higher above the respective aggregation concentrations. We believe that, in addition to surfactant lamellae around bubbles, the bilayers within vesicles contain surfactant chains in a solidlike state yielding indestructible aggregates that jam the aqueous films between bubbles, reducing the drainage rate and both bubble coalescence and gas-transfer between bubbles. In mixtures of particles and surfactant, the adsorption of SSL monomers occurs on particle surfaces, leading to an increase in their hydrophobicity, promoting particle adsorption to bubble surfaces. Ultrastable foams result with half-lives of around an order of magnitude higher again at low concentrations and foams which lose only around 30% of their volume within a year at high concentrations. In the latter case, we evidence a high surface density of discrete surfactant-coated particles at bubble surfaces, rendering them stable to coalescence and disproportionation.

Assembly of jammed colloidal shells onto micron-sized bubbles by ultrasound

Stabilization of gas bubbles in water by applying solid particles is a promising technique to ensure long-term stability of the dispersion against coarsening. However, the production of large quantities of particle stabilized bubbles is challenging. The delivery of particles to the interface must occur rapidly compared to the typical time scale of coarsening during production. Furthermore, the production route must be able to overcome the energy barriers for interfacial adsorption of particles. Here we demonstrate that ultrasound can be applied to agitate a colloidal dispersion and supply sufficient energy to ensure particle adsorption onto the air-water interface. With this technique we are able to produce micron-sized bubbles, solely stabilized by particles. The interface of these bubbles is characterized by a colloidal shell, a monolayer of particles which adopt a hexagonal packing. The particles are anchored to the interface owing to partial wetting and experience lateral compression due to bubble shrinkage. The combination of both effects stops coarsening once the interface is jammed with particles. As a result, stable bubbles are formed. Individual particles can desorb from the interface upon surfactant addition, though. The latter fact confirms that the particle shell is not covalently linked due to thermal sintering, but is solely held together by capillary interaction. In summary, we show that our ultrasound approach allows for the straightforward creation of micron-sized particle stabilized bubbles with high stability towards coarsening.

Capillary tube wetting induced by particles: towards armoured bubbles tailoring

In this paper, we report on the strongly modified dynamics of a liquid finger pushed inside a capillary tube, when partially wettable particles are lying on the walls. Particles promote the appearance of new regimes and enable the tailored synthesis of bubbles encapsulated in a monolayer of particles (so-called "armoured bubbles"). This remarkable behavior arises due to the collection of particles at the air-liquid interface, which modify the global energy balance and stabilize the interface. Armoured-bubbles are of primary interest in industrial processes since they display increased stability, interfacial rigidity and can even sustain non-spherical shapes. This work opens perspective for a low cost bubbles-on-demand technology enabling the synthesis of armoured bubbles with specific sizes, shapes and composition.

Mixtures of latex particles and the surfactant of opposite charge used as interface stabilizers--influence of particle contact angle, zeta potential, flocculation and shear energy

We investigate the stabilization of air-water interfaces by mixtures of negatively charged latex particles (sulfate polystyrene) and cationic surfactants (alkyl trimethylammonium bromides). First we report results concerning the binding of surfactant molecules to the latex particles. As the surfactant concentration increases, the charge of the particles reverses, from negative to positive, because CnTAB first binds electrostatically to the latex particles and then through hydrophobic interaction with the monolayer already adsorbed on the particles as well as directly with the hydrophobic surface of the latex. Over a large range of surfactant concentrations around the charge inversion, a strong flocculation is observed and 100 μm large aggregates form in the suspension. Unlike previous studies published on mixtures of inorganic particles with oppositely charged surfactants, we show that we can vary the sign of the zeta potential of the particles without changing the contact angle of the particles over a large range of surfactant concentrations. Indeed, the latex particles that we study are more hydrophobic than inorganic particles, hence adding moderate concentrations of the surfactant results in a weak variation of the contact angle while the charge of the particles can be reversed. This enables decoupling of the effect of zeta potential and contact angle on the interfacial properties of the mixtures. Our study shows that the contact angle and the charge of the particles are not sufficient parameters to control the foam properties, and the key-parameters are the flocculation state and the shear energy applied to produce the foam. Indeed, flocculated samples, whatever the sign of the zeta potential, enable production of a stable armour at the interface. The large aggregates do not adsorb spontaneously at the interface because of their large size, however when a large shear energy is used to produce the foam very stable foam is obtained, where particles are trapped at interfaces. We suggest that the large aggregates may be broken during shear and may reform at the interface to form a solid armour. A simple calculation taking into account the adsorption dynamics of the aggregates as a function of their size is consistent with this hypothesis.

Armoring a droplet: soft jamming of a dense granular interface

Droplets and bubbles protected by armors of particles have found vast applications in encapsulation, stabilization of emulsions and foams, or flotation processes. The liquid phase stores capillary energy, while concurrently the solid contacts of the granular network induce friction and energy dissipation, leading to hybrid interfaces of combined properties. By means of nonintrusive tensiometric methods and structural measurements, we distinguish three surface phases of increasing rigidity during the evaporation of armored droplets. The emergence of surface rigidity is reminiscent of jamming of granular matter, but it occurs differently since it is marked by a step by step hardening under surface compression. These results show that the concept of the effective surface tension remains useful only below the first jamming transition. Beyond this point, the surface stresses become anisotropic.

Noncontact orientation of objects in three-dimensional space using magnetic levitation

This paper describes several noncontact methods of orienting objects in 3D space using Magnetic Levitation (MagLev). The methods use two permanent magnets arranged coaxially with like poles facing and a container containing a paramagnetic liquid in which the objects are suspended. Absent external forcing, objects levitating in the device adopt predictable static orientations; the orientation depends on the shape and distribution of mass within the objects. The orientation of objects of uniform density in the MagLev device shows a sharp geometry-dependent transition: an analytical theory rationalizes this transition and predicts the orientation of objects in the MagLev device. Manipulation of the orientation of the levitating objects in space is achieved in two ways: (i) by rotating and/or translating the MagLev device while the objects are suspended in the paramagnetic solution between the magnets; (ii) by moving a small external magnet close to the levitating objects while keeping the device stationary. Unlike mechanical agitation or robotic selection, orienting using MagLev is possible for objects having a range of different physical characteristics (e.g., different shapes, sizes, and mechanical properties from hard polymers to gels and fluids). MagLev thus has the potential to be useful for sorting and positioning components in 3D space, orienting objects for assembly, constructing noncontact devices, and assembling objects composed of soft materials such as hydrogels, elastomers, and jammed granular media.

Unusually stable liquid foams

Obtaining stable liquid foams is an important issue in view of their numerous applications. In some of these, the liquid foam in itself is of interest, in others, the liquid foam acts as a precursor for the generation of solid foam. In this short review, we will make a survey of the existing results in the area. This will include foams stabilised by surfactants, proteins and particles. The origin of the stability is related to the slowing down of coarsening, drainage or coalescence, and eventually to their arrest. The three effects are frequently coupled and in many cases, they act simultaneously and enhance one another. Drainage can be arrested if the liquid of the foam either gels or solidifies. Coalescence is slowed down by gelified foam films, and it can be arrested if the films become very thick and/or rigid. These mechanisms are thus qualitatively easy to identify, but they are less easy to model in order to obtain quantitative predictions. The slowing down of coarsening requests either very thick or small films, and its arrest was observed in cases where the surface compression modulus was large. The detail of the mechanisms at play remains unclear.

Dry oil powders and oil foams stabilised by fluorinated clay platelet particles

A series of platelet sericite particles coated to different extents with a fluorinating agent has been characterised and their behaviour in mixtures with air and oil studied. The material which forms by vigorous shaking depends on both the surface tension of the oil and the surface energy of the particles which control their degree of wetting. Oil dispersions are formed in liquids of relatively low tension (<22 mN m(-1)), e.g. hexane and cyclomethicone, for all particles. Particle-stabilised air-in-oil foams form in liquids of higher tension, e.g. dodecane and phenyl silicone, where the advancing three-phase contact angle θ, measured on a planar substrate composed of the particles into the liquid, lies between ca. 65° and 120°. For oils of tension above 27 mN m(-1) like squalane and liquid paraffin with particles for which θ > 70°, we have discovered that dry oil powders in which oil drops stabilised by particles dispersed in air (oil-in-air) can be prepared by gentle mixing up to a critical oil : particle ratio (COPR) and do not leak oil. These powders, containing up to 80 wt% oil, release the encapsulated oil when sheared on a substrate. For many of the systems forming oil powders, stable liquid oil marbles can also be prepared. Above the COPR, catastrophic phase inversion occurs yielding an ultra-stable air-in-oil foam. We thus demonstrate the ability to disperse oil drops or air bubbles coated with particles within novel materials.

Generation of antibubbles from core-shell double emulsion templates produced by microfluidics

We report the preparation of antibubbles by microfluidic methods. More specifically, we demonstrate a two-step approach, wherein a monodisperse water-in-oil-in-water (W/O/W) emulsion of core-shell construction is first generated via microfluidics and freeze-dried thereafter to yield, upon subsequent reconstitution, an aqueous dispersion of antibubbles. Stable antibubbles are attained by stabilization of the air-water interfaces through a combination of adsorbed particles and polymeric surfactant. The antibubbles strongly resemble the double emulsion templates from which they were formed. When triggered to release, antibubbles show complete release of their cores within about 100 ms.

Arrested coalescence of viscoelastic droplets with internal microstructure

There are many new approaches to designing complex anisotropic colloids, often using droplets as templates. However, droplets themselves can be designed to form anisotropic shapes without any external templates. One approach is to arrest binary droplet coalescence at an intermediate stage before a spherical shape is formed. Further shape relaxation of such anisotropic, arrested structures is retarded by droplet elasticity, either interfacial or internal. In this article we study coalescence of structured droplets, containing a network of anisotropic colloids, whose internal elasticity provides a resistance to full shape relaxation and interfacial energy minimization during coalescence. Precise tuning of droplet elasticity arrests coalescence at different stages and leads to various anisotropic shapes, ranging from doublets to ellipsoids. A simple model balancing interfacial and elastic energy is used to explain experimentally observed coalescence arrest in viscoelastic droplets. During coalescence of structured droplets the interfacial energy is continuously reduced while the elastic energy is increased by compression of the internal structure and, when the two processes balance one another, coalescence is arrested. Experimentally we observe that if either interfacial energy or elasticity dominates, total coalescence or total stability of droplets results. The stabilization mechanism is directly analogous to that in a Pickering emulsion, though here the resistance to coalescence is provided via an internal volume-based, rather than surface, structure. This study provides guidelines for designing anisotropic droplets by arrested coalescence but also explains some observations of "partial" coalescence observed in commercial foods like ice cream and whipped cream.

Field-induced breakup of emulsion droplets stabilized by colloidal particles

We simulate the response of a particle-stabilized emulsion droplet in an external force field, such as gravity, acting equally on all N particles. We show that the field strength required for breakup (at fixed initial area fraction) decreases markedly with droplet size, because the forces act cumulatively, not individually, to detach the interfacial particles. The breakup mode involves the collective destabilization of a solidified particle raft occupying the lower part of the droplet, leading to a critical force per particle that scales approximately as N(-1/2).

Multiple pickering emulsions stabilized by microbowls

Some researchers have focused on the adsorption of solid particles at fluid-fluid interfaces and prepared emulsions and foams called "Pickering emulsions/foams". However, while several reports exist on simple spherical emulsions, few reports are available on the formation of more complex structures. Here, we show that holes on particle surfaces are a key factor in establishing the variety and complexity of mesoscale structures. Microbowls, which are hollow particles with holes on their surfaces, form multiple emulsions (water-in-oil-in-water and oil-in-water-in-oil emulsions) by simply mixing them with water and oil. Furthermore, stable potato-like or coffee-bean-like emulsions are also obtained, although nonspherical emulsions are usually unstable because of their larger interfacial energies. These findings are useful in designing the building blocks of complex supracolloidal systems for pharmaceutical, food, and cosmetic products.