The self-assembly of solid particles at fluid-fluid interfaces is widely exploited to stabilize emulsions and foams, and in materials synthesis. The self-assembly mechanism is very robust owing to the large capillary energy associated with particle adsorption, of the order of millions of times the thermal energy for micrometer-sized colloids. The microstructure of the interfacial colloid monolayer can also favor stability, for instance in the case of particle-stabilized bubbles, which can be indefinitely stable against dissolution due to jamming of the colloid monolayer. As a result, significant challenges arise when destabilization and particle removal are a requirement. Here we demonstrate ultrafast desorption of colloid monolayers from the interface of particle-stabilized bubbles. We drive the bubbles into periodic compression-expansion using ultrasound waves, causing significant deformation and microstructural changes in the particle monolayer. Using high-speed microscopy we uncover different particle expulsion scenarios depending on the mode of bubble deformation, including highly directional patterns of particle release during shape oscillations. Complete removal of colloid monolayers from bubbles is achieved in under a millisecond. Our method should find a broad range of applications, from nanoparticle recycling in sustainable processes to programmable particle delivery in lab-on-achip applications.self-assembly | colloidal interactions | Pickering | interfacial assemblies C olloidal particles can adsorb to fluid-fluid interfaces and confer outstanding stability to emulsions and foams (1, 2). The interfacial self-assembly of colloids has been used to create novel materials, for instance colloidosomes (3) and bijels (4). These applications rely on the large decrease in free energy accompanying particle adsorption, ΔE = −γ 0 πa 2 ð1 − cos θÞ 2 , which depends on the surface tension γ 0 , the particle size a, and the three-phase contact angle θ (5), and ranges from hundreds to millions of times the thermal energy for nanometer-to micrometer-sized particles. The microstructure that the colloidal particles form at the interface has also been shown to enhance stability. For instance, jamming of an interfacial colloid monolayer prevents coarsening in bicontinuous emulsions (4) and can arrest the dissolution of particle-stabilized bubbles (6). Particle removal from fluid-fluid interfaces is a significant challenge in emerging applications of functional nanoparticles in interfacial biocatalysis (7), gas storage (8), and biomass conversion (9), where the ability to recover and regenerate the nanoparticles at the end of the process is a key requirement. The most common approaches to particle removal from fluid interfaces are based on the physicochemical modification of the fluid phases or the interface. Desorption has been obtained for instance by the addition of a surface-active agent (10, 11). In addition, by tuning the strength of electrostatic repulsion between charged particles at an interface through pH and el...

Emulsions and foams that remain stable under varying environmental conditions are central in the food, personal care, and other formulated products industries. Foams stabilized by solid particles can provide longer-term stability than surfactant-stabilized foams. This stability is partly ascribed to the observation that solid particles can arrest bubble dissolution, which is driven by the Laplace pressure across the curved gas-liquid interface. We studied experimentally the effect of changes in temperature on the lifetime of particle-coated air microbubbles in water. We found that a decrease in temperature destabilizes particle-coated microbubbles beyond dissolution arrest. A quasi-steady model describing the effect of the change in temperature on mass transfer suggests that the dominant mechanism of destabilization is the increased solubility of the gas in the liquid, leading to a condition of undersaturation. Experiments at constant temperature confirmed that undersaturation alone can drive destabilization of particle-coated bubbles, even for vanishing Laplace pressure. We also found that dissolution of a particle-coated bubble can lead either to buckling of the coating or to gradual expulsion of particles, depending on the particle-to-bubble size ratio, with potential implications for controlled release.

Ultrasound waves drive shape oscillations of particle-coated microbubbles. During the ultrafast, non-uniform deformation of the interface, particles are expelled from the antinodes of the shape oscillations.

Biomedical microbubbles stabilized by a coating of magnetic or drug-containing nanoparticles show great potential for theranostics applications. Nanoparticle-coated microbubbles can be made to be stable, echogenic, and to release the cargo of drugcontaining nanoparticles with an ultrasound trigger. This article reviews the design principles of nanoparticle-coated microbubbles for ultrasound imaging and drug delivery, with a particular focus on the physical chemistry of nanoparticle-coated interfaces; the formation, stability and dynamics of nanoparticle-coated bubbles; and the conditions for controlled nanoparticle release in ultrasound. The emerging understanding of the modes of nanoparticle expulsion and of the transport of expelled material by microbubble-induced flow is paving the way towards more efficient nanoparticlemediated drug delivery. The article highlights the knowledge gap that still remains to be addressed before we can control these phenomena.

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