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Mardi 18 juillet 2017 - 14:00

Shrinking Armored Droplets to Advance New Materials for Applications

François Sicard (University College London)

par Revaz Ramazashvili - 18 juillet

Pickering emulsions, i.e. particle­‐stabilized emulsions, have been studied intensively in recent years owning to their wide range of applications including biofuel processing and food preservation. They have also been developed as precursors to magnetic particles for imaging and drug delivery systems. In some important processes that involve emulsions, it can be required to reduce the volume of the dispersed droplets. The interface may undergo large deformations that produce compressive stresses, causing localized mechanical instabilities. The proliferation of these localized instabilities may then result in a variety of collapse mechanisms. Numerous experiments have been conducted to link the rheological response of particle-­laden interfaces to the stability of emulsions and foams. However, due to their inherent limited resolution, direct access to local observables, such as the particles three­-phase contact angle distribution, θC, remains out of reach. This crucial information can be accessed by numerical simulations, sometimes with approximations. All-atom molecular dynamics (MD) simulations have then become a widely employed computational technique. However, they remain computationally expensive. Mesoscopic simulations, in which the structural unit is a coarse-grained representation of a large number of molecules, allow us to overcome this limitation.

In this talk, the issue of the buckling mechanism in droplets stabilized by solid particles (armored droplets) is tackled at a mesoscopic level using Dissipative Particle Dynamics (DPD) simulations. We consider spherical water droplet in a decane solvent coated with nanoparticle monolayers of two different types : Janus and homogeneous. The chosen particles yield comparable initial three-­phase contact angles, chosen to maximize the adsorption energy at the interface. We study the interplay between the evolution of droplet shape, layering of the particles, and their distribution at the interface when the volume of the droplets is reduced. We show that Janus particles affect strongly the shape of the droplet with the formation of a crater-­like depression. This evolution is actively controlled by a close-­packed particle monolayer at the curved interface. On the contrary, homogeneous particles follow passively the volume reduction of the droplet, whose shape does not deviate too much from spherical, even when a nanoparticle monolayer/bilayer transition is detected at the interface. We discuss how these buckled armored droplets might be of relevance in various applications including potential drug delivery systems and biomimetic design of functional surfaces.

Post-scriptum :

contact : Nicolas Destainville