Buckling in armored droplets

Abstract

The issue of the buckling mechanism in droplets stabilized by solid particles (armored droplets) is tackled at a mesoscopic level using dissipative particle dynamics 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.Keywords: Pickering emulsions, DPD simulations, Janus/homogeneous nanoparticles, bucklingPickering emulsions [1], i.e. particle-stabilized emulsions, have been studied intensively in recent years owning to their wide range of applications including biofuel processing [2] and food preservation [3,4]. They have also been developed as precursors to magnetic particles for imaging [5] and drug delivery systems [6]. Even with their widespread use, they remain, however, underutilized. In Pickering emulsions, particles and/or nanoparticles (NPs) with suitable surface chemistries adsorb at the droplet surfaces, with an adsorption energy of up to thousands of times the thermal energy. The characteristics of Pickering emulsions pose a number of intriguing fundamental physical questions including a thorough understanding of the perennial lack of detail about how particles arrange at the liquid/liquid interface. Other not completely answered questions include particle effects on interfacial tension [7], layering [8], buckling [9-11] and particle release [8,12].In some important processes that involve emulsions, it can be required to reduce the volume of the dispersed droplets [9,[13][14][15]. 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 [8,10,11]. Despite the vast interest in particle-laden interfaces, the key factors that determine the collapse of curved particle-laden interfaces are still subject of debate. Indeed, although linear elasticity describes successfully the morphology of buckled particle-laden droplets, it is still unclear whether the onset of buckling can be explained in terms of classic elastic buckling...

Collapse mechanisms and extreme deformation of particle-laden interfaces

“…It was also found that, in the absence of bending rigidity, buckling requires Π/γ 0 = 1, while desorption can occur for Π/γ 0 < 1, depending on the contact angle. Dissipative particle dynamics simulations of nanoparticles at the interface of a dissolving droplet [70] reveal a bimodal distribution of contact angles characteristic of a bilayer, as shown in Figure 3(d). of phopsholipids was also imaged using high-speed fluorescence [73].…”

Particle-covered drops in electric fields: drop deformation and surface particle organization

“…[1][2][3][4] This is because they are promising for a variety of practical applications, such as in food technology, 5 the oil industry, 6 biofuel processing, 7 and for improving pharmaceutical products. 8 Moreover, such drops possess characteristics that make them useful as experimental model systems for studying, for example, particle effects on interfacial tension, 9 particle crystal growth and ordering or particle layer buckling on curved interfaces, [10][11][12][13] particle assembly and rearrangement on drop surfaces, 14,15 and particle detachment from drops. 16 Particle-covered drops can additionally be employed for fabricating porous structures, 17 granular or colloidal capsules of different mechanical properties, morphologies, or shapes, 18,19 and adaptive structures.…”

Buckling vs. particle desorption in a particle-covered drop subject to compressive surface stresses: a simulation study

“…The change in morphology of drops covered with homogeneous and Janus particles for a/R = 0.1 and subject to volume reduction was studied by Dissipative Particle Dynamics in Ref. 19 . These simulations include Brownian fluctuations.…”