Self-assembled nanoparticle-coated interfaces: Capillary pressure, shell formation and buckling

Abstract

Hypothesis: Particle accumulation at liquid-liquid or liquid-gas interfaces can significantly alter capillary behavior and give rise to unusual interfacial phenomena including the asymmetric macroscopic mechanical response of the interface.

Experiments: This study explores the accumulation of cetyltrimethylammonium bromide-modified nanoparticles at fluid interfaces and the subsequent mechanical response of nanoparticle-coated droplets during contraction and expansion. Droplet tests involve the simultaneous recording of the droplet shape and the capillary pressure. Complementary single-pore experiments examine the response of particle-coated interfaces as they traverse a pore constriction.

Findings: Interfaces promote order. The time-dependent nanoparticle accumulation at the interface is diffusion-controlled. The nanoparticle coated droplets can sustain negative capillary pressure before they buckle. Buckling patterns strongly depend on the boundary conditions: non-slip boundary conditions lead to crumples while slip boundary conditions result in just a few depressions. The particle-coated interface exhibits asymmetric behavior in response to particle-level capillary forces: an "oil droplet in a nanofluid bath" withstands a significantly higher capillary pressure difference than a "nanofluid droplet in an oil bath". A first-order equilibrium analysis of interaction forces explains the asymmetric response. Single-constriction experiments show that the formation of particle-coated interfaces has a pronounced effect on fluid displacement in porous media.

Keywords: Buckling patterns; Capillarity; Multi-phase flow; Oil displacement; Particle-coated interface.