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 particlecoated
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.