Continuum and Molecular Dynamic Studies of The Hydrodynamics of Colloid Particles at a Fluid Interface
Charles Maldarelli, City College of New York

Colloidal-sized particles (10 nm – 10 ?m in size) adsorb onto  fluid interfaces (i.e. a gas/liquid or a liquid/liquid interface) from immiscible continuous phases surrounding the surface. At the surface they become trapped due to a reduction in their interfacial energy, forming a two dimensional monolayer.  Colloid monolayers which are adsorbed onto the dispersed phase of emulsions and foams are  used  in stabilizing dispersions from coalescence.  Emerging technologies focus on the self-organization of colloidal monolayers formed on the fluid interface of liquid films on solid substrates. Control over lateral forces (e.g. by capillary attraction and electrostatic or magnetic repulsion) allows the formation of 2D crystalline phases which can be used as templates for materials fabrication, and textured surface topologies for super-hydrophobic surfaces.

The self-organization of colloids in a monolayer is a balance between the  surface forces and the viscous resistance to particle motion along the surface. This presentation focuses on the surface hydrodynamics. A continuum analytical theory is presented for the drag force on a colloid at a vapor/liquid interface as a function of its immersion depth into the liquid phase, and the theory is extended by numerical calculation to colloids on the fluid interface of a thin film. A hydrodynamic theory is also developed for the viscous resistance due to  the mutual approach of two colloids, and Brownian dynamics simulations are presented to understand the role of thermal fluctuations and hydrodynamic interactions in the capillary attraction of colloid pairs. Molecular dynamics calculations are detailed for the drag force on nano-sized colloids translating at a vapor/liquid interface, and a significant reduction in drag is obtained as the nanoparticle translates within the finite-width interfacial zone of the surface.

Experiments are presented to demonstrate how the calculated drag force can be used to accurately model the capillary attraction of two colloids.  Experiments measuring the Brownian diffusion coefficient of a colloid at an interface are detailed, and used with the drag force calculation to obtain the colloid immersion depth and three phase contact angle.