Colloidal-sized particles (10 nm – 10 μm in size) adsorb onto a fluid interface (i.e. a gas/liquid or a liquid/liquid interface) from the continuous phases surrounding the surface and become trapped due to a reduction in their interfacial energy, forming a two dimensional monolayer. Colloid monolayers adsorbed onto the dispersed phase of emulsions and foams are traditionally used in stabilizing dispersions from coalescence. Emerging technologies focus on the self-organization of colloid 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 monolayer phases on substrates as templates for materials fabrication, and textured surface topologies for super-hydrophobic surfaces.
The 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.
Seminars are open to alumni, friends of the Department, and the general public.