Bryn Mawr College Physics Colloquium
Directed Assembly by Capillarity
Prof. Kathleen J. Stebe
Department of Chemical and Biomolecular Engineering
University of Pennsylvania, 220 S 33 St, Philadelphia, PA 19072
Monday 11 February 2013
Abstract: Directed assembly is a field in which discrete objects are directed to assemble with given configurations or at given locations. Many researchers exploit applied electric or magnetic fields to direct the assembly of micro- or nano-particles. In our research, we have been focusing on energy fields that arise spontaneously between particles located at fluid interfaces owing strictly to their wetting boundary conditions. Thus far, we have focused on microparticle assembly. Scaling arguments suggest that these assembly mechanics should play a role for sub-micron particles.
In this presentation, I summarize our work on microparticles with well-defined shapes, which assemble into complex structures by capillarity. We have studied these systems to understand the `rules' for capillary assembly as a function of microparticle and interface shape. In this talk, I describe three related themes of recent research. First, I discuss the assembly of microparticles with well-defined shapes on otherwise planar interfaces to form structures with preferred orientations; these assemblies have micromechanics that depend strongly on subtle changes of particle shape. For example, cylindrical microparticles assemble end-to-end to form rigid chains, while ellipsoidal microparticles with similar aspect ratios and wetting conditions assemble side-by-side to form flexible chains. The strength of attraction between pairs of cylinders is orders of magnitude greater than that between ellipsoids; both particle types assemble in apparent contact and form structures that are kinetically trapped. Second, I discuss particles on curved interfaces. Particles attach to interfaces, align along principle axes and migrate to sites of high curvature. Interface curvature can be harnessed as an applied field to drive capillary assembly at well-defined locations. Third, I describe experiments in which we create near field repulsive capillary interactions which allow particles to assemble at well-defined equilibrium distances.