Seminar: Kathleen Stebe, University of Pennsylvania
Professor Kate Stebe of University of Pennsylvania, presents a seminar in Chemical & Biomolecular Engineering, "Oriented Assembly by Capillarity."
Abstract: Microparticles with well‐defined shapes can be directed to assemble into complex structures bycapillarity; a delineation of the ‘rules’ for assembly as a function of particle shape paves the way toassemble particles into structures at interfaces with novel material properties. Here we explore twothemes. First, we explore the assembly of microparticles with well‐defined shapes on otherwise planarinterfaces to form structures with preferred orientations and with mechanical responses that dependsubtly on particle shape. Second, we study particles on curved interfaces. Interface curvature isharnessed as an applied field to drive capillary assembly at well defined locations.
Capillary attraction between colloidal particles at fluid interfaces is ubiquitous. A particle at a fluidinterface deforms the interface to satisfy its wetting boundary condition at the three phase contact linewhere solid, liquid and vapor phase meet. Capillary attraction occurs when deformation fields fromneighboring particles overlap. As particles approach, the surface area can decrease, lowering thesurface energy of the system. The resulting attractive interactions can be remarkably large; the surfacetension of an aqueous‐air interface is 72 mN/m or 18 kT/nm2, so the elimination of even 1 nm2 ofsurface area translates into significant energy reduction in particle assembly. Anisotropically shapedparticles create deformations that bear the signature of the particle shape. When these particlesinteract by capillarity, they can orient, align, and assemble into complex structures and networks. In thistalk, progress in developing a quantitative understanding of pair interactions and mechanics ofassemblies between particles is described and compared to experiment. Interactions between particlesnear contact are studied to understand the properties of capillary bonds as a function of particle surfacefeatures. On curved interfaces, particle‐induced deformations interact with interface curvature field.The resulting capillary energy drives particle migration and alignment, an effect that is explored as ameans to direct particles to docking sites, and to mold particle structures.