Oriented Assembly by Capillarity

Lecture by Kathleen J. Stebe

Department of Chemical and Biomolecular Engineering
University of Pennsylvania

Tuesday, March 13, 2012
Room 1610 Engineering Hall
Lecture at 4:00 p.m.

Reception at 3:30 p.m. Engineering Hall Lobby

Particles with well defined shapes can be directed to assemble into complex structures by capillarity. Here we explore two themes. First, we explore the assembly of microparticles with well-defined shapes on otherwise planar interfaces to form structures with preferred orientations and with mechanical responses that depend subtly on particle shape. Second, we study particles on curved interfaces. Interface curvature is harnessed 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 fluid interface deforms the interface to satisfy its wetting boundary condition at the three phase contact line where solid, liquid and vapor phase meet. Capillary attraction occurs when deformation fields from neighboring particles overlap. As particles approach, the surface area can decrease, lowering the surface energy of the system. The resulting attractive interactions can be remarkably large; the surface tension of an aqueous-air interface is 72 mN/m or 18 kT/nm2, so the elimination of even 1 nm2 of surface area translates into significant energy reduction in particle assembly.

Anisotropically shaped particles create deformations that bear the signature of the particle shape. When these particles interact by capillarity, they can orient, align, and assemble into complex structures and networks that depend subtly on the particle shape. For example, ellipsoidal and cylindrical microparticles assemble with different preferred orientations, and form structures with differing mechanics. In this talk, progress in developing a quantitative understanding of pair interactions and mechanics of assemblies between rod-like particles is described and compared to experiment. Experiments using microparticles with a variety of particle shapes are presented to illustrate a range of possibilities including control over preferred face for assembly and the assembly of particles with complex features in registry.

On curved interfaces, particle-induced deformations interact with interface curvature field. The resulting capillary energy forces particle migration along curvature gradients and particle alignment along principal axes. Capillary driven migration is explored as a means to direct particles to docking sites, and to mold particle structures.

Kathleen J. Stebe
Kathleen J. Stebe received a B.A. in Economics from the City College of New York, and a Ph.D. in Chemical Engineering at the Levich Institute, also at CCNY. After a post-doctoral year in Compiegne, France, Professor Stebe joined the Department of Chemical Engineering at Johns Hopkins University, where she rose through the ranks to become a tenured Professor and to serve as the department chair. Professor Stebe has been a Fellow at the Radcliffe Institute for Advanced Studies; she has received the Robert S. Pond Excellence in Teaching Award at JHU, the Frenkiel Award from the Division of Fluid Dynamics of the American Physical Society, and was recently names a Fellow of the APS. Since 2008, Professor Stebe has served as the department chair of Chemical and Biomolecular Engineering at the University of Pennsylvania and as the Richer and Elizabeth M. Goodwin Professor of Engineering and Applied Science. Professor Stebe's research focuses on non-equilibrium fluid interfaces, including capillary phenomena, surfactants, and interfacial flows.