Orientation and Assembly of Anisotropic Particles by Capillary Interactions

Kathleen J. Stebe, Department of Chemical and Biomolecular Engineering, University of Pennsylvania

Friday, March 5, 2010

Spanos Auditorium

This seminar is part of the Jones Seminars on Science, Technology, and Society series

There is significant scientific and technological potential if reliable means are developed to assemble anisotropic particles into ordered structures. Capillary attraction holds tremendous promise as an important means of orienting, assembling and positioning particles. These interactions arise spontaneously between partially wet particles at fluid interfaces. Particles at interfaces deform the interface to satisfy their wetting boundary conditions. The deformations expand the area of the interface relative to a planar case. The product of this excess area and the surface tension is an excess energy associated with the particle. Capillary attractions arise when deformation fields from neighboring particles overlap; the excess area created by the particles decreases as the particles approach each other. Capillary interactions are 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. Here, particles with shape anisotropy create undulations with excess area that can be locally elevated at certain locations around the particle. The local elevation of excess area (and therefore excess energy) makes these sites locations for preferred assembly, causing particles to orient and aggregate in preferred orientations. We present means to dictate object orientation, alignment, and the sites for preferred assembly, including means of promoting registry of features on particles. These ideas are developed for the example of a right circular cylinder using analysis, experiment and numerics. A series of other shapes are then studied to illustrate the generality of the concepts developed.

About the Speaker

Kathleen J. Stebe recently joined the Department of Chemical and Biomolecular Engineering at the University of Pennsylvania as the Richer and Elizabeth M. Goodwin Professor of Engineering and Applied Science. She also serves as department chair. Professor Stebe studies non-equilibrium interfaces, including interfacial flows, surfactant effects, and capillary interactions, with applications ranging from microfluidics to nanotechnology. Professor Stebe received a B.A. in Economics from the City College of New York. After she earned a Ph.D. at the Levich Institute of the City University of New York, Professor Stebe spent a post-doctoral year in Compiegne, France. She 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. In 2008, Professor Stebe joined the faculty at the University of Pennsylvania. She has been a Fellow at the Radcliffe Institute for Advanced Studies, and has received the Robert S. Pond Excellence in Teaching Award at JHU, and the Frenkiel Award from the Division of Fluid Dynamics of the American Physical Society.