Fall 2014
Colloquia are held on Fridays at 11:30 a.m. in Cullimore Lecture Hall II, unless noted otherwise. Refreshments are served at 11:30 am. For questions about the seminar schedule, please contact Yassine Boubendir.
Date: September 5, 2014
Speaker: Kate Stebe
Department of Chemical and Biomolecular Engineering,
University of Pennsylvania
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Title: "Energy Stored in Deformation Fields: Opportunities for Directed Assembly in Soft Matter"
Abstract:
Colloidal particles are often directed to assemble by use of applied external fields-e.g. by exploiting particle charge or ferromagnetism, and by applying electro-magnetic fields to induce interactions and to steer the particles into well-defined structures at given locations. Here, we exploit fields that arise spontaneously when microparticles are placed in contact with deformable matter. In particular, we have been exploring energy stored in deformation fields around microparticles as a means of directing colloidal assembly.
In one context, we use capillary interactions that occur between anisotropic microparticles at fluid interfaces. The microparticles have undulated contact lines owing to wetting boundary conditions; the fluid interface deforms, creating an area field around the particle that bears the signature of the particle shape and wetting. The product of this area and surface tension is an energy field, which we exploit to direct particles to migrate, orient and assemble.
In another context, we exploit elastic energies and defect fields that arise in confined liquid crystals. For example, when a nematic liquid crystal is confined using surfaces with well-defined anchoring energies, the director field and associated defect fields can be molded to store elastic energy. This energy can be used to steer particles within the bulk or particles that are trapped at the nematic-air interface. We explore this theme using topographically patterned solid surfaces to define defect fields that steer particles trapped at fluid interfaces into assemblies mimicking the defect texture. Related examples for particle migration in smectic films, with either free surfaces or on topographically complex surfaces are discussed.