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Séminaire de Kathleen J. Stebe

Energy Stored in Deformation Fields : Opportunities for Directed Assembly in Soft Matter


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

Energy Stored in Deformation Fields : Opportunities for Directed Assembly in Soft Matter

Colloidal particles are often directed to assemble by use of applied 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. Non-spherical microparticles have undulated contact lines owing to wetting boundary conditions that make the interface deform, creating an excess area field around the particle that bears the signature of particle shape and wetting. The product of this excess area and surface tension is an energy field, which we exploit to direct particles to migrate, orient and assemble into structures that depend on particle shape. Microcylinders at fluid interfaces assemble end-to-end into rigid chains, ellipsoids assemble side-to-side into flexible chains. The mechanics of these assemblies can be related to the capillary energy landscape. If particle-induced deformations are “capillary charges” with analogies to electric charges, then curvature fields are analogous to electric fields. They can be used to direct the migration, orientation and assembly at preferred locations. The importance of curvature-induced deformations in these phenomena is examined.
In another context, we exploit elastic energies that arise in confined liquid crystals. For example, a nematic liquid crystal can be confined using surfaces with well-defined anchoring energies to mold the director field and associated defect fields 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 and using anisotropic microparticles with well-defined anchoring conditions which assemble in preferred orientations.