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Mechanics of Soft Materials: Elasticity, Dynamics, and Geometry

Wednesday, March 30, 2011
11:00–12:00 PM
209 Norris Hall, (GTC)
Dr. Douglas Holmes
Princeton University, Princeton, NJ

Abstract: Soft materials exhibit remarkably complex shapes and patterns due to their nonlinear response to large stresses and strains. Thin films wrinkle, the leaves of the Venus flytrap rapidly close around their prey, and a can of soda will crumple under sufficient loads. These behaviors become biologically relevant when considering morphogenesis and the instabilities that occur during the growth of soft tissues. Elastic gels undergo similar volume changes and deformations when subjected to external stimuli. Building upon fundamental ideas based on beam bending, Euler buckling, and poroelasticity, I will highlight the dynamic instabilities that occur by the non-homogenous swelling of an elastic gel and then demonstrate their use in the development of advanced materials.

The dynamics of elastic instabilities significantly depend on the system's geometry and boundary conditions. In the simplest case, a thin beam will bend due to non-homogenous swelling. I will show that the time-dependent response of this deformation can be captured by drawing an analogy to the thermal bending of beams. When the material's geometry becomes more complex, as in the case of a circular disc, the dynamics change dramatically and the disc undergoes bending and twisting due to the axisymmetry.

Similarly, changing the confinement of the material results in an entirely different dynamical response. In this case, a clamped beam will undergo a rapid, snap-buckling instability from one stable equilibrium to another. The timescale of this snap-through is on the order of 10ms. This elastic instability, inspired by the rapid closing of the Venus flytrap's leaves, provides a unique design paradigm for the creation of responsive, advanced materials. I will demonstrate the use of this instability in the development of a biomimetic responsive surface based on an array of microlens shells that reversibly snap between curvatures, and function as a switchable optics device. The ongoing development of a quantitative understanding of these phenomena will allow the mechanics community to explore fundamental problems like of the morphogenesis of soft growing tissues, and to design advanced elastic materials that can adapt to stimuli and change shape, optic properties, adhesion, and directions of flow.

Bio: Douglas Holmes is currently a postdoctoral research associate in the Mechanical & Aerospace Engineering Department at Princeton University. He works in the Complex Fluids Group under Professor Howard Stone where his research focuses on elasto-fluid interactions. At the University of Massachusetts, Amherst, Douglas did his doctoral research in the Polymer Science & Engineering Department with Professor Al Crosby, where his work focused on elastic instabilities of thin, soft materials for the development of responsive polymer surfaces. His work has been highlighted in Discovery News, Wired Magazine, and Science News.