Structural dynamics of an actin spring.

Actin-based motility in cells is usually associated with either polymerization/depolymerization in the presence of cross-linkers or contractility in the presence of myosin motors. Here, we focus on a third distinct mechanism involving actin in motility, seen in the dynamics of an active actin spring that powers the acrosomal reaction of the horseshoe crab (Limulus polyphemus) sperm. During this process, a 60-µm bent and twisted bundle of cross-linked actin uncoils and becomes straight in a few seconds in the presence of Ca(2+). This straightening, which occurs at a constant velocity, allows the acrosome to forcefully penetrate the egg. Synthesizing ultrastructural information with the kinetics, energetics, and imaging of calcium binding allows us to construct a dynamical theory for this mechanochemical engine consistent with our experimental observations. It also illuminates the general mechanism by which energy may be stored in conformational changes and released cooperatively in ordered macromolecular assemblies.

Onset of buckling in drying droplets of colloidal suspensions.

Minute concentrations of suspended particles can dramatically alter the behavior of a drying droplet. After a period of isotropic shrinkage, similar to droplets of a pure liquid, these droplets suddenly buckle like an elastic shell. While linear elasticity is able to describe the morphology of the buckled droplets, it fails to predict the onset of buckling. Instead, we find that buckling is coincident with a stress-induced fluid to solid transition in a shell of particles at a droplet's surface, occurring when attractive capillary forces overcome stabilizing electrostatic forces between particles.

First-order Fréedericksz transition in the presence of light-driven feedback in nematic liquid crystals.

We show that feedback, which introduces a dependence of the electric field on the liquid-crystal director, renders the Fréedericksz transition first order. Experimentally, the feedback is introduced as a light loop in a liquid-crystal light valve. Theoretically, we include the feedback term into the Frank free energy and we derive an amplitude equation that is valid close to the transition. The theoretical description is in qualitative agreement with the experimental observations. Depending on the values of the feedback parameters, both theory and experiment exhibit bistability, propagation of fronts, and a Maxwell point.