Reproductive success among ascomycete fungi requires that spores ejected from the parent fungus be borne by the wind far from the originating fruiting body. Before reaching vigorous wind currents, spores must pass through a thin layer of still air that clings to the fruiting body. Spores are tiny fast moving objects, and therefore suffer enormous fluid drag within this fluid boundary layer. To ensure that their spores pass through the boundary layer, ascomycetes have evolved an elaborate apparatus to launch spores at very high speeds. We have shown that the need to pass through this boundary layer also constrains the shape of the spores, and quantified the stiffness of this constraint by comparing spores with perfect projectiles: bodies of prescribed volume that are designed to experience minimum possible drag in flight. An innocuous observation about the symmetry of the perfect projectile points to a mechanism by which a developing spore that has never left the ascus or encountered a moving fluid may yet be templated to grow in accordance with drag-minimising principles.

Millimeter-sized swimmers often employ different sets of limbs or locomotory gaits for fast and slow swimming. It is believed that these bifurcations in swimming behavior reflect fundamental constraints upon how propulsive force may be generated in the world of small Reynolds numbers inhabited by such swimmers.I have been probing these constraints using a rigid foil flapped in a time-reversible manner as a simulacrum of a propulsive limb. If appropriately shaped the limb is always capable of generating useful thrust by imparting momentum to coherent masses of fluid, and continues to do so even if the rate of energy expenditure in flapping is allowed to become arbitrarily low. However, the most effective targets of this momentum transfer shift from steady coherent eddies to vortices shed from the fin edges as the foil is scaled up.

“Locomotion with a reversible stroke in a weakly inertial or weakly viscoelastic fluid” M. Roper and H.A. Stone shortly-to-be-submitted

"Regimes of force generation from a slowly flapped foil" M. Roper, H.A. Stone and J. Wilkening in preparation

Recent experiments (Pine, Nature 2005) have identified a finite-shear amplitude transition to irreversible motion for the particles in a periodically sheared suspension -- apparent evidence for the finite-shear-amplitude onset of chaotic dynamics. We present an alternative explanation in which the irreversibility persists down to arbitrarily weak shear amplitudes, highlighting the roles played by long-lived clusters of particles in near-contac.