Non-Brownian particles in reversibly sheared suspensions should follow reversible trajectories: the motion that is induced by dialling up the shear should be undone when the shear is reversed. Recent experiments (Pine et al, Nature 2005) show, remarkably, a finite shear onset of irreversibility. I will offer one explanation for this, in which the onset of irreversibility is marked by the formation of long chains of jammed particles.
Time permitting, I'll discuss our efforts to model a micrometer sized swimming device, built by collaborators at ESPCI. The device consists of a filament self-assembled from superparamagnetic micron-sized beads and driven by an applied magnetic field. The swimming mechanics are deciphered in order to show how actuation by a spatially-homogeneous but temporally-varying torque leads to propagation of a bending wave along the filament. This bending wave can only be harnessed for propulsion if the lateral symmetry of the filament is broken, for instance by harnessing one end to a high drag object such as a red blood cell. Finally I explain the extent to which the magnetic micro-swimmer behaves like a flagellated eukaryotic cell.