RESONANT STICK-SLIP MOTION IN A COLLOIDAL CRYSTAL

THE frictional properties of solid bodies can often be described in te rms of stick-slip motion, in which one body slides against another, at constant driving force, in an alternating series of sticking and slip ping events, Stick-slip motion between surfaces separated by a very th in fluid layer was recently observed on the molecular scale1. At the o ther extreme, macroscopic bodies can exhibit a coupling between stick- slip motion and mechanical (such as acoustic) resonances. The stick-sl ip motion of a bow on a violin string, for example, excites resonant v ibration of the string2. Here we describe the observation of such reso nant stick-slip behaviour on the microscopic scale. We have studied th e flow behaviour of colloidal crystals in a rectangular tube. These cr ystals, comprising a dense suspension of sub-micrometre-sized polystyr ene spheres, are known to exhibit shear-induced melting at high flow r ates3-6. We find that, at lower flow velocity, stick-slip processes at the interface between the crystal and the cell wall can excite resona nces in the crystal, detectable as periodic shifts of the Bragg angle in optical diffraction. We show that the resonances can be tuned by va rying the density of the suspension, which is equivalent to altering t he tension in a violin string.