Resonant stick–slip motion in a colloidal crystal

Abstract

THE frictional properties of solid bodies can often be described in terms of stick–slip motion, in which one body slides against another, at constant driving force, in an alternating series of sticking and slipping events. Stick–slip motion between surfaces separated by a very thin fluid layer was recently observed on the molecular scale¹. At the other extreme, macroscopic bodies can exhibit a coupling between stick-slip motion and mechanical (such as acoustic) resonances. The stick–slip motion of a bow on a violin string, for example, excites resonant vibration of the string². Here we describe the observation of such resonant stick–slip behaviour on the microscopic scale. We have studied the flow behaviour of colloidal crystals in a rectangular tube. These crystals, comprising a dense suspension of sub-micrometre-sized polystyrene spheres, are known to exhibit shear-induced melting at high flow rates^3–6. We find that, at lower flow velocity, stick–slip processes at the interface between the crystal and the cell wall can excite resonances in the crystal, detectable as periodic shifts of the Bragg angle in optical diffraction. We show that the resonances can be tuned by varying the density of the suspension, which is equivalent to altering the tension in a violin string.