Mechanical properties of colloidal crystals at fluid interfaces

Abstract We characterise the local mechanical properties of two-dimensional colloidal crystals with hexagonal symmetry assembled at the flat interface between oil and water. Our experiments elucidate the conditions under which the material behaves isotropically, as opposed to those where the microstructure plays a major role. Brownian fluctuations are used to extract the stiffness of the lattice under the continuum approximation, whereas at larger displacements, obtained by optically driving one particle through the structure, the mechanical resistance of the lattice depends on both the area fraction and the direction of the applied force. Remarkably, the minimum resistance does not necessarily correspond to a probe being driven between neighbours, i.e. at 30° with respect to the crystal axes.