Dynamic transitions and oscillatory melting of a two-dimensional crystal subjected to shear flow

The effect of a delicate balance of forces on the interparticle dynamics and structure of monodisperse spherical polystyrene particles suspended at the interface between decane and water was observed as shear flow was applied to the system. A strong dipole-dipole repulsion, due to ionizable surface sulfate groups, induces the particles to arrange themselves on a hexagonal lattice under quiescent conditions. The application of a shear flow to the interface, however, forces the lattice into a new semiordered, anisotropic state over which great control is exerted by particle concentration and applied shear rate. At low particle concentrations or high shear rates, the forces applied by the flow dominate the system and cause strings of particles to align in the flow direction to facilitate their movement past each other. A remarkable contrast to this behavior is seen at high concentrations or low shear rates, where the interparticle forces gain importance and tend to more strongly keep the particles in their lattice positions. Consequently, domains of particles are forced to rotate in the flow. The transition between these two regimes and the nature of this rotation, including an associated cyclic melting and crystallization of the lattice, is discussed. (C) 2004 The Society of Rheology.