12
Langmuir 1993,9, 12-15
Reversible Solidification of Hard Sphere-like Dispersions Due to Colloidal Crystallization B. E. Rodriguez* and M. S. Wolfe* DuPont Experimental Station, P.O. Box 80356, Wilmington, Delaware 19880-0356
Eric W. Kaler Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716 Received June 22, 1992. In Final Form: September 25,1992
When subjected to low stresses, an abrupt cessation of flow (solidification) is observed in concentrated dispersions of hard sphere-like colloidal particles. This is due to the formation of colloidal crystals, as evidenced by opalescence when illuminated by white light. The elastic polycrystalline structure is stable if unperturbed but can be destroyed by exceeding a yield stress.
Introduction Flow-induced changes in the dispersion structure are generally considered to be central to understanding the rheology of concentrated colloidal dispersions. However, due to the great complexity of colloidal interactions, the dispersion structure-rheology connection has been clearly demonstrated in only a few systems in which the interaction potential is well-defined. A dramatic demonstration of this connection was made for the case of the shear thickening of concentrated, monodisperse, polymer latex dispersions.' For high volume fraction suspensions at shear rates slightly below the critical shear rate for shear thickening, ordering into two-dimensional close-packed sheets which slide over one another was indicated by the presence of a 6-fold light diffraction pattern. Exceeding the critical shear rate resulted in the disappearance of the light diffraction, i.e., destruction of the ordered sheets, thereby eliminating the facile mechanism of particle flow. In turn, this change in flow mechanism resulted in a pronounced increase in the viscosity. More recently]both shear-induced ordering233and shearinduced melting4t5of colloidal dispersions in the range of shear rates in which shear thinning occurs have been demonstrated experimentally by scattering techniques] as well as by computer simulations.6 For the most part, studies have focused on constant shear rate experiments in which the reduced stress, UT, is comparable to or greater than unity. The reduced stress' is defined as ur = a%/kT, where a, u, k, and T are the particle radius, stress, Boltzman's constant, and temperature] respectively. The reduced stress reflects the relative time scales of convective motion, given by the reciprocal of the shear rate, to that of Brownian motion, which is taken as the time required for a particle to diffuse a distance comparable to its radius (a2/6D,where D is the particle self-diffusion constant). For ur > 1,convective motion is fast compared to particle diffusion and the equilibrium dispersion structure is expected to be distorted by flow. As a consequence, it has been observed that shear-induced ordering occurs for ur 1and the ordered arrays of particles are oriented with respect to the direction of shear.213 In the present study, the observation of an abrupt cessation of the flow of hard sphere-like dispersions under
an imposed stress provided an impetus for understanding this phenomenon in terms of the dispersion structure. This unexpected solidification would not be observed in the more conventional constant shear rate experiments. Since there are many physically important processes that occur under constant stress conditions, such as those involving gravity-induced flow, this observation highlights the importance of controlled stress studies of the flow of colloidal dispersions. An additional contrast with studies of shear-induced ordering of liquid-like dispersions is that the observed anomalous flow is present only for a,
