No abstract

Suspensions in a liquid of "nearly hard" colloidal spheres were subjected to steady and oscillatory shear flows and studied by light scattering. Samples which exhibit a fluidlike ordering of the particles in equilibrium were induced to a solidlike order by oscillatory shear of strain amplitude -1. In steady shear flow the suspensions showed evidence of "string" structures similar to but less complete than those found in computer simulations of simple liquids. PACS numbers: 82.70.Dd, 05.40,+j, 05.70.Ln, In this Letter we describe studies by light scattering of some nonequilibrium structures of concentrated suspensions of colloidal spheres formed by the application of both oscillatory and steady shear flows. Probably our most interesting finding is that the application of a small-amplitude oscillatory shear, imposing a strain of order 1 on a suspension in which the equilibrium arrangement of particles is fluidlike, can cause strong particle ordering: Essentially an oscillating, three-dimensionally ordered colloidal crystal is formed. With increasing amplitude of applied oscillatory shear a range of structures, showing less pronounced order, is observed. Finally, with steady applied shear a weaker ordering remains. We compare the steady shear results with computer experiments on simple fluids and show also that the observed ordering of the particle structure is correlated with sheer-thinning Theological behavior.The particles used in these experiments consisted of polymethylmethacrylate (PMMA) cores stabilized sterically by a thin layer, =10 nm, of poly-(12-hydroxystearic acid). 1 Measurements by light scattering gave a particle diameter of 0.99 pirn with a relative standard deviation less than 0.05. The particles were dispersed either in pure decalin or in a mixture of decalin and tetralin in proportion (3.06:1, by weight) chosen to nearly match the particle refractive index (-1.51), thus providing nearly transparent samples with strong single scattering of light but relatively weak multiple scattering. The turbidity was =0.2 cm -1 but could be reached further by adjustment of the solvent mixture. Rather the mixture was chosen to place the first minimum in the particle form factor at larger values of scattered wave vector than that of the first Debye-Scherrer ring. Multiple scattering does not influence the features described in this communication, these features being the same for the mixture and pure decalin solvents. Samples covering a range of concentration were prepared as described previously. l As in two earlier studies of similar but smaller particles, 1 we found the phase behavior with increasing particle volume fraction in the following sequence: colloidal fluid-•colloidal fluid + colloidal crystal-* fully colloidal crystal-* colloidal glass. As before we assume the interparticle interaction to be close to "hard sphere" and identify the concentration at which crystallization is first observed with an effective hard-sphere volume fraction E ^0.494, the freezing concentration found in computer simula...