Flow-induced anisotropy and reversible aggregation in two-dimensional suspensions

The time evolution of the flow-induced changes in structure of two-dimensional suspensions have been studied by means of video microscopy. The interparticle forces were tailored to produce a two-dimensional 12D) particulate network that could be reversibly broken down by means of a shear flow. Two types of suspensions have been investigated: systems with fairly strong attractive potentials in which essentially rigid bonds develop and systems with weaker attractions in which particles can still slide over each other at contact. For both suspensions interfacial shear flow causes the flocs and their spatial organization to become anisotropic at various length scales. The FFT of the real space images is used to characterize the anisotropy at large length scales. Structural anisotropy at smaller length scales is deduced from the orientational dependence of the pair distribution function and from an harmonic expansion of g(r). The mechanism leading to the anisotropy during shear flow is shown to be related to a directional dependence of breakup and re-formation of flocs. Interfacial flow also affects the density of the flocs. The evolution of the distribution of coordination numbers with shear rate indicates that shear flow densifies the rigid flocs whereas the opposite occurs for the mobile ones.