The hard and soft of dense suspensions

A complex fluid, whether blood or mud, can exhibit a tremendous range of mechanical properties determined, among other things, by the liquid’s viscosity, the particles’ composition and chemistry, and the concentration of particles. Other effects being equal, whether the particles are hard or soft doesn’t matter much at low or intermediate concentrations. But that should change at the highest concentrations, more in the realm of cosmetics (as shown here) than of paint. In a conceptually simple model with hard spheres (HSs), a particle’s motion is confined to a cage formed by its nearest neighbors. As the concentration increases, the cage shrinks and motion decreases until, at a HS volume fraction of about 0.64, there remains no more available space for a particle to move. Soft spheres (SSs) are expected to behave differently—they deform each other at close quarters—but the details and implications of that behavior have been unclear. A team led by Peter Schall at the University of Amsterdam now provides a detailed experimental comparison of both types of complex fluid. The physicists applied an oscillating shear with steadily increasing amplitude to dense suspensions of either hard or soft spherical particles, while simultaneously imaging a small region with confocal microscopy. Among their findings: The shrunken HS cages eventually yielded to the shear and ruptured, after which the initially arrested material began to flow once again. In contrast, the SS material deformed more elastically and dissipated energy in long-range relaxation modes, which resulted in a much higher maximum cumulative stress prior to yielding. (K. van der Vaart et al., J. Rheol. 57, 1195, 2013, doi:10.1122/1.4808054 .)