A defining characteristic of soft materials is their structural disorder at small length scales. Indeed, the special geometric properties of these materials have a profound effect on their behavior. Yet it is becoming increasingly clear that it is not only geometry that is important, also microscopic interactions are critically relevant in determining the most salient features of their surprising behavior. This is evident in a highly idealized type of soft matter: suspensions. These mixtures of solid particles in a fluid have very surprising and counter-intuitive rheological behavior, and are thus an ideal test bed for a wide range of soft matter experiments.
One aspect of suspensions is very intuitive: they become more viscous when their solid fraction is increased. Yet even simply quantifying this behavior has remained a huge challenge. However, Boyer et al. have recently accurately established this divergence in a series of very elegant experiments . Although Boyer has provided a comprehensive description of the suspension rheophysics, the origin of the divergence remains largely obscure.
To fill this gap, we aim to find and illuminate this source of rigidity by answering critical questions: what is the role of flow structure and the microscopic force structure in the emergent rigidity? Furthermore, how do interparticle friction and other particle properties, like shape or stiffness, influence the rheology of these materials?
To answer these questions, we investigate the microstructure of the suspension in two ways: both suspension structure and forces are probed. Imaging structure is done via established techniques that make use of index matching . To measure internal forces, we have developed an optical rheometry technique. We use birefringent particles (Figure 1) in an index matched fluid. Such birefringent particles give complete access to the spatial stress field inside the materials, and have been used very successfully for dry granular materials [3,4]. This technique therefore gives access to the anisotropy of the force structures that are likely responsible for generating the rigidity.
 Boyer, F., É. Guazzelli, and O. Pouliquen, Physical Review Letters, 107 188301 (2011)
 Dijksman, J.A., et al. Rev Sci Instrum, 83 011301. (2012)
 Ren, J., J. Dijksman, and R. Behringer, Physical Review Letters, 110(1) 018302 (2013)
 T.S. Majumdar, R. P. Behringer, Nature, 435 1079 (2005)