Nonlinear Elasticity of Microsphere Heaps

Thermal fluctuations, geometric exclusion, and external driving all govern the mechanical response of dense particulate suspensions. Here, we describe a microfluidic device which measures the stress-strain response of quasi-two-dimensional flow-stabilized microsphere heaps in a regime in which all three effects are present. We observe that the elastic modulus of the heaps is tunable via the confining stress provided by the fluid flow. Furthermore, the measured stress-strain curves exhibit a universal nonlinear shape which can be predicted from a thermal excluded volume equation of state. These results provide a new technique for measuring the material properties of soft solids, and open the door for designing soft solids with particular characteristics. PACS numbers: 47.57.J-, 47.61.Jd, 05.40.Jc, 82.70.Dd The emergent mechanical properties of particulate matter play a central role in many physical processes and biolog-ical systems. Hard-sphere Brownian suspensions allow di-rect study of important condensed matter phenomena, such as the emergence of irreversibility from reversible equations of motion [1, 2], the glass transition [3–6], crystal nucleation and dislocation [7, 8], and dynamical heterogeneities [9, 10]