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Flow-stabilized solids are a class of fragile matter that forms when a dense suspension of colloids accumulates against a semi-permeable barrier, for flow rates above a critical value. In order to probe the effect of particle size on the formation of these solids, we perform experiments on micron-sized monodisperse spherical polystyrene spheres in a Hele-Shaw geometry. We examine the spatial extent, internal fluctuations, and fluid permeability of the solids deposited against the barrier, and find that these do not scale with the P\'eclet number. Instead, we find distinct behaviors at higher Peclet numbers, suggesting a transition from thermal- to athermal-solids which we connect to particle-scale fluctuations in the liquid-like layer at the upstream surface of the solid. We further observe that while the Carman-Kozeny model does not accurately predict the permeability of flow-stabilized solids, we do find a new scaling which predicts the permeability.

Thermal fluctuations, geometric exclusion, and external driving all govern the mechanical response of dense particulate suspensions. Here, we measure the stress-strain response of quasi-two-dimensional flow-stabilized microsphere heaps in a regime in which all three effects are present using a microfluidic device. We observe that the elastic modulus and the mean interparticle separation of the heaps are 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 van der Waals equation of state with excluded volume. This analysis indicates that many-body interactions contribute a significant fraction of the stress supported by the heap.