  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.
