  The interaction of solidification fronts with objects such as particles,
droplets, cells, or bubbles is a phenomenon with many natural and technological
occurrences. For an object facing the front, it may yield various fates, from
trapping to rejection, with large implications regarding the solidification
pattern. However, whereas most situations involve multiple particles
interacting with each other and the front, attention has focused almost
exclusively on the interaction of a single, isolated object with the front.
Here we address experimentally the interaction of multiple particles with a
solidification front by performing solidification experiments of a monodisperse
particle suspension in a Hele-Shaw cell, with precise control of growth
conditions and real-time visualization. We evidence the growth of a particle
layer ahead of the front at a close-packing volume fraction and we document its
steady state value at various solidification velocities. We then extend single
particle models to the situation of multiple particles by taking into account
the additional force induced on an entering particle by viscous friction in the
compacted particle layer. By a force balance model, this provides an indirect
measure of the repelling mean thermomolecular pressure over a particle entering
the front. The presence of multiple particles is found to increase it following
a reduction of the thickness of the thin liquid film that separates particles
and front. We anticipate the findings reported here to provide a relevant basis
to understand many complex solidification situations in geophysics,
engineering, biology, or food engineering, where multiple objects interact with
the front and control the resulting solidification patterns.
