Alain Pocheau

Cell tip undercooling in directional solidification

Undercooling Δt of cell tips in directional solidification of impure succinonitrile is measured with respect to pulling velocity V, thermal gradient and cell spacing. Denoting Vc as the critical velocity of planar front destabilization and ν as the reduced velocity V/Vc, the relation νΔt=1 is evidenced in the cell to near dendritic regime 1⩽ν⩽17 with a high accuracy. Some of its consequences are drawn.

Interaction of Multiple Particles with a Solidification Front: From Compacted Particle Layer to Particle Trapping

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.

Coherence of dendritic sidebranching in directional solidification

Dendrites stand as the main solidification microstructure of dilute alloys. They are characterized by repetitive emissions of sidebranches on a growing needle form whose regularity conditions the solid structure at micrometric scales. We experimentally evidence here the coherence of this dendritic sidebranching in directional solidification. Sidebranches are found to be emitted by bursts of typically four to fifteen branches. The correlation functions of the signal extracted by cuts of the interface profile reveals a burst distribution that is uncorrelated in length, in time, and in phase of sidebranching, but a sidebranch distribution that is remarkably coherent inside each burst. The former feature may explain the appearance of uncorrelated sidebranching in former experiments and the latter feature reveals a puzzling self-organization of dendrites. Both their origins and mechanisms remain to be understood.