Boundary-induced inhomogeneity of particle layers in the solidification of suspensions

When a suspension freezes, a compacted particle layer builds up at the solidification front with noticeable implications on the freezing process. In a directional solidification experiment of monodispersed suspensions in thin samples, we evidence a link between the thickness of this layer and the sample depth. We attribute it to an inhomogeneity of particle density induced by the sample plates. A mechanical model enables us to relate it to the layer thickness with a dependency on the sample depth and to select the distribution of particle density that yields the best fit to our data. This distribution involves an influence length of sample plates of about nine particle diameters. These results clarify the implications of boundaries on suspension freezing. They may be useful to model polydispersed suspensions since large particles could play the role of smooth boundaries with respect to small ones.Comment: 16 pages, 13 figure

Dynamique du décollement d'une vésicule en adhésion faible sur un substrat

A l'équilibre thermodynamique, la forme d'une vésicule est régie par l'énergie de courbure, d'adhésion et son poids. La configuration expérimentale permet l'observation de sa forme et la mesure de l'épaisseur du film d'eau entre la membrane et le substrat. Avant la phase finale de décollement, deux régimes sont principalement observés en diminuant le volume réduit: une phase à aire de contact constante et épaississement du film ou bien une phase à forte diminution d'aire et épaisseur constante

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.Comment: 13 pages, 10 figures, submitted to Langmui

Characterization of cell tip curvature in directional solidification

We experimentally characterize the geometry of cell tips in directional solidification by determining their curvature radius. Observations are made on a dilute alloy of a plastic crystal (SCN) confined in a thin sample. They go from the cellular regime to the weakly dendritic regime. Attention is drawn on providing an objective measure, insensitive to the irrelevant features of the measurement procedure and capable of characterizing the overall cell tip region. This is achieved by fitting tip shapes to suitably selected parabolas. We obtain this way accurate and coherent measures that give sense to the evolution of tip curvature radius with control parameters. A scan over large ranges of velocity V, cell spacing ? and thermal gradient G reveals no significant change at the cell to dendrite transition but scaling laws different than those expected at low or large Péclet numbers. They thus provide definite information for understanding cell tip geometry over the cell to dendrite transition regime

Cell shapes in directional solidification : a global study

Extended Summary on CD-RomWe experimentally characterize the whole shape of growth cells in directional solidification, from their tip to their grooves, and in a large domain of control parameter. For this a library of cell shapes is determined and fitted to a class of definite shape functions. This first global characterization of cell geometry in both the real space and the control parameter space provides a firm ground for testing or improving theories or simulations of directional growth in the cellular to near dendritic regime

Solidification en lames minces

National audienc

Cellular arrays in binary alloys : from geometry to stability

In directional solidification of binary alloys, cellular arrays undergo various instabilities that affect both the average width of growth cells and their dynamics. A fundamental issue in the formation of microstructures consists in understanding the occurrence of these instabilities from the evolution of cell geometry. Towards this goal, we experimentally address the instability diagram, the undercooling and the geometry of growth cells in directional solidification of succinonitrile alloys. In particular, we determine a global parametrization of cell boundaries, from their grooves to their tip, over the whole control parameter space of the system. The implications of these determinations for the onset of sidebranching are discussed