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Biomedical subjects

C Allain

Publications and source records attributed to C Allain.

At least 19 recordsLinked to original sources
PubMed2006

Imbibition, desiccation and mechanical deformations of zein pills in relation to their porosity.

This paper deals with the interaction between zein (the main protein component of corn grain) and water. It induces macroscopic properties changes and may allow for the understanding of the basis of zein endosperm structure: vitreous endosperm is compact and floury endosperm is porous, giving the endosperm its hard and soft textures, respectively. In that aim porous pills made by compaction of zein powder submitted to different hydration/dehydration processes have been prepared and studied. In particular, imbibition measurements of a pure-water drop deposited onto a zein pill were performed. Also, desiccation of a zein pill previously imbibed induces strong mechanical stresses leading to crack formation and/or large deformations.

Mechanics↗
PubMed2005

Structuring sedimentation in a shear-thinning fluid.

We study the sedimentation of suspensions of monodisperse non-Brownian particles in a shear-thinning polymeric fluid. We observe the formation of particle-rich structures and show that they are associated with a well-defined flow pattern. The results suggest that the observed structuring is a consequence of particle aggregation that amplifies concentration heterogeneities, the flow pattern being further driven by a gravitational instability.

Journal Article↗
PubMed2003

Stable and unstable surface evolution during the drying of a polymer solution drop.

Drying of a sessile drop of a complex liquid can lead to intriguing complex shapes. We report here a study dealing with a model system, made of a hydrosoluble polymer that is glassy when pure. Under solvent evaporation, polymers accumulate near the vapor/drop interface and may form a glassy skin, which bends as the volume of liquid it encloses decreases. The conditions for the occurrence of this buckling instability have been investigated; the experimental results are well explained by a model that compares the characteristic times for drying and for the formation of a glassy skin. Depending on the experimental conditions, different types of shape distortion take place; secondary instabilities that break the axisymmetry are also observed.

Journal Article↗
PubMed2003

Rapid settling of a colloidal gel.

We study the rapid collapse of gels formed from strongly aggregating colloidal suspensions. This gravity-driven collapse is associated with the apparition of fractures in the bulk of the gels that provide an easy route to the gel-supernatant interface for the solvent and are the cause for the strong increase of the settling velocity. We propose a model that connects the apparition of a fracture in the gel to the settling velocity of the interface. This description takes into account the microscopic structure of the gel and is consistent with the experimental results.

Journal Article↗
PubMed2003

Morphologies resulting from the directional propagation of fractures.

When growing in a stress gradient, cracks have a directional growth. We investigate here this type of instability in the case of a colloidal gel deposited on a substrate and left to dry. The use of various materials reveals the existence of two distinct types of dynamics. When the crack nucleation is easy a well known situation is reached: an array of periodic fractures forms, which grow parallel to each other and move quasistatically with the stressed region. In contrast, in materials where the crack nucleation is difficult, a subcritical process is observed with the retarded formation of isolated cracks which move faster and which display an arch shaped trajectory. This type of process appears to be generic in all cases where there is delayed nucleation. This is confirmed by experiments on the directional propagation of cracks in thermally stressed glass.

Journal Article↗
PubMed2002

Aggregation of particles settling in shear-thinning fluids. Part 2. Three-particle aggregation.

We have experimentally studied the coaxial settling of three identical non-Brownian spheres in a shear-thinning fluid at small Reynolds numbers. While settling, the particles create corridors of reduced viscosity in their wake and, if they are initially close enough to one another, they can form stable clusters. By analogy with previous results obtained on two-particle interaction in the first part of this work, we show that the particle velocities can be satisfactorily described using a first-order expression and assuming that the reduced viscosity remains constant. We report systematic experiments performed at different initial separation distances between particles and the use of our simple model allows the prediction of the settling behaviour and in particular the conditions for clusters formation. We thus show that particle aggregation can occur even for large initial distances between particles and within times that are small compared to the time scales in Newtonian fluids.

Journal Article↗
PubMed1997

Viscosity and elasticity during collagen assembly in vitro: relevance to matrix-driven translocation.

In order to better understand the gelation process associated with collagen assembly, and the mechanism of the in vitro morphogenetic phenomenon of "matrix-driven translocation" [S.A. Newman et al. (1985) Science, 228, 885-889], the viscosity and elastic modulus of assembling collagen matrices in the presence and absence of polystyrene latex beads was investigated. Viscosity measurements at very low shear rates (0.016-0.0549 s(-1)) were performed over a range of temperatures (6.9-11.5 degrees C) in a Couette viscometer. A magnetic levitation sphere rheometer was used to measure the shear elastic modulus of the assembling matrices during the late phase of the gelation process. Gelation was detected by the rapid increase in viscosity that occurred after a lag time tL that varied between O and approximately 500 s. After a rise in viscosity that occurred over an additional approximately 500 s, the collagen matrix was characterized by an elastic modulus of the order of several Pa. The lag time of the assembly process was relatively insensitive to differences in shear rate within the variability of the sample preparation, but was inversely proportional to the time the sample spent on ice before being raised to the test temperature, for test temperatures > 9 degrees C. This suggests that structures important for fibrillogenesis are capable of forming at 0 degrees C. The time dependence of the gelation process is well-described by an exponential law with a rate constant K approximately 0.1 s(-1). Significantly, K was consistently larger in collagen preparations that contained cell-sized polystyrene beads. From these results, along with prior information on effective surface tension differences of bead-containing and bead-lacking collagen matrices, we conclude that changes in matrix organization contributing to matrix-driven translocation are initiated during the lag phase of fibrillogenesis when the viscosity is < or = 0.1 Poise. The phenomenon may make use of small differentials in viscosity and/or elasticity, resulting from the interaction of the beads with the assembling matrix. These properties are well described by standard models of concentrated solutions.

Animals↗
PubMed1993

Neonatal vestibular stimulation and mating in cerebellar mutants.

Two cerebellar mutants, staggerer and reeler, and their congenic nonmutants were used in this experiment. Experimental animals were subjected to intense rotational stimulation on a tilted plane during the first 3 weeks of life, while controls were left nonstimulated. The capacity for mating, as evidenced by vaginal plugs or the occurrence of pregnancy, was assayed during two periods: between 36 and 89 days of age (Experiment A) and between 90 and 120 days of age (Experiment B). During Experiment A the mutants as well as the normals were caged inter se with partners of the opposite sex. During Experiment B the animals were caged with intact, sexually experienced partners. The animals were examined daily for evidence of mating. During Experiment A, only 3 of the 89 couples participating in this study showed evidence of mating. During Experiment B, the number of males of both strains which had mated increased significantly. The staggerer females showed a relatively high level of mating activity, whether stimulated or not. The reeler females, in contrast, rarely mated, although early stimulation significantly increased the level of sexual efficiency. The majority of the normal males and females mated, whether stimulated or not. It was concluded that massive motor-sensory stimulation in infancy, improving gait and body balance in staggerer and reeler mice, may also improve mating efficiency.

Animals↗