Instability and morphology of polymer solutions coating a fiber

We report an experimental study on the dynamics of a thin film of polymer solution coating a vertical fiber. The liquid film has first a constant thickness and then undergoes the Rayleigh-Plateau instability which leads to the formation of sequences of drops, separated by a thin film, moving down at a constant velocity. Different polymer solutions are used, i.e. xanthan solutions and polyacrylamide (PAAm) solutions. These solutions both exhibit shear-rate dependence of the viscosity, but for PAAm solutions, there are strong normal stresses in addition of the shear-thinning effect. We characterize experimentally and separately the effects of these two non-Newtonian properties on the flow on the fiber. Thus, in the flat film observed before the emergence of the drops, only shear-thinning effect plays a role and tends to thin the film compared to the Newtonian case. The effect of the non-Newtonian rheology on the Rayleigh-Plateau instability is then investigated through the measurements of the growth rate and the wavelength of the instability. Results are in good agreement with linear stability analysis for a shear-thinning fluid. The effect of normal stress can be taken into account by considering an effective surface tension which tends to decrease the growth rate of the instability. Finally, the dependence of the morphology of the drops with the normal stress is investigated and a simplified model including the normal stress within the lubrication approximation provides good quantitative results on the shape of the drops.Comment: Accepted in Journal of Fluid Mechanic

Annular cracks in thin films of nanoparticle suspensions drying on a fiber

We report an experimental study of the crack pattern formed during the drying of a colloidal suspension. A horizontal fiber, which provides a one dimensional, boundary-free substrate, is coated by a film of micronic thickness. The geometry imposes a remarkable annular crack pattern and allowing precise measurements of the crack spacing over a short range of film thickness (between 2 and 10 $\mu$m) which varies linearly with the film height. We compare our experimental data with a model proposed by Kitsunezaki which suggests that the variation of the crack spacing with the film thickness depends on the ratio between a critical stress at cracking and a critical stress for slipping on the substrate. By measuring the friction force of the colloidal gels on a hydrophobic surface through a cantilever technique, we can deduce the critical crack stress for these colloidal gels simply by measuring the crack spacing of the pattern.Comment: Accepted in EP

Craquelures dans les couches picturales des peintures d'art

Reflets de la Physique, vol. 3 (Mars 2007).Les craquelures dans les couches picturales présentent un intérêt nouveau dans l'étude des peintures d'art. Par la complexité de leur morphologie, elles sont depuis longtemps un moyen de juger l'authenticité des peintures. Les figures de craquelures retracent également la manière dont l'artiste a œuvré. En étroite collaboration avec le C2RMF du musée du Louvre, nous avons étudié les morphologies de craquelures dans la série de peintures consacrées aux Apôtres de Georges de La Tour, pour laquelle peintures de l'artiste et copies n'ont pas été clairement dissociées jusqu'à présent. Nous nous baserons sur des expériences modèles de séchage de suspensions colloïdales qui peuvent aider à comprendre les morphologies de craquelures observées sur les œuvres réelles

Solute mechanical properties impact on the drying of dairy and model colloidal systems

The evaporation of colloidal solutions is frequently observed in nature and in everyday life. The investigation of the mechanisms taking place during the desiccation of biological fluids is currently a scientific challenge with potential biomedical and industrial applications. In last decades, seminal works have been performed mostly on dried droplets of saliva, urine and plasma. However, the full understanding of the drying process in biocolloids is far to be achieved and, notably, the impact of solute properties on the morphological characteristics of the evaporating droplets, such as colloid segregation, skin formation and crack pattern development, is still to be elucidated. To this purpose, the use of model colloidal solutions, whose rheological behavior is more easily deducible, could represent a significant boost. In this work, we compare the drying of droplets of whey proteins and casein micelles, the two main milk protein classes, to that of dispersionsof silica particles and polymer-coated silica particles, respectively. The mechanical behavior of such biological colloids and model silica dispersions was investigated through the analysis of crack formation, and the measurements of their mechanical properties using indentation testing. The study reveals numerous analogies between dairy and corresponding model systems, thus confirming the latter as plausible powerful tool to highlight the signature of the matter at the molecular scale on the drying process

Structural anisotropy of directionally dried colloids

Aqueous colloidal dispersions of silica particles become anisotropic when they are dried through evaporation. This anisotropy is generated by a uniaxial strain of the liquid dispersions as they are compressed by the flow of water toward a solidification front. Part of the strain produced by the compression is relaxed, and part of it is stored and transferred to the solid. This stored elastic strain has consequences for the properties of the solid, where it may facilitate the growth of shear bands, and generate birefringence