Structuration and deformation of colloidal hydrogels.

The aim of the present paper is to determine the optimum conditions for the formation of homogeneous colloidal silica hydrogels by aggregation and drying processes, avoiding mechanical instabilities at the surface. Aggregation is controlled by adding monovalent salt to the silica nano-particle suspension while the drying of the sol is also modulated by changing the evaporation rate. A phase diagram reveals two regions in the parameter plane, ionic strength versus evaporation rate: a region where the drop undergoes an isotropic shrinkage and forms the required homogeneous gel and a region where mechanical instabilities appear due to the formation of a solid skin at the gel surface. The frontier between these two regions can be determined by equating the following two characteristic times: the gelation time and the time for skin formation. Permeability measurements of the final gel provide an estimate of the drying stress which is compared to the yield stress of the material. In accordance with the determined phase diagram, our study shows that instabilities appear when the drying stress is larger than the yield stress.

Drying drops : Drying drops containing solutes: From hydrodynamical to mechanical instabilities.

The drying of complex fluids involves a large number of microscopic phenomena (transport and organization of non-volatile solutes) as well as hydrodynamic and mechanical instabilities. These phenomena can be captured in drying sessile drops where different domains can be identified: strong concentration gradients, formation of a glassy or porous envelope that withstands mechanical stress, and consolidation of a layer strongly adhering to the substrate at the drop edge. In colloidal systems, we quantify the evolution of the particle volume fraction at a nanometric scale and microscopic scale and identify the conditions for the envelope formation at the free surface by balancing the effect of diffusion and evaporation. When a solid envelope is formed at a drop surface, the mechanical instabilities induced by the drying result in different drop shapes. Finally, large drying stresses build up in the solid layer adhering on the substrate, and possibly cause crack formation. In particular, we study how crack patterns are affected by the contact angle of drops and the drying conditions. A particular interest of the review is devoted to drying pattern of solutes.

Striped patterns induced by delamination of drying colloidal films.

The drying of a dispersion of nanoparticles on a solid substrate can result in the formation of spontaneous well-ordered stripe patterns left on the substrate. The evaporation of solvent yields large stresses in the material which usually cause crack formation and delamination from the substrate. The formation of these stripes results from a balance between the drying stress which drives the delamination crack front propagation and the cohesive properties of the material. These solid residues arise behind the crack front and can be perpendicular or parallel to the front. It is then possible to inhibit these structures by modifying the cohesive properties of the material. This self-assembly into an ordered pattern can offer an efficient method to produce a patterned surface in a simple way.

Surface patterns in drying films of silica colloidal dispersions.

We report an experimental study on the drying of silica colloidal dispersions. Here we focus on surface instability occurring in a drying paste phase before crack formation which affects the final film quality. Observations at macroscopic and microscopic scales reveal the occurrence of instability, and the morphology of the film surface. Furthermore, we show that the addition of adsorbing polymers on silica particles can be used to suppress the instability under particular conditions of molecular weight and concentration. We relate this suppression to the increase of the paste elastic modulus.

Elapsed time for crack formation during drying.

The drying of colloidal films usually leads to mechanical instabilities that affect the uniformity of the final deposit. The resulting patterns are the signature of the mechanical stress, and reveal the way the system consolidates. We report experimental results on the crack patterns induced by the drying of sessile drops of concentrated dispersions. Crack patterns exhibit a well-defined spatial order, and a regular temporal periodicity. In addition, the onset of cracking occurs after a well-defined elapsed time that depends on the mechanical properties of the gel, and on the drying kinetics. The estimation of the time elapsed before cracks form is related to the elastic properties of the material. This is supported by quantitative measurements using indentation testing and by a simple scaling law derived from poro-elastic theory.

Direct observation of concentration profiles induced by drying of a 2D colloidal dispersion drop.

We report experimental results on the drying of a colloidal dispersion drop in a circular thin cell. This confined geometry is well adapted to quantify concentration profiles inside the drop using fluorescence microscopy. Two stages have been identified in the drop evolution. In the first one the drop is shrinking such as if a pure drop, keeping axisymmetry. In the second one strong distortions occur and result in the appearance of a local depression at the drop surface. This process results in the spontaneous formation of a complex drop shape with both concave and convex interfaces. The influence of the interface concavity on the concentration profiles inside the drop and the drying kinetics are investigated. Particularly, concentration profiles are related to the nonuniform evaporation rate at the distorted drop surface.

Liquid film coating a fiber as a model system for the formation of bound states in active dispersive-dissipative nonlinear media.

We analyze the coherent-structure interaction and the formation of bound states in active dispersive-dissipative nonlinear media using a viscous film coating a vertical fiber as a prototype. The coherent structures in this case are droplike pulses that dominate the evolution of the film. We study experimentally the interaction dynamics and show evidence for formation of bound states. A theoretical explanation is provided through a coherent-structures theory of a simple model for the flow.

Absolute and convective instabilities of a viscous film flowing down a vertical fiber.

The stability of a viscous film flowing down a vertical fiber under the action of gravity is analyzed both experimentally and theoretically. At large or small film thicknesses, the instability is convective, whereas an absolute instability mode is observed in an intermediate range of film thicknesses for fibers of small enough radius. The onset of the experimental irregular wavy regime corresponds precisely to the theoretical prediction of the threshold of the convective instability.