A drying drop containing solid particles like tea or coffee leaves a ring stain resulting from the accumulation of the particles near the contact line. In most of industrial applications such as printing, coating or biological microtechnologies, these inhomogeneities must be avoided. A significant range of new technologies involves soft materials. Among these applications, we can cite soft contact lenses coated with silver particles which have antimicrobial properties to prevent eye diseases. Homogeneous coatings of particles, bacteria or cells could also be of practical interest for biological research and diagnosis.
To suppress the coffee stain effect, different strategies have been developed. A physicochemical route consists in adding surfactant that reverses the internal flow with a Marangoni effect. This suppression has also been observed in mixtures of spherical and ellipsoidal particles, which introduce shape-dependent capillary interactions. All of these strategies involve the liquid phase or the particles, and are combined with evaporation to remove the solvent.
In the present work, I propose to substitute the drying by absorption in hydrogels to extract the solvent of a colloidal drop. We study the deposition mechanisms of micrometer-sized particles on the surface of swelling hydrogels. To the best of our knowledge, we show for the first time that the particle deposition on these gels is homogeneous. Using fluorescence microscopy coupled with particle tracking techniques, we record the flow field inside the droplet and analyze the particle deposition mechanism. We rationalize our findings with a theoretical model on the absorption and the particle deposition
dynamics that enables the measurement of the diffusion coefficient in the gels.
A second part of my presentation will be devoted to get more insights in the drying of droplets. Most of the studies focus on a drop on a dry surface. I will address the question of the similarities and differences on the drying of a « coffee drop » on dry and wet tables. This work combines a theoretical and experimental approach on the evaporation dynamics and the particle density at the contact line.