Homogeneous deposition of particles by absorption on hydrogels

When a drop containing colloidal particles evaporates on a surface, a circular stain made of these particles is often observed due to an internal flow toward the contact line. To hinder this effect, several approaches have been proposed such as flow modification by addition of surfactants or control of the interactions between the particles. All of these strategies involve the liquid phase while maintaining the drying process. However, substitution of evaporation by absorption into the substrate of the solvent has been investigated less. Here, we show that a droplet containing colloidal particles deposited on swelling hydrogels can lead to a nearly uniform coating. We report experiments and theory to explore the relation between the gel swelling, uniformity of deposition and the adsorption dynamics of the particles at the substrate. Our findings suggest that draining the solvent by absorption provides a robust route to homogeneous coatings. It is well known that the accumulation of particles at a contact line can be explained by liquid flow in a droplet [1, 2, 3], which rationalizes the formation of a so called coffee stain. The combination of an anchored contact line and the liquid evaporation induces a radial flow, which is enhanced by the larger evaporation rate at the corner. To suppress the coffee stain effect, different strategies have been proposed [4]. For instance, a surfactant initially added in the liquid phase [5, 6] or produced by bacteria [7] can reverse the internal flow field through Marangoni stresses related to surface tension gradients. Moreover, the shape of the colloidal particles can change the topology of the menisci at the liquid-air interface, which modifies the particle-particle interactions and suppresses the coffee stain effect for mixtures of spheres and ellipsoids [8]. Also, carefully chosen electric fields can be used to manipulate the motion of the contact line and to apply an electrophoretic force on the particles to suppress their accumulation at the contact line [9]. At present, methods for suppressing the coffee stain effect mainly focus on the properties of the liquid phase, which evaporates. In contrast, our approach consists in removing the liquid with absorption by the substrate, which leaves the particles at the surface of the substrate. This strategy can be achieved with different materials such as solid porous networks [10, 11, 12, 13, 14] for which the absorption is driven by capillarity. However, ~ J(r, t) h(0, t)