Séminaire Liquides/Mat. molle

Mardi 17 Septembre 2019 à 11h00.

Wetting on soft gels

''How the liquid wets the surface of a solid substrate has been long studied, and is still attracting researchers from different fields because of the fundamental nature of the topic and its wide industrial applications, such as biofouling, printing, coating. Here, I am going to show our experimental and theoretical studies on the wetting statics and dynamics of liquids on soft gels. I will firstly show our home-made observation technique: a quantitative Schlieren optics to measure the surface deformation of a transparent gel film with a high precision over large areas in real time. The long-range surface deformation of soft PDMS films is found to be dependent on the sessile droplet size, and the thickness and elasticity
 of the soft films. Next, I will talk about our model based on the linear elasticity theory with the integration of the surface tension of soft materials that predicts the long-range surface deformation in excellent agreement with the experiments. Then I will give the experimental proof and theoretical analysis of the importance of contact angle hysteresis in the description of the deformation of the surface of the gel and argue that the tangential component of the liquid-vapor surface tension at the contact line, whose contribution are often neglected, significantly affects the surface deformation. Besides, I am going to disscuss our investigations on the wetting dynamics by deflating droplets on PDMS films with well-controlled thickness. Experimentally, I am going to show how the energy dissipation in the soft gel depends on the thickness. Then I will introduce how to rationalize those observations with a model based on the theory of linear viscoelasticity and a simple scaling law accounting for the thickness effect. Finally, I will demonstrate that we are able to guide moving droplets with coatings having a gradient of their thickness.''