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Abstract

Dynamic wetting of a liquid deposited on a soft solid is very common in many biological, medical, and industrial processes. Thus, understanding the interaction between a moving liquid drop and a soft surface remains crucial, and yet poorly understood. In this context, this thesis focuses on the wetting dynamics of both biological and simple liquid systems on soft tailored surfaces. To do so, we first develop and produce soft PDMS solids of tunable stiffness with a in-house formulation by using pre-polymers of different molecular weight, and crosslinkers of various silane group concentration. We fully characterise the mechanical properties of our different gels with classical rheological tools. We use such soft PDMS surface as a model system with a stiffness comparable to the brain tissue to study the coalescence of phase separating tau protein droplets. We find that the tau protein droplets behave similarly to viscous liquid droplets and therefore that their coalescence dynamics can be described by using the same scaling law. Beyond the relevance of soft and deformable PDMS surfaces for biological applications, they can also be used to tackle more fundamental questions. For instance, we also address recent controversies on the underlying theoretical description of static and dynamic wetting of soft polymer gels. We present measurements of the shapes of moving wetting ridges obtained with high spatio-temporal resolution, combining different liquid systems on top of different soft PDMS gels. We find that the ridge shapes fail to collapse with the commonly used elastocapillary scaling, but for small normal forces, yields a viable prediction of the the dynamic ridge angles. We demonstrate that neither of the debated theoretical models delivers a quantitative description, while the capillary extraction of an oil skirt appears to be the most promising.