Dynamic wetting on soft solids (DyWeSS)

On rainy days, water droplets can be observed sliding across solid surfaces such as windows and leaves. One might wonder what factors determine the droplet shapes and how fast they move. To address these questions, we need to understand the physics governing liquid flow within the droplet, as well as the interactions between the fluid flow and the solid. The material properties of the solid indeed play a crucial role. Some solids, known as rigid solids, resist deformation. In contrast, materials such as gels, rubbers, and biological tissues are classified as soft solids, which undergo significant deformation under stresses. Certain soft materials such as gels also exhibit poroelastic behavior, meaning that solvents can diffuse through them. Recent studies have found that the properties of softness and poroelasticity are key factors influencing droplet motion on soft solids and solvent separation from the gel at the point where the droplet, solid, and surrounding fluid meet. In fact, dynamic wetting of a liquid displacing air or another immiscible liquid on soft solids is a process common to many biological, medical, and industrial applications. However, our understanding of it is far less than what we know for rigid solids. In this project, we aim to fill this gap in knowledge by developing a ground-breaking model to investigate problems of dynamical soft wetting. The success of our project will provide fundamental insights for critical applications, such as controlling droplet movement in channels and fluid coating on soft materials. Through our research, we hope to manipulate the filling time and the thickness of the coated film by adjusting the softness and poroelasticity of solids. Understanding how poro-viscoelasticity affects wetting dynamics will offer new perspectives for designing fluidic devices with specific functions and manufacturing soft industrial products, such as those used in medical applications.

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Many biological and industrial processes involve a fluid displacing another immiscible fluid over soft solids such as biological tissues and gels. This fluid mechanics phenomenon is known as the dynamical soft wetting. Current experiments have shown some fundamentally different dynamical wetting phenomena on soft as compared to rigid solids. From a theoretical modelling perspective, the ambiguity remains due to different types of approximation used. The main challenge is that it is a multi-scaled problem. The underlying physical mechanisms involve length scales ranging from the droplet size to the microscopic lengths at play at the three-phase contact line, where the solid, liquid and gas meet. In this project, we aim to fill this gap in knowledge by developing a ground-breaking model that incorporates both viscous flow in the liquid and the viscoelasticity of the soft solid, so that we can compute the evolution of both the liquid-air and solid-liquid interfaces with resolution that covers all the relevant length scales. We will also characterize the soft wetting at the nano-scale by molecular dynamics simulations, where comparison with the continuum models will allow us to describe nano-scale and finite size effects as well as help clarify the local boundary conditions at the contact line. Using our new models, we will investigate the dynamics of the interfaces in geometrically simple setups such as 2-phase fluid flow in a soft channel and droplet motion on a soft fiber. We will elucidate the physical mechanisms that dictate the spontaneous motion of droplets on soft substrates along stiffness gradients and the stick-slip motion of the contact line, in terms of the high resolution of the interfacial dynamics, as well as uncover the features of dynamical soft wettings for very small droplets, very soft substrates and completely wetting fluids, which have not been investigated so far.