In rainy days, water droplets are seen sliding on solid surfaces such as those of windows and leaves. One may wonder what factors determine the droplet shapes and how fast they move. To answer these questions, we need to understand the physics that governs the flow of liquid in the droplet, as well as the interactions between the fluid flow and the solid. The material properties of solid indeed play an important role. Some solids are hard to deform, which are often named as rigid solids. Materials like gels, rubbers and biological tissues are soft solids, which respond with significant deformation under stresses. The property of softness has been found in recent studies to be a key factor for the motion of droplets on soft solids, which are very different from that on rigid solids. In fact, the dynamic wetting of a liquid, displacing another immiscible liquid or air, on soft solids is a commonality to many biological, medical and industrial processes. 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 knowledge for important applications such as droplet control in channels and fluid coating on soft material. Through our research, we hope to be able to control the time of the filling process and the thickness of the coated film, by varying the softness of solids. Understanding the influence of softness on wetting dynamics will provide us new knowledge for designing fluidic devices that can achieve certain specific functions, as well as manufacturing soft industrial products for e.g. medical applications.
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.