Wetting, Bouncing, Drying and Freezing of Drops on Soft Materials

Interactions of liquids with soft materials occur abundantly in nature and technological applications, be it water wetting the surface of plant leaves, rain drops that impact on textiles, or liquids that are filtrated through membranes. Often these interactions involve complex simultaneous phase change phenomena, such as liquid-vapor and vapor-liquid transitions in drying, boiling, or condensation applications, or during freezing and melting. Traditionally, research focused on the interaction of liquids with rigid materials, largely overlooking the fact, that many relevant materials are soft, a fact that can significantly alter system behavior. Furthermore, employing thin layers of soft coatings to rigid materials can possibly result in desirable substrate properties, such as liquid repellency or low adhesion strength to ice. Hence, the knowledge base of wetting and interfacial phase change involving rigid materials, need to be significantly expanded into the area of soft materials, which is the focus of this thesis. First, droplet drying on soft materials was investigated in depth and by focusing on microscopic level phenomena at the contact line region. We observed a transition towards lower contact angles, when the evaporative flux was high and the receding contact line speed exceeded a characteristic rate. We employed an advanced measurement technique, 4D reference-free traction force microscopy, to quantify the microscale substrate deformation field at the contact line, the so-called wetting ridge, and the related stress field during drying. We observed high and asymmetric local substrate deformations and stresses at elevated receding contact line speeds, leading to contact line motion retardation and inward tilting of the wetting ridge, resulting in smaller contact angles. These findings underpin a rate-dependent wettability on viscoelastic solids and are important for understanding liquid removal from soft materials and associated surface design considerations. The developed methodology has great potential to study a wide range of complex dynamic soft substrate phenomena. Next the focus of the dissertation shifted to the drying of suspension droplets on soft materials and the resulting particle deposit. One well-known type of particle footprint on rigid surfaces is the so-called “coffee ring”, that is when a particle ring is formed after the carrying liquid has evaporated. In addition to lots of interesting physics, enhancing or suppressing the “coffee ring” is relevant to printing, coating and microfabrication industries. We studied this phenomenon on soft materials and found that we can control the topography of the deposit by simply adjusting the environment humidity, regulating the evaporative flux and receding contact line speed. By performing particle tracking, we discovered, that at low environment humidity advection of particles towards the contact line occurs, characteristic to the coffee ring effect, even for a non-pinned contact line, which we attribute to viscous dissipation in the soft substrate. Eventually, pinning occurred for expedited contact line speeds, resulting in a ring deposit as opposed to a circular spot. We confirmed our findings in a contact printing configuration, showing the ability to trigger line bifurcation on soft substrates by regulating the evaporative flux. The subsequent part of this dissertation focused on freezing of supercooled water and ice adhesion on soft materials. This being a large research topic on its own, the results in this study constitute a feasibility study. Soft coatings are known to have low adhesion strength to ice, which is why they are promising icephobic materials and why understanding the mechanisms of ice detachment is important. We found, that when applying a shear force to an icicle attached to a soft material, very high substrate deformations and interfacial slippage occurred. On the other hand, when applying a normal, tensile force, we observed elastic instabilities, that are known to reduce the critical stress needed for fracture of the interface. Gaining knowledge of these distinct modes of ice removal from soft substrates will be the goal of future research activities. Finally, we investigated how substrate flexibility can enhance surface superhydrophobicity. We showed, that additionally to the surface micro- and nanotexture, flexibility can boost liquid repellency. The underlying mechanism is the immediate acceleration and intrinsic responsiveness of flexible materials to impacting droplets, mitigating the collision and lowering the impalement probability. These findings were confirmed on materials ranging from man-made (thin steel or polymer sheets) to natural (butterfly wings). Taken together, the investigations presented in this thesis address important aspects of the broad and complex role of substrate compliance and flexibility in wetting and dewetting of liquids, in particular in the presence of drying, and freezing. In doing so, the work makes a significant contribution to the knowledge base in this area and paves the way for future research activities and applications within the field