CAREER: Wetting and Dynamics on Soft and Swollen Polymeric Surfaces

CAREER: Wetting and dynamics on soft and swollen polymeric surfaces Overview: The wetting and dynamics of liquid drops on soft surfaces is critical for many emerging technologies, from advanced coatings and adhesives to biomaterials and soft robotics. Over the last decade, wetting has been investigated on slippery lubricant-infused porous surfaces (SLIPS) and on soft crosslinked surfaces. When a drop is placed on SLIPS, the lubricant is pulled from the porous surface. For soft solids, the drop drives an out-of-plane deformation of the crosslinked network. However, wetting is not well understood for elastomers swollen with fluids having similar chemical makeup. When a water drop is placed onto a swollen elastomer, the network can deform, the swelling fluid can separate from the network, or a combination of both can occur. Hence, this combines concepts from both SLIPS and soft surfaces. Yet it is unclear what governs the wetting geometry. Moreover, it is not understood how the swelling fluid and the soft network synergistically control wetting dynamics compared to the unswollen case. The main hypothesis is that equilibrium wetting on soft swollen networks is governed by a balance of network deformation, interfacial tensions, and mixing thermodynamics of the swollen network. For dynamic cases where either a wetting ridge is growing or the contact line is moving, we hypothesize that viscoelasticity of the network and poroelasticity of the swelling fluid flowing to the contact line will need to be considered. In order to systematically cover a wide range of properties, including the viscoelasticity and the amount and molecular weight of swelling fluid, we will exploit both linear and bottlebrush polymers. Confocal microscopy and fluorescent molecules will be employed to visualize the network and swelling fluid simultaneously and independently. The proposed work focuses on four aims. 1) How does a soft swollen elastomer accommodate a drop’s surface tension? When does the swelling fluid separate? 2) What is the time- dependence of a growing wetting ridge? How does the swelling fluid change the geometry of a moving contact line compared to an unswollen case? 3) How does an applied global strain affect the contact line wetting behavior? 4) A fourth aim focuses on an international educational experience that investigates how insects interact with these well-characterized swollen surfaces. Intellectual merit: The proposed project is expected to provide a better understanding of how a drop interacts with soft polymeric surfaces having a range of properties and liquid/solid components. This will be beneficial for the fields of polymer science, soft matter physics/mechanics, surface science, and bio- and bioinspired materials. For example, understanding the coupling of swelling fluid and polymer network properties will provide insight into ‘wet’ adhesion, which is important for climbing organisms, like lizards and insects, as well as insight into lubricated manufacturing processes. It is anticipated that the proposed work will provide knowledge on the phase behavior and mechanical response of soft interfaces. This knowledge is a necessary stepping stone for developing models that account for different material properties and for developing new guidelines on advanced materials design. The proposed work is original because it introduces molecular scale physics into continuum scale mechanics to bridge the fields of SLIPS and soft elastomeric surfaces. Broader impacts: The expected outcomes will guide new advances in coatings, adhesives, biomaterials, and organogels, which are important for applications in synthetic tissues, cosmetics, water recovery, soft robotics, and daily-use products. Moreover, the project will establish an advanced experimental approach for researchers to investigate multi-phase soft or biological materials, (bio)adhesion, and soft interfaces. In addition, the project affords an avenue to broaden the scientific literacy of diverse students in polymers, interfacial science, and mechanics – a critical set of skills for today’s science and engineering landscape. Training will be accomplished through development of a high school summer camp, a workshop of regional research groups working on soft materials and interfaces, an international graduate student research experience, and new curriculum development. The PI aims to continue to provide an equal opportunity for success of diverse and underrepresented students. The emphasis on both fundamental and practical science of the proposed program will enable the future success of students joining the modern workforce.