NSF EPSCoR: Soft, Superhydrophobic Surfaces

Overview: Superhydrophobic surfaces are critical for a gamut of applications, ranging from biofouling of marine vessels and of medical devices to self-cleaning surfaces. For typical superhydrophobic surfaces to function correctly, a stable layer of air is required that separates water from the main part of the underlying surface. One of the key challenges in developing superhydrophobic surfaces it to make sure that such a requirement can be achieved consistently. Limited mechanical properties of the superhydrophobic layer can lead to easy penetration of water into the surface microstructures, which leads to collapse of the air layer. In addition, another open challenge is to minimize the contact time of an impacting water drop onto the surface - for example a situation that occurs during raining on windows, airplanes, and automobiles. Therefore, it is important to understand how to optimize both of these situations with a fabrication process that is simple enough for commercial viability. By combining superhydrophobic layers on substrates having different modulus values, will investigate the wetting behavior of a static water drop placed on top, as well as the impact behavior of a water drop impinging the surface at a particular velocity. We hypothesize that the surface deformability will offer a route to decrease the contact time of an impinging drop, while also decreasing the splashing efficiency. To test this hypothesis, we will prepare simple, polydimethylsiloxane (e.g. silicone rubber) surfaces with different crosslinking densities. These surfaces will then be coated with a candle-soot layer, followed by a silica shell and fluorination of the surface. To investigate the static wettability, water contact angle measurements will be taken on the difference surfaces. To investigate the repelling properties, the surfaces will then be tilted until the drop rolls - this will be taken as the roll-off angle and provides a method semi-quantitative method to capture the adhesion between the drop and the surface. Finally, drop impact measurements will be taken by positioning a syringe above the surface and observing the drop bouncing and splashing. Varying the height will allow for systematic control over the incoming velocity to investigate the robustness of the surface properties. Intellectual Merit: The proposed experiments will lead to a better understanding of the interaction between a water drop and a soft, superhydrophobic surface. This will have a positive impact on the fields of solid and fluid mechanics, hydrodynamics, coatings, soft matter physics, drop impact physics, polymer and interfacial science, and the growing field of soft capillarity. Understanding the deformation and how that affects the drop interactions with the underlying surface will provide information on how to design better surfaces for anti-icing, anti-fouling, anti-wetting, and self-cleaning. Moreover, the information that is gained from this work will provide insight into the best fabrication practices for superhydrophobic layers on soft surfaces, and if soft silicone molecules affect the overall water-repellent behavior. It is expected that this study will provide the necessary information to spark the interest of new studies related to soft, superhydrophobic surfaces. Broader Impacts: The proposed project will have a significant impact on our understanding of soft, superhydrophobic surface and how to control water-repellent properties. The expect outcomes will guide the development of biomaterials or coatings for applications in corrosion protection, water filtration, medical devices, and commodity products. Moreover, the project will enable the ultimate goal, which is to provide an opportunity for an underrepresented undergraduate student to conduct a short-term research project in a STEM area, which in this case is soft materials fabrication and characterization. The PI has a record of mentoring women in science and aims to provide an equal opportunity for success of diverse and underrepresented groups. The emphasis on both fundamental science and practical application of this project will encourage the success of this undergraduate student in joining the workforce, whether government, industry or academia.