In the research outlined within this proposal, a new superhydrophobic surface design will be developed that is capable of actively controlling both the slip length and overall drag reduction under environmental conditions inaccessible to current superhydrophobic surfaces. Superhydrophobic surfaces are engineered by taking materials with micron or nanoscale surfaces roughness and chemically treating them to make them hydrophobic. Because of the hydrophobicity of these microscale and nanoscale protrusions, when water is brought in contact with a superhydrophobic surface, it does not fully wet the surface. Instead, it remains in contact with only the peaks of the surface topology resulting in a shear-free air-water interface. In this proposal, the focus will be on how the interface shape and deformation affects drag reduction, slip length and the potential of these surfaces to be used as novel filters.
Intellectual Merit : One of the challenges of implementation of superhydrophobic surfaces in real-world applications is that the air-water interface is not robust. Under even modest static or dynamic pressures, the air-water interface can collapse, fully wetting the superhydrophobic surface and eliminating the desired drag reduction. In this proposal, a new microfluidic design is presented that will allow to extend the range and lifetime of superhydrophobic surface. This is achieved through an active back pressurization scheme that will stabilize the air-water interface even under large static pressures. The proposed microfluidic design will allow the investigation of the role of interface curvature on drag reduction, slip velocity and slip length. Access to the air trapped within the superhydrophobic surface will allow the probing of the stability and dynamics of the air water interface under unsteady flow conditions akin to those experienced in turbulent flows through the imposition of a periodic pressure pulse of variable amplitude and frequency. The design will also make it possible to replace the air with incompressible oil that is immiscible in water. With this liquid infused superhydrophobic surfaces the importance of the viscosity ratio between the two liquid phases can be studied on drag reduction while simultaneously maintaining the desired interface shape at arbitrary pressures without the need for back pressurization. Finally, the principles and strategy of back pressurization will be used to develop a series of low-porosity, high-permeability microfluidic filters from a regular array of posts, where the posts are uniquely designed such that their sides are superhydrophobic and support a shear-free air-water interface. The effect of post design and air interface shape will be studied on the permeability of the filter and its effectiveness at removing large and small contaminants from the flow.
Broader Impacts: The proposed research program bridges fundamental experimental wetting phenomena and fluid dynamics with commercial and industrial applications where drag reduction could significantly reduce costs. Investigating paths towards back pressurized superhydrophobic surfaces will allow them to be adapted for high-pressure applications such as pumping and commercial shipping where the large static pressures make implementation a significant challenge. Finally, the proposal will develop a new class of high permeability, low porosity filters that can be quickly and broadly implemented in a number of commercial, biomedical and academic applications. The project has several educational components including involvement of undergraduates in the research through the hiring of three REU students who will work closely with the PI and graduate students on the grant. Additionally, we propose the continuation and further development of an ongoing multifaceted K-12 outreach program. This program will include the development of instructional modules, presentations and videos on fluid dynamics, surface tension and superhydrophobicity that will be presented and distributed to both middle and high school teachers and students through a series of weekend and weeklong outreach activities organized through the UMASS STEM program.
