Superhydrophobic Surfaces for Liquid Drag Reduction: Design, Fabrication, and Slip Testing.

摘要

It is well established that certain superhydrophobic surfaces can effectively reduce frictional drag of liquid flows in nano- and micro-scale fluidic systems. However, whether or not they can eventually be developed to be effective for regular-scale (i.e., millimeters and above) fluidic systems has remained uncertain. The goal of the research described in this dissertation is to advance the understanding of superhydrophobic surfaces for liquid slips and ultimately develop slip surfaces that reduce drag even for macro (i.e., large) fluidic systems.;To obtain a meaningful drag reduction in a macro fluidic system, a slip length comparable to the length scale of the macro fluidic system is required. Considering the large gap between the state of the art (i.e., ∼20 microns) and the required (i.e., > 100 microns), we start by studying how superhydrophobic surfaces produce the slip in a systematic and quantitative manner. First, we investigate how surface parameters affect slip length and provide the design guideline to maximize slip effect. It is shown that slip length increases exponentially with gas fraction and linearly with pitch of posts or grates on two model surfaces. To obtain experimental data that can confirm or disprove the theoretical predictions, however, superhydrophobic surfaces with no defect over the entire sample area (i.e., 6 cm diameter) are necessary --- achieved by developing a specialized lithography process in house. By pushing the surface parameters (i.e., gas fraction and pitch) to the thermodynamic limit of non-wetted (i.e., Cassie) state, we achieve a giant liquid slip of 187 microm.;An investigation into the effect of a surface hierarchy of superhydrophobic surfaces helps us expand the Cassie range and increase the liquid slip even further. Specifically, nanostructures added only onto the sidewalls of microstructures significantly reinforces the stability of the non-wetted state such that we can further increase gas fraction and pitch to achieve slip lengths as large as 400 microm. Interestingly, adding nanostructures on top of the microstructures as well, i.e., nanostructures uniformly over the entire microstructure surface, is found detrimental to the slip length. Giant slip lengths (>100 mum) obtained in the present studies are expected large enough to directly benefit macro fluidic systems.;In real flow conditions, a superhydrophobic surface can be easily impregnated with water by various external instigators (e.g., high liquid pressure, pressure variations, debris), causing the drag reducing effect to be lost (for high pressure) or deteriorate over time (for pressure variation and debris effect). To address this issue, we develop new slip surfaces, on which the non-wetted state can be indefinitely maintained by restoring superhydrophobicity if and when the surface becomes wet, i.e., transitions to Wenzel condition. Our surface consists of hydrophobic microstructures upon a hydrophobic nanostructured bottom surface. Furthermore, the surface has electrodes patterned on the nanostructures for a self-limited electrolytic gas generation. The gas is generated to recover the superhydrophobicity only when and where the microstructures are wet, i.e., with minimal energy consumption. This surface architecture reliably works even under high liquid pressures and on defective surfaces, which can be commonly encountered in practical applications. We expect the approach developed here will lead us to drag reduction in real flow environments.