Superhydrophobic surfaces for liquid drag reduction

- 제목: Superhydrophobic surfaces for liquid drag reduction
- 연사: Dr. Choongyeop Lee
- 소속: Liquid @ Interface Group, Condensed Matter Lab, University Claude Bernard Lyon 1, France

- 일시: 9월 1일(목) 11시
- 장소: 301동 1512-2호

Abstract:
It is well established that certain superhydrophobic surfaces can effectively reduce frictional drag of liquid flows in nano- and microscale 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. 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 in good agreement of theoretical predictions. 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 μm.

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 μm. 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.

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 with electrodes patterned for a self-limited electrolytic gas generation.