Sustained Superhydrophobic Friction Reduction at High Liquid Pressures and Large Flows

Abstract

This Article introduces and experimentally explores a novel self-regulating method for reducing the friction losses in large microchannels at high liquid pressures and large liquid flows, overcoming previous limitations with regard to sustainable liquid pressure on a superhydrophobic surface. Our design of the superhydrophobic channel automatically adjusts the gas pressure in the lubricating air layer to the local liquid pressure in the channel. This is achieved by pneumatically connecting the liquid in the microchannel to the gas-pockets trapped at the channel wall through a pressure feedback channel. When liquid enters the feedback channel, it compresses the air and increases the pressure in the gas-pocket. This reduces the pressure drop over the gas-liquid interface and increases the maximum sustainable liquid pressure. We define a dimensionless figure of merit for superhydropbic flows, W(F) = P(L)D/γ cos(θ(c)), which expresses the fluidic energy carrying capacity of a superhydrophobic microchannel. We experimentally verify that our geometry can sustain three times higher liquid pressure before collapsing, and we measured better friction-reducing properties at higher W(F) values than in previous works. The design is ultimately limited in time by the gas-exchange over the gas-liquid interface at pressures exceeding the Laplace pressure. This method could be applicable for reducing near-wall laminar friction in both micro and macro scale flows.

Superhydrophobic drag reduction in laminar flows: a critical review

“…The design approach to help keeping the plastron on a SHPo surface includes making the surface resistant to a wetting transition (Carlborg et al 2008;Lee and Kim 2009;Barthlott et al 2010;Carlborg and van der Wijngaart 2011) and adding a built-in mechanism that replenishes the gas on the surface on demand Kim 2011b, 2012;Lee and Yong 2015). The approach can be categorized depending on whether or not external energy is used, i.e., passive versus active approach, as summarized in Fig.…”

“…To sustain liquid pressure without collapsing the plastron, a side channel was added under the SHPo surface in a way that the air layer on the SHPo surface was automatically compressed by a flowing liquid in the side channel (Carlborg and van der Wijngaart 2011). In this method, a gas layer was compressed between two liquid flows-one in the main channel and the other in the side channel.…”

“…Hydrophilic patches at the tip of the surface structures pinned the liquid-air meniscus on the tip, preventing the gas departure from the surface structures (Barthlott et al 2010); a-3 self-regulating wetting control. An air layer on SHPo surface is pressurized by the intentional wetting transition outside the main channel (Carlborg and van der Wijngaart 2011). b Active approaches.…”

Modification of wettability property of titanium by laser texturing

“…Superhydrophobic surface has wide applications in self-cleaning, anti-fog snow, anti-corrosion resistance, microftuidic chip, nondestructive liquid output [2][3][4][5][6][7][8], etc. Therefore, recent researches mainly focus on improving the wettability of solid surface, especially manufacturing hydrophobic surface.…”

Wetting and Active Dewetting Processes of Hierarchically Constructed Superhydrophobic Surfaces Fully Immersed in Water

“…As described in the experimental section, we could easily pattern electrodes on the substrate of the microposts. Compared with pneumatic methods, which would require a pump, feeding lines, and a source of gas, for example, [34], generation of gases by electrolysis using the built-in electrodes and the surrounding water is much simpler. Compared with chemical methods, which would require specific combinations of materials that would be consumed, for example, [35], the electrolytic method would be more practical.…”