Sustained superhydrophobic friction reduction at high liquid pressures and large flows.

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.