Delayed lubricant depletion on liquid-infused randomly rough surfaces

Abstract

In this study, pressure drops on liquid-infused superhydrophobic surfaces were measured through a microchannel. A number of different superhydrophobic surfaces were prepared and tested. These surfaces included several PDMS surfaces containing precisely patterned microposts and microridges as well as a number of PTFE surfaces with random surface roughness created by sanding the PTFE with different sandpapers. Silicone oil was selected as the lubricant fluid and infused into the microstructures of the superhydrophobic surfaces. Several aqueous glycerin solutions with different viscosities were used as working fluids so that the viscosity ratio between the lubricant and the working fluid could be varied. The lubricant layer trapped within the precisely patterned superhydrophobic PDMS surfaces was found to be easily depleted over a short period of time even in limit of low flow rates and capillary numbers. On the other hand, the randomly rough superhydrophobic PTFE surfaces tested were found to maintain the layer of lubricant oil even at moderately high capillary numbers resulting in drag reduction that was found to increase with increasing viscosity ratio. The pressure drops on the liquid-infused PTFE surfaces were measured over time to determine the longevity of the lubricant layer. The pressure drops for the randomly rough PTFE surfaces were found to initially diminish with time before reaching a short-time plateau which is equivalent to maximum drag reduction. This minimum pressure drop was maintained for at least three hours in all cases regardless of feature size. However, as the depletion of the oil from the lubricant layer was initiated, the pressure drop was observed to grow slowly before reaching a second long-time asymptote which was equivalent to a Wenzel state.