Atomic force microscopy measurement of boundary slip on hydrophilic, hydrophobic, and superhydrophobic surfaces

Abstract

Reduction in drag is important in fluid flow applications. So called boundary slip, a measure of relative fluid velocity at the solid-fluid interface, affects the drag. The slip is a function of the degree of hydrophobicity. In this study, boundary slip was studied through slip length measurements on hydrophilic, hydrophobic, and superhydrophobic surfaces in de-ionized water with atomic force microscopy. On the hydrophilic surface, the experimental data are consistent with no-slip boundary conditions. However, boundary slip is observed on hydrophobic and superhydrophobic surfaces. Experimental results obtained with different squeezing velocities show that the slip length is independent of squeezing velocity. Moreover, the degree of boundary slip is observed to increase when the surface was changed from the hydrophobic surface to the superhydrophobic one. The increasing degree of boundary slip from a hydrophobic surface to a superhydrophobic surface is believed to be because the increasing hydrophobicity favors nanobubble formation. Nanobubbles with a diameter of about 150 nm and a height of about 6 nm were observed on the hydrophobic surface and were verified through observation of nanobubble coalescence.