Flow of Water Adjacent to Smooth Hydrophobic Solids

The boundary flow condition for water near a hydrophobic plate was determined by measuring the force acting on a hydrophilic sphere as it was driven toward a hydrophobic plate in water. Comparison to the theoretical forces for different boundary conditions showed that the measured force was weaker than expected for the no-slip boundary condition and consistent with partial slip with a magnitude of tens of nanometers on the hydrophobic solid. The slip length increased steeply when the water contact angle on the solid increased above 90°, with the most hydrophobic plate (terminated with a perfluorinated alkane) having a contact angle of about 120° and a fitted slip length in the range 40–100 nm. The latter was fitted assuming that the hydrophilic solid retained zero slip length. Analysis of a probe driven by thermal oscillations produced similar results. The sharp increase in slip length when the water contact angle is above 90° has been previously predicted, but the magnitude is much greater than in previous measurements or simulations.