effect of hydrophobic coating on hagen-poiseuille flows

The effect of a hydrophobic coating on the flow through circular pipes with newtonian fluids has been investigated. velocity fields inside a pipe were experimentally determined by the particle image velocimetry (piv) technique. the test fluid presented a viscosity of about sixty times higher than water viscosity. two glass pipe configurations were used: one uncoated and another covered with an extremely hydrophobic commercial product. comparisons between coated and uncoated pipes were made at similar reynolds (re) numbers, all in the laminar regime (70-250). results show that the hydrophobic effect consists in an observable slip velocity at the wall, with a reduction in shear rate near the pipe boundary. pressure drop values were estimated from a modified hagen-poiseuille equation, taking into consideration the non-zero velocity at the boundary for both set of experiments, and the results show a 20% reduction in the pressure drop for the hydrophobic wall compared with the uncoated pipe case.