Centro de Ciencias Aplicadas y Desarrollo Tecnológico, Universidad Nacional Autónoma de México, Circuito Exterior, Ciudad Universitaria, DF 04510, México.
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.