Experimental and theoretical investigations on the flow resistance reduction and slip flow in super-hydrophobic micro tubes

Super-hydrophobic micro tubes with contact angles larger than 150° on inner walls are obtained by solidifying a hydrophobic solution prepared by adding 2% perfluorinated octyl fluorine silane and other additives to modified silicone dilute solution, and diameters of micro tubes are 0.447 mm, 0.728 mm and 0.873 mm, respectively. The changing coefficients of the pressure drops and friction factors in micro tubes due to the super-hydrophobic processing are experimentally measured with Reynolds number ranging from 0 to 2500. A theoretical method is employed to obtain the theoretical correlations of the slip length and velocity in super-hydrophobic micro tubes, so the relationships between the flow resistance reduction and slip flow are quantitatively analyzed. The results illustrate that the changing coefficient of the pressure drop and the friction factor reaches over 60% due to super-hydrophobic surfaces, but the reduction of the pressure drop decreases gradually with the increase of Re and the decrease of tube diameter. The theoretical predictions to changing coefficients of pressure drops and friction factors are consistent with the experimental results. Moreover, the reduction rate of slip length and slip velocity with the increase of pressure and Re is the dominant factor to decide the total resistance reduction in super-hydrophobic micro tubes.