Effect of Pipe Surface Wettability on Flow Slip Property

The effect of surface wettability on the slip property continues to be a controversial subject. In this regard, the contact angles and sliding angles of ethylene glycol, tap water, and #26 white oil deposited on plexiglass pipe, 304 stainless-steel pipe, polytetrafluoroethylene pipe, and polypropylene pipe surfaces were determined using a contact angle meter. The slip velocity, slip length, shear stress, and flow increment of the three liquids flowing in the four pipes were calculated using the slip boundary condition, which refers to the laminar flow resistance of a liquid in a fully developed section of a no-slip pipe under a constant pressure drop. The results show that the main characteristic of slip flow at a solid–liquid interface is that at a constant average velocity of the liquid, the wall shear stress decreases with an increase of the contact angle and a decrease of the sliding angle. This in turn, increases the slip velocity and the slip length and results in lower flow resistance of the liquid. The negative slip phenomenon exists at low-speed flow for part of the liquid, which is different from the linear slip length model. The slip length does not tend to be constant until the average velocity increases to a certain value.