An analysis of the effects of substrate hydrophobicity on Poiseuille flow in capillaries

Charles E. Eckert

doi:10.17918/00000696

The ability to predict laminar flow behavior in tube and packed bed geometries for Newtonian liquids at high wetting coefficients is well established. During the analytical treatment of such systems, the no slip hydrodynamic boundary condition is presumptively applied. It has been reported in the literature, however, that fluid flow may be significantly impacted upon by the wetting characteristics of the fluid/solid interface. In hydrophobic aqueous systems, a flow rate enhancement beyond predicted values reportedly occurs, and has been ascribed to interfacial slip. It is not clear whether slip occurs due to the direct sliding of fluid over the substrate, or through the presence of an interfacial discontinuity. The existance of real slip in molten metals would have a significant impact on melt filtration and the production of metal matrix composites. In this investigation, Poiseuille flow through hydrophobic silica capillaries is used to examine slip in aqueous systems, where the principle independent variable was contact angle. A series of high precision experiments were conducted using capillary tube diameters of 0.1042-0.2068 cm, over the Reynolds number range of 80 to 3500. Hydrophobization was accomplished by silanization using vinyltrimethoxysilane, dimethyldichlorosilane and dimethylpolysiloxane agents that resulted in measured contact angles of 30[degrees]-110[degrees]. The capillary tube contact angles were also manipulated by adjustment of the surface tension of water. Intrusion of the capillary tubes was accomplished both in vacuum and in air for comparative purposes. It was shown by using two independent experimental techniques that hydrophobization by silanization results in a flow retardation of water through vacuum intruded capillaries. The extent of retardation increases with increasing surface hydrophobicity, up to a maximum flow rate reduction of about 13 percent. Decreasing the surface tension of water from 70 to 40 dynes-cm-1 did not have a measurable effect on flow. It was also shown that surface hydrophobicity extends the laminar flow regime. A flow rate enhancement of approximately 4 percent was observed using hydrophobic capillary tubes that were intruded in air, and only above a wall shear stress value of 6.80 dynes [times] cm-2. The observations indicate that Newtonian fluid slip reported on in the literature is caused by an interfacial air layer. A viscous retardation layer is the most plausible explanation for the observed reduction of flow rate in this investigation.

An analysis of the effects of substrate hydrophobicity on Poiseuille flow in capillaries

Files and links (1)

Abstract

Metrics

Details

An analysis of the effects of substrate hydrophobicity on Poiseuille flow in capillaries

Files and links (1)

Abstract

Metrics

Details

Drexel University Social media