Boundary slip and wetting properties of interfaces: Correlation of the contact angle with the slip length

Abstract

Correlations between contact angle, a measure of the wetting of surfaces, and slip length are developed using nonequilibrium molecular dynamics for a Lennard-Jones fluid in Couette flow between graphitelike hexagonal-lattice walls. The fluid-wall interaction is varied by modulating the interfacial energy parameter εr=εsf∕εff and the size parameter σr=σsf∕σff, (s=solid, f=fluid) to achieve hydrophobicity (solvophobicity) or hydrophilicity (solvophilicity). The effects of surface chemistry, as well as the effects of temperature and shear rate on the slip length are determined. The contact angle increases from 25° to 147° on highly hydrophobic surfaces (as εr decreases from 0.5 to 0.1), as expected. The slip length is functionally dependent on the affinity strength parameters εr and σr: increasing logarithmically with decreasing surface energy εr (i.e., more hydrophobic), while decreasing with power law with decreasing size σr. The mechanism for the latter is different from the energetic case. While weak wall forces (small εr) produce hydrophobicity, larger σr smoothes out the surface roughness. Both tend to increase the slip. The slip length grows rapidly with a high shear rate, as wall velocity increases three decades from 100to105m∕s. We demonstrate that fluid-solid interfaces with low εr and high σr should be chosen to increase slip and are prime candidates for drag reduction.