Boundary Slip and Wetting Properties of Interfaces: Correlation of the Contact Angle with the Slip Length

R. S. Voronov, D. V. Papavassiliou, and L. L. Lee, J Chem Phys, 124, 204701 (2006).

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 100 to 100,000 m/ 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.

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