Boundary slip and wetting properties of interfaces: Correlation of the contact angle with the slip length
Voronov, RS; Papavassiliou, DV; Lee, LL
JOURNAL OF CHEMICAL PHYSICS, 124 (20): Art. No. 204701 MAY 28 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 epsilon(r)=epsilon(sf)/epsilon(ff) and the size parameter sigma(r)=sigma(sf)/sigma(ff), (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 degrees to 147 degrees on highly hydrophobic surfaces (as epsilon(r) decreases from 0.5 to 0.1), as expected. The slip length is functionally dependent on the affinity strength parameters epsilon(r) and sigma(r): increasing logarithmically with decreasing surface energy epsilon(r) (i.e., more hydrophobic), while decreasing with power law with decreasing size sigma(r). The mechanism for the latter is different from the energetic case. While weak wall forces (small epsilon(r)) produce hydrophobicity, larger sigma(r) 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 10(5) m/s. We demonstrate that fluid-solid interfaces with low epsilon(r) and high sigma(r) should be chosen to increase slip and are prime candidates for drag reduction. (c) 2006 American Institute of Physics.
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