Hidden microscopic life of the moving contact line of a waterlike liquid
JC Fernandez-Toledano and TD Blake and J De Coninck and M Kanduc, PHYSICAL REVIEW FLUIDS, 5, 104004 (2020).
We have used large-scale molecular dynamics (MD) simulations to investigate the contact-line friction of a waterlike liquid interface moving across a flat solid surface. The dynamic contact angle theta(D) has been measured as a function of contact-line speed U-cl using two configurations: a liquid drop spreading spontaneously on a stationary solid surface and a liquid bridge in Couette flow between two solid surfaces moving in opposite directions at a range of steady speeds. The wettability of the solid surface was systematically varied to give a range of equilibrium contact angles theta(0) from 38 to 126 deg. The coefficient of contact-line friction zeta was determined from the expression zeta U-cl= gamma(L)(cos theta(0)- cos theta(D)), where gamma(L) is the surface tension of the liquid. This simple linear equation, which maybe derived from the molecular-kinetic theory (MKT) of dynamic wetting, is predicted to apply at sufficiently low values of U-cl. In addition, we have applied a Langevin formalism to extract the coefficients of contact-line friction directly from the equilibrium fluctuations of the contact line, without any additional theoretical interpretation or model. Both approaches yield the same values for the coefficient within the probable uncertainty, confirming that this intriguing property is determined only by equilibrium properties. Overall, our results show that contact-line friction is a real phenomenon, intrinsic to wetting and applicable to real liquids such as water. Thus, models of dynamic wetting that ignore it are incomplete. From a practical perspective, the concept of a simple linear relationship between surface tension driving force and contact-line velocity may provide a useful simplification in many areas such as nanotechnology where contact-line speeds can be low. Another finding from our study is that slip between water and a molecularly flat, partially wetted solid surface is minimal, with slip lengths of little more than the diameter of a water molecule for equilibrium contact angles of 90 deg or less. Furthermore, we have confirmed a mechanistic link between the coefficients of slip and contact-line friction, specifically beta = zeta/delta, where delta is the thickness of the contact line when viewed at the molecular scale. Thus measurements of the dynamic contact angle or contact-line fluctuations may potentially be used to predict slip and vice versa.
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