Surface contact line, roughness

Ai, X., "Numerical Analyses of Elastohydrodynamically Lubricated Line and Point Contacts with Rough Surfaces by Using Semi-System and Multi-Grid Methods, Ph.D. Thesis, Northwestern University, 1993. 

It has been shown that contact angle hysteresis might arise as a result of inhomogeneities of the surface wetted by the liquid phase or surface roughness [70]. When surface roughness plays a considerable role, the observed contact angle may depend on the exact position of the contact line with respect to the microscopic or... 

Contact angle hysteresis can be caused by surface roughness, heterogeneity, dissolved substances, and structural changes of the solid at the three-phase contact line. 

The evaporation of a droplet on a rough surface behaved similarly except that the second regime was not entered the diameter stayed constant due to the pinning of the droplet s contact line by the rough surface. 

Wenzel s relation has been confirmed in terms of the first two laws of thermodynamics. Huh and Mason, in 1976, used a perturbation method for solving the Young-Laplace equation while applying Wenzel s equation to the surface texture. Their results can be reduced to Wenzel s equation for random roughness of small amplitude. They assume that hysteresis was caused by nonisotropic equilibrium positions of the three-phase contact line, and its movement was predicted to occur in jumps. On the other hand, in 1966, Timmons and Zisman attributed hysteresis to microporosity of solids, because they found that hysteresis was dependent on the size of the liquid molecules or associated cluster of molecules (like water behaves as an associated cluster of six molecules). 

Recent analyses of contact angle hysteresis has treated surface heterogeneity in terms of random fluctuations see the excellent review by deGennes The approach is to treat both surface roughness and variations in surface composition as weak fluctuations, i.e. deviations from the ideally smooth surface, du/dy (Fig. 11) and from the difference in solid/liquid and solid/vapor surface energies, — yj - Both fluctuations are considered to be equivalent in perturbing the contact line and are analyzed in terms of their effect on the elastic line energy. 

Air entrainment] = [Dynamic contact line approaches 180P] web speed too fast/ too little vacuum/gap too large/viscosity too high/Ca too high > 0.1/rough sur-face/surface tension too low. 

Contact angle hysteresis can be attributed to a departure from the conditions of ideality of the solid, namely, surface roughness, chemical inhomogeneities, or contamination, or the presence of solute (polymers or surfactants) deposits on the surface [2]. Figure 7 shows a plan view illustration of the hysteresis mechanism in which the receding contact line of a drop is locally pinned when the drop moves over a localized surface inheterogeneity which is more hydrophilic than... 

Wetting and Spreading, Fig. 8 Schematic depiction of the contact line region on a microscopic level illustrating the distinction between the apparent (macroscopic) contact angle 6 and the intrinsic (microscopic) contact angle 6 due to surface roughness effects on the substrate... 

Wetting and Spreading, Fig. 9 Contact line region of a drop on a (a) physically rough but chemically homogeneous surface and (b) a physically smooth but chemically inhomogeneous surface consisting of two different... 

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