Wetting rough solid surface

The calculations, which have been made for periodic and random surfaces [6,104-106], demonstrated that if the liquid fully wets the solid surface, the roughness reduces slip and shifts the position of the effective surface plane (the plane where the liquid and substrate velocities are equal) in the direction of the liquid phase [6,107]. The authors of [39,108] suggested intro-... 

From the thermodynamic point of view, wetting of a rough solid surface is governed by Young, Wenzel, and Cassie equations that relate the contact angle between liquid and solid to interface free energies and to surface roughness. 

We consider here the structure of such a thin liquid film which completely wets a rough solid surface. By rough we mean any surface that is not atomically smooth and that can be described by its height Cs p) above a two-dimensional reference plane defined by the vector p. The free energy associated with such a thin film depends on the local height of the liquid-vapor interface taken with respect to the same reference plane (l p) and can be expressed as... 

Wetting of rough solid surfaces by liquids D. Andelman. J.-F. Joanny and M.O. Robbins... 

However, no matter how rough the surface, the forces will be the same as those that exist between a solid and a liquid. The surface roughness may show contact angle hysteresis if one makes the drop move, but this will arise from other parameters (e.g., wetting and dewetting). Further, in practice, the surface roughness is not easy to define. A fractal approach has been used to achieve a better understanding (Feder, 1988 Birdi, 1993). 

In this work, ordered arrays of core-shell particles were used as model surfaces to study the water wetting behaviour of these surfaces. Two factors were varied in the wetting experiments (i) the shell chemistry and hence the solid surface tension of the organic shell, and (ii) the height roughness from sub- xm up to xm roughness values whereas the Wenzel roughness factor was kept constant. 

The roughness of solid surfaces affects wetting owing to two different effects the first is the fact that the actual surface area is increased and the second is pinning of the triple line by sharp edges. 

To sum up, the effects on static contact angles of the departures from ideality of solid surfaces are qualitatively well understood and some of these effects are used in practice to improve or reduce wettability. Moreover, for simple geometries, a semi-quantitative agreement is obtained between experimental results and theoretical predictions. For surfaces with random roughness, predictions of wetting hysteresis present a great difficulty because the relevant size of defects is not yet well-established. 

At present, when working with high-purity materials, smooth solid surfaces and low P02 atmospheres, the thermodynamic contact angle in a particular system can be determined at best within about five degrees. Roughness must be very low, particularly in non-wetting systems in order to obtain such an accuracy. The control of P02 is critical for oxidisable liquids and solids, specially at relatively low temperatures, and dynamic vacuum is often preferable to a static neutral gas atmosphere. 

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