EDGE PROFILES AND DYNAMIC CONTACT ANGLES OF A SPREADING DROP

The spreading dynamics of a drop edge is studied experimentally using a laser light interference microscopy method. From the movie of the interference fringes, the spreading speed at the drop edge is measured and the edge profile is reconstructed. The experimental edge profiles at different speeds agree very well with those predicted by Hervet and de Gennes' theory on a one-dimensional thin spreading edge with a constant and uniform surface tension. The edge profiles at different capillary numbers, C, can be collapsed into one dimensionless curve, using their scaling laws. The agreement between experiments and theory suggests that in our experiments the surface tension of the drop remains constant and uniform during spreading. The dependence of the dynamic contact angle, θ, on distance and C also agrees with their theory. The edge profile is almost a straight line at a large distance from the three-phase contact line and becomes concave toward the air phase as the distance decreases. The profile becomes flatter as C decreases. As a result, θ decreases with decreasing distance and decreasing C. The dynamic contact angle at large distance away from the three-phase contact line, θ1, at different C is compared with two other contact angles, θ0 and θRH, obtained from photographs at 42× magnification of the silhouette of a spreading drop. θ0 is the angle between the solid plane and the tangent to the silhouette at the drop edge, and θRH is the angle at the edge assuming the drop shape is a spherical cap. θ1, θ0, and θRH are also compared with a correlation for a meniscus moving in a capillary tube as functions of C. For the same C, there is a good agreement between θ1, θ0, θRH, the theory, and the correlation. Data also suggest that θ1 and θ0 are the same angle. © 1992.