MICROSCOPIC AND MACROSCOPIC DYNAMIC INTERFACE SHAPES AND THE INTERPRETATION OF DYNAMIC CONTACT ANGLES

We have studied shapes of dynamic fluid interfaces at distances less t han or equal to 1700 mu m from the moving contact line at capillary nu mbers (Ca) ranging from 10(-3) to 10(-1). Near the moving contact line where viscous deformation is important, an analysis valid to O(1) in Ca describes the shape of the fluid interface. Static capillarity shou ld describe the interface shape far from the contact line. We have qua ntitatively determined the extent of the regions described by the anal ysis with viscous deformation and bq a static shape as a function of C a. We observe a third portion of the interface between the two regions cited above, which is not described by either the analysis with visco us deformation or a static shape. In this third region the interface s hape is controlled by viscous and gravitational forces of comparable m agnitude. We detect significant viscous deformation even far from the contact line at Ca greater than or similar to 0.01, Our measured dynam ic contact angle parameter extracted by fitting the analysis with visc ous deformation to the shape near the moving contact line coincides wi th the contact angle of the static-like shape far from the contact lin e. We measure and explain the discrepancy between this dynamic contact angle parameter and the apparent contact angles based on meniscus or apex: heights. Our observations of viscous effects at large distances from the contact line have implications for dynamic contact angle meas urements in capillary tubes. (C) 1996 Academic Press, Inc.