QUASI-STATIC SHAPE OF THE STEADY MOVING MENISCUS - COMPARISON OF THE APPARENT AND EXTRAPOLATED DYNAMIC CONTACT ANGLES

The present investigation compares apparent dynamic contact angles, ev aluated from dynamic capillary heights via the corresponding static re lationships, and extrapolated contact angles deduced from dynamic meni scus profiles fitted with the Laplace equation. A siliconized glass cy linder and a thin filament vertically withdrawn from glycerol-water mi xtures are studied as being representative of the two limiting cases o f very low and high azimuthal curvature. Published data for plunging o f a hydrophobic tape (flat wall) into similar mixtures are also discus sed. It is found that all steady menisci considered are adequately des cribed by the Laplace equation with the exception of the point at the solid wall. Thus, apparent contact angles, which are larger than the e xtrapolated ones, are found in all cases. This difference is small, bu t significantly exceeds the experimental error; it indicates a hydrody namic deformation close to the contact line. The thickness of this def ormation is estimated from the deviation between the extrapolated Lapl acian fitting curve and the actual dynamic profile of the fluid interf ace. For the systems and conditions studied (capillary number, Ca < 0. 15) the thickness of this deformation is 3-5% of the maximum static ca pillary height.