Sessile drops of surfactant solutions on nonhorizontal solid surfaces

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The effects of surface active agents on contact angle hysteresis is examined by photographing sessile drops at the point of incipient motion on an inclined solid. For solutions of anionic and cationic fluorocarbon surfactants Zonyl FSA and FSC and nonionic surfactant octanol on Teflon, contact angle hysteresis is substantially higher than that for pure water. Hydrophobic bonding of the surfactant molecules at the solid-liquid interface appears to be responsible for this change. Although inhomegeneities on the Teflon surface cause the advancing and receding contact angles to vary at a fixed surfactant concentration, their difference displays significantly lower scatter, indicating that hysteresis is a more reproducible material property than the individual contact angles. Experimental dimensionless sessile drop volumes at the critical inclination angle are compared to an analytical prediction by E. B. Dussan. (J. Fluid Mech. 151, 1 (1985)). For the cationic and anionic surfactant solutions these volumes are between three and four times the prediction from the analysis. This disagreement is especially surprising because the analysis represents a force balance on the drop at the critical configuration, and a reasonable match was obtained for pure liquids by H. V. Nguyen et al. (J. Colloid Interface Sci. 115, 410 (1987)). For solutions of the weak surfactant 1-octanol, the experimentally observed drop volumes at the critical inclination angle match the analytical predictions, indicating that the surfactant strength has an important bearing on the results. One possible explanation is that as the solid substrate is tilted from the horizontal, the sessile drop morphology is modified and the contact line shape changes from circular to one with parallel sides. Such a change must be accompanied by fluid motion. The presence of strong surfactants imposes some hydrodynamic rigidity to the solution-vapor interface and opposes this movement. Therefore, for a fixed drop volume, the inclination angle required for the drop to achieve its critical shape is greater than that predicted by the analysis. © 1988.

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Journal of Colloid And Interface Science