MEASUREMENT OF CONTACT ANGLES FROM THE SHAPE OF A DROP ON A VERTICAL FIBER

Photomicrographs were taken of the organic drops formed on surfaces of vertical cylindrical fibers in water. The organic liquids used were 96% paraffin oil + 4% tetrabromoethane, paraffin oil, and 80% paraffin oil + 20% heptane. The fibers studied consised of plyester, a fluoroethylene-propylene copolymer (FEP), and nylon. The upper and lower contact angles, θt and θb, formed by the drops on the fiber surface were measured as a function of the dimensionless maximum drop radius, N, and length, L, from the projected images. As N increased so did θt, whereas θb only initially increased and then became more or less constant. Furthermore, θb decreased slightly close to the highest investigated values of N for systems involving FEP fiber. For a given system, the difference between the values of θt and θb increased as N increased, confirming that gravity forces affect the drop shape and contact angles. Good agreement is found between the measured values of θt and θb, and those obtained by using the theory for the shape of a drop on a vertical fiber [A. Kumar and S. Hartland, J. Colloid Interface Sci. 124, 67 (1988)]. © 1990.