Measurement of contact angles from the shape of a drop on a vertical fiber

Abstract

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, θ t and θ b , 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 θ t , whereas θ b only initially increased and then became more or less constant. Furthermore, θ b 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 θ t and θ b increased as N increased, confirming that gravity forces affect the drop shape and contact angles. Good agreement is found between the measured values of θ t and θ b , 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)].