Using the Flotation of a Single Sphere to Measure and Model Capillary Forces

The flotation of small polymer spheres in lower density liquids was studied to better understand the capillary forces associated with curved surfaces. A sphere composed of poly(tetrafluoroethylene) (PTFE) or polycarbonate (PC) was placed in a clear container, and liquid (water, formamide, or ethylene glycol) was slowly added. The progression of liquid rising and ultimately floating or engulfing the sphere was observed. Capillary forces aided buoyancy, allowing some of the spheres to float. A combination of greater lyophobicity and surface tension enabled water to suspend larger spheres than the other two liquids. As compared to PTFE, a smaller density difference between water and PC permitted much larger PC spheres to be buoyed. The largest PTFE that floated was approximately 5 mm in diameter versus 10 mm for PC. Maximum flotation diameters were estimated by an iterative method as well as a closed solution. Both gave reasonable estimates, falling within the range bracketed by the largest floater and the smallest sinker.