Capillary force with a concave probe-tip for micromanipulation

This letter describes the measurement of capillary force with a concave probe-tip for micromanipulation. Using an electric balance, we measure the capillary force for a given gap distance between a spherical object and a concave surface coaxially fabricated on a cylinder. We used three different materials (glass, stainless steel, and polytetrafluoroethylene) to check the influence of contact angles. The results are in good agreement with the theoretical prediction expressed in the normalized form; these indicate that a concave tip can generate a much larger capillary force than a flat one, provided the tip is designed to fit the convex surface of the object; moreover the results clarify that the more wettable a material is, the greater capillary force it can generate. This suggests micromanipulation by capillary force should be more practical by using probes with concave tips specifically designed for the object.