Torsion balance for measurement of capillary immersion forces

Particle-particle and particle-wall capillary interactions were measured as a function of the separation distance. The ''particles'' were vertical thin glass cylinders and/or small glass spheres, protruding from an air/liquid interface. The particles attract each other due to the overlapping of the menisci formed around each of them. The force of interaction is detected by a sensitive torsion microbalance. It is based on counterbalancing the moment of a couple of forces, acting between two pairs of particles, by the torsion moment of a thin platinum wire. By varying the wire diameter, we accessed forces differing by several orders of magnitude, from about 5 dyn at small separation between the particles down to 0.001 dyn at large separation. The smallest force was measured with two cylinders of diameters about 300 mu m. For two spheres (diameters 1.2 mm) we obtained difference in the forces corresponding to different heights of protrusion from the liquid surface. For interacting sphere and glass cylinder the force follows similar trends as the forces between two spheres or two cylinders. In the case of sphere and glass wall, however, the force first increases with decreasing the distance and then decreases close to the wall passing through a maximum. The predictions of the theory of capillary immersion forces are in quantitative agreement with the experimental results.