NOTE Direct Measurement of Nanonewton Capillary Forces
torsion force balance has comparable sensitivity with the balances for surface forces, which makes it quite appropriate for measurement of small
We measured experimentally very weak interactions between forces between distant microparticles.
microscopic particles by means of a torsion microbalance of sensi-
To prove this possibility we apply here the torsion microbalance to meativity better than 10 09 N. The model system consists of two thin sure very weak capillary interactions of two microscopic particles (very glass cylinders of radii 50 mm immersed vertically in a liquid at the thin cylinders). The two cylinders are partially immersed in water as shown liquid/air interface. The cylinders attract each other by a capillary in Fig. 1 and are vertically immobilized. The menisci are formed due to immersion force due to deformation of the liquid surface around the cylinders' wettability; hence, they interact via capillary immersion force them. The magnitude of the measured force ranges between 4 1 (4). This leads to attraction between the cylinders when their menisci 10 07 and 10 09 N, depending on the separation distance and the overlap. surface tension. At a fixed interparticle distance the force for solu-
The immersion force is much stronger than the flotation force acting tion of sodium dodecyl sulfate is about twice as small as that for on floating particles which deform the liquid interface by their own weight ( 4 ). Except for particles on the fluid interface, capillary immersion forces pure water due to the lower surface tension. The experimental are important also for the formation of two-dimensional arrays of spherical data are well described by the theory of lateral capillary forces.
particles in a wetting film on substrate (8 -12 ) . In this case the particles
The presented results can be applied to the explanation of the protrude from the water /air interface when the film thickness becomes interaction between spherical particles forming two-dimensional less than the particle diameter. The menisci around the particles overlap, arrays on substrate.