  Micron-sized objects confined in thin liquid films interact through forces
mediated by the deformed liquid-air interface. This capillary interactions
provide a powerful driving mechanism for the self-assembly of ordered
structures such as photonic materials or protein crystals. Direct probing of
capillary interactions requires a controlled force field to independently
manipulate small objects while avoiding any physical contact with the
interface. We demonstrate how optical micro-manipulation allows the direct
measurement of capillary interactions between two micron sized spheres in a
free standing liquid film. The force falls off as an inverse power law in
particles separation. We derive and validate an explicit expression for this
exponent whose magnitude is mainly governed by particles size. For micron-sized
objects we found an exponent close to, but smaller than one, making capillary
interactions a unique example of strong and very long ranged forces in the
mesoscopic world.
