Measurement of the force between uncharged colloidal particles trapped at a flat air/water interface

The radial attraction between microspheres straddling at the air/water interface (Bond number ≪1), whose origin is the irregular shape of the contact line and its concomitant distortion of the water surface, is measured using two light beams of a time-sharing optical tweezer. The colloidal particles used to make the measurements are microspheres made of hydrophobically covered silica to reduce the electrostatic interactions to a minimum. The measured radial force goes as a quadrupolar power law, r⁻ⁿ, with n = 5.02 ± 0.18 and n = 5.04 ± 0.18 for particles of 3 μm and 5 μm, respectively. In both cases, the electrostatic interaction is negligible.