Measurement of the capillary interaction force between Janus colloidal particles trapped at a flat air/water interface

The capillary interaction force between spherical Janus particles trapped at the air-water interface is measured using a time-sharing optical tweezer (bond number < 1). One face of the particles is hydrophilic, and the other one, hydrophobic. Measured force goes from almost pure quadrupolar to almost pure hexapolar interaction due to the three-phase contact line corrugation. Measured force curves are modeled as a sum of power laws, Ar − α + Br − β + Cr − γ , obtained from an expansion in capillary multipoles. The mean values for the exponents of particle pairs of 3 μm are 〈 α 〉 = 5.05 ± 0.12, 〈 β 〉 = 7.02 ± 0.03, and 〈 γ 〉 = 5.96 ± 0.03. For particles pairs of 5 μm, we find 〈 α 〉 = 5.02 ± 0.04, 〈 β 〉 = 6.94 ± 0.06, and 〈 γ 〉 = 5.80 ± 0.05. In both cases, A < 0, B < 0, and C > 0. The capillary interaction force between spherical Janus particles trapped at the air-water interface is a sum of power laws.