Brownian diffusion of a partially wetted colloid

The dynamics of colloidal particles at interfaces between two fluids plays a central role in microrheology(1), encapsulation(2), emulsification(3), biofilm formation(4), water remediation(5) and the interface-driven assembly of materials(6). Common intuition corroborated by hydrodynamic theories(7-9) suggests that such dynamics is governed by a viscous force lower than that observed in the more viscous fluid. Here, we show experimentally that a particle straddling an air/water interface feels a large viscous drag that is unexpectedly larger than that measured in the bulk. We suggest that such a result arises from thermally activated fluctuations of the interface at the solid/air/liquid triple line and their coupling to the particle drag through the fluctuation-dissipation theorem. Our findings should inform approaches for improved control of the kinetically driven assembly of anisotropic particles(10) with a large triple-line-length/particle-size ratio, and help to understand the formation and structure of such arrested materials.