Capillary Forces on a Janus Sphere Straddling a Liquid-Gas Interface

Particles with a patterned surface can exhibit interesting packing and self-assembly behavior. One class of such particles is Janus particles whose surfaces are divided into two halves with distinct physical properties. We compute the capillary forces on a Janus sphere, one side of which is solvophobic while the other is solvophilic, straddling a liquid-gas interface via molecular dynamics simulations. In equilibrium, the liquid-gas interface is flat in the horizontal plane and intersects with the equator of the Janus sphere, with its solvophilic side immersed in the liquid. When the Janus sphere is pulled or pushed out of its equilibrium position along the vertical direction, a capillary rise or fall occurs but the contact line is first pinned at the equator. The contact line only starts to slide when the apparent contact angle becomes equal to the acute (obtuse) contact angle on the solvophilic (solvophobic) side for the Janus sphere pulled upward (pushed downward). An analytical model is developed to explain the observation. The capillary force on a Janus sphere oriented upside-down with the solvophobic side submerged in the liquid is also computed with simulations and possible metastable configurations are found.