Squirmers adhered to fluid interfaces

We theoretically investigate active colloids adhered to a clean fluid-fluid interface of arbitrary viscosity ratio in Stokes flow. We adopt the squirmer model in which a spherical colloid is made to self-propel by assigning a tangential slip velocity to its surface. We assume that the fluid maintains a 90° contact angle at the three-phase contact line with the colloid, the contact line is fixed, and the capillary number is small. Thus, we assume a flat fluid-fluid interface bisecting the squirmer at its meridian, and the squirmer is only allowed to translate and rotate in the interfacial plane. Unlike a self-propelled colloid in the bulk, a swimmer attached to an interface may exert a net force and/or torque on the fluid, which are balanced by capillary forces on the swimmer itself. Thus, in the far field, the flow is dominated by a stresslet that can be aligned at an arbitrary angle to the interface. As a result, squirmers that push (pull) against the interface create purely attractive (repulsive) interactions with neighboring bodies. Additionally, by including rotational modes, two adhered squirmers can orbit about a common center or exhibit cooperative self-propulsion.