Self-propulsion of a rigid shape in active gels and active nematics

Since active liquids flow spontaneously, it is natural to ask if a passive rigid object can propel itself using the forces of a surrounding active liquid. We present simple theoretical arguments showing that a Janus particle in an active nematic or an active gel moves spontaneously. For example, imposing boundary conditions that favor parallel alignment of the active nematic rods on one side of the particle and that favor perpendicular (homeotropic) alignment on the other side leads to a net active force on the particle, and propulsion. Likewise, imposing a weak homeotropic boundary condition on only one side of a particle in active gel also leads to net active force on the particle. In both cases, the propulsion mechanism is analogous to the way that Marangoni forces propel a camphor boat on a water surface.