Spontaneous autophoretic motion of isotropic particles

Suspended colloidal particles interacting chemically with a solute are able to self-propel by autophoretic motion when they are asymmetrically patterned (Janus colloids). Here we demonstrate that the chemical anisotropy is not a necessary condition to achieve locomotion. The non linear interplay between surface osmotic flows and solute advection can produce spontaneous, and self-sustained motion of isotropic particles. We solve, for a spherical particle, the classical nonlinear autophoretic theoretical framework at arbitrary P\'eclet number. For a given set of material parameters, there exists a critical particle size, or equivalently a critical Peclet number, above which spontaneous autophoretic motion occurs. The flow induced by the particle further displays a hierarchy of instabilities associated with quantized critical Peclet numbers. Using numerical solutions of the full (unsteady) diffusiophoretic problem we confirm our analytical predictions and show that, above the instability threshold, the isotropic particles reach a steady swimming state with broken front-back symmetry in the concentration field and the hydrodynamic signature of a "pusher" swimmer. This instability to propulsion could be relevant to the high-throughput production of self-propelled particles.