 Flow-free rotation of a spherical squirmer

A spherical squirmer can rotate without disturbing the surrounding fluid if its prescribed surface slip exactly cancels the velocity of rigid rotation: $u^{\prime }=-\Omega \times an$. The laboratory-frame boundary velocity is then zero. Uniqueness of Stokes flow with decay at infinity gives identically zero exterior velocity. In particular $u^{\prime }=b_0\sin \theta e_{\varphi }$ gives $\Omega =-(b_0/a)e_z$. The cancellation concerns the total fluid boundary velocity, not the solid material's angular velocity.