Fig. 2.

Cellular shape control for active boundaries with passive interior. (A) Simulations in the microscopic model with 512 interior spinners and 80 boundary spinners reveal a symmetric buckling of the colloidal cell. The buckling is suppressed for low driving torque τ (Top Row) but appears if the driving torque is sufficiently high (Bottom Row). We can control the symmetry by changing the number of alternately driven segments on the active boundary, varied horizontally. (B) Simulations in the continuum model with fixed boundaries reveal convective flows of the interior spinners, which we visualize via the vorticity field w. (C) Simulations in the continuum model with free boundaries confirm the shape changes observed in the microscopic model. Again, we show results for two different levels of activity ${\tau }^{\prime }$ for frictional damping ${\gamma }^{\prime }=0.1$ and boundary tension $\kappa =80$ in a cell of size $R=20$. The case of $n=1$ exhibits a cusp-like singularity that cannot be captured by the perturbation analysis used to compute the cell shape (Materials and Methods).