Symmetry Breaking in Overdamped Colloidal Chiral Matter

Many material properties emerge from underlying symmetries. By inducing controlled rotation, magnetic colloids offer a platform to break some of these symmetries in the lab, creating fascinating materials with fewer constraints. This work presents experimental, computational, and theoretical studies of rotating magnetic colloids and the aggregate two-dimensional material they form. When colloids are made to rotate continuously, time-reversal and parity symmetry are broken, resulting in the emergence of odd stress and odd elastic and viscous moduli, which are not possible for equilibrium systems. These rotationally induced odd stresses also drive the motility of dislocations in the material, causing its structure to be in a polycrystallized, dynamically fluctuating, whorly state. The controlled drive further allows the incorporation of anisotropy as an additional type of symmetry breaking. It is shown that anisotropic driving results in nontrivial dynamics, connected to the inner degree of freedom of the material —in this case, the dipole moment of the colloids. By employing a different anisotropic drive, it is possible to induce an exciting instability mechanism that promotes the formation of material edges in specific directions while suppressing it in others. The observed morphology cannot be readily explained from energetic considerations alone, highlighting the fundamentally active nature of the material.