Elastic colloidal monopoles and reconfigurable self-assembly in liquid crystals

Monopole-like electrostatic interactions are ubiquitous in biology(1) and condensed matter(2-4), but they are often screened by counter-ions and cannot be switched from attractive to repulsive. In colloidal science, where the main goal is to develop colloidal particles(2,3) that mimic and exceed the diversity and length scales of atomic and molecular assembly, electrostatically charged particles cannot change the sign of their surface charge or transform from monopoles to higher-order multipoles(4). In liquid-crystal colloids(5-7), elastic interactions between particles arise to minimize the free energy associated with elastic distortions in the long-range alignment of rod-like molecules around the particles(5). In dipolar(6,8), quadrupolar(8-12) and hexadecapolar(13) nematic colloids, the symmetries of such elastic distortions mimic both electrostatic multipoles(14) and the outermost occupied electron shells of atoms(7,15,16). Electric and magnetic switching(17,18), spontaneous transformations(19) and optical control(20) of elastic multipoles, as well as their interactions with topological defects and surface boundary conditions, have been demonstrated in such colloids(21-23). However, it has long been understood(5,24) that elastic monopoles should relax to uniform or higher-order multipole states because of the elastic torques that they induce(5,7). Here we develop nematic colloids with strong elastic monopole moments and with elastic torques balanced by the optical torques induced by ambient light. We demonstrate the monopole-to-quadrupole reconfiguration of these colloidal particles by unstructured light, which resembles the driving of atoms between the ground state and various excited states. We show that the sign of the elastic monopoles can be switched, and that like-charged monopoles attract whereas oppositely charged ones repel, unlike in electrostatics(14). We also demonstrate the out-of-equilibrium dynamic assembly of these colloidal particles. This diverse and surprising behaviour is explained using a model that considers the balance of the optical and elastic torques that are responsible for the excited-state elastic monopoles and may lead to light-powered active-matter systems and self-assembled nanomachines.