  Monopole-like electrostatic interactions are ubiquitous in biology and
condensed matter, but they are often screened by counter-ions and cannot be
switched from attractive to repulsive. In colloidal science, where the prime
goal is to develop colloidal particles 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. In liquid-crystal colloids, 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. In dipolar, quadrupolar and hexadecapolar nematic
colloids, the symmetries of such elastic distortions mimic both electrostatic
multipoles and the outmost occupied electron shells of atoms. Electric and
magnetic switching, spontaneous transformations and optical control of elastic
multipoles, as well as their interactions with topological defects and surface
boundary conditions, have been demonstrated in such colloids. However, it has
long been understood that elastic monopoles should relax to uniform or
higher-order multipole states because of the elastic torques that they induce.
Here we develop nematic colloids with strong elastic monopole moments and with
elastic torques balanced by optical torques exerted 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. We also
demonstrate the out-of-equilibrium dynamic assembly of these colloidal
particles.
