Colloidal Flatlands Confronted with Urge for the Third Dimension

Two-dimensional sheets are a relatively neglected form of soft matter, interesting because of their capability to deform into the third dimension with little energy cost. Here, we confront colloidal sheets with an abruptly imposed potential tending to produce strings normal to the plane. Experimentally, this is implemented first by using ultrasound-induced acoustic levitation to produce planar sheets and then by abruptly imposing AC electric fields that introduce dipolar interactions. Seeking to identify the microscopic mechanisms underlying the observed collective behavior, we find that the patterns quantified from our fast confocal experimental imaging are reproduced by our Brownian dynamics simulations. We follow the evolution of these patterns, including their structure factor, from start to final steady state, and from successful parametrization we predict simulation phases not yet observed in experiment. The transient-state evolution toward final outcome includes monocrystalline hexagonal lattice, polycrystalline body-centered tetragonal lattice with grain boundaries, interconnected rings, serpentine zigzag chains, and columns vertical to the plane, and a "fat worm" serpentine pattern. To explain the counterintuitive findings presented here, we map dependence on softness of the confining potential.