Drying solvent from a solution containing nano-particles changes their density as well as effective interactions among them. Dramatic rearrangements can follow, and the resulting mesoscopic patterns of particle density and orientation can be much more intricate than in either the initial or the terminal state. These nonequilibrium patterns are nonetheless derived from the nature of the two limiting equilibrium states. Professor Geissler and graduate student JiYeon Ku use computer simulations to explore how fine details of materials prepared in this way can be tuned through simple particle properties, thermodynamic parameters, and other controllable external conditions. Snapshots from a Monte Carlo simulation of magnetic nanoparticles (red circles) in evaporating solution are shown in (a). Particle correlations due to dipolar interactions, and changes in the relative strengths of short- and long-ranged forces, shape this self-assembly in many competing ways that require further study. Transient structures generated by model simulations share several nontrivial qualitative features with preliminary experimental results obtained by Professor Paul Alivisatos's group for ε-Co nanocrystals (b).