Self-Assembly of Gradient Concentric Rings via Solvent Evaporation from a Capillary Bridge

A drop of solution containing nonvolatile solute is allowed to evaporate from a sphere-on-flat geometry. Left behind is a striking pattern of gradient concentric rings with unprecedented regularity. The center-to-center distance between adjacent rings, ${\lambda }_{C\mathrm{\text{−}}C}$, and the height of the ring, $h_d$, are strongly affected by the concentration of the solution and the properties of the solvent. The nature of the formation of regular gradient ring patterns during the course of irreversible solvent evaporation is revealed through theoretical calculations based on the mass conservation of the solution.