Guided organization of λ-DNA into microring arrays from liquid capillary bridges.

A drying droplet on a solid surface (i.e., unbound solution) often leads to the formation of irregular structures, including coffee rings, [ 1 ] fi ngering instabilities, [ 2 ] and polygonal networks, [ 3 ] owing to the capillary fl ow induced by nonuniform evaporation fl ux or temperature-gradient-induced Marangoni convection. Hence, the ability to precisely control the capillary fl ow and convective fl ux in dryingmediated self-assembly to create spatially well-defi ned surface structures has received considerable attention. To date, a few elegant studies have demonstrated intriguing means based on the controlled evaporation in confi ned geometries (i.e., bound solution) to delicately tailor the evaporative self-organization process, thereby yielding highly ordered structures. [ 4–23 ] For example, by rationally designing the upper curved surface in curve-on-fl at geometries to accommodate different shapes, a wide range of complex surface patterns with unprecedented regularity were readily produced during the course of solvent evaporation by controlled, repetitive pinning–depinning cycles of the three-phase contact line of the droplet constrained in curve-on-fl at geometries. [ 24 ] Recently, chemically patterned surface[ 25–27 ] and stamp-assisted deposition methods [ 28–35 ] have been exploited for evaporative self-assembly of nonvolatile solutes (e.g., nanoparticles and biomolecules). The use of patterned surfaces or polydimethylsiloxane (PDMS) stamps minimizes uncontrolled droplet shape and possible structural instabilities that may otherwise result in a lack