Spontaneous Formation of Mesoscale Polymer Patterns in an Evaporating Bound Solution

The use of spontaneous self-assembly as a lithographyand external-fields-free means to construct well-ordered, often intriguing structures, has received much attention owing to the ease of producing complex structures with small feature sizes. Drying mediated self-assembly of nonvolatile solutes (polymers, nanoparticles, and colloids) through irreversible solvent evaporation of a sessile droplet on a solid substrate (unbound solution) represents one such case. However, irregular polygonal network structures (Benard cells) and stochastically distributed concentric ‘coffee rings’ are often observed. The irregular multirings (‘coffee rings’) are formed via repeated pinning and depinning events (i.e., ‘stickslip’ motion) of the contact line. The evaporation flux varies spatially, with the highest flux observed at the edge of the drop. Therefore, to form spatially periodic patterns at the microscopic scale, the flow field in an evaporating liquid must be delicately harnessed. In this regard, recently, a few attempts have been made to guide the droplet evaporation in a confined geometry with or without the use of external fields. Patterns of remarkably high fidelity and regularity have been produced. However, interfacial interactions between nonvolatile solutes and substrates govern the stability of thin films and have not been explored in these studies. The synergy of controlled self-assemblies of solutes, and their destabilization mediated by the interaction between solutes and substrates during the solvent evaporation, can lead to the formation of intriguing, ordered structures. Herein, we report on the spontaneous formation of well-organized mesoscale polymer patterns during the course of solvent evaporation by constraining polymer solutions in a sphere-on-Si geometry, as illustrated in Figure 1 (bound solution, i.e., capillary bridge). Gradient concentric rings and selforganized punch–holelike structures were obtained via mediating interfacial interactions between the polymer and the substrate. This facile approach opens up a new avenue for producing yet more complex patterns in a simple, controllable, and cost-effective manner. Poly(methyl methacrylate) (PMMA), polystyrene (PS), and PS-b-PMMA diblock copolymers were used as nonvolatile solutes to prepare PMMA, PS, and PS-b-PMMA toluene solutions, respectively. The concentration of all the solutions was 0.25 mg mL. The evaporation, in general, took less than 30 min to complete. The pattern formation was monitored in situ by using optical microscopy (OM). After the evaporation was complete, two surfaces (spherical lens and Si) were separated and examined by using OM and atomic force microscopy (AFM). Only the patterns on Si were evaluated. Highly ordered gradient concentric rings of PMMA, persisting toward the sphere/Si contact center, were obtained over the entire surfaces of the sphere and Si except the region where the sphere was in contact with Si (Fig. 2a). A typical OM image of a small region of entire rings of PMMA is shown in Figure 2b. The formation of periodic, gradient rings was a direct consequence of controlled, repetitive ‘stick-slip’ motion of the contact line, resulting from the competition of linear pinning force and nonlinear depinning force (i.e., capillary force) in the sphere-on-Si geometry. This is in sharp contrast with irregular concentric rings formed in an unbound liquid by stochastic ‘stick-slip’ motion of the contact line, suggesting that the use of the sphere-on-Si geometry rendered control over the evaporation rate, and is effective in improving the stability against the convection. Representative 3D AFM height images of PMMA rings at different radial distances, X (Fig. 1), away from the center of the sphere/Si contact are shown in Figure 2c–e. The recession (Fig. 2c–e) of the center-to-center distance between adjacent rings, kC–C, and the height of the ring, h, was clearly evident. As the solution front moved toward the center of the sphere/ Si contact due to evaporative loss of toluene (Fig. 2a), both kC–C and h decreased progressively from kC–C = 35.2 lm and C O M M U N IC A IO N