Spontaneous Formation of Mesoscale Polymer Patterns in an Evaporating Bound Solution

Abstract

The use of spontaneous self-assembly as a lithography-and 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. [1][2][3] 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. [3][4][5][6][7][8][9][10][11][12][13][14][15][16] However, irregular polygonal network structures (Benard cells) [14,15] and stochastically distributed concentric 'coffee rings' [4][5][6]10] are often observed. The irregular multirings ('coffee rings') are formed via repeated pinning and depinning events (i.e., 'stickslip' motion) of the contact line. [4][5][6]10] 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 [17][18][19][20] with [17] or without [18][19][20][21][22] the use of external fields. Patterns of remarkably high fidelity and regularity have been produced. [18][19][20][21][22] 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 -1 . 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 ...

Dynamical Model for the Formation of Patterned Deposits at Receding Contact Lines

“…The model incorporates wettability, capillarity, evaporation, convective transport of the solution and diffusion of the solute and has been derived employing a long-wave approximation. We find that a strong nonlinear dependence of viscosity (i.e., the front mobility) on concentration triggers, in an intricate interaction with evaporation and diffusion, the deposition of periodic and aperiodic line patterns as observed in experiments for many different materials and settings [2,[5][6][7][8][9][10][11]30]. We believe that the model explains a basic mechanism for the formation of regular line patterns.…”

“…The occurrence of regular stripe patterns is a somewhat generic phenomenon, that is not only observed for different combinations of substances but also in a variety of experimental setups that allow for slow evaporation. Examples include the meniscus technique in a sphere-on-flat geometry [7,9], a controlled continuous supply of liquid between two sliding plates to maintain a meniscus-like surface [5] and dewetting forced by a pressure gradient [10]. Interestingly, besides the stripes parallel to the receding contact line, a variety of other patterns are observed, including regular orthogonal stripes [9], superpositions of orthogonal and parallel stripes [5], regular arrays of drops [5,14] and irregularly branched structures [14,15].…”

“…Other observed structures include crack [3] and chevron [4] patterns. Recently it has been shown that evaporating polymer solutions [5][6][7] and (nano)particle suspensions [8-10] may be used to fabricate strikingly regular stripe patterns, where the deposited stripes are parallel to the receding contact line and have typical distances ranging from 10-100µm. The goal is to use this effect as a non-lithographic technique for covering large areas with regular arrays of small-scale structures, such as, e.g., concentric gold rings with potential uses as resonators in advanced optical communications systems [11].…”

Mesoscale Patterns Formed by Evaporation of a Polymer Solution in the Proximity of a Sphere on a Smooth Substrate: Molecular Weight and Curvature Effects

“…8 In our previous work, we reported that concentric rings of electrically conducting polymer and organometallic polymer of high regularity were formed naturally and spontaneously via controlled, repetitive "stick-slip" motion of the three-phase contact line when a drop of polymer solution was confined either between two crossed cylindrical mounts covered with single crystals of mica sheets 10 or between a spherical lens made of silica and a Si substrate (sphere-on-flat geometry), resulting in a capillary-held polymer solution (i.e., capillary bridge). [11][12][13][14][15][16][17] The evaporation in this geometry was restricted to the edge of the droplet, and the "stick-slip" cycles resulted in hundreds of concentric rings with regular spacing, very much resembling a miniature archery target. Each ring was nanometers high and several microns wide.…”

“…[10][11][12] By tuning the interfacial interaction between the polymer and the substrate that governed the stability of thin films, intriguing, ordered dissipative structures can be produced as a result of synergy of controlled self-assemblies of the polymer and its destabilization mediated by the interfacial interaction. 15 We have reported that the use of solutions with different concentrations and different solvents effectively mediated the pattern formation in an evaporating droplet containing nonvolatile solutes. 11 In this paper, we extend our previous work to investigate the molecular weight (MW) effect on the mesoscale polymer patterns formed by drying a drop of polymer solution in a sphere-on-flat geometry (i.e., a spherical lens (or a pushpin) on a Si substrate), as depicted in Figure 1.…”