Robust Self‐Assembly of Highly Ordered Complex Structures by Controlled Evaporation of Confined Microfluids

Abstract

The evaporative self-assembly of nonvolatile solutes such as polymers, nanocrystals, and carbon nanotubes has been widely recognized as a nonlithographic means of producing a diverse range of intriguing complex structures. [1][2][3][4][5] The spatial variation of evaporative flux and possible convection mean, however, that these non-equilibrium dissipative structures (e.g., coffee rings, [6] fingering patterns, [7] and polygonal network structures [8] ) are often irregular and stochastically organized. Yet for many applications in microelectronics, data storage devices, and biotechnology, it is highly desirable to achieve surface patterns that have a well-controlled spatial arrangement. To date, only a few elegant studies have centered on the precise control of the evaporation process to produce ordered structures. [9][10][11][12][13][14][15][16][17] When compared with conventional lithographic techniques, surface patterning by controlled solvent evaporation is simple, cost-effective, and offers a lithography and external-field-free means of organizing nonvolatile materials into ordered microscopic structures over large surface areas. For example, it has been recently demonstrated that constraining a drop of solution in a restricted geometry formed by placing a sphere against a flat substrate results in controlled evaporation. Consequently, the repetitive pinning and depinning of the solutions contact line produces a lateral surface pattern that consists of hundreds of concentric, highly ordered "coffee rings", the gradients of which vary in width and height. [12,13,17] The ability to engineer an evaporative self-assembly process that yields a wide range of complex, self-organizing structures over large areas other than strictly concentric rings [12,13,17] offers tremendous potential for applications in electronics, optoelectronics, and sensors. The formation of periodic assemblies of polymeric squares, triangular contour lines, and ellipses would be effectively mediated by controlled solvent evaporation that is precisely guided by the shape of the curved upper surface of a confined curve-on-flat geometry. Herein, we demonstrate a facile, robust, and one-step method of evaporating polymer solutions in curve-on-flat geometries to create versatile, highly regular microstructures in a precisely controlled environment, as well as offering a comprehensive study of the influence of different upper surfaces on complex structure formation by controlled evaporation. Our method further enhances current fabrication approaches to creating highly ordered structures in a simple and cost-effective manner, with the potential to be tailored for use in photonics, [18] electronics, [19,20] optoelectronics, [21] microfluidic devices, [22] nanotechnology, [23] and biotechnology. [24] A linear conjugated polymer, poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV, molecular weight = 50-300 kg mol À1 ) was used as the nonvolatile solute. The choice of system was motivated by its numerous potential applications in the ar...

Controlled evaporative assembly of polymers from confined solutions

“…In this context, to take advantage of the extreme simplicity of this nonlithographic, external field‐free, top‐down technique and exploit its full potential for developing miniaturized optical, electronic, optoelectronic, magnetic devices, and so forth, control of the evaporative flux, solution concentration, and interfacial interactions among the solvent, solute, and substrate is necessary to construct highly ordered, complex structures rapidly and cheaply over large areas. To date, a few impressive studies have centered on the precise manipulation of the evaporation process, including controlled anisotropic wetting/dewetting processes15, 16 controlled dewetting by dip‐coating,17 convective assembly in evaporating menisci,18 evaporation‐induced assembly in restricted geometries,19–29 and evaporative lithography using a mask,30, 31 to create complex deposit patterns with unprecedented regularity.…”

“…To date, several elegant approaches have been successfully exploited to control droplet evaporation in confined geometries, which include two plates with one plate placed at a certain angle against another horizontal substrate,18, 36 two parallel plates with the top one sliding on the lower stationary substrate,19, 20 cylindrical tube,28, 29 two crossed cylindrical mounts covered with single crystals of mica sheets,21 and curve‐on‐flat geometries22–26, 37, 38 as illustrated in Figure 1. The confined geometries provide a unique environment for remarkable control over the flow within an evaporating droplet, which, in turn, regulates structure formation.…”

“…with permission from The American Chemical Society, Copyright 2004. (d) Representative fluorescence micrographs of concentric square stripes formed from controlled evaporative assembly of poly[2‐methoxy‐5‐(2‐ethylhexyloxy)‐1,4‐phenylenevinylene] (MEH‐PPV) toluene solution in a pyramid‐on‐flat geometry 25. Scale bars are 600 μm in the upper left and 300 μm in the upper and lower right images.…”

Meniscus Lithography: Evaporation-Induced Self-Organization of Pillar Arrays into Moiré Patterns

“…For example, the ''coffee ring'' effect [7] has been shown to generate a variety of ringed and banded colloidal deposits via evaporation-induced fluid flows that carry particles to the periphery of a droplet. Beyond simple rings and bands, this strategy has been used to produce complex patterned films by placing a shadow mask over the droplet to create regions of free and hindered evaporation [8] or by confining the meniscus to control the ''stick and slip'' motion of capillary flow [9]. In addition to flow-based mechanisms, the capillary forces generated by the menisci between individual particles have attracted interest for their ability to assemble nanopillars [10][11][12][13] and other building blocks [14,15] into intricately patterned structures.…”

Photosensitive self-assembled nanoarchitectures containing surfactant-free Si nanocrystals produced by laser fragmentation in water

“…Such self-assembled nanostructures can lead to functional nanoarchitectures with a higher degree of complexity. For instance, a carefully controlled dewetting process allows the formation of one-dimensional (1D) architectures at low cost [4,1,5]. In other cases the heteroepitaxial growth or chemical synthesis have been observed to produce nanocrystals assembled in determined arrangements [6][7][8][9][10].…”