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

We demonstrate a self-organizing system that generates patterns by dynamic feedback: two periodic surfaces collectively structure an intervening liquid sandwiched between them, which then reconfigures the original surface features into moiré patterns as it evaporates. Like the conventional moiré phenomenon, the patterns are deterministic and tunable by mismatch angle, yet additional behaviors—chirality from achiral starting motifs and preservation of the patterns after the surfaces are separated—emerge uniquely from the feedback process. Patterning menisci based on this principle provides a simple, scalable approach for making a series of complex, long-range-ordered structures.

Figure 1

Pattern formation in pillar arrays upon liquid evaporation. (a)–(c) Maskless process: (a) Schematics of the pillar arrays (side view) before (left) and after (right) evaporative assembly. Dark-colored pillars have deterministic bending directions, while light-colored pillars have random bending directions. Curved blue lines indicate menisci formed between pillars. Arrows indicate the direction of the capillary forces, with relative force magnitudes indicated by length. Note that the final assemblies shown represent only a sampling of the possible patterns that can form, since all the bending directions are random. (b) SEM image of the periodic nanopillar array used in all the experiments. (c) SEM image of the assembled pillar array following evaporation in the absence of a mask. Note the absence of any long-range order. All assembly experiments were performed as described in Ref. 13. (d)–(f) Evaporation under a superimposed mask with feature spacing larger than that of the array: (d) Schematic of the pattern formation (see text for details); the wetting liquid fills the space between the array and the mask. (e) SEM image of an exemplary honeycomb mask. (f) SEM image of pillars shown in (b) assembled under a honeycomb mask. Note the long-range symmetry determined by the mask, with random patterns within each assembled cell. (g)–(i) Masked evaporation and pattern formation in the sandwich system composed of two identical arrays of pillars: (g) Schematic of the process depicting fully deterministic bending directions in such a system (see text for details). (h) (Left) Schematic of two superimposed pillar arrays (top and side views) corresponding to the case shown in (g). $\theta$ is the mismatch angle between the top and bottom lattice axes. (Right) Schematic of the individual pillar positions (top view). Red and blue dots correspond to pillars from top and bottom samples, respectively. Note the appearance of the moiré pattern. (i) SEM image showing the pattern generated from the two identical superimposed arrays after evaporation and remaining as a permanent imprint of the moiré interference on the substrates after separation. The red outline shows a unit cell of the generated pattern. The scale bar is $10\mu \mathrm{m}$ for (b) and $100\mu \mathrm{m}$ for (c), (e), (f), and (i).

Figure 2

Plot of the period of the pattern vs the mismatch angle. The dotted line is based on theory (Ref. 21) and shows good agreement with the experimental results. Error bars indicate standard deviations from at least 5 independent measurements. Representative SEM images of the assembled patterns are shown with their corresponding data points; the number at the upper right indicates the mismatch angle. The red lines indicate unit cells of the generated patterns. The scale bar is $50\mu \mathrm{m}$.

Figure 3

Appearance of chirality in pillar arrays assembled by meniscus lithography. (a) SEM image showing a spiral collapse pattern. The scale bar is $20\mu \mathrm{m}$. (b) Schematic showing that the patterns formed on the top and bottom arrays have the same chirality. (c),(d) Schematic of the array alignment (top views) and resulting pillar positions depending on the sign of the mismatch angle. A $7°$ mismatch angle was used as an example. Red hollow circles represent pillars from the top array, while blue solid circles correspond to those from the bottom array. Small arrows indicate the bending directions of the bottom pillars during evaporation, and the large arrows indicate the chirality of the generated patterns.

Figure 4

Bright-field snapshots of the drying process (first five panels, clockwise from the top left panel) and an SEM image of the resulting pattern (last panel). Dark regions are dry, and light regions are wet. Snapshots were focused at a height approximately corresponding to the pillar tips from the bottom array. The first panel shows the optical micrograph of the moiré interference pattern due to superposition of the top and bottom pillar arrays before drying. The inset in the SEM panel is a zoom-in of the pattern after evaporation, with the square showing a unit cell of the pattern.