Formation of Twisted Bundles from Small Regular Arrays of Polymer Nanopillars

Elastocapillary assembly has been used to form twisted bundles in large arrays of nanopillars, often placed in square lattices. We study how the emergence of twisting can be controlled by both the dimensions of individual pillars and the local bundle geometry. Using direct laser writing, we fabricate nanopillars with high spatial resolution in arbitrary lattices. The bundles are composed of a central pillar surrounded by 4 to 7 outer pillars that are evenly spaced from one another. Although no chirality is programed into the design, we find that twisted bundles form under certain combinations of pillar spacing (D) and pillar height (H). Interestingly, we find that achiral bundles always form when D/H is small, even with long and flexible pillars. We find a transition from achiral to chiral bundles when D/H is 0.2 to 0.3. To understand this transition, we use a scaling model and numerical simulations.