Research Article

Shaping colloidal bananas to reveal biaxial, splay-bend nematic, and smectic phases

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Science  21 Aug 2020:
Vol. 369, Issue 6506, pp. 950-955
DOI: 10.1126/science.abb4536

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Controlling the curvature

Molecular chirality is often required to make chiral liquid crystalline phases, but liquid crystallinity has also been obtained using curved elongated rods known as bent-core or banana-shaped molecules. Fernández-Rico et al. developed a method to controllably alter the curvature of the rods using ultraviolet light and a photoresponsive polymer (see the Perspective by Godinho). From a single starting batch can come a family of rods with different curvatures but similar overall rod thickness, length, and length distribution. The researchers explored a range of liquid crystalline phases, including the splay-bend nematic phase that was predicted more than 40 years ago.

Science, this issue p. 950; see also p. 918

Abstract

Understanding the impact of curvature on the self-assembly of elongated microscopic building blocks, such as molecules and proteins, is key to engineering functional materials with predesigned structure. We develop model “banana-shaped” colloidal particles with tunable dimensions and curvature, whose structure and dynamics are accessible at the particle level. By heating initially straight rods made of SU-8 photoresist, we induce a controllable shape deformation that causes the rods to buckle into banana-shaped particles. We elucidate the phase behavior of differently curved colloidal bananas using confocal microscopy. Although highly curved bananas only form isotropic phases, less curved bananas exhibit very rich phase behavior, including biaxial nematic phases, polar and antipolar smectic-like phases, and even the long-predicted, elusive splay-bend nematic phase.

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