Colloidal bananas form a new phase

Most liquid crystals are made up of rigid, rod-like molecules in a nematic phase, meaning their long axes are aligned. But mesogens—molecules that form liquid crystals—also come in other shapes, such as disks and bowls, and their assemblies can exhibit unexpected optical and electronic properties (see, for example, Physics Today, April 2020, page 17 ). Kinked linear molecules, known as bent-core mesogens, are of particular interest because they form unique phases that exhibit spontaneous chirality. Understanding when and why those phases form, though, has remained a challenge. Existing bent-core mesogens exhibit only a few bend angles, which limits how much of phase space can be studied, and the mesogens’ assemblies are observed indirectly through birefringence and x-ray diffraction imaging.

Now Carla Fernández-Rico and coworkers at the University of Oxford in the UK, in collaboration with researchers at Utrecht University in the Netherlands, have synthesized banana-shaped colloids that mimic bent-core mesogens. Unlike their molecular counterparts, the fluorescent, micron-scale polymer particles come in a wide range of curvatures and can be observed using confocal imaging. By varying particle curvature and packing fraction, the researchers produced a range of ordered phases, including one that was previously unobserved in molecular liquid crystals.

Coloring the fluorescently imaged particles by orientation, as shown in the figure, highlights their phase behavior. Particles with the highest curvature (top row; scale bar is 10 μm) were always isotropic no matter how densely packed. But those with intermediate and low curvature (middle and bottom rows) became ordered as their packing fractions increased (left to right in the figure). Both types of particles first entered a biaxial nematic phase, in which they were aligned along two axes (panels B2, C2, and C3). At still higher packing fractions, they formed a smectic phase—the particles were both aligned and separated into layers (panels B3, B4, and C4).

A careful analysis of the low-curvature particles at packing fractions around 0.7, which fall between panels C3 and C4, showed an additional transition: On their way from biaxial nematic phase to smectic the particles passed through a splay–bend nematic phase in which particle orientations vary periodically in space. It was predicted in 1976 but has never been seen in molecular liquid crystals. The colloids are smoother and less uniform in size than molecular mesogens, and the researchers suspect that those differences might explain the new phase’s appearance. (C. Fernández-Rico et al., Science 369, 950, 2020 .)