While a lot of everyday materials can be naively understood through a primary-school representation of molecules as simple marbles, some have orientation, like compass needles or weather vanes. A well-known example from solid-state physics are ferromagnetic materials with spins domains, which supported the majority of data storage throughout the analog and digital history [1]. In soft materials, many building blocks have direction, such as cells and muscle fibers, molecules of liquid crystals, and particles of a ferrofluid [2]. Liquid crystals are especially versatile, as they can be persuaded by geometry of the container, applied voltage, or properties of surrounding materials, to assume various states. Liquid crystals are known to reorient due to shear flow, as the elongated molecules can pivot with the swirling fluid. When the liquid crystal is in contact with another fluid, the motion of the other liquid can influence it across the interface [3].
In Kara et al. [4], a piece of silicon was etched into a regularly bumpy surface, similar to a keypad of a button accordion, with a liquid crystal poured into the spaces between the hexagonally-ordered cylindrical posts, and a layer of water on top. The liquid crystal molecules orient perpendicular to the surface of the posts, and lie flat next to the water layer. Wherever the water is present on top, either as a uniform layer, or just an isolated droplet, no matter how the directions are followed through the material, there will be for each post, one so called “defect”, where no direction fits. Around the posts and defects, the directions of molecules snake around as smooth as possible, shaping polarized light into distinctive pattern under the microscope (see Figure 1).
Each defect can pick one of the 6 positions around its post, so we can say each post has a direction, like a weather vane. If the droplet is deposited when the liquid crystal is disordered, then upon cooling we get randomized directions. This orientational order exists not on the level of a single molecule, but as two-dimensional pattern of directions on the posts, clearly visible under the microscope. The comparison with the weather vane goes further, as the defects react to the flow of the water on top. The defects move to the trailing direction of the flow, so that we can read out the direction directly from the patterns in liquid crystals. This pattern stays intact after the flow stops, so it records a memory of past motion, such as movement of droplets, their collisions, or just squirting water around with a syringe. Using a tiny cantilever tool, individual weather vanes can be turned at will, like painting a picture out of individual pixels (see Figure 2). The pattern can be reset to a random state by heating above the nematic-isotropic transition, and colling it back down.
As a simple form of soft memory, the liquid crystal on a patterned surface cannot compete in speed, reliability and miniaturization with any existing solid state storage. However, with micron-sized features, we can imagine it tracking microscopic living organisms swimming in the water layer, staying safe from the chemistry of the liquid crystals below them, reacting indirectly to the tiny flutters of water. With additional changes in material, geometry and optics, such micro-devices with a free liquid-liquid interface offer capabilities that cannot be seen when liquid crystals are confined to a rigid container.
[1] C. Chappert, A. Fert, F. Nguyen Van Dau, The emergence of spin electronics in data storage. Nat. Mater. 6, 813 (2007).
[2] S. R. Nagel, Experimental soft-matter science. Rev. Mod. Phys. 89, 025002 (2017).
[3] G. Ilhan, L. N. Carenza, E. Bukusoglu, Shear-induced structural transitions in confined nematic soft interfaces. Commun. Phys. 8, 143 (2025).
[4] U. I. Kara et al., Multistable polar textures in geometrically frustrated nematic liquid crystals. Nat. Phys. (2025). https://doi.org/10.1038/s41567-025-02966-x