UNIST Researchers Uncover Revolutionary Phenomenon in Liquid Crystals

Abstract
Swimming in low-Reynolds-number fluids requires the breaking of time-reversal symmetry and centrosymmetry. Microswimmers, often with asymmetric shapes, exhibit nonreciprocal motions or exploit nonequilibrium processes to propel. The role of the surrounding fluid has also attracted attention because viscoelastic, non-Newtonian, and anisotropic properties of fluids matter in propulsion efficiency and navigation. Here, we experimentally demonstrate that anisotropic fluids, nematic liquid crystals (NLC), can make a pulsating spherical bubble swim despite its centrosymmetric shape and time-symmetric motion. The NLC breaks the centrosymmetry by a deformed nematic director field with a topological defect accompanying the bubble. The nematodynamics renders the nonreciprocity in the pulsation-induced fluid flow. We also report speed enhancement by confinement and the propulsion of another symmetry-broken bubble dressed by a bent disclination. Our experiments and theory propose another possible mechanism of moving bodies in complex fluids by spatiotemporal symmetry breaking.

A research team, affiliated with UNIST, has unveiled for the first time a new principle of motion in the microworld, where objects can move in a directed manner simply by changing their sizes periodically within a substance known as liquid crystal. Led by Professor Joonwoo Jeong and his research team in the Department of Physics at UNIST, this discovery is poised to have far-reaching implications across various research fields, including the potential development of miniature robots in the future.

In their research, the team observed that air bubbles within the liquid crystal could move in one direction by altering their sizes periodically, contrary to the symmetrical growth or contraction typically seen in air bubbles in other mediums. By introducing air bubbles, comparable in size to a human hair, into the liquid crystal and manipulating the pressure, the researchers were able to demonstrate this extraordinary phenomenon.

Figure 1. Pulsating bubbles dispersed in NLC.

The key to this phenomenon lies in the creation of phase defects within the liquid crystal structure next to the air bubbles. These defects disrupt the symmetrical nature of the bubbles, enabling them to experience a unidirectional force despite their symmetrical shape. As the air bubbles fluctuate in size, pushing and pulling the surrounding liquid crystal, they are propelled in a consistent direction, defying conventional laws of physics.

Sung-Jo Kim, the first author of the study, remarked, “This groundbreaking observation showcases the ability of symmetrical objects to exhibit directed motion through symmetrical movements, a phenomenon previously unseen.” He further highlighted the potential applicability of this principle to a wide range of complex fluids beyond liquid crystals.

Figure 2. Polarised optical microscopy observations of HHB during a single pulsation cycle (left) and a schematic diagram of a pulsating HHB as an oscillating dumbbell (right).

Professor Jeong commented, “This intriguing result underscores the significance of symmetry breaking in both time and space in driving motion at the microscopic level. Moreover, it holds promise for advancing research in the development of microscopic robots.”

Their findings have been published in the online version of Nature Communications on February 9, 2024. This research has been supported by the National Research Foundation of Korea (NRF), the Institute of Basic Science (IBS), and the Slovenian Research Agency (ARRS).

Journal Reference
Sung-Jo Kim, Žiga Kos, Eujin Um, and Joonwoo Jeong, “Symmetrically pulsating bubbles swim in an anisotropic fluid by nematodynamics,” Nat. Commun., (2024).