Microfluidic control over topological states in nematic liquid crystal flow

Researchers from the Faculty of Medicine and the Faculty of Mathematics and Physics of the University of Ljubljana became the first in the world to discover phase transitions to a nonequilibrium chiral state in liquid crystals and explain this in theoretical terms.

The orientational order in liquid crystals has a key influence on the remarkable optical properties of these anisotropic liquids and is at the same time responsible for their unusual rheological response. The structural regulation of liquid-crystal molecules in microfluidic channels can be precisely controlled using modern research equipment by surface treatment, temperature change and flow regulation with adjustable pressure difference.

The research group from the University of Ljubljana (Simon Čopar (lead author), assistant professor of physics at the Faculty of Mathematics and Physics; Žiga Kos, postdoctoral researcher at the Faculty of Mathematics and Physics and at MIT (USA), Tadej Emeršič, former junior researcher at the Faculty of Medicine and current postdoctoral researcher at the University of Chicago (USA) and Asst. Prof. Uroš Tkalec (corresponding author), research associate at the Faculty of Medicine), which was led by Dr. Tkalec, reported in Nature Communications (impact factor 12.12) on the experimental establishment, dynamic stabilisation and theoretical analysis of previously unobserved but topologically significant chiral states which they had previously only studied under equilibrium conditions. The authors of the research explained the mechanism of the sequence of phase transitions between topologically different states in the pressure-driven flow of a liquid crystal using a phenomenological model based on the anisotropy of elastic constants that logically complements the phase diagram of all possible hydrodynamic states. The work thus completes a decade-long period of research in the field of nematofluidics and promises the application of these concepts in lyotropic and active liquids with nematic order.

The research has met with a notable response from other experts, with Dr. Tkalec lecturing on it at the University of Chicago, ESPCI Paris and Johns Hopkins University and at the Gordon Research Conference in the USA. At the invitation of the editors, the paper has been presented in popular form on the Nature Research Device and Materials Engineering Community website and additionally promoted on the Nature Research Instagram feed. A review article on these findings has also been published by the French research equipment manufacturer Elveflow on the Microfluidic Reviews page of its website. Last but not least, the achievement has contributed to enabling two of paper’s co-authors to continue their postdoctoral research at two prestigious institutions: MIT and the Pritzker School of Molecular Engineering at the University of Chicago.

Reference: ČOPAR, Simon, KOS, Žiga, EMERŠIČ, Tadej, TKALEC, Uroš. Microfluidic control over topological states in channel-confined nematic flows. Nature Communications, ISSN 2041-1723, Jan 2020, Vol. 11, Art. No. 59: https://www.nature.com/articles/s41467-019-13789-9.

The figure shows the experimental equipment used to establish and analyse nonequilibrium states in a nematic liquid crystal and a sample with a microfluidic channel. Chiral topological states, which can most easily be recognised polarised light, are particularly highlighted.
Figure created by: Uroš Tkalec