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Molecular insights into structures, thermodynamics and dynamics of topological defects

Molecular simulations can provide microscopic details and physical insights into topological defect related phenomena. We rely on a coarse grained molecular model, namely Gay-Berne model, to study microscopic structures, thermodynamics and annihilation dynamics of topological defects in a thin-film nematic. Our results show good match with the continuum theory and experiments. Based on the agreement, we have proposed several convenient methods to measure orientational elastic constants of the nematic. We also find that defects tend to move to hotter areas.

Relevant publication:

- Huang, Wang, Whitmer and Zhang#, Structures, Thermodynamics and Dynamics of Topological Defects in Gay-Berne Nematic Liquid Crystals, Soft Matter, in press (2022).

Active and Programmable Transformations of Novel Topological Defect Structures in Nematic Liquid Crystals

Topological structures can autonomously move and transform in active matter, making it difficult to engineer for applications. We collaborate with experimental partner, Chenhui Peng group from USTC to propose using optical and mechanical means to realize programmable transformations of novel topological defects (left movie). Our approach can lead to highly controllable defects and their transformation, paving the way toward defect-directed self-assembly, active photonic devices and others.

Relevant publications:

- Jiang*, Ranabhat*, Wang*, Zhang# and Peng#, Active Transformations of Topological Structures in Light-Driven Nematic Disclination Networks, Proc. Natl. Acad. Sci. U. S. A 119 (23) e2122226119 (2022).

- Jiang*, Wang*, Akomolafe*, Tang, Zhawure, Ranabhat, Zhang# and Peng#, Proc. Natl. Acad. Sci. U. S. A. accepted (2023).

- Wang et al. submitted.

Pattern Formation in Lyotropic Chromonic Liquid Crystals under External Field and Active Stress

The majority of nematic liquid crystals are flow-tumbling nematics, in which the directors will be constantly rotating under a shear flow. Examples include liquid crystal polymers. Recent interest is on lyotropic chromonic liquid crystals (LCLCs), in which disk-like molecules can self assemble into stacked cylinders, which can form nematic and columnar phases. Study shows that certain LCLCs exhibit low twist constant and tumbling character. Collaborating with Irmgard Bischofberger group and Peter So group from MIT, we investigate defect structures and resulting retardance map in tumbling nematic LCLC under a pressure driven flow. We also use continuum simulations to study the interplay of driven flow and extensile active stress in this special nematic (right movie).

Relevant publications:

- Wang and Zhang#, Interplay of Active Stress and Driven Flow in Self-Assembled, Tumbling Active Nematics, Crystals 11, 1071 (2021) (invited article).​

- Zhang*, Zhang*, Ge*, Yaqoob, So and Bischofberger, Structures and topological defects in pressure-driven lyotropic chromonic liquid crystals, Proc. Natl. Acad. Sci. U. S. A. 118(35) e2108361118 (2021).

- Zhang, Zhou, Zhang and Bischofberger, Dendritic patterns from shear-enhanced anisotropy in nematic liquid crystals, Science Advances, in press (2023).

Use Activity Patterning to Control Active Matter for Logic Operations

Current study of active nematics is focused on characterisations of topological defects and spontaneous flows. Its further applications are hindered by the difficulty in controlling these dynamics. Our collaborators Margaret Gardel group from UChicago and Zev Bryant group from Stanford have realized a new actin-based active nematics whose local myosin activity is responsive to light. Through a combination of simulation, theory and experimental works, we have demonstrated a high level of control over defects and flows in this novel active nematic systems through a judicious design of spatial activity pattern. We further show that patterned activity can give rise to rich defect dynamics, and can even be harnessed to achieve logic operations over defects, which pave the way towards designing autonomous materials.

Relevant publications:

- Zhang*, Redford*, Ruijrok, Kumar, Mozaffari, Zemsky, Dinner, Vitelli, Bryant, Gardel and de Pablo, Spatiotemporal Control of Liquid Crystal Structure and Dynamics Through Activity Patterning, Nature Materials 20, 875-882 (2021).

- Mozaffari*, Zhang*, Atzin and de Pablo, Defect Spirograph: Dynamical Behavior of Defects in Spatially Patterned Active Nematics, Phys. Rev. Lett. 126, 227801 (2021).

- Zhang#, Mozaffari and de Pablo#, Logic Operations with Active Topological Defects, Science Advances, 8, eabg9060 (2022).

Dynamics of Topological Defects in Actin-Based Liquid Crystals

Topological defects are characteristics of nematic liquid crystals (LCs). In a two-dimensional (2D) system, the stable defects are +1/2 and -1/2 defects. Their dynamics are set by the interplay of the material's elasticity and the hydrodynamic effect. In the movie we show that the hydrodynamic simulations of nematic LCs (left) can quantitatively capture the annihilation events in the 2D sheet of short actin-based lyotropic LCs (right).

Relevant publications:

- Zhang*, Kumar*, Ross, Gardel and de Pablo, Interplay of Structure, Elasticity and Dynamics in Actin-Based Nematic Materials, PNAS, 115 (2) E124-E133 (2018).

- Kumar*, Zhang*, de Pablo and Gardel, Tunable Structure and Dynamics of Active Liquid Crystals, Science Advances 4 (10), eaat7779 (2018) (* equal contribution).


Pattern Formation in Composite Active Matter

Bacteria dispersed in biocompatible nematic LCs, the so-called living nematic, is an emerging type of semi-synthetic, composite active matter. Recent works showed that the swimming behaviors of bacteria can be controlled by the continuous LC phase. Our experimental collaborators at Argonne National Lab inserted a magnetic particle to the living nematic. When the particle is rotating in response to an external magnetic field, it aligns the surrounding nematic circularly (a, e). As the particle rotation ceases, the circular texture develops undulations (b-d), which are quantitatively captured by the continuum simulations of active liquid crystals (f-h). 

Relevant publication:

Sokolov*, Mozaffari*, Zhang*, de Pablo and Snezhko, Emergence of radial tree of elastic bands in active nematics, Phys. Rev. X 9(3), 031014 (2019) (* equal contribution).

Controlled Deformation of Deformable Particles

In nature, droplets usually favour round shape to minimize interfacial energy. For example, a giant vesicle is usually in a spherical shape when submerged in water. However, when it is immersed in a lyotropic LC in its nematic phase, it deforms into an elongated shape. We have developed a computational model to calculate how the interplay of elastic force and anchoring condition dictates the equilibrium shape of the vesicle. The simulations predicts that if the anchoring is planar, the vesicle elongates along the nematic far field; if the anchoring is homeotropic, it will deform into a pancake shape.

Relevant publication:

Zhang, Zhou, Martinez-Gonzalez, Hernandez-Ortiz, Abbott and de Pablo, Science Advances 2(8), e1600978 (2016).


Nano Droplet Impact on Solid Surfaces

Droplet splashing is an everyday life phenomenon whose mechanism is difficult to elucidate to date. Nano droplet "splashing" is accessible to molecular dynamics simulations, and can help us understand the difference between nano and macro fluid mechanics. In the following figure, we show simulation snapshots of nano droplet impact on an atomic wall. Different from macroscopic splashing during which the spreading lamella remains intact, nano splashing appears as disintegration of the whole droplet.

Relevant publications:

- Zhang, Farokhirad, Lee and Koplik, Multiscale liquid drop impact on wettable and textured surfaces, Phys. Fluids 26, 082003 (2014).

- Koplik and Zhang, Nanodrop Impact on Solid Surfaces, Phys. Fluids 25, 022003 (2013). 

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