Active matter encompasses a wide spectrum of non-equilibrium systems, the constituents of which can convert energy into local mechanical work, giving rise to long-range collective dynamics. Examples include bird flocking, fish schooling and cytoplasmic streaming. The study of active matter can help us better understand biological systems and design new biomimetic materials and applications. A paradigmatic active matter system, namely active nematic liquid crystals (LCs), consist of anisotropic units which can form a nematic phase at equilibrium. Recent experiments have unveiled active nematics in many cell-based systems, including epithelial tissues and dense bacterial films. In these active nematics, intriguing phenomena emerge, such as spontaneous flows and nucleation and self-propulsion of topological defects. Controlled dynamics of these defects can lead to applications in microfluidics and photonics. However, this is difficult so far. 

　　In this talk, I will introduce several measures through which we can achieve a high control over topological defects in active nematics. In one, I show that a judicious choice of a spatial pattern of active stress can lead to defect confinement effect and guide the nucleation and trajectory of +1/2 defects; we further show that we can use these patterns to realize logic operations. In the other measure, the active nematic is placed on a spheroidal droplet, which leads to a periodic dynamic pattern. These measures are demonstrated through a combination of biopolymer-based experiments and hydrodynamic simulations. In the end, I will also emphasize the importance of material properties in terms of hydrodynamic flows in driven and active nematics. Specifically, for lyotropic chromonic LCs, we show that its low twist modulus and tumbling character can give rise to new flow state and director pattern that are not seen in other types of LCs. Taken together, our work sheds light on the understanding the behavior of topological defects in non-equilibrium settings and paves the way towards applications in microfluidics and photonic devices. 

　　References: 

Zhang, Redford, et al., Spatiotemporal Control of Liquid Crystal Structure and Dynamics Through Activity Patterning, Nature Materials (2021). 

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

Zhang, Mozaffari and de Pablo, Logic Operations with Active Topological Defects, Science Advances, in press (2022). 

Zhang, Zhang, et al. Structures and Topological Defects in Pressure-driven lyotropic Chromonic Liquid Crystals, PNAS (2021).