1. Defect Dynamics in 2D Active Nematic Liquid Crystals
S. DeCamp, G. Redner, M. Hagan, and Z. Dogic
Brandeis MRSEC ( )
Unbinding of a pair of defects
While conventional materials are assembled from inanimate building blocks, we are exploring the behavior of soft materials in which the constituent components consume energy and spontaneously coordinate their microscopic behavior and form novel materials such as active gels, crawling emulsion droplets, and living liquid crystals.
In active liquid crystals, defects proliferate to a dynamical steady state through unconventional creation and annihilation processes. Pairs of defects stream across the nematic liquid crystal exhibiting tunable dynamics not readily observed in equilibrium liquid crystals.
Characteristic Defects of a Nematic Liquid Crystal
+1/2
-1/2
Tracking defects – Tunable Defect Dynamics
Behavior of materials assembled from inanimate molecules is constrained by the laws of equilibrium of statistical mechanics. We have assembled animate materials built from filamentous microtubules and molecular motors. Being actively driven, these materials are not constrained by the laws of equilibrium statistical mechanics and they can exhibit sought-after properties such as autonomous motility, internally generated flows and self-organized beating. Starting from extensile microtubule bundles, we hierarchically assemble far-from-equilibrium analogues of conventional polymer gels, emulsions, and liquid crystals.
When confined near a 2D oil/water interface, the microtubules spontaneously adsorb onto the surface to produce highly active two-dimensional nematic liquid crystal whose streaming flows are controlled by internally generated fractures and self-healing, as well as unbinding and annihilation of oppositely charged disclination defects. Through image processing defect tracking algorithms combined with the optical technique, quantitative polarization light microscopy, we are able to track defects to study their dynamics and understand their motion. With this bottom-up approach, we are able to tune the dynamics of the defects by varying the amount of the chemical fuel ATP, which the kinesin motors use to walk along microtubules. The resulting active liquid crystals exhibit interesting properties, such as tunable defect streaming speeds and complex orientationally dependent interactions. Additionally, a bimodal orientation of +1/2 defects spontaneously arises and suggests the possibility of engineering directed fluid flows using active nematic liquid crystals. These observations exemplify how assemblages of animate microscopic objects exhibit collective biomimetic properties that are very different from those found in materials assembled from inanimate building blocks, challenging us to develop a theoretical framework that would allow for a systematic engineering of their far-from-equilibrium material properties.