Microtubule-based Active Matter: Controlling Microscopic Properties and Characterizing Instabilities
Creators: name: Chandrakar, Pooja
Publisher: name: Brandeis University
Year: 2021
Subjects: Active Liquid Crystal
Active Matter
Active matter under confinement
Bend Instability
Kinesin
Microtubule
Bioengineering

description: Active matter are internally driven non-equilibrium systems, which are made of energy consuming motile entities. They exhibit long-range self-organization phenomena such as pattern formation, collective motion and spontaneous flows. Understanding the physical principles governing these phenomena are of both fundamental and practical interest. They offer promise of improving the fundamental knowledge of non-equilibrium statistical mechanics while also providing a platform for creating novel, programmable synthetic materials with life-like properties. Studying active matter systems is difficult owing to the complexity in controlling their constituents. We demonstrate experimental methods to control a biological active matter system and characterize its behavior. We describe a bottom-up approach for assembling microtubule (MT) and kinesin motor-protein-based active fluids that are versatile and highly tunable. We start with demonstrating four different formulations of MT-based active fluid that improve previously developed MT-based active fluids and characterize their dynamics by measuring their average speed, lifetime and velocity-velocity correlation length. First, we show that a non-processive kinesin motor improves the temporal stability of the active fluid. Unlike the processive motors which take 100 steps before detaching from the MTs, processive motors detach from the microtubule after a single step reducing effective motor induces cross-linking. Second, we develop an active fluid driven by kinesin motors that can be permanently attached to the backbone of the MTs. It yields similar dynamic steady state as previously developed MT-based active fluid and enables patterning the motor-distribution on microtubules and altering MT-MT interactions. These motors significantly improve the longevity of the active fluid from hours to a week. Third, the motors discussed in the second formulation surprisingly forms clusters which, in their native state without permanently attaching to the MT-backbone, can generate spontaneous flows like traditional active fluids. Fourth, we provide a depletant free formulation with MT-specific crosslinker protein which allows addition of other passive soft-materials such as liquid crystals and colloids to the MT-based active fluids.
description: Next, we study the fundamental instabilities of microtubule-based active fluid in 3D nematic state. We characterize the shear aligned 3D isotropic active fluids undergoing the universal bend instability. We experimentally study the bend-instability in 3D active nematic in different rectangular confinements and observe that the characteristic instability length-scale is strongly dependent on the dimensions of the confinement signifying that the boundaries control the bulk behavior of the active nematic as well. We further study the emergent length-scale associated with this instability and its dependence on the constituents such as kinesin motor, microtubule cross-linker and the chemical fuel for the kinesin motors, Adenosine triphosphate (ATP). These observations aid in understanding of the microscopic parameters associated with generating active and elastic stresses in the active nematic. These studies will improve the understanding of extensile active nematic systems and mapping theories of active matter to the experiments.
description: Finally, we develop a membrane-based microfluidic system which enables variation of ATP in a 2D active nematic system dynamically. We provide a proof-of-concept by dynamically varying the ATP concentration in same samples. This method enables study of MT-based active nematics at ATP ranges that were not accessible via previous formulations. Exploring a broader range of ATP variation would aid in robust mapping of experiments onto theoretical models and subsequently understanding the underlying physical principles of 2D active nematic systems.