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1. From chaos to order: Boundary-driven flow transitions in microtubule-kinesin active fluid

   Dickie, J.H.; Weng, T.; Chen, Y.-C.; He, Y.; Saxena, S; Pelcovits, R.; Powers, T.R.; Wu, K.-T. (March 2024, Bulletin of the American Physical Society)

   Microtubule-kinesin active fluids are distinguished from conventional passive fluids by their unique ability to consume local fuel, ATP, to generate internal active stress. This stress drives internal flow autonomously and promotes micromixing, without the need for external pumps. When confined within a looped boundary, these active fluids can spontaneously self-organize into river-like flows. However, the influence of a moving boundary on these flow behaviors has remained elusive. Here, we investigate the role of a moving boundary on the flow kinematics of active fluids. We confined the active fluid within a thin cuboidal boundary with one side serving as a mobile boundary. Our data reveals that when the boundary's moving speed does not exceed the intrinsic flow speed of the active fluid, the fluid is dominated by chaotic, turbulence-like flows. The velocity correlation length of the flow is close to the intrinsic vortex size induced by the internal active stress. Conversely, as the boundary's moving speed greatly exceeds that of the active fluid, the flow gradually transitions to a conventional cavity flow pattern. In this regime, the velocity correlation length increases and saturates to those of water. Our work elucidates the intricate interplay between a moving boundary and active fluid behavior. *We acknowledge support from the National Science Foundation (NSF-CBET-2045621). 

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2. Confinement Controls the Bend Instability of Three-Dimensional Active Liquid Crystals

   https://doi.org/10.1103/PhysRevLett.125.257801  

   Chandrakar, Pooja; Varghese, Minu; Aghvami, S.Ali; Baskaran, Aparna; Dogic, Zvonimir; Duclos, Guillaume (December 2020, Physical Review Letters)

   Spontaneous growth of long-wavelength deformations is a defining feature of active liquid crystals. We investigate the effect of confinement on the instability of 3D active liquid crystals in the isotropic phase composed of extensile microtubule bundles and kinesin molecular motors. When shear aligned, such fluids exhibit finite-wavelength self-amplifying bend deformations. By systematically changing the channel size we elucidate how the instability wavelength and its growth rate depend on the channel dimensions. Experimental findings are qualitatively consistent with a minimal hydrodynamic model, where the fastest growing deformation is set by a balance of active driving and elastic relaxation. Our results demonstrate that confinement determines the structure and dynamics of active fluids on all experimentally accessible length scales. 

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3. Motor guidance by long-range communication on the microtubule highway

   https://doi.org/10.1073/pnas.2120193119  

   Wijeratne, Sithara S.; Fiorenza, Shane A.; Neary, Alex E.; Subramanian, Radhika; Betterton, Meredith D. (July 2022, Proceedings of the National Academy of Sciences)

   Coupling of motor proteins within arrays drives muscle contraction, flagellar beating, chromosome segregation, and other biological processes. Current models of motor coupling invoke either direct mechanical linkage or protein crowding, which rely on short-range motor–motor interactions. In contrast, coupling mechanisms that act at longer length scales remain largely unexplored. Here we report that microtubules can physically couple motor movement in the absence of detectable short-range interactions. The human kinesin-4 Kif4A changes the run length and velocity of other motors on the same microtubule in the dilute binding limit, when approximately 10-nm–sized motors are much farther apart than the motor size. This effect does not depend on specific motor–motor interactions because similar changes in Kif4A motility are induced by kinesin-1 motors. A micrometer-scale attractive interaction potential between motors is sufficient to recreate the experimental results in a biophysical model. Unexpectedly, our theory suggests that long-range microtubule-mediated coupling affects not only binding kinetics but also motor mechanochemistry. Therefore, the model predicts that motors can sense and respond to motors bound several micrometers away on a microtubule. Our results are consistent with a paradigm in which long-range motor interactions along the microtubule enable additional forms of collective motor behavior, possibly due to changes in the microtubule lattice. 

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4. Self-organization and mixing of microtubule-kinesin active fluid in an activity gradient

   Bate, Teagan E.; Wu, Kun-Ta (January 2021, Bulletin of the American Physical Society)

   Active fluid, composed of kinesin-driven extensile bundles of microtubules, consumes ATP locally to create a self-mixing flow. Mean speed of microtubule-kinesin active fluid was shown to be tunable by varying its components’ concentrations. Such tunability demonstrated the controllability of active fluid with uniform activity. However, how active fluid self-organizes when its activity is non-uniform remains poorly understood. Here, we characterized active fluid behavior and its associated mixing performance in an activity gradient. The activity gradient was created by imposing a temperature gradient because our previous work showed that microtubule-kinesin active fluid exhibited an Arrhenius response to temperature: Increasing temperature sped up active fluid flow, and thus, along a temperature gradient, active fluid flowed faster on one side and slower on the other, forming an activity gradient. We characterized how such a gradient influenced the mixing performance of active fluid in terms of mixing efficiency, stretching rate, and mean squared displacement, comparing with an activity-uniform sample. Our work suggests that applying an activity gradient can serve as a new in-situ method for controlling self-organization and mixing performance of microtubule-kinesin active fluid. 

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5. Mixing of active and passive fluids in microtubule-kinesin active fluid system

   Bate, Teagan; Wu, Kun-Ta (January 2022, Bulletin of the American Physical Society)

   Fluid mixing is driven by the passive process of diffusion and the active process of stretching and folding, which homogenize the system's constituents. Conventionally, the active process is applied via external shearing machines such as a kitchen stand mixer. However, applying external shearing becomes more challenging in mesoscopic fluid systems due to the increasing difficulty of controlling the injection of energy on the micron scale. To overcome this challenge, we introduced microtubule-kinesin active fluid to power the active mixing process. To demonstrate its mixing capability, we created a multi-fluid system where active fluid is adjacent to an inactivated, passive fluid and allowed the active fluid to blend with the passive fluid until the system reaches a homogeneous state. We found that the mixing dynamics of such active-passive fluid mixing was dominated by the passive process of diffusion, until the activity of active fluid was tuned to be sufficiently high and the active processes of active fluid began to dominate the mixing process. Our work will stimulate the development of utilizing active fluid to accomplish mesoscale mixing tasks in multi-fluid systems at the micron scale. *We acknowledge support from the National Science Foundation (NSF-CBET-2045621). 

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