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1. Competition of convection and diffusion in the self-mixing of microtubule-kinesin active fluid with non-uniform activity: Experiment

   Bate, Teagan; Varney, Megan; Taylor, Ezra; Dickie, Joshua; Chueh, Chih-Che; Norton, Michael M; Wu, Kun-Ta (March 2023, Bulletin of the American Physical Society)

   Active fluids have potential applications in micromixing, but little is known about the mixing kinematics of such systems with spatiotemporally-varying activity. To investigate, UV-activated caged ATP was used to activate controlled regions of microtubule-kinesin active fluid inducing a propagating active-passive interface. The mixing process of the system from non-uniform to uniform activity as the interface advanced was observed with fluorescent tracers and molecular dyes. At low Péclet numbers (diffusive transport), the active-inactive interface progressed toward the inactive area in a diffusion-like manner and at high Péclet numbers (convective transport), the active-inactive interface progressed in a superdiffusion-like manner. The results show mixing in non-uniform active fluid systems evolve from a complex interplay between the spatial distribution of ATP and its active transport. This active transport may be diffusion-like or superdiffusion-like depending on Péclet number and couples the spatiotemporal distribution of ATP and the subsequent localized active stresses of active fluid. Our work will inform the design of future microfluidic mixing applications and provide insight into intracellular mixing processes. *T.E.B., E.H.T., J.H.D., and K.-T.W. acknowledge support from the National Science Foundation (NSF-CBET-2045621). C.-C. C. was supported through the National Science and Technology Council (NSTC), Taiwan (111-2221-E-006-102-MY3). M.M.N. was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences (DE-SC0022280). 

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2. Competition of convection and diffusion in the self-mixing of microtubule-kinesin active fluid with non-uniform activity: Simulation

   Dickie, Joshua H; Bate, Teagan; Varney, Megan; Taylor, Ezra; Chueh, Chih-Che; Norton, Michael M; Wu, Kun-Ta (March 2023, Bulletin of the American Physical Society)

   Active fluids with spatiotemporally varying activity have potential applications to micromixing; however previously existing active fluids models are not prepared to account for spatiotemporally-varying active stresses. Our experimental work used UV-activated caged ATP to activate controlled regions of microtubule-kinesin active fluid inducing a propagating active-passive interface. Here, we recapitulate our experimental results with two models. The first model redistributes an initial ATP distribution by Fick's law and translates the ATP distribution into a velocity profile by Michaelis-Menton kinetics. This model reproduces our experimental measurements for the low-Péclet number limit within 10% error without fitting parameters. However, as the model is diffusion based, it fails to capture the convective based superdiffusive-like behaviour at high Péclet numbers. Our second model introduces a spatiotemporally varying ATP field to an existing nematohydrodynamic active fluid model and then couples the active stresses to local ATP concentrations. This model is successful in qualitatively capturing the superdiffusive-like progression of the active-inactive interface for high Peclet number (convective transport) experimental cases. Our results show that new model frameworks are necessary for capturing the behaviour of active fluid with spatiotemporally varying activity. *T.E.B., E.H.T., J.H.D., and K.-T.W. acknowledge support from the National Science Foundation (NSF-CBET-2045621). C.-C. C. was supported through the National Science and Technology Council (NSTC), Taiwan (111-2221-E-006-102-MY3). M.M.N. was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences (DE-SC0022280). 

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3. 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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4. VLE-Based Phase Field Method to Simulate High-Pressure Diffuse Interface with Phase Change

   https://doi.org/10.2514/6.2024-1635  

   Srinivasan, Navneeth; Zhang, Hongyuan; Yang, Suo (January 2024, American Institute of Aeronautics and Astronautics)

   Supercritical fluids, often present in modern high-performance propulsion systems, result from elevated operating pressures. When these systems utilize fluid mixtures as fuel or oxidizers, a transcritical effect often occurs. This effect can lead to misjudgments, as mixture critical points exceed those of individual components. Fluid mixing may induce phase separation, creating liquid and vapor phases due to the transcritical multi-component effect. Consequently, two-phase modeling is essential for transcritical and supercritical fluids. Traditional interface capturing methods, like Volume of Fluid (VOF) and Level Set (LS), present challenges such as computational expense and lack of conservatism. The Phase Field (PF) method, or the Diffuse Interface (DI) method which uses a phase fraction transport equation, emerges as a conservative alternative. Despite the absence of an initial interface in transcritical fluids, phase separation from mixing may form liquid droplets, necessitating multiphase modeling. To address these complexities, a Vapor-Liquid Equilibrium (VLE) model, coupled with the PR equation of state, is introduced. This model estimates phase fractions, liquid and vapor compositions, densities, and enthalpies through a flash problem solution. The conventional PF model is enhanced by replacing the phase fraction transport equation with VLE-derived values. The resulting VLE-based PF method is implemented into an OpenFOAM compressible solver, ensuring numerical stability with explicit phase field terms and a new CFL criterion. Test cases involve 1D interface convection and 2D droplet convection. In the 1D test, the VLE-based PF model adeptly captures interfaces, adjusting thickness as needed. The 2D droplet case, challenging due to a non-aligned Cartesian grid, exhibits uniform interface thickness and preserves droplet shape. The VLE-based PF model demonstrates versatility and reliability in capturing complex fluid behaviors, offering promising prospects for future research. 

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5. Driven and Active Colloids at Fluid Interfaces

   https://doi.org/: https://doi.org/10.1017/jfm.2020.708  

   Chisholm, Nicholas G; Stebe, Kathleen J (January 2021, Journal of fluid mechanics)

   null (Ed.)

   We derive expressions for the leading-order far-field flows generated by externally driven and active (swimming) colloids at planar fluid–fluid interfaces. We consider colloids adjacent to the interface or adhered to the interface with a pinned contact line. The Reynolds and capillary numbers are assumed much less than unity, in line with typical micron-scale colloids involving air– or alkane–aqueous interfaces. For driven colloids, the leading-order flow is given by the point-force (and/or torque) response of this system. For active colloids, the force-dipole (stresslet) response occurs at leading order. At clean (surfactant-free) interfaces, these hydrodynamic modes are essentially a restricted set of the usual Stokes multipoles in a bulk fluid. To leading order, driven colloids exert Stokeslets parallel to the interface, while active colloids drive differently oriented stresslets depending on the colloid's orientation. We then consider how these modes are altered by the presence of an incompressible interface, a typical circumstance for colloidal systems at small capillary numbers in the presence of surfactant. The leading-order modes for driven and active colloids are restructured dramatically. For driven colloids, interfacial incompressibility substantially weakens the far-field flow normal to the interface; the point-force response drives flow only parallel to the interface. However, Marangoni stresses induce a new dipolar mode, which lacks an analogue on a clean interface. Surface-viscous stresses, if present, potentially generate very long-ranged flow on the interface and the surrounding fluids. Our results have important implications for colloid assembly and advective mass transport enhancement near fluid boundaries. 

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