ABSTRACT
The Active River
Characterizing Active Fluid Response to External Shear and Geometric Confinement
Active fluids are a class of soft matter systems capable of self-propulsion through the conversion of microscopic energy sources into mechanical motion. These materials are ubiquitous in nature; for example, the cytoplasm of eukaryotic cells can be described as a form of active fluid which converts chemical energy (ATP) into the mechanical motion known as cytoplasmic streaming. Cytoplasmic streaming is a phase of coherent active fluid which transports proteins within the cell far more efficiently than diffusion alone. When eukaryotic cells move through their environment, they experience external shear, which then affects their cytoplasm. Despite the ubiquity of this interaction, active fluid’s response to external shear has been relatively unexplored. Therefore, to understand the interplay between internal and external shear we use a biologically derived model active fluid system, microtubule kinesin-based active fluid, to probe the behavior.
Utilizing our active fluid system, we demonstrate that the response of active fluid to external shear is influenced by geometric confinement. Specifically, we examine three distinct geometries: a thin slab-like confinement, a toroidal confinement, and a connected toroidal confinement. Across these configurations, the behavior of the active fluid ranges from resisting externally applied shear to cooperating with it, resulting in a diverse set of dynamical responses. Slab-like geometries were found to suppress the influence of shear until the externally applied stress became comparable to the internally generated stress at ~1.5mPa. Above this critical point, the system’s flow structure became water-like with the correlation length matching the inactive, passive fluid system and fluid dynamics dominated by the external driving force.
Toroidal geometries, which can form coherent transport phases, were instead found to promote cooperation with external forcing even at low driving speeds of (~50 µm/s). It was found that under external forcing the local active fluid network is reoriented by the external shear. This reorientation then propagates across the entire interlinked network, exceeding the length scales accessible in passive fluid systems, and reversing the direction of flow. Further examination of the interconnected toroidal system found it capable of self-correcting behavior. When a notch or ratchet is inscribed into the wall of a toroidal system, it had previously been shown that the spontaneous flow direction in a toroidal system would be directed by the ratchets orientation. This work expands upon these previous results, showing if the fluid is driven against its preferred orientation the ratchets can reverse the flow after driving has ceased.
These results suggest that the competition between internally generated active stress and externally applied shear depends on confinement geometries. By leveraging the role of geometry in active fluid response to external stimuli, its flow phases can be programmed and externally controlled, allowing novel microfluidic chips with self-correcting flow dynamics after mechanical forcing. More broadly this work can connect to cellular systems, demonstrating that geometry influences the behavior of active fluids, such as cytoplasm, in the presence of mechanical shear. Understanding how geometry influences the interaction between internally and externally generated stress can provide new control mechanisms for systems which are driven by force-sensitive self-organization.
Advisor: Kun-Ta Wu
Committee Members:
Chair: Qi Wen
Member: Robert Pelcovits
Member: Kun-Ta Wu
Member: Germano Iannachione
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Meeting ID: 923 1153 0319