Geometry-dependent transmission of externally imposed shear stress in confined microtubule-kinesin active fluids

Abstract

Active fluids generate internal active stress and exhibit unique responses to external forces such as superfluid-like flow and self-yielding transitions. However, how confinement geometry influences these responses remains poorly understood. Here, we investigate microtubule-kinesin active fluids under external shear stresses in three geometries. In a thin slab-like container with a translating wall, we observed a kinematic transition from activity-dominated chaotic flow to lid-driven cavity flow when the applied shear stress exceeds ~1.5 mPa, comparable to the intrinsic active stress magnitude. Simulations supported that this transition arises from competition between internal active stress and imposed shear stress. In contrast, in a ratcheted toroidal confinement, the imposed shear remains localized near the driven boundary and does not propagate through the bulk. Nevertheless, this localized perturbation cooperatively couples with internal active stress to reverse the global circulation. This cooperative mechanism is further demonstrated in a connected-toroid geometry: driving one toroid reorganizes flow in a second, indirectly connected toroid, while no such influence occurs in a passive fluid. Together, these findings show that the response of active fluids to external forcing depends not only on the magnitude of applied stress but also on how confinement geometry mediates whether stresses interact through bulk competition or local-to-global cooperative reorganization, revealing a new approach to combining static geometrical design with dynamic external stimuli for real-time modulation of flow patterns. Such strategies may be applied to microfluidics, where micromechanical actuators dynamically tune active fluid behavior within fixed geometries, enabling transitions between chaotic and coherent flows for mixing, sorting, or transport.