Confinement geometry governs the impact of external shear stress on active stress-driven flows in microtubule-kinesin active fluids

Abstract

Active fluids generate internal active stress and exhibit unique responses to external forces such as superfluidity 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 two geometries. In slab-like confinement (a narrow-gap cavity), external stresses propagated throughout the system, leading to stress competition and a kinematic transition that shifted dynamics from active stress-dominated to shear stress-dominated flow. At the transition, we estimate the active stress to be ~1.5 mPa. Simulation supported that this transition arises from stress competition. In contrast, in ring-like confinement (a toroidal system), external forces acted locally, inducing a mini cavity flow that triggered self-organized reconfiguration rather than direct entrainment. These findings show that the response of active fluids to external forcing depends not only on the magnitude of the applied stress but also on how confinement geometry directs and redistributes that stress, revealing a new approach to controlling active fluid behavior by combining static geometrical design with dynamic external stimuli for real-time modulation of flow patterns. Such control strategies may be applied to microfluidic systems, where external inputs such as micromechanical actuators can dynamically tune active fluid behavior within fixed device geometries, enabling transitions between chaotic and coherent flows for tasks such as mixing, sorting, or directed transport.