Measuring chaotic advection in a biological active nematic in viscous environments*

Active flows are commonly found in nature ranging from macro-scale (bird flocks and schools of fish), to the micro-scale (cytoplasmic streaming and bacterial colonies). The common theme among these active materials is the individual entities consume energy leading to large-scale collective motion and flows. A widely studied example is the microtubule/kinesin based active network which is highly tunable. When this non-equilibrium system is confined in 2D at the oil/water interface, active topological defects emerge generating chaotic flows where the microtubules are advected within the network. An avenue of interest in nonequilibrium systems is to observe the material’s response to external changes in its environment. Prior research has shown that changing the viscosity of the oil in contact with the active network changes the velocity and morphology of the network. Increasing the viscosity increases the number density of defects and decreases the network velocity. We investigate how the external changes from viscosity affect various quantitative parameters from chaotic advection theory, such as local fluid stretching rates within the network and topological entropy calculated from defect braiding.