Rheology and pattern formation in an active suspension of swimming nematodes

We explore the rheological properties of suspensions of the Turbatrix aceti nematodes, a model active fluid system. Prior studies on such an active fluid of nematodes have found interesting synchronized motion or metachronal waves as well as time-dependent viscosities when exposed to low shear rates. Here, we use a rheo-optical setup to observe synchronized motion of swimming nematodes as the suspension is being sheared. We simultaneously collect rheological data while performing shear rate sweeps, where we observe hysteresis, and perform other tests to establish relationships between viscosity and concentration. We observe metachronal waves that form for certain concentrations and shear rates. We also study the pattern formation in lifting Hele-Shaw flows. We see and quantify the fingering instabilities and other patterns that form as we separate the parallel plates confining the active nematode suspension. Our observations offer potential new directions for understanding active matter and collective motion in response to external forces.