On the oscillatory behavior of living particles suspension under step strain

In the present work, we investigated the response of a C. elegans suspension under step strain. Most experimental measurements and theoretical models published so far have focused on the steady-flow rheology. However, under transient conditions, we expected to observe competition between flow alignment and orientational relaxation. Upon startup of rotation, an initial viscous stress jump occurred primarily due to the solvent viscosity. After this initial jump, particle orientations relaxed leading to a decrease in the measured viscosity as a result of the extensile stresslet. When rotation stopped, the negative active stress persisted for a certain time, leading to a negative undershoot in the stress response, which corresponds to a retrograde torque. These mechanics led to an oscillatory response. Based on these experimental findings, we postulate an expression for the equivalent relaxation function based on the Landau-de Gennes theory and physics of nematic liquid crystals.