Evaluation of the mass transfer effect of the stalk contraction cycle of \textit{Vorticella}

\textit{Vorticella} is a protozoan with a contractile stalk that can contract pulling the cell body toward the substrate in less than 10 ms and return to the extended state in a few seconds. Although this stalk contraction is one of the fastest cellular motions, it is unknown why \textit{Vorticella} contracts. Because the flow field induced by \textit{Vorticella} shows different characteristics between contraction and relaxation, it has been suggested that \textit{Vorticella} augments mass transfer near the substrate based on its stalk contraction-relaxation. We investigate this hypothesis using computational fluid dynamics (CFD) simulations and particle image velocimetry (PIV) experiments. In both approaches, \textit{Vorticella} is modelled as a solid sphere that translates perpendicular to a solid surface in liquid based on the measured stalk length changes of \textit{Vorticella}. Based on the computationally and experimentally simulated flow, we evaluate the mass transfer capability of \textit{Vorticella}, for a possible application of the stalk contraction of \textit{Vorticella} as a biomimetic model system for microfluidic mixers.