Estimating the pressure force around swimming plankton using micro particle image velocimetry

Obtaining pressure force for freely swimming microorganisms is a challenging yet important problem. Here, we report the swimming kinematics and dynamics of the zooplankton Acartia tonsa nauplius investigated using Micro Particle Image Velocimetry (mu PIV). Using rigid object tracking, we obtain sub-pixel accurate localization of freely swimming A. tonsa, revealing its highly periodic locomotion. We exploit this periodicity to obtain phase-locked averaged kinematics for position, speed, and acceleration. The swimming speed profile of A. tonsa has a distinct double peak, due to its two power strokes. Next, we investigate the flow field around swimming A. tonsa using mu PIV. We dynamically mask A. tonsa in mu PIV images using an object-fixed coordinate transformation, leveraging the sub-pixel accurate localization. Our analysis shows of a pair of attached vortices during the two power strokes, which are pushed away during the recovery stroke. Finally, using a semi-implicit pressure velocity algorithm, we calculate the pressure force from the time-dependent flow fields. These calculations indicate a low-pressure region ahead of the A. tonsa during the peak of the power strokes. The vertical pressure force correlates well with the vertical swimming speed.