Swimming and transport of the photosynthetic alga Chlamydomonas reinhardtii in steady and oscillatory shear flows were investigated. Sinusoidal flow of algal suspension was generated in the microchannels using a microfluidic pressure-driven controller. The oscillating flow was expressed as Q (t) = A. sin (ωt) where Q (t), t, A, and ω, represent the instantaneous flow rate, time, amplitude and angular frequency of the oscillating flow, respectively. The different amplitudes considered in the study were A = 0.075, 0.15, 0.22, 0.3 and 0.5 μl/min. Maximum mean velocities corresponding to this range of amplitudes varies between 14.7 μm/s and 392 μm/s depending on the microchannel used in the experiment. At each amplitude, four frequencies were applied: f = 0.5, 1, 2 and 3.3 Hz. The motion of algal cells was recorded and their trajectories in the flow were analyzed. It was shown that the dynamics of cells in the steady flow varies from a random motion to a zigzag motion depending on the flow shear rate. At low shear rates, the average value of the angle (α) between the cell trajectory and flow stream was 40°, and a zigzag motion of the cells was observed between the lateral walls. However, at higher flow shear rates, the mean value of this angle was about 10° with a smaller distribution range (0° < α < 30°) compared to the low shear regime (10° < α < 70°). When the flow condition was turned to a sinusoidal flow, two strong symmetric peaks appeared close to ± 90° in the probability distribution of α. The visualizations of cell trajectories showed that the cells swim perpendicular to the flow direction in wavy-form trajectories such that regardless of the oscillation conditions, their average vertical velocity (Vy) is equal to their motility (130 μm/s).

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