PMC

Instantaneous x₁–x₂ plane of the streamwise (top), wall-normal (center) and spanwise (bottom) velocities in a spanwise rotating channel flow with slip boundary conditions at the walls for Re_τ = 395 and Ro_τ = 10. The colorbar shows velocity in wall units.

Heat maps of B. subtilis instantaneous velocity analyzed by track the position of each cell moving along hyphae from . We tracked the positions of each cell moving along hyphae and from the instantaneous position of the center of mass of a cell. We generated heat maps of the instantaneous velocity, where darker colors represent slower speeds and lighter colors represent higher velocities, respectively. The heat maps indicate that there is a weak oscillation in the instantaneous velocity over time (). The reason for the oscillations that we see in the instantaneous velocity of bacteria moving along hyphae may be the result of stick-slip motion mediated by the flagella. Because of the low water content and narrow gaps between hyphae and agar, the cells may momentarily become wedged in tight gaps; however, the flagellar motors may exert sufficient force to free the stuck cells.

“Footprints” and instantaneous velocities of neutrophils rolling through L-selectin on PNAd. (A–C) Footprints of representative rolling neutrophils on PNAd (1:10) at 2, 8, and 32 dyn/cm² of wall shear stress, respectively. Neutrophils rolled with a jerky, step-wise motion. Tightly packed collections of white pixels show the center zone of cells during pauses. (D–F) Instantaneous velocities of representative, individual neutrophils on PNAd (1:10) at 2, 8, and 32 dyn/cm² of wall shear stress, respectively. Velocities were determined by measuring the displacement of the cell within successive video frames (0.033 s) in the flow direction (V _x) and in the direction perpendicular to flow (V _y). Dashed line, threshold velocity used in method 1 to define the beginning and end of pauses during rolling.

Velocities of VeGe synaptic clusters. A, Segmentation and individualization of VeGe synaptic clusters (1–4) in the field shown in the inset (arrowheads; VeGe in green, FM4–64 staining in red; same field as in A–C). Individual clusters are identified by a color code. B, Trajectory of the fluorescence “mass center” of the four synaptic clusters highlighted in D. C, Instantaneous velocities (V_inst) as a function of time of four VeGe synaptic clusters in a control cell (same clusters as in A and B) and in a cell exposed to 3 μm latrunculin. Insets, Fluctuations with time of V_inst relative to average value on each trajectory. Note the reduced dynamics of VeGe clusters after latrunculin treatment.