Figure 4.

Calculated trajectories of a particle lighter than the fluid (d_p = 60 μm, ρ_p = 1.05 g/mL, ρ_f = 1.075 g/mL) in a rotating disc. (a) The orange dashed line shows a trajectory of a free particle in a disc rotating around the point (0,0) in the counterclockwise direction with angular velocity ω = 800 rpm. The red line shows the trajectory of a particle in a rectangular chamber of 4 mm width (the grey empty box). After reaching the lateral wall of the chamber, the particle keeps moving in the direction of the centrifugal force. (b) The red line shows the trajectory of a particle in the absence of the Euler force. The particle starts moving from the initial position (x = 0, y = 5 mm) and reaches the bottom of the chamber. When the Euler force (when ω linearly increases from 0 to 800 rpm in 0.5 s) is exerted on the particle, it achieves the wall and then moves along the wall in the direction of the centrifugal force (the blue line). A stronger Euler force (ω increases from 0 to 800 rpm in 0.33 s) leads to the backward motion (the light blue line). When the trajectory of the particle starts at a closer position to the center of rotation (x = 0, y = 2 mm), a weaker Euler force (ω = 0 to 800 rpm in 0.5 s) is sufficient for the backward motion (the green line).