At the beginning of each experiment, 12-18 µM tubulin and DMSO were imaged as the first negative control. Next, the flow chamber was extensively washed with the imaging buffer, and a new solution of tubulin with a tested compound was introduced. Up to 3 compounds were tested sequentially in the same flow chamber, with washes in between. At the end of each experiment, the second negative control, identical in composition to the first one, was imaged to make sure that the chamber did not degrade over time. Each compound was tested in N = 3 independent experiments.