a, b Still images of tracer beads before and c–f during application of a 15 Hz, 17.4 kA/m field at the time indicated (fluid height h = 544 µm). Ellipses highlight selected bead clusters and dashed lines demarcate 250 µm steps. g, h Statistical comparison of flow speed before and during application of g 17.4 kA/m and h 1.9 kA/m fields (15 Hz, h = 544 µm); * = p < 0.05 (one-tailed paired t-test), data bars are the average flow (error bars show standard deviation) from n = 3 separate field applications (individual data points; average flow for each field application measured from 50 beads). Effect of i field amplitude (at 15 Hz) and j frequency (at 11.2 kA/m) on flow speed at different h. The line in j is a linear fit to the h = 357 µm data for frequencies ≤ 20 Hz. Data obtained by calculating the magnitude of the average velocity vector from 58–187 beads, correcting for ambient drift by subtracting the average pre-field measurement (error bars derived from the standard deviation of the x- and y-components of the velocity vector).