Shear forces are regulated by well fluid volume.

Three consecutive frames of 400 msec duration captured beads before (a), during (b), and after (c) the application of a ∼11 atm peak pressure blast with (180 µl fluid volume) and without shear (380 µl fluid volume). Significant bead motion due to shear is registered in top frame b. Note, a single bead did not move (arrow); this bead, presumably immobilized due to adhesion to the surface, allows one to check for stability of the stage during the blast. In the absence of shear, the application of the same peak pressure blast does not show any bead displacement (bottom frame b).