Figure 2.

A hydrophobic barrier uses a small hydrophobic capillary to keep liquid in the larger channel until sufficient centrifugal pressure is applied to overcome the capillary pressure of the small restriction. A hydrophilic barrier is a metastable configuration in which all the channel walls have the same contact angle. The capillary pressure keeps the liquid in the small tube until sufficient centrifugal pressure is applied to reverse the curvature to allow expansion into the large channel. Redrawn from Ref. 27.