Lab-on-a-chip (LOC) is a minute chemical processing plant that integrates on a single substrate common laboratory procedures ranging from ‰ltration and mixing to separation and detection. To achieve these tasks, it is often necessary to propel, stir, and control uids. Because in many applications one uses buffers and solutions that are electrically conductive, one can transmit electric currents through these solutions. When the device is subjected to an external magnetic ‰eld provided by either a permanent magnet or an electromagnet, the electric current interacts with the magnetic ‰eld to produce Lorentz body forces, which, in turn, drive uid motion. The phenomenon is commonly referred to as magnetohydrodynamics (MHD) and has been used, among other things, to pump ows in microuidic conduits (Qian and Bau, 2005a), to control uid ow in networks (Bau et al., 2003) without a need for mechanical pumps and valves, to stir and mix uids (Bau et al., 2001; Qian et al., 2002; Yi et al., 2002), and to enhance mass transfer next to electrodes’ surfaces (Boum and Alemany, 1999; Lioubashevski et al., 2004; Alemany and Chopart, 2007). For a recent review of a few applications of MHD in microuidics, see Qian and Bau (2009).