Pushing fluids through micro- and nano-channels is an essential task for microfluidic devices but is notoriously difficult because of large viscous forces. Another strong force available at microscale is the explosive power of vapor bubbles generated by microheaters in contact with the fluid. We have discovered that when bubbles are created closer to one end of a micro-channel, the expansion-collapse cycle leads to net flow from the shorter arm of the channel toward its longer arm. Pumping action is attributed to unequal inertia of the two columns of fluid. An essential role is played by bulk reservoirs which act as momentum sinks and absorb or supply mechanical momentum during the cycle. In this talk, I will first review existing micropump technologies and then describe physical principles behind the inertial pump. Full-scale, three-dimensional numerical simulations that reveal vortex formation in the reservoir and non-uniform flows inside the channel will be presented. An effective, nonlinear, one-dimensional model of fluid motion will be derived and solved with emphasis on the boundary condition at the channel-reservoir interface. Experimental demonstrations of inertial pumps in action will be shown. Finally, potential extensions and applications of this new technology will be briefly discussed.
