  Bubble-driven inertial pumps are a novel method of moving liquids through
microchannels. We combine high-speed imaging, computational fluid dynamics
(CFD) simulations and an effective one-dimensional model to study the
fundamentals of inertial pumping. For the first time, single-pulse transient
flow through U-shaped microchannels is imaged over the entire pump cycle with
4-microsecond temporal resolution. Observations confirm the fundamental N-shape
flow profile predicted earlier by theory and simulations. Experimental flow
rates are used to calibrate the CFD and one-dimensional models to extract an
effective bubble strength. Then the frequency dependence of inertial pumping is
studied both experimentally and numerically. The pump efficiency is found to
gradually decrease once the successive pulses start to overlap in time.
