Measuring acoustofluidic phenomena at the nanoscale using holographic microscopy

To understand the mechanisms driving fluid flow behavior in nanofluidics so that they may be used for on-chip biomedical and chemical applications, the fluid’s motion itself needs to be observable and measurable, a difficult challenge at the nanoscale, and especially so at the speeds made possible with acoustically driven propulsion. We present a new method for measuring both slow and fast flows in nanofluidics using high-speed digital holographic microscopy. We measure the evaporation-driven flow in 25- and 7-nm tall nanoslit channels, showing that the consequent flow speed is about 15 times slower than open atmospheric evaporation due to the confinement of the nanoslit channel. We also measured the surface acoustic wave-driven flow in the 25-nm channel, showing flow at a speed of 0.12 m/s from acoustic wave propagation at 39.7 MHz interacting with the fluid in the channel. A process to eliminate the many sources of noise to produce these results are provided, showing that—in particular—spatial averaging is useful to determine the fluid flow and the dewetting of the fluid in the nanoslit channel over time.