Fast nanofluidics by travelling surface waves

During the past two decades, we see the fast development and wide applications of nanotechnologies, biological chips and lab-on-a-chip. Nanoscale transport governs the behaviour of a wide range of nanofluidic systems, but it remains less understood due to the enormous hydraulic resistance associated with the nano-confinement and the resulting minuscule flow rates in MEMS/NEMS. In addition, the huge surface-volume ratio (up to 106−109) signifcantly affects the mass and momentum transport in micro/nanoelements and makes this type of research challenging.

    Obviously, the challenge is to overcome the large surface and viscous forces that prevent the fluid from flowing at the nanoscale, wherein other driving forces can be ignored in most microfluidic systems. In microfluidic biochip engineering, a driving force for driving microscale fluid motion has been introduced by employing the surface waves. The key of this novel technology is to make a micropump that is able to position reagents on the surface of chips or in microfluidic channels without the mechanical contact. This is implemented in terms of the surface acoustic waves (SAW) that are induced using radio frequency electric signals. These waves arise through the use of piezoelectric substrate materials in the chip. It is interesting that the SAW-induced effect has similarity in live nature, for example with the skin features of fast-swimming sea animals, such as dolphins. Dolphins use travelling waves on their skin surface to damp the turbulence in the boundary layers near the skin surface.

    Our purpose is to present that the travelling surface waves propagating on the walls of nanochannels can offer a powerful method for inducing a host of extremely fast nanofluidic flow. We find that the flow rate is enlarged by increasing the amplitude of travelling surface waves and can be up to a sevenfold increase. However, the flow rate is only enhanced in the limited range of frequency of travelling surface waves such as low frequencies, and a maximum fivefold increase in flow rate is pronounced. In addition, the fluid-wall interaction (surface wettability) plays an important role in the nanoscale transport phenomena, and the flow rate is signifcantly increased under a strong fluid-wall interaction (hydrophilicity) in the presence of travelling surface waves. Moreover, the friction coefficient on the wall of nanochannels is decreased obviously due to the large slip length, and the shear viscosity of fluid on the hydrophobic surface is increased by travelling surface waves. It can be concluded that the travelling surface wave has a potential function to facilitate the fow in nanochannels with respect to the decrease in surface friction on the walls. Our results allow to defne better strategies for the fast nanofluidics by travelling surface waves. (more details please see: https://doi.org/10.1007/s10404-017-1946-z)

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R Pillai, JD Berry, DJE Harvie, MR Davidson (2017) Electrophoretically mediated partial coalescence of a charged microdropChemical Engineering Science, 169: 273-283. (access here)

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