Microfluidics and Nanofluidics

, Volume 14, Issue 3–4, pp 469–477 | Cite as

Selective particle and cell clustering at air–liquid interfaces within ultrasonic microfluidic systems

  • Priscilla Rogers
  • Ian Gralinski
  • Cameron Galtry
  • Adrian Neild
Research Paper


In this study, we demonstrate particle and cell clustering in distinct patterns on the free surface of microfluidic volumes. Employing ultrasonic actuation, submersed microparticles are forced to two principal positions: nodal lines (pressure minima) of a standing wave within the liquid bulk, and distinct locations on the air–liquid interface (free surface); the latter of which has not been previously demonstrated using ultrasonic standing waves. As such, we unravel the fundamental mechanisms behind such patterns, showing that the contribution of fluid particle velocity variations on the free surface (acoustic radiation force) results in patterned particle clustering. In addition, by varying the size and density of the microparticles (3.5–31 μm polystyrene and 1–5 μm silica), acoustic streaming is found to increase the tendency for a smaller and lighter particle to cluster at the air–liquid interface. This selectivity is exploited for the isolation of multiple microparticle and cell types on the free surface from their nodally aligned counterparts. Free surface clustering is demonstrated in both an open microfluidic chamber and a sessile droplet, as well as using a range of biological species Escherichia coli, blood cells, Ragweed pollen and Paper Mulberry pollen). The ability to selectively cluster submersed microparticles and cells in distinct patterns on the free surface showcases the excellent suitability of this method to lab-on-a-chip systems.


Particle manipulation Air–liquid interface Ultrasonic standing waves Acoustic radiation force Acoustic streaming 



The authors would like to thank the Australian Research Council (No. DP110104010) for their kind support of this research.

Supplementary material

10404_2012_1065_MOESM1_ESM.pdf (1.8 mb)
ESM1 (PDF 1810 kb)


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Copyright information

© Springer-Verlag Berlin Heidelberg 2012

Authors and Affiliations

  • Priscilla Rogers
    • 1
  • Ian Gralinski
    • 1
  • Cameron Galtry
    • 1
  • Adrian Neild
    • 1
  1. 1.Department of Mechanical and Aerospace EngineeringMonash UniversityClaytonAustralia

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