CEA-Leti developed a new acousto-microfluidic technology for manipulating micro- and nanoscale samples using evanescent sound waves. The technique is expected to replace existing technology, which uses high-frequency propagative surface acoustic waves (leaky SAW) to move microscopic samples on complex substrates. CEA-Leti’s technique generates an evanescent Bessel beam in the low-frequency ultrasound (kHz range). Using the enhanced radiation force coming from the evanescent field, it is possible to pattern living organisms as small as bacteria along concentric circles on a simple substrate.
“The novelty of our approach is the ability to do exactly the same thing that people did in the propagative SAW domain using high frequency,” said CEA-Leti scientist Cédric Poulain. “In that sense, we followed the same route as in optics, where conventional (propagative) optical tweezers were replaced by the improved evanescent optical versions (using plasmon nano-optical tweezers). Here, in acoustics, the use of a very thin substrate in the low-frequency range favors the emission of evanescent sound waves.’’
Evanescent waves are localized waves in the vicinity of the emitting substrate with a very small wavelength gradient – and therefore a high level of force. To achieve such forces, conventional techniques (mainly SAW) require ultrasound waves in the MHz range, which are difficult to generate. In existing propagative wave technology, waves radiate into the fluid and thus decay very quickly as they propagate along the substrate. Because the radiation of these waves wastes energy, researchers commonly refer to them as “leaky waves”. Evanescent waves do not radiate, and consequently do not decay as they propagate along the substrate.
For more information, visit www.leti-cea.com.