Until now, particle size has been a major limiting factor in the use of acoustic fields for manipulation and sorting applications. Unfortunately, being able to handle such particles would be useful in fields such as biology (small bioparticle enrichment), sensing (plasmonic nanoparticles), and optics (micro-lenses). But now, an interdisciplinary research team from Singapore University of Technology and Design and MIT has developed a novel acoustofluidic technology that can trap these massively multiplexed submicron particles within nanocavities at the single-particle level.
Acoustofluidics is the fusion of acoustics and fluid mechanics. It provides a rapid, contact-free, and active manipulation of suspended particles and the fluid they’re in. It works by creating an acoustic wave in the liquid. The wave produces a non-zero time-averaged pressure field, which exerts an acoustic radiation force on the particles suspended in the microfluidic channel. The reason this doesn’t work for particles below a critical size is that the viscous drag force dominates over the acoustic radiation forces. This happens because when the acoustic energy dissipates in the fluid, it causes robust acoustic streaming. Meaning, there’s too much movement to handle the particles.
Not only did the research team find a solution to this challenge, but they also demonstrated unique nanoparticle positioning into discrete traps using nanoacoustic fields in the megahertz (MHz) frequencies. Their novel acoustofluidic device uses surface acoustic waves (SAWs) as the actuation source. Additionally, it contains an elastic nanocavity layer located at the interface of the acoustic transducer and the microfluidic channel.
When SAW is activated, it causes acoustically-driven deformations in the nanocavities and produces a time-averaged acoustic field. As a result, the nanoscale acoustic force gradients along the channel. This unique nanoscale acoustic force overcomes the Brownian motion and acoustic streaming. Therefore, the team was able to manipulate millions of individual nano and submicron scale particles towards the nanocavities.
The combination of implementing a nanocavity layer with the SAW actuator made it possible to trap individual nanoparticles. The particles can then be released by turning off SAW. The system has great potential for widespread application in patterning, sorting, and size-selective capture of sub-micron and nanoscale objects.
