This paper demonstrates successful on-line particle separation in
silicon microchips using acoustic forces. Ultrasonic excitation of
the micro fluidic channel, matching the ultrasound frequency to the
channel width, enabled the focusing of particles to defined flow
lines in the streaming fluid and thus the collection of a particle
enriched fluid in the side channel outlets. 90% of the particles were
recovered in 67% of the original volume. By increasing the
frequency to higher resonance harmonics, controlled higher order
particle band formation (3 and 4 bands) was demonstrated. The chip
based separation approach will in future versions enable serial
connection of separators for a sequential enrichment and at the end
a net total higher particle enrichment factor than currently obtained.
In order to improve the separation efficiency it would be desirable
to confine the particles to a smaller zone. By reducing the channel
width the particles are expected to be focused in a narrower band as
the force on the particles increases with increasing frequency since
a narrower channel has a higher resonance frequency.