Particle Separation inside a Sessile Droplet with Variable Contact Angle Using Surface Acoustic Waves

A sessile
droplet of water carrying polystyrene microparticles
of different diameters was uniformly exposed to high frequency surface
acoustic waves (SAWs) produced by an interdigitated transducer (IDT).
We investigated the concentration behavior of the microparticles as
the SAWs generated a strong acoustic streaming flow (ASF) inside the
water droplet and exerted a direct acoustic radiation force (ARF)
on the suspended particles, the magnitude of which depended upon the
particle diameter. As a result of the ARF, the microparticles were
concentrated according to their diameters at different positions inside
the sessile droplet placed in the path of the SAW, right in front
of the IDT. The microparticle concentration behavior changed as the
sessile droplet contact angle with the substrate was varied by adding
surfactant to the water or by gradually evaporating the water. The
positions at which the smaller and larger microparticles were concentrated
remained distinguishable, even at very different experimental conditions.
The long-term exposure of the droplets to the SAWs was accompanied
by the gradual evaporation of the carrier fluid, which dynamically
changed the droplet contact angle as well as the concentration of
particles. Complete evaporation of the fluid left behind several concentrated
yet separated clusters of particles on the substrate surface. The
effect of the droplet contact angle on particles’ concentration
behavior and consequent separation of particles has been uniquely
studied in this SAW-based report.