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Atomisation and capillary wave generation in ultrasonically-actuated microfluidics
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posted on 15.02.2017by Blamey, Jeremy Kim
Ultrasonic surface acoustic wave (SAW) actuation is a promising technique for generating fluid flow in microfluidic devices, with many potential applications in fluid and particle manipulation. One particular application is ultrasonic atomisation, which has already been proposed as an effective means of generating fine particle mists for pulmonary drug delivery. The mechanism behind ultrasonic SAW-induced atomisation is the generation of large capillary waves on the fluid surface which, when at sufficient amplitude, can break off, forming small droplets.
Capillary waves appear across a huge range of frequency and length scales in ultrasonic microfluidic systems. The physics behind the generation of these waves is not well understood. For the unstable waves that produce atomised droplets, the process of breaking and droplet ejection has been previously described, and some research has been done in determining the size of droplets ejected. However, the rate of atomisation, and its dependence on excitation level and frequency, is not known.
In this thesis we experimentally measure the capillary wave vibrations induced by ultrasonic SAW actuation of a sessile drop, and the rate of atomisation from the drop. Our vibration measurements agree very precisely with the predictions of capillary wave turbulence theory, which predicts a broadband “cascade” power-law relation in the spectrum of vibration. Ours is the first description of wave turbulent behaviour in microfluidic systems; we demonstrate the power-law behaviour over three decades of frequency. We also identify an oscillation at the fundamental resonance frequency of the drop near 200 Hz, which we demonstrate to be directly induced by the excitation at 20 MHz. This fundamental resonance drives the wave turbulent cascade.
The atomisation rate is directly connected to this resonance. We show that atomisation does not take place when the excitation level is below a particular threshold; beyond this threshold, atomisation rate steadily increases with excitation amplitude. The threshold behaviour of the resonance is the opposite: steadily increasing with excitation level up to the threshold, then remaining constant above the threshold.