Abstract
The combination of a bulk acoustic wave device and an optical trap allows for studying the buildup time of the respective acoustic forces. In particular, we are interested in the time it takes to build up the acoustic radiation force and acoustic streaming. For that, we measure the trajectory of a spherical particle in an acoustic field over time. The shape of the trajectory is determined by the acoustic radiation force and by acoustic streaming, both acting on different time scales. For that, we utilize the high temporal resolution (Δt=0.8μs) of an optical trapping setup. With our experimental parameters the acoustic radiation force on the particle and the acoustic streaming field theoretically have characteristic buildup times of 1.4μs and 1.44ms, respectively. By choosing a resonance mode and a measurement position where the acoustic radiation force and acoustic streaming induced viscous drag force act in orthogonal directions, we can measure the evolution of these effects separately. Our results show that the particle is accelerated nearly instantaneously by the acoustic radiation force to a constant velocity, whereas the acceleration phase to a constant velocity by the acoustic streaming field takes significantly longer. We find that the acceleration to a constant velocity induced by streaming takes in average about 17 500 excitation periods (≈4.4ms) longer to develop than the one induced by the acoustic radiation force. This duration is about four times larger than the so-called momentum diffusion time which is used to estimate the streaming buildup. In addition, this rather large difference in time can explain why a pulsed acoustic excitation can indeed prevent acoustic streaming as it has been shown in some previous experiments.
Highlights
In recent years, acoustofluidics has provided many powerful tools
For the stationary force measurements the particle is fixed in place by the optical potential and the Brownian motion is negligible
In this work we presented the measurement of the temporal evolution of the acoustic streaming (AS) field and the acoustic radiation force (ARF) in a bulk acoustic wave (BAW) device utilizing an optical tweezer (OT)
Summary
Acoustofluidics has provided many powerful tools. Due to being contactless, label-free, and biocompatible [1,2,3,4,5], acoustofluidic manipulation can be used in medical applications for cancer research [1,2,3,4], Alzheimer research [5], targeted drug delivery [6], and for pumping medical fluids [7]. The ARF is a second-order time-averaged effect that arises from the interaction of an acoustic field scattered at an object surface and a background acoustic field [13,14,15,16,17]. These objects can be solid particles, air bubbles, fluid droplets, and biological samples, as long as their material properties (density ρ and speed of sound c) are different from the surrounding medium. This motion can arise either from viscous losses in the fluid (Eckhart type streaming [21]) or it can arise in the viscous boundary layer at a fluid to wall interface (Schlichting and Rayleigh streaming [22,23])
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