Diversity of 2D Acoustofluidic Fields in an Ultrasonic Cavity Generated by Multiple Vibration Sources

Abstract

Two-dimensional acoustofluidic fields in an ultrasonic chamber actuated by segmented ring-shaped vibration sources with different excitation phases are simulated by COMSOL Multiphysics. Diverse acoustic streaming patterns, including aggregation and rotational modes, can be feasibly generated by the excitation of several sessile ultrasonic sources which only vibrate along radial direction. Numerical simulation of particle trajectory driven by acoustic radiation force and streaming-induced drag force also demonstrates that micro-scale particles suspended in the acoustofluidic chamber can be trapped in the velocity potential well of fluid flow or can rotate around the cavity center with the circumferential acoustic streaming field. Preliminary investigation of simple Russian doll- or Matryoshka-type configurations (double-layer vibration sources) provide a novel method of multifarious structure design in future researches on the combination of phononic crystals and acoustic streaming fields. The implementation of multiple segmented ring-shaped vibration sources offers flexibility for the control of acoustic streaming fields in microfluidic devices for various applications. We believe that this kind of acoustofluidic design is expected to be a promising tool for the investigation of rapid microfluidic mixing on a chip and contactless rotational manipulation of biosamples, such as cells or nematodes.

Rotational manipulation of massive particles in a 2D acoustofluidic chamber constituted by multiple nonlinear vibration sources

“…The computational process of the acoustofluidic field consists of three simulation steps using the commercial finite element analysis software COMSOL Multiphysics. [40,46] In the first step, combining Eqs. ( 6) and ( 7) with appropriate acoustic boundary conditions, the first-order sound pressure and velocity field generated by multiple nonlinear vibration sources with different initial phases can be calculated by the 'Thermoviscous Acoustics, Frequency Domain' module.…”

“…The inertial force ρ 0 (𝑢 2 •∇)𝑢 2 is usually negligible compared with the mass source term and the volume force term in a flow field with a small Reynolds number and is neglected in our simulation. [56,57] All of the flow boundaries are defined as non-slip. To ensure the convergence of flow field calculation, weak contributions of mass source and acoustic streaming pressure are also indispensable.…”

“…To calculate the movement trajectory of micro-scale particles in the 2D microfluidic chamber under the combined influence of an acoustic radiation force 𝐹 rad (acoustophoretic force) and Stokesian drag force 𝐹 drag (drag force induced by acoustic streaming), the 'Particle Tracing for Fluid' module needs to be taken into consideration. [40,46]…”

“…The rotational direction of the main acoustic streaming vortex is consistent with the increasing direction of the initial phases of the nonlinear vibration sources. [40] To ensure the calculation accuracy of the particle trajectory in Fig. 2(c), the acoustic-streaming-induced drag force and the acoustic radiation force are both considered in the simulation process of 1-µm-diameter particle (polystyrene bead) movement at 5 MHz, while the simulated particle trajectory with comet tails is qualitatively consistent with the stream-line of the acoustic streaming field (shown in Fig.…”

“…[29,30,38,39] Considering that the spatial variations in sound intensity and Reynolds stress in the Navier-Stokes equations are the results of time averaging of the sound field distribution, the acoustic radiation force and acoustic streaming field are significantly influenced by the input frequency, oscillation amplitude, and initial phase differences of multiple vibration sources, the acoustofluidic characteristics of different media, and the shape and distribution of fluid-solid interfaces. [40] In general, there are two main types of acoustic streaming: [41,42] one is boundary-layerdriven streaming, which originates from the sound energy attenuation and dissipation occurring in the acoustic viscous boundary layer and can be categorized as inner streaming (i.e., Schlichting streaming) and outer streaming (i.e., Rayleigh streaming); the other is bulk-wave-driven acoustic streaming (i.e., Eckart streaming), which is a time-independent net flow along the acoustic propagation path formed by the absorption of sound energy inside the fluidic medium.…”