Numerical simulation of 3D boundary-driven acoustic streaming in microfluidic devices

Abstract

This article discusses three-dimensional (3D) boundary-driven streaming in acoustofluidic devices. Firstly, the 3D Rayleigh streaming pattern in a microchannel is simulated and its effect on the movement of microparticles of various sizes is demonstrated. The results obtained from this model show good comparisons with 3D experimental visualisations and demonstrate the fully 3D nature of the acoustic streaming field and the associated acoustophoretic motion of microparticles in acoustofluidic devices. This method is then applied to another acoustofluidic device in order to gain insights into an unusual in-plane streaming pattern. The origin of this streaming has not been fully described and its characteristics cannot be explained from the classical theory of Rayleigh streaming. The simulated in-plane streaming pattern was in good agreement with the experimental visualisation. The mechanism behind it is shown to be related to the active sound intensity field, which supports our previous findings on the mechanism of the in-plane acoustic streaming pattern visualised and modelled in a thin-layered capillary device.

Theoretical study of time-dependent, ultrasound-induced acoustic streaming in microchannels

“…Unfortunately, their study is not directly comparable to our analysis, as we treat the buildup of Rayleigh streaming perpendicular to the pressure nodal plane, whereas Hoyos et al studied the streaming flow in this plane. Such in-nodal-plane streaming flows have been studied numerically by Lei et al [12,13], though only with steady actuation. The contradicting results of our numerical study and the experimental study of Hoyos et al may thus rely on the differences of the phenomena studied.…”

“…This scheme was later extended to take into account the thermoviscous effects arising from the dependence of the fluid viscosity on the oscillating temperature field [11]. Lei et al [12,13] have developed a numerical scheme based on the effective slip-velocity equations, originally proposed by Nyborg in 1953 [14,15], which avoid the resolution of the thin * peter.b.muller@gmail.com † bruus@fysik.dtu.dk boundary layers but still enable qualitative predictions of the three-dimensional streaming flows observed in microchannels and flat microfluidic chambers. To obtain quantitative results from such models that do not resolve the acoustic boundary layers, Hahn et al [16] developed an effective model to determine the loss associated with the viscous stresses inside the thermoacoustic boundary layers, and apply this loss as an artificial bulk absorption coefficient.…”

“…Numerical solutions of the time-domain acoustic equations were used by Wang and Dual [20] to calculate the timeaveraged radiation force on a cylinder and the steady streaming 1539-3755/2015/92(6)/063018 (13) 063018-1 ©2015 American Physical Society around a cylinder, both in a steady oscillating acoustic field. However, they did not present an analysis of the unsteady buildup of the acoustic resonance and the streaming flow.…”

Modal Rayleigh-like streaming in layered acoustofluidic devices

“…In practical acoustofluidic particle manipulation devices working in the MHz region this approximation is usually valid, and since the fluid channel dimensions are typically several orders larger than the acoustic boundary layer thicknesses only the acoustic streaming field in the bulk of the fluid is usually of interest. While Rayleigh streaming has been recently extensively studied within the field of acoustic particle trapping and manipulation, [23][24][25][26][27][28][29][30] there are acoustic streaming patterns observed experimentally in acoustofluidic particle manipulation devices that cannot be explained by Rayleigh's classical theory. 3,31-34 Recently, we have explained the mechanism behind the four-quadrant transducer-plane streaming, which has a vortex pattern parallel to the transducer face and is driven by the limiting velocity on the walls perpendicular to the axis of main acoustic propagation.…”

“…For a given application, however, a full model may be required to capture more complex combinations of boundary movement to determine which resonance is excited in the fluid layer. 28 In this case, we excite a particular cavity mode (see below) through applying a normal acceleration boundary condition on the bottom surfaces of the fluid channels.…”