The importance of travelling wave components in standing surface acoustic wave (SSAW) systems

Abstract

The use of ultrasonic fields to manipulate particles, cells and droplets has become widespread in lab on a chip (LOC) systems. There are two dominant actuation methods, the use of bulk acoustic waves (BAW) or surface acoustic waves (SAW). The development of BAW actuated systems have been underpinned by a robust understanding of the link between the ultrasonic field and forces which can be generated. In this work, we examine this link for standing surface acoustic waves (SSAW) comparing the relative strengths of streaming induced drag and acoustic radiation forces on suspended particles. To achieve this we have employed boundary conditions which accurately capture the travelling wave components of the pseudo-standing wave field, describe the key features of the acoustic radiation force fields and the acoustic streaming fields which can be generated, and finally we show that the relative importance of these two mechanisms is spatially dependant within a fluid chamber. The boundary condition used models the SSAW as two counter-propagating travelling waves, rather than assuming a standing wave directly. This allows the accurate inclusion of energy decay as the SAW couples into the fluid chamber and the resulting travelling wave component. This study shows that this previously neglected complexity of the SAW field is a critical factor in the nature of the resultant streaming field, as it gives rise to strong streaming rolls at the channel walls, which we validate experimentally. These rolls result in spatial variations of the dominant forces which in turn varies particle migration patterns spatially across the fluid domain.

Three-Dimensional Numerical Modeling of Acoustic Trapping in Glass Capillaries

“…The presence of the TBAWs reduces the Q value of the resonance state from previously reported values of 200-500 [41] to approximately 50, a value that, however, is large enough to ensure a well-defined resonance. While combinations of standing and traveling waves have been studied for surface-acoustic-wave systems [52][53][54][55], it is remarkable that they appear here in a study of bulkacoustic-wave (BAW) systems.…”

Patterning and manipulating microparticles into a three-dimensional matrix using standing surface acoustic waves

“…Citsabehsan et al described the boundary conditions in full. 12 The parameters used in the numerical study are given in Table S1 in the SM. In the COMSOL Multiphysics software, we used a module called "Pressure Acoustic" to solve for the acoustic pressure field and first-order velocity in frequency domain, then we used the "Particle Tracing" module to track the microparticles.…”

Formation of cell spheroids using Standing Surface Acoustic Wave (SSAW)

“…The magnitude and distribution of these pressure nodes in the central region of the cavity are quite uniform, but not at the edge [53] . Those cells accumulated at the middle region of pressure nodes are in the suspension and may have low possibility of attachment to the cavity.…”

“…Cells attachment on the surface of microchannel is a common issue due to complicated surface properties [59,60] . Briefly, the attractive forces from the surface are stronger than the combination of electrostatic repulsion force and acoustic streaming forces applied to the cell [61] . Subsequently, cells could not move and follow the acoustic radiation force.…”