Modelling of particle paths passing through an ultrasonic standing wave

Abstract

Within an ultrasonic standing wave particles experience acoustic radiation forces causing agglomeration at the nodal planes of the wave. The technique can be used to agglomerate, suspend, or manipulate particles within a flow. To control agglomeration rate it is important to balance forces on the particles and, in the case where a fluid/particle mix flows across the applied acoustic field, it is also necessary to optimise fluid flow rate.To investigate the acoustic and fluid forces in such a system a particle model has been developed, extending an earlier model used to characterise the 1-dimensional field in a layered resonator. In order to simulate fluid drag forces, CFD software has been used to determine the velocity profile of the fluid/particle mix passing through the acoustic device. The profile is then incorporated into a MATLAB model. Based on particle force components, a numerical approach has been used to determine particle paths. Using particle coordinates, both particle concentration across the fluid channel and concentration through multiple outlets are calculated.Such an approach has been used to analyse the operation of a microfluidic flow-through separator, which uses a half wavelength standing wave across the main channel of the device. This causes particles to converge near the axial plane of the channel, delivering high and low particle concentrated flow through two outlets, respectively. By extending the model to analyse particle separation over a frequency range, it is possible to identify the resonant frequencies of the device and associated separation performance. This approach will also be used to improve the geometric design of the microengineered fluid channels, where the particle model can determine the limiting fluid flow rate for separation to occur, the value of which is then applied to a CFD model of the device geometry.

Modelling for the robust design of layered resonators for ultrasonic particle manipulation

“…Hence there is an optimum positioning of the node that is dependent on the reflector thickness and a significant decrease in capture efficiency on each side of this thickness. When flow and particle tracking were added to the acoustic model [23] it was possible to predict the nature of the dependence of particle capture on reflector depth, as shown in Fig. 6b.…”

A thin-reflector microfluidic resonator for continuous-flow concentration of microorganisms: a new approach to water quality analysis using acoustofluidics

“…44 In order to obtain a significant increase in particle concentration, the fluid flow must be split into the particle-rich and the particle-depleted fractions. 44,45 Based on this functioning principle, concentration of a range of particles including bacterial spores, biological cells and polystyrene spherical micro-beads has been demonstrated. 15,38,[46][47][48][49][50][51] Nordin and Laurell have recently demonstrated a very significant increase in concentration of red blood cells and prostate cancer cells (up to a maximum of ~200-fold), using a two-stages half-wavelength resonator.…”

“…15 However, direct manipulation of bodies as small as bacteria by means of ARFs represents a considerable physical challenge. 43 This is mainly due (i) to the fact that the acoustic primary radiation force scales with particle's volume, 45 and (ii) to the increased particle susceptibility with respect to hydrodynamic drag forces resulting from acoustic streaming or thermal convection. 52,53 It has been found that at diameters of around 1 μm there is a transition to drag-dominated behaviour for operating frequencies of ~2 MHz.…”

Performance of a quarter-wavelength particle concentrator

“…An acoustic impedance transfer model can be used to predict the occurrence of various modes and their strength [6] in a multi-layered device, as is being considered here. When coupled with particle simulations [7], it is a useful tool to assess the likely particle separation.…”