Particle separation by phase modulated surface acoustic waves

Abstract

High efficiency isolation of cells or particles from a heterogeneous mixture is a critical processing step in lab-on-a-chip devices. Acoustic techniques offer contactless and label-free manipulation, preserve viability of biological cells, and provide versatility as the applied electrical signal can be adapted to various scenarios. Conventional acoustic separation methods use time-of-flight and achieve separation up to distances of quarter wavelength with limited separation power due to slow gradients in the force. The method proposed here allows separation by half of the wavelength and can be extended by repeating the modulation pattern and can ensure maximum force acting on the particles. In this work, we propose an optimised phase modulation scheme for particle separation in a surface acoustic wave microfluidic device. An expression for the acoustic radiation force arising from the interaction between acoustic waves in the fluid was derived. We demonstrated, for the first time, that the expression of the acoustic radiation force differs in surface acoustic wave and bulk devices, due to the presence of a geometric scaling factor. Two phase modulation schemes are investigated theoretically and experimentally. Theoretical findings were experimentally validated for different mixtures of polystyrene particles confirming that the method offers high selectivity. A Monte-Carlo simulation enabled us to assess performance in real situations, including the effects of particle size variation and non-uniform acoustic field on sorting efficiency and purity, validating the ability to separate particles with high purity and high resolution.

Massively Multiplexed Submicron Particle Patterning in Acoustically Driven Oscillating Nanocavities

“…While deterministic single‐particle trapping has recently been demonstrated for 7–10 µm diameter particles [ 59,79–82 ] and cells [ 54,83,84 ] with acoustic wavelengths on the order of 20–30 µm, this work represents an order of magnitude reduction in particle size and allows a level of control over nanocavity locations (thus trapped particles) that is not possible with SAWs alone. Furthermore, our work improves on recent reports of nanoparticle capture such as nanoparticle aggregates formed using BAWs [ 49 ] and via interference patterns using air cavity waveguides, [ 50 ] by patterning particles at the single‐particle level.…”

Theoretical Framework of Radiation Force in Surface Acoustic Waves for Modulated Particle Sorting

“…An expanded version of the figure illustrating phase modulation and the resulting trapping positions can be found in [18].…”

“…In our previous work [18], we presented an analytical expression for the acoustic radiation force in the horizontal direction within a microchannel. In this section, we expand this theoretical investigation for the vertical direction and validate the horizontal trapping force against a numerical model.…”

“…In most cases, the contrast factor is positive, forcing the particles to collect at the pressure nodes. Although standing waves in surface wave devices are similar to bulk standing waves, we proposed a different radiation force describing the effect of standing surface acoustic waves on particles [18]. The interplay of the acoustic radiation force and hydrodynamic forces can be used for sorting.…”

“…The structure of the paper is as follows: in Section 2 we discuss the radiation force equation proposed in our previous work [18] for the horizontal direction in a standing surface acoustic wave and expand our research with description of the force in the vertical direction within the microchannel. We describe a finite element model and its implementation in COMSOL to verify the proposed horizontal force equation.…”