Self-Aligned Acoustofluidic Particle Focusing and Patterning in Microfluidic Channels from Channel-Based Acoustic Waveguides

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Self-Aligned Acoustofluidic Particle Focusing and Patterning in Microfluidic Channels from Channel-Based Acoustic Waveguides

David J. Collins, Richard O’Rorke, Citsabehsan Devendran, Zhichao Ma, Jongyoon Han, Adrian Neild, and Ye Ai

Phys. Rev. Lett. 120, 074502 – Published 15 February 2018

See Synopsis: Acoustic Waves Direct Particles in Microchannels

Abstract

Acoustic fields have been widely used for manipulation of particles and cells within microfluidic systems. In this Letter, we explore a novel acoustofluidic phenomenon for particle patterning and focusing, where a periodic acoustic pressure field is produced parallel to internal channel boundaries with the imposition of either a traveling or standing surface acoustic wave (SAW). This effect results from the propagation and intersection of edge waves from the channel walls according to the Huygens-Fresnel principle and classical wave fronts from the substrate-fluid interface. We demonstrate versatile control over this effect to produce both one- and two-dimensional acoustic patterning from one-dimensional SAW fields and its utility for continuous particle focusing. Uniquely, this channel-guided acoustic focusing permits the generation of robust acoustic fields without channel resonance conditions and particle focusing positions that are difficult or impossible to produce otherwise.

Received 25 October 2017

DOI:https://doi.org/10.1103/PhysRevLett.120.074502

Physics Subject Headings (PhySH)

Acoustic wave phenomena Microfluidic devices Ultrasonics

Fluid DynamicsGeneral Physics

Synopsis

Acoustic Waves Direct Particles in Microchannels

Published 15 February 2018

Acoustic waves guided by the channels of a microfluidic device can precisely manipulate microscopic particles suspended in the liquid flowing through the device.

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Authors & Affiliations

David J. Collins^1,3,4, Richard O’Rorke¹, Citsabehsan Devendran², Zhichao Ma¹, Jongyoon Han^3,4,5, Adrian Neild², and Ye Ai^1,*

¹Pillar of Engineering Product Development, Singapore University of Technology and Design, Singapore 487372, Singapore
²Department of Mechanical and Aerospace Engineering, Monash University, Melbourne, Victoria 3800, Australia
³Department of Biological Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
⁴Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
⁵BioSystems and Micromechanics (BioSyM) IRG, Singapore-MIT Alliance for Research and Technology (SMART) Centre, Singapore 138102, Singapore

^*Corresponding author. aiye@sutd.edu.sg

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Issue

Vol. 120, Iss. 7 — 16 February 2018

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Images

Figure 1
Principle of channel-based acoustofluidic waveguides. (a) A time-averaged pressure distribution occurs near the boundaries of a spatially finite wave source in the $x − y$ plane. With a SAW, this is readily achieved by affixing a channel wall to the substrate. The acoustic pressure field near a single channel wall is shown. (b) Placing two channel walls near one another yields a complex acoustic field, here mapped for a wave source with a width of $4 λ_{l}$ in the $y − z$ plane. Insets show (i) a time-domain image of the pressure field with a $4 λ_{l}$ and (ii) $100 λ_{l}$ transducer width over the same domain. (c) In the $y − x$ plane (at $h = λ_{l}$ ), periodicity develops parallel to the channel waveguides.
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Figure 2
Particle patterning via SAW and channel feature interactions. (a) Two-dimensional particle patterning is accomplished with a one-dimensional SSAW. Gray region in the bottom left incorporates air gaps in the channel walls, with no effect on nodal spacing vs solid PDMS walls. (b) Lateral patterning is independently observed with the imposition of a TSAW. Dashed inset (right) shows a channel feature at the edge of the domain, demonstrating patterning in an (effectively) unbounded domain (scale bar is $500 μ m$ ). Other insets show selected regions with 200 and $520 μ m$ widths. $λ_{SAW} = 80 μ m$ and applied $power = 1.6 W$ . Scale bar (lower right) is 1 mm. Left enlarged inset scale bars are $100 μ m$ .
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Figure 3
Control over lateral periodicity. (a) The simulated periodicity perpendicular to the SAW propagation direction $λ_{⊥}$ matches the predictions from Eq. (1) and the experimental results. (b) Setting this width in terms of the calculated lateral periodicity, we show that the number of lateral node-antinode locations at a given height (here mapped from $z = 0$ to $λ_{l}$ ) corresponds directly with the width when cast in terms of different $λ_{⊥}$ .
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Figure 4
Continuous particle focusing and negative sorting via channel-based waveguides. (a) A series of loops ( $15 \times$ ) with $800 μ m$ long focusing channels are subjected to SAWs to generate acoustic force gradients parallel to the channel waveguides. (b) The modeled acoustic force shows the appearance of either two or three local minima in the $x − y$ plane (upper view), assessed here at $z = 3$ and $19 μ m$ , and the $y − z$ plane (lower view) for two applied frequencies (53.3 and 46.2 MHz). (c) At 53.3 MHz, three lateral focusing positions develop [denoted as 1, 2, and 3 in (b)], with fluorescent $2 μ m$ particle migration to the center and edges of the channel. (d) At 46.2 MHz, all particles are pushed to the edges of the channel. Here shown for a $λ_{SAW} = 78 μ m$ , 100 mW applied power, and $2 μ l / \min$ flow rate. Scale bars are $500 μ m$ .
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