Highly Localized Acoustic Streaming and Size-Selective

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Highly localized acoustic streaming and size-selective sub-micron particle concentration using high frequency microscale focused acoustic fields David J. Collins, Zhichao Ma, and Ye Ai Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b01069 • Publication Date (Web): 22 Apr 2016 Downloaded from http://pubs.acs.org on April 24, 2016

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Analytical Chemistry

Highly localized acoustic streaming and size-selective sub-micron particle concentration using high frequency microscale focused acoustic fields

David J. Collins, Zhichao Ma and Ye Ai*

Pillar of Engineering Product Development, Singapore University of Technology and Design, Singapore 487372, Singapore

* Corresponding author. Email: [email protected]; Tel: (+65) 6499 4553

ACS Paragon Plus Environment


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Abstract Concentration and separation of particles and biological specimens is a fundamental function of micro/nanofluidic systems. Acoustic streaming is an effective and biocompatible way to create rapid microscale fluid motion and induce particle capture, though the >100 MHz frequencies required to directly generate acoustic body forces on the microscale have traditionally been difficult to generate and localize in a way that is amenable to efficient generation of streaming. Moreover, acoustic, hydrodynamic and electrical forces as typically applied have difficulty manipulating specimens in the sub-micron regime. In this work, we introduce highly focused travelling surface acoustic waves (SAW) at high frequencies between 193-636 MHz for efficient and highly localized production of acoustic streaming vortices on microfluidic length scales. Concentration occurs via a novel mechanism, whereby the combined acoustic radiation and streaming field results in size-selective aggregation in fluid streamlines in the vicinity of a high-amplitude acoustic beam, as opposed to previous acoustic radiation induced particle concentration where objects typically migrate towards minimum pressure locations. Though the acoustic streaming is induced by a travelling wave, we are able to manipulate particles an order of magnitude smaller than possible using the travelling wave force alone. We experimentally and theoretically examine the range of particle sizes that can be captured in fluid streamlines using this technique, with rapid particle concentration demonstrated down to 300 nm diameters. We also demonstrate that locations of trapping and concentration are size-dependent, which is attributed to the combined effects of the acoustic streaming and acoustic forces.

Keywords: microfluidics, surface acoustic waves, acoustic streaming, particle concentration, nanoparticles


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1 Introduction Acoustic streaming is a practical means of efficiently inducing fluid motion in microfluidic devices, where the propagation of compression/rarefaction waves through in a liquid results in time-averaged fluid flow without the need for moving parts1-3. This phenomenon has been successfully demonstrated for efficient microfluidic pumping4,5, mixing6,7, gradient generation8,9 and particle concentration10,11, where the geometries and flow conditions determine which of these effects is realized. Compared to other methods for generating fluid flow in sub-mm systems, such as electrohydrodynamic forcing12 and/or thermal gradients13, high-frequency acoustic fields are generally biocompatible and are not sensitive to the ionic composition of the fluid14,15. Despite the decades over which acoustic streaming has been a field of study, it is only relatively recently that microfabrication techniques have been applied to generate acoustic field gradients on a scale relevant to microfluidic systems.

Earlier microfluidic acoustic transducers including microfabricated capacitive diaphragms16,17 require complex fabrication processes, and have thus not been widely applied. An increasingly popular and relatively simple method for driving localized microscale acoustic streaming utilizes oscillating microbubbles immersed in an acoustic field, which respond in phase to the pressure oscillations in the fluid, and where oscillation amplitude is maximized at bubble resonance conditions18. The formation, patterning and maintenance of these bubbles, however, are non-trivial tasks, where pre-patterned air-trapping regions are used to form microbubbles during device filling19-21. A substantive issue with this method is the longterm stability of microbubbles, whose dimensions are highly sensitive to the pressure conditions in the fluid and whose contents are subject to gaseous diffusion across the air/liquid and air/solid interfaces22,23. The ubiquitous microfluidic channel material, polydimethylsiloxane (PDMS), for example, is highly gas-permeable24. Moreover, the entire



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microfluidic device is typically subject to bulk acoustic waves (BAW) for generating the microbubble oscillations, with the resulting non-localized acoustic forces potentially hindering integration with other microfluidic processes.

In contrast to lower frequency, typically kHz order bubble oscillations25, high frequencies can be used to directly induce acoustic streaming in the fluid via body forces without a bubble intermediary. High frequencies are required for this mode of operation in microscale systems, where the coefficient of energy transfer (per unit length) scales with the square of this parameter26. Different acoustic actuation technologies are appropriate for directly generating streaming on the microscale, though their suitability is limited by the frequencies they typically generate. BAW actuators, for example, have been used to drive circulatory motion in open and closed flow chambers27-30 and droplets2. However, strong acoustic streaming effects ideally occur when large and highly local acoustic displacement gradients are generated at smaller wavelengths, characteristics that are not best realized in BAW systems that are typically limited to mm-scale transducers and ~1 MHz order frequencies31. Previous work with surface acoustic waves (SAW)32 and flexural plate waves33,34 has demonstrated the potential for surface-bound waves to generate streaming at frequencies substantially greater than those available to most BAW transducers, including for localized streaming. While the enhancement of acoustic field gradients for streaming can be achieved through acousticstructure interactions in a sessile droplet subject to high-frequency SAW10,35, localization of acoustic forces is further enhanced by directly focusing acoustic energy, most often generated by applying an alternating potential across conductive focused interdigital transducers (FIDTs) on a piezoelectric substrate that are patterned along the outlines of concentric circles or elipses7,33,34,36-43, though phononic arrays have also been used to redirect travelling SAWs into a locally confined area44,45. Typical SAW frequencies used for microfluidic actuation are


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on the order of ~10-1000 MHz, corresponding to fluid wavelengths directly comparable to the length scales of microfluidic systems (~1-100 µm) 35,46. As acoustic force gradients scale with the inverse of the acoustic wavelength, recent work has increasingly utilized higher frequencies for refined actuation47. At higher frequencies individual particle and cell patterns can be generated with acoustic wavelengths on the order of their dimensions (down to 5 µm diameters)14, smaller droplet dimensions are produced during atomization48-50, the minimum particle size that can be displaced using travelling acoustic waves is smaller32,51, and the width of a focused acoustic beam can be reduced33,36. Shilton et al. explored the relationship between the size of a fluid body and the acoustic wavelength dimensions, generating streaming in a sessile 0.6 nl (~100 µm wide) droplet at more than 1 GHz35, though streaming and other acoustofluidic effects and their relationship to acoustic frequency have also been demonstrated in droplets for larger droplet dimensions10,52,53. Though these relationships have not been as thoroughly examined in cases where an acoustic field is oriented orthogonal to a closed channel, the acoustic frequency utilized is also vitally important to the dominance of streaming-based effects in these systems, where these higher frequencies available to SAW permit the efficient generation of localized streaming in typical microfluidic channel dimensions (~100 µm).

Streaming is not the only acoustofluidic effect that results from the application of a high frequency acoustic field, where a travelling wave will also displace particles in the direction of propagation and acoustic field gradients at the edges of an acoustic beam will displace particles towards the beam periphery, from regions of high to low acoustic energy density54,55. While the travelling wave force has been extensively explored and utilized for particle manipulation56,57, this latter effect has not. As particle dimensions must be larger than a specific ratio of the acoustic wavelength for significant forces to be generated51, the



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acoustic force arising from field gradients offers a path towards circumventing this size limitation to manipulate particles smaller than those accessible using the travelling wave force alone. Also not previously examined is the nature of interaction between suspended objects and a combined acoustic radiation and streaming field in the vicinity of a high frequency acoustic beam and how it might be used for selective particle manipulation.

Here we propose the use of highly focused SAW at high frequencies to efficiently drive localized fluid flow for microfluidic capture and segregation of microscale and sub-micron particles. This focusing effect maximizes both the acoustic force gradients at the edges of the acoustic field and the efficient generation of acoustic streaming, and therefore the ability to manipulate particles using these combined acoustofluidic effects. SAW-based microfluidics is a rapidly expanding research field47, where a series of IDTs on a piezoelectric substrate generates a substrate bound wave that efficiently couples into a fluid placed in its path58. The ability to arbitrarily and precisely define the dimensions and location of the acoustic field is a defining characteristic of SAW, permitting its use in diverse applications such as particle and cell separation59,60, concentration, droplet production and manipulation40,61, encapsulation62, active sorting36,41, controlled heating63-65, nanotube alignment66,67, viscosity measurement68 and aerosol production69,70. In this work, we demonstrate the capability of focused SAW to generate highly localized streaming fields for selective particle capture and show theoretical work that both accurately describes the acoustic body force generated by SAW and predicts the range of particle sizes that can be concentrated. The high degree of spatial localization is made possible by the high SAW frequencies (193-636 MHz) and small acoustic wavelengths (6-20 µm) that are utilized, where the minimum acoustic beam width scales with the acoustic wavelength. The aggregation mechanism presented here is substantively different from streaming induced by an oscillating microbubble, where the acoustic radiation force (instead


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of mechanical displacement) serves to translate particles toward size-selective streamlines, or previous work in acoustic particle concentration, where concentration occurs at nodal positions in a standing acoustic field71. The combined effects of shear stress induced by acoustic streaming and direct force on particles subject to size-selective acoustic forces serve to reinforce particle concentration, which permits highly localized aggregation of particles here with diameters as small as 300 nm. This combined acoustic radiation and streaming field produced by a highly focused travelling wave beam is able to manipulate particle dimensions an order of magnitude smaller than those accessible to the travelling wave force alone, which to date has only demonstrated the direct displacement of objects as small as 2-3µm32,36.

2 System principles and Theory Fluid and particles suspended within it are subject to body forces and interfacial pressures, respectively, when acted on by an acoustic field. In the case of a fluid, a collimated acoustic beam will give rise to acoustic streaming, where the nonlinear attenuation of oscillating displacements in a dispersive media results in a finite time-averaged momentum flux, or body force, FB, pushing the fluid in the direction of acoustic propagation72. In the case of a substrate-bound SAW coupling energy into the fluid, the momentum flux that results in the body force is brought about by both amplitude attenuations in the fluid and along the substrate/fluid interface. The attenuation coefficients along the substrate/fluid interface (α) and in the fluid (β) are given by26 =

,

= ,

(1) (2)

where λ is the SAW wavelength in the substrate, ω is the angular frequency, ρl,s and cl,s are the density and sound speed in the liquid and substrate, respectively (ρs = 4628 kg m-3 and cs



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= 3931 m s-1 for the substrate used in this study)73,74, and b = 4/3µ + µ' is a function of the dynamic and bulk viscosity. Though previous work has examined the generation of acoustic streaming induced by SAW48,74-76, these models have not accounted for the attenuation length in the fluid, increasingly important at higher frequencies, nor the acoustic reflections off interfaces in a confined channel. As such, we develop and examine here a model that takes these considerations into account. The displacement magnitudes (the spatial extent of oscillatory motion) in the substrate and the fluid in the arbitrary direction xi are given by = ,

(3)

= .

(4)

These attenuation values yield characteristic attenuation lengths α-1 and β-1 where decays to 1/e of its initial value, i.e. =

= / plotted in Figure 1a. In a

confined channel, a necessary condition for microfluidic devices to contain and direct fluid while avoiding evaporation, the calculation of acoustic displacement amplitude is complicated by the infinite series of reflections and re-reflections off the channel upper and lower boundaries, with reflection coefficients given by

" "

= !"# $"% , where the acoustic #

impedances (Z1, Z2, where Zi = ρici) are those of the fluid/roof or fluid/substrate interfaces. Accounting for this, the displacement magnitude in the x–z plane is given by , & = ' , & + ' , &

+ ' , &

+ ⋯ .

(5)

The attenuation function f(x,z) takes into account the attenuation in both the substrate and the fluid, where an attenuated displacement at a point x along the substrate produces a beam propagating at θR and attenuating thereafter in the fluid. Incorporating up the first order (the first reflection with initial magnitude

' , ℎ),

this function is given by


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' , & + ' , &

= + , -./ 01 , 234 01 +

5, -./ 01 5, 234 01

(6) 6,

where h is the height of the channel roof. Finally, the body force (in N/m3) produced at a point (x, z) in the fluid, directed at θR from the substrate/fluid interface, is given by 78 = 9, & ,

(7)

where the displacement velocity is 9 = .

(8)

Figure 1a shows the attenuation lengths along the substrate/fluid interface and in the fluid according to Eqns. 1 and 2, where Figure 1b shows the effect of wavelength on displacement amplitude attenuation along the substrate/fluid interface (here for λ = 6 µm and 20 µm). These displacements couple into the fluid and attenuate along θR, resulting in a body force, computed in Figure 1c according to Eqn. 7. The body force is more strongly concentrated in both the horizontal and vertical planes for decreasing λ. For reference, Figure 1d geometrically represents the distances used to determine f0(x,z) from Eqn. 6.

In the case where the particle is smaller than the characteristic length scale of a non-uniform acoustic field, the direct acoustic force on a particle is determined by the spatial gradient in the force potential field U, with particle translation from locations of high force potential to regions of low force potential. The acoustic radiation force is given by 7: = − α-1, where in a 2D system with Lx,y >> Lz (e.g., with channel dimensions ~100–1000 µm width/length and ~10 µm high), the displacement magnitude is roughly uniform in the z-direction, with



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attenuation along the axis of wave propagation determining the shape of the fluid streamlines. Figure 3 examines the effects of acoustic wavelength and acoustic beam width on the scale of the acoustic streaming, showing the scale of the particle collection with respect to the body

force attenuation length in the substrate, K , where K = . Experimental results

(Figure 3a, 3c, and 3e) demonstrate the locations where particles aggregate, reflected in the locations of the closed inner streamlines predicted by the simulation results (Figure 3b, 3d and 3f), simulated as per a single-reflection implementation of Eqn. 7. Because the attenuation length in the fluid and the substrate both decrease with increasing frequencies, smaller wavelengths thus reduce the scale of acoustic streaming flow pattern, with particle collection occurring increasingly closer to the SAW source. Increasing wavelengths result in correspondingly increased trapping region dimensions and distances from the source (λ = 20 µm in Supplementary Video 1), though this effect is also limited by the channel dimensions; for K larger than the channel width, the maximum velocity locations (and thus the particle trapping positions) will not occur more than halfway along the channel. The distance between opposing capture locations is approximately that of the terminal IDT aperture, with Lw~W; the most tightly focused particle trapping occurs with the smallest wavelength and device aperture (6 µm and 14 µm, respectively). Though the particle aggregation occurs primarily in the x-y plane, flow also occurs in the z-direction due to the body force projecting at θR from the substrate, as noted in the inset of Figure 3d, showing a three-dimensional trace of fluid streamlines from the simulation in Figure 3c. Though difficult to visualize experimentally, these flow patterns can be observed soon after the application of SAW directly on either side of a tightly focused λ = 8 µm beam in Supplementary Video 2.

Applied Power


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The expression for the acoustic body force in Eqn. 7 is validated in Figure 4, which shows the measured and simulated maximum particle velocities for particles directly in the path of a 10 µm wavelength, 386 MHz acoustic beam over two orders of applied power magnitude, result in mm s-1 velocities for 10 mW order applied powers. Experimental velocities in Figure 4b are derived from recorded 1 µm particle trajectories in a 2 mm × 2 mm channel (Figure 4a) using academically published tracking software78, tracked from a random particle distribution at rest immediately prior to application of SAW. As the maximum velocity occurs in the path of the beam, only trajectories from this region are considered.

Figure 4b demonstrates the linear relationship between the second order streaming velocities, v2, and the applied power, P, arising from the relationship of these parameters with the first order substrate displacement velocities, v1. For Mach numbers M = v1/c

Highly Localized Acoustic Streaming and Size-Selective

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