STANDING SURFACE ACOUSTIC WAVE (SSAW) BASED THREE-DIMENSIONAL (3D) PARTICLE FOCUSING

J. Shi, S. Yazdi, S.-C. S. Lin, X. Ding, and T. J. Huang* Department of Engineering Science and Mechanics, The Pennsylvania State University

University Park, PA 16802 USA ABSTRACT

Three-dimensional (3D) continuous microparticle focusing has been achieved in a single-layer polydimethylsiloxane (PDMS) microfluidic channel using a standing surface acoustic wave (SSAW). The SSAW was generated by the interference of two identical surface acoustic waves (SAWs) created by two parallel interdigital transducers (IDTs) on a piezoelectric substrate with a microchannel precisely bonded between them. Compared with other 3D focusing techniques, this method is non-invasive, energy-efficient, easy to implement, and applicable to nearly all types of microparticles. KEYWORDS: 3D focusing, standing surface acoustic wave

INTRODUCTION

The ability to focus particles and cells into a confined, accurately controlled region is important for many lab-on-a-chip applications such as flow cytometry, single molecule detection, protein folding, and enzymatic kinetics.1 To focus particles to a narrow stream, however, is not a trivial task. Due to the laminar nature of the microfluidic flows, particles suspended in the fluid medium tend to follow the fluidic streamlines unless a lateral force moves them from their original paths. To date several methods that have been utilized to demonstrate particle focusing in microfluidic channels are hydrodynamics, electroosmosis, dielectrophoresis, magnetophoresis, and acoustics. In this work, through experimental and computational studies, we demonstrate that utilizing the SSAW method,2 3D particle focusing in a continuously flowing microchannel can be achieved. This acoustic force-based 3D focusing technique is non-invasive, energy-efficient, applicable to virtually any microparticles, and easy to fabricate and implement. Utilizing this method, microparticles suspended in the liquid media can be focused to a predesigned location, regardless of particle properties.

PARTICLE FOCUSING IN THE LATERAL DIRECTION

Figure 1a shows the device used in our study. Two sets of IDTs with a period of 100 µm were arranged parallel to each other on a 128° Y-cut LiNbO3 substrate. Once a RF signal is applied to both IDTs, two series of SAW were generated and formed a SSAW in between the IDTs, causing a periodic distribution of the pressure nodes (minimum pressure amplitude) and anti-nodes (maximum pressure amplitude) on the substrate. A PDMS microchannel with 50 µm width and 100 µm height was bonded on the LiNbO3 substrate in between the IDTs such that a pressure node of the SSAW was located in the middle of the channel width. Due to acoustic energy absorption, a finite acoustic pressure difference is produced in the fluid and lead to the generation of a serious of longitudinal sound waves in the liquid, which propagates in a specific angle (Rayleigh angle) with respect to the substrate surface (Fig. 1b).3

We calculated the resultant acoustic pressure field of the leakage longitudinal waves in the cross-section of the microchannel. At two time snapshots with a time difference of half a period (τ), the simulated pressure field is shown in Figs. 2a and 2b. Due to the interference of the radiated and reflected longitudinal waves, the resultant pressure field is not uniform. The strong discontinuity in the pressure field in Figs. 2a and 2b is due to the destructive interference of the radiated longitudinal waves and their reflections from the channel walls. During this time difference (τ/2), the amplitude of the pressure in the y-z plane of the channel at x = 25 µm remains zero (dashed line), while the pressure in the other analogous regions simply changes sign. These facts indicate that a standing wave is formed in the lateral direction and a pressure node exists in the middle plane of the channel (x = 25 µm), as denoted by the dashed lines in Figs. 2a and 2b.

Figure 1: (a) A photograph of the experimental. (b) A cross-section schematic of the microchannel and IDTs. The leakage waves are radiated into the liquid phase under the Rayleigh angle.

978-0-9798064-4-5/µTAS 2011/$20©11CBMS-0001 1242 15th International Conference onMiniaturized Systems for Chemistry and Life Sciences

October 2-6, 2011, Seattle, Washington, USA

Figure 2: Resultant acoustic pressure field in the cross-section of the channel at (a) t = t0 and (b) t = t0 + τ/2. (c) Top-view fluorescent images at sites (I–IV) showing the lateral focusing of particles.

The longitudinal waves-induced pressure fluctuations in the liquid generate axial primary acoustic radiation forces on

suspended particles. As a results of radiation forces, polystyrene beads (1.9 µm) in the liquid aggregate at the pressure node (in the middle plane of the channel). Fig. 2c shows the fluorescent images at different sites recorded from top of the microchannel. Randomly distributed particles (site I) were driven by the acoustic radiation forces toward the center of the channel (site II) and formed a narrow stream with a width of approximately 5 µm (sites III & IV)—less than three times the diameter of a single particle, 5% of the SSAW wavelength, and 10% of the channel width. Note that the focusing process takes only several milliseconds.

PARTICLE FOCUSING IN THE VERTICAL DIRECTION

Once the particles are moved, by the axial primary radiation forces, to the pressure nodal (y-z) plane in the center of the microchannel, the transverse primary radiation forces move them to particular points with maximum acoustic kinetic energy within the nodal plane. Because time-averaged kinetic energy KE is proportional to the square of velocity (V2),

!!!" = !!!!/2 , (1) the distribution of the velocity field is required in order to find the maximum kinetic energy point(s) within the pressure nodal plane. Figs. 3a and 3b demonstrate the time-averaged velocity squared field in the x and y directions, respectively. Note that we are merely interested in the square of the velocity’s magnitude within the pressure nodal plane. Fig. 3c shows the time-averaged distribution of velocity squared in x and y directions within the pressure nodal plane. The discontinuities near y = 20 µm and y = 60 µm are due to destructive interference of the longitudinal waves. As shown in Fig. 3a, the magnitude of the time-averaged velocity squared in the x-direction in the nodal plane is zero (black dashed contour in Fig. 3c). Hence, the magnitude of the time-averaged velocity squared is equal to the time-averaged velocity squared in the y-direction (Vy

2), represented by the red solid curve. The local maximum of the Vy

2 contour, which from eqn. (1) is also the local maximum kinetic energy, is approximately at y = 46 µm. Hence, after the particles are focused to the middle of the microchannel due to the axial primary radiation forces, transverse primary radiation forces push them to a point around 46 µm above the bottom of

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the channel: the “acoustic hot spot.”4 Hence, suspended particles are focused in both horizontal and vertical directions. In order to observe focusing in the vertical direction, a triangular 45-degree prism was positioned next to the PDMS

channel sidewall to visualize the y-z plane. When the fluorescent polystyrene particles passed through the microchannel section adjacent to the prism, their movement in the vertical direction (y-z plane) was captured through the prism using a CCD camera. When no SSAW is generated, the distribution of the fluorescent particles across the microchannel is random (Fig. 4b). Upon SSAW generation, the particles experience the acoustic radiation forces from the surface acoustic wave-induced longitudinal leakage waves in the microchannel. As a result, they are focused to a balance line, located about 42 µm above the bottom of the channel (Fig. 4c). Alternatively, we can confirm the 3D focusing using recorded top-view images. As shown in different time snapshots in Fig. 4d, three particles in dotted contours were driven by the acoustic radiation forces to the middle of the channel along the x-direction (the lateral direction). The solid contours illustrate focusing in the vertical direction (y-direction): the particles out of the focal plane of the microscope, which appear large and blurry, were gradually driven to the focal plane of the microscope and aligned in the middle of the channel. The enlarged view of the selected particles in solid contours is shown on the right-side column in Fig. 4d.

Figure 4: (a) Schematic of a prism adjacent to the experimental device used to observe migration of particles in the y-direction. Recorded side view fluorescent images before (b) and after (c) the SSAW was generated. (d) Top view images of the fluorescent particles recorded at different time snapshots.

CONCLUSION

We have introduced a novel acoustic manipulation technique, SSAW, to enable fast and effective 3D continuous particle focusing in a microfluidic channel using SSAWs. Simulation results predict the existence and location of a focusing point, which is in excellent agreement with our experimental observations. Compared with many existing 3D particle focusing techniques, our SSAW-based method is non-invasive, reversible, energy-efficient, easy to implement, and applicable to almost any microparticle. With these advantages, this method is expected to be valuable in many lab-on-a-chip applications. ACKNOWLEDGEMENTS

This research was supported by was supported by NIH Director’s New Innovator Award (1DP2OD007209-01), the National Science Foundation (0348149, 0824183, and 0801922), and the Penn State Center for Nanoscale Science (MRSEC). REFERENCES [1] M. M. Wang, E. Tu, D. E. Raymond, J. M. Yang, H. Zhang, N. Hagen, B. Dees, E. M. Mercer, A. H. Forster, I. Kariv,

P. J. Marchand and W. F. Butler, Nat. Biotechnol, 2004, 23, 83-87. [2] J. Shi, X. Mao, D. Ahmed, A. Colletti, T. J. Huang, Lab Chip, 2008. 8, 221-223. [3] C.-H. Chou, B. T. Khuri-Yakub and G. S. Kino, IEEE Trans. Ultrson. Ferroelectr. Freq. Control, 1988, 35, 464-469. [4] C. J. Schram, in Advances in Sonochemistry: Manipulation of particles in an acoustic field, T.J. Mason, Elsevier, 1991,

pp. 293-322. CONTACT *T. J. Huang, tel: +1-814-863-4209; junhuang@psu.edu

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