Meacham, J. Mark

Wang, Ray

English

Washington University St. Louis: Open Scholarship

Washington University in St. Louis Washington University Open Scholarship Mechanical Engineering and Materials Science Independent Study Mechanical Engineering & Materials Science 5-9-2017 Evaluation of a high-throughput acoustic particle sorter Ray Wang Washington University in St. Louis J. Mark Meacham Washington University in St. Louis Follow this and additional works at: https://openscholarship.wustl.edu/mems500 Recommended Citation Wang, Ray and Meacham, J. Mark, "Evaluation of a high-throughput acoustic particle sorter" (2017). Mechanical Engineering and Materials Science Independent Study. 36. https://openscholarship.wustl.edu/mems500/36 This Final Report is brought to you for free and open access by the Mechanical Engineering & Materials Science at Washington University Open Scholarship. It has been accepted for inclusion in Mechanical Engineering and Materials Science Independent Study by an authorized administrator of Washington University Open Scholarship. For more information, please contact digital@wumail.wustl.edu. Evaluation of a high-throughput acoustic particle sorter Student: Ray Wang Advisor: Dr. J. Mark Meacham MEMS400 Independent Study Report Dept. of Mechanical Engineering & Materials Science Washington University in St. Louis December 2016   Introduction The goal of this independent study was to determine whether the standing pressure field that drives fluid transport from a resonant ultrasonic jet/droplet generator could preferen-tially retain (or trap) suspended particles during fluid ejection. Meacham et al. have re-ported thorough characterization of the physics underlying the droplet generator (e.g., resonant operation and pressure field focusing at frequencies corresponding to the sam-ple reservoir geometry, as well as phenomena that predict whether ejection occurs as discrete droplets or continuous jets) using a variety of computational and experimental visualization methods [1-3]. These results suggest that at particular resonant frequencies of a fluid-filled reservoir a standing pressure field develops and is focused within pyrami-dal nozzles of the device, leading to fluid ejection from microscopic orifices.  Significant research efforts have explored use of various particle characteristics (size, density, compressibility, magnetic susceptibility, etc.) to separate microparticles and cells from heterogeneous mixtures. Among available separation techniques, acoustophoretic methods provide high biocompatibility due to the gentle noncontact nature of the applied forces. For example, it is well established that acoustic radiation forces cause suspended particles to migrate to the nodes (pressure minima) or antinodes (pressure maxima) of an applied standing acoustic pressure field. In addition to field properties (amplitude and frequency), particle size/shape and material density and compressibility relative to the suspending medium dictate the amplitude and direction (toward/away from zero-pressure nodes) of these forces.  Here, I explore use of the standing acoustic pressure field that drives ultrasonic droplet generation as a means of separation of cell-sized particles. Working with M. Kim (Meacham lab PhD student), I first confirmed ejection of water from device orifices for a range of chamber heights (corresponding to different available frequencies of operation). After identifying suitable operating modes, the sample chamber was loaded with suspen-sions of 5 and/or 10 µm polystyrene (PS) beads, and particle concentration was meas-ured before and after treatment to determine retention (enrichment capability) based on particle size. Though results were inconclusive, data suggested that 5 µm beads were less likely to be retained in the device versus 10 µm beads. Further exploration of the parameters governing separation in the ultrasonic droplet generator is needed before such a device can be applied to more complicated biological mixtures (e.g., diluted whole blood). Procedures and Apparatus The device concept is shown in Fig. 1. Briefly, a sample reservoir is sandwiched between an ultrasonic resonator (piezoelectric transducer with/without an acoustic coupling layer of aluminum) and an array of pyramidal horns wet etched into silicon. When the reservoir is driven at a longitudinal resonant frequency (i.e., pressure wave oriented parallel to the ejection direction; nodal field lines located perpendicular to the ejection direction), a high pressure gradient is established near the tips of the nozzles. This pressure gradient pro-pels fluid from the orifices, and for a mixture of suspended particles, a fraction of the particles is carried along with the fluid due to viscous drag. In the realized device, the ejection rate (and thus overall flow rate) is not ‘on’ or ‘off’, and the degree to which nozzles are ejecting has a significant impact on resultant flow drag and particle retention. There-fore, initial experiments used water only to identify optimal ejection frequencies evaluated based on quality of ejection. Ejection was qualitatively assessed using a three-level sys-tem: not spraying, misting and robust ejection. For each reservoir height, the first 3–4 device resonances were typically found (see Fig. 2), though only a subset of these exhib-ited robust ejection. Available device assemblies included polycarbonate reservoirs var-ying in height from 0.9 to 1.3 mm with resonances covering an almost continuous range of frequencies from 500 kHz to 2.5 MHz. Transducer temperature was monitored during experiments to ensure operation at reasonable temperatures (<50°C). The piezoelectric transducer temperature can increase rapidly when operating at a non-optimal frequency.  Figure 1. Micro machined ultrasonic droplet generator: A schematic of device assembly and B concept for particle retention at nodes of a resonant acoustic pressure field that drives droplet ejection.  Figure 2. Device operation: spraying from a 32‐orifice nozzle array (including high‐speed imaging of droplet breakup) and reso‐nant operation illustrating the first three half‐wave resonances corresponding to fluid ejection from a 1.0 mm height reservoir. In addition to drag induced by the bulk flow of sample from the nozzle orifices, particles in the standing acoustic pressure field are acted upon by the primary acoustic radiation force Fac                                                                                 ,   where Fac scales with particle volume (radius cubed, ~a3), field parameters (pin and vin), and parameters f1 and f2, which are functions of the particle compressibility κp and density ρp, respectively. Importantly, the expression for Fac indicates that the magnitude of the force has a strong dependence on particle size with larger particles experiencing much larger trapping forces than smaller particles. To explore the relationship between particle size and collection (complement of retention) rate, various bead solutions were run through the ultrasonic droplet generator and particle concentrations of the stock and collected samples were compared. Beads were sus-pended in phosphate-buffered saline (PBS) with 5 μm beads at approximately double the concentration of the 10 μm beads. Bead concentrations were measured by counting par-ticles in the grid of a hemocytometer. Beads on edges were only counted on the top and left edges to avoid repetition. C1 identifies concentration of the stock solution before ejec-tion. C2 is the concentration of collected sample. If d and h are the side length and height of a grid square volume, then the collection rate Rcoll is found as follows: C1= Npred2h  and C2=Npostd2h Rcoll= C2C1 =ቆNpostd2hቇ ቆNpred2hቇ ൌ Npost Npre⁄൘  Measured particle concentration is influenced by bead settling from solution so samples were vortexed before each trial. The stock concentration of 10 μm beads was around 120 per grid and that of the 5 μm beads was approximately 220 per grid. Results As mentioned above, five different reservoirs with heights ranging from 0.9 mm to 1.3 mm were evaluated. For each test case, I recorded function generator amplitude, and forward and reverse power of the power amplifier. All operating conditions are reported in Table 1. Of note, Table 1 indicates that operating frequencies of less than 1 MHz require higher voltage amplitudes, which is logical as the natural longitudinal resonance of the 1.5 mm thick piezoelectric transducer is approximately 1.4 MHz. As operation moves away from this resonance, the impedance of the piezoelectric increases requiring higher voltage to achieve appreciable displacements. Table 1. Summary of experimental data for ejection from various ultrasonic droplet generators.  Table 2 and Fig. 3 summarize collection data obtained using the above listed systems. Very little can be gleaned from the recorded data as all cases exhibit collection efficiency of 55 to 73 percent. At 1.17 MHz, a slight separation was observed between the 10 μm bead collection (~65%) and that for 5 μm (73%), though that was the only set of data for 5 μm beads that was valid. No clear dependence on frequency of operating is seen. Instability of the ejection was a significant hindrance to obtaining repeatable data during these experiments. Often, it was not possible to achieve robust ejection, which lowers drag forces on suspended particles (as drag is directly related to the number of firing nozzles) and favors retention in the device. In addition, the solution concentration was not easily assessed due to measurement errors or perhaps settling issues with samples rest-ing between experiments. Both issues need to be addressed before additional experi-ments are able to better characterize the retention capabilities of the ultrasonic droplet generator. Table 2. Collection rate (retention complement) data. Reservoir Orifice Freq Particle # Trials Pre 1 Post 1 Percent 1 Pre 2 Post 2 Percent 2 Pre 3 Post 3 Percent 3 Pre 4 Post 4 Percent 4 Avg Pct1 65 1.17 5um PS 228.5 166.75 72.975931 65 1.17 10um PS 115.25 76 65.9436 120.875 80.25 66.3909 118.75 74.5 62.73684 150.75 98.875 65.58872 65.165021.1 45 1.13 10um PS 4 104.75 68.875 65.75179 100.5 64.625 64.30348 99.5 64.875 65.20101 94.75 62.5 65.96306 65.304831.2 45 1.07 10um PS 4 119.75 65.125 54.38413 125.75 73.375 58.3499 111.25 60.875 54.7191 107 68.125 63.66822 57.780341.3 45 1.03 10um PS 4 109.5 74.75 68.26484 144.375 101.875 70.56277 137.375 91 66.24204 104.25 74.75 71.70264 69.19307   Figure 3. Collection rates for devices operating across a small frequency range. Future Work In addition to solving problems with concentration measurement and consistency of op-eration, additional experiments including 20 μm beads are needed to better assess the relationship between retention rate and particle size. Since temperature effects are im-portant to ejection, temperature measurement and perhaps some form of temperature control would improve experimental outcomes. Acknowledgement I would like to express my appreciation to Dr. J. Mark Meacham and Minji Kim for their time and significant contributions to my independent study. I would also like to thank the Mechanical Engineering and Material Science department at Washington University in St. Louis for providing high-quality undergraduate research opportunities.    5055606570751.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.1 1.11 1.12 1.13 1.14 1.15 1.16 1.17 1.18Retention Rate(%)FrequenciesRetention Rate vs. FrequenciesReservoir(1)Orifice(65)‐5um Reservoir(1)Orifice(65)‐10umReservoir(1.1)Orifice(45)‐10um Reservoir(1.2)Orifice(45)‐10umReservoir(1.3)Orifice(45)‐10umReferences 1. J. M. Meacham, C. Ejimofor, S. Kumar, F. L. Degertekin and A. G. Fedorov, “A micromachined ultra‐sonic droplet generator based on a liquid horn structure,” Review of Scientific Instruments 75, 1347–1352 (2004). Also in Virtual Journal of Nanoscale Science & Technology 9 (18), May 10, 2004. 2. J. M. Meacham, M. J. Varady, F. L. Degertekin and A. G. Fedorov, “Droplet formation and ejection from a micromachined ultrasonic droplet generator: visualization and scaling,” Physics of Fluids 17, 100605‐1 (2005). 3. J. M. Meacham, M. J. Varady, D. Esposito, F. L. Degertekin and A. G. Fedorov, “Micromachined ultra‐sonic atomizer for liquid fuels,” Atomization and Sprays 18, 163–190 (2008). 

Evaluation of a high-throughput acoustic particle sorter

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Evaluation of a high-throughput acoustic particle sorter

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