Millimetre scale ultrasonic concentrator for microparticles in fluid

MILLIMETRE-SCALE ULTRASONIC CONCENTRATOR FOR MICROPARTICLES IN FLUID THEIN MIN HTIKE NATIONAL UNIVERSITY OF SINGAPORE 2011 MILLIMETRE-SCALE ULTRASONIC CONCENTRATOR FOR MICROPARTICLES IN FLUID THEIN MIN HTIKE (M. ENG., Bandung Institute of Technology) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF MECHANICAL ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2011 Acknowledgements _____________________________________________________________________ Acknowledgements Firstly, I would like to take this opportunity to express my sincere gratitude to A/Prof. Lim Kian Meng for his constant encouragement, support, invaluable guidance on both academic matters and personal concerns. His profound academic knowledge, technical excellence and clear-cut example in mechanical engineering and other related disciplines have benefitted and will benefit and influence my life. This work would have never been finished without his great guidance and prompt responses whenever I needed his help and advice. I am also very grateful to A/Prof. Lim Siak Piang and A/Prof. Lee Heow Pueh for their guidance and help in my research. Secondly, I would like to send my sincere acknowledgements to National University of Singapore and AUN/SEED.Net/ JICA for their financial support. I would also like to offer my special thanks to the staff and friends in Dynamics Laboratory for their help, encouragement, caring and sharing during my study here. Special thanks must go to Dr. Liu Yang, who helped me a lot during my earlier of years of this study. Finally, I would like to thank my parents and friends for their constant support during my difficult times and all of my teachers and lecturers from my state schools, AGTI (Chauk), GTU (Kyaukse), MTU, YTU, ITB and NUS for their teaching and sharing the knowledge on subject matters and other inspiring acts. _____________________________________________________________________ I Table of Contents _____________________________________________________________________ Table of Contents Acknowledgements I Table of Contents . II Abstract . V List of Figures VII List of Tables . XII 1. Introduction . 1.1 Separation Methods 1.2 Acoustic Separation . 1.3 Objectives and Scope…………………………………………… .…………4 1.4 Original Contributions . 1.5 Thesis Organization . 2. Literature Review . 2.1 Brief History 2.2 Different Applications . (A) Size-Based Fractionation ……… .……………………………………. (B) Property-Based Separation 10 (C) Gravity-Aided Separation 10 (D) Separation with The Aid of Porous Medium . 11 (E) Carrier Medium Exchange . 11 2.3 Geometrical Design and Mode of Operation 12 (A) Cylindrical Resonator . 12 (B) H-Shaped Separator 13 2.4 Modeling . 15 (A) Transfer Matrix Model 15 (B) Electro-Acoustic Model . 15 (C) Finite Element Model 15 (D) Particle Trajectory Model 16 2.5 Studies on Effect of Layer Thicknesses 17 2.6 Studies on Particle Trapping . 19 _____________________________________________________________________ II Table of Contents _____________________________________________________________________ 3. Theory 22 3.1 Basic theory of sound 22 (a) Travelling wave . 24 (b) Standing Wave 25 3.2 Acoustic Radiation Force . 26 (a) Primary Radiation Force 26 (b) Secondary Inter-particle Forces 32 (c) Acoustic Concentration in Simple Rectangular Channel 34 (d) Acoustic Streaming . 35 3.3 One-Dimensional Layered Resonator Model 37 (a) Boundary Conditions . 40 (b) Criterion for Selection of Layer Thicknesses 44 (c) Losses in the System 45 (d) Effect of Adhesive Layer……………………………………………….45 4. Experimental Setup 49 4.1 Design and Mode of Operation of Acoustic Concentrator 49 4.2 Acoustic Concentrator . 50 4.3 Fluid and Particles . 54 4.4 Experimental Setup and Procedure . 54 (a) Experimental System Setup . 54 (b) Measurement of Separation Height . 56 (c) Measurement of Particle Concentration . 57 5. Experimental Results and Discussion . 59 5.1 Separation Height in Channel A 59 5.2 Separation Height in Channel B 62 5.3 5.4 Measurements of Temperature Rise……………………………………… .65 Particle Concentration in Channel B . 69 5.5 Effect of Inlet Particle Concentration 77 6. Numerical Modeling and Analysis 84 6.1 Measurement of Acoustic Energy Density 84 (a) Methodology for Eac Measurement 84 (b) Results and Discussions 88 _____________________________________________________________________ III Table of Contents _____________________________________________________________________ 6.2 Numerical Modeling and Simulation Results . 89 (a) Acoustic Model . 90 (b) Flow Model . 104 (c) Particle Trajectory Model 108 (d) Effect of Channel Width………………………………………………117 (e) Effect of Three-Dimensional Variations …………………………… 122 7. Conclusion . 128 7.1 Design and Performance Characterization 128 7.2 Significance of Particle Trapping 129 7.3 Numerical Modeling and Analysis 130 7.4 Directions of Future Work 131 References 133 Appendix 142 Publications……………………………………………………………………… .145 _____________________________________________________________________ IV Abstract _____________________________________________________________________ Abstract Previous studies on particle concentration by the ultrasonic standing wave technology show that microparticles can easily and effectively be concentrated in the micrometerscale concentrators. However, microparticles are difficult to concentrate in millimeter-scale concentrators because of wide disparity between the small particle size and large channel width. One of the millimeter-scale devices that can concentrate microparticles at a relatively large volume flow rate is the h-shaped acoustic concentrator. Previous studies on the h-shaped concentrator have presented design guidelines, performance characterization and some design improvements. However, more studies are still needed to fully understand the insights of device’s operation and the behaviour of microparticles inside the concentrator. This study conducts systematic investigation into the operation of the h-shaped concentrator by measuring separation heights and particle concentrations at different voltage and flow rates. Specifically, this study (1) performs the characterization of concentration effectiveness of the h-shaped concentrator and (2) investigates the existence of particle trapping due to the lateral radiation forces. One-dimensional layered piezoelectric model was first used to obtain the design criterion for the layer thicknesses. Next, two h-shaped concentrators were constructed, one with nominally chosen layer thicknesses and one with properly designed layer thicknesses. Separation height measurements were performed to characterize the concentration effectiveness of the devices. The results validated that correct choice of layer thicknesses would help to improve the maximum achievable flow rate compared to nominally designed channel. _____________________________________________________________________ V Abstract _____________________________________________________________________ Secondly, concentration effectiveness of the h-shaped concentrator was characterized by particle concentration experiments at inlet and two outlets using turbidity measurements. The results showed that particle trapping exists in the chamber and can be very significant at high voltage and high inlet particle concentration. These results shed the light into the insights of the device’s operation since particle trapping can be beneficial or detrimental depending on the required mode of operation. The results suggested that the device can be used as a particle trapping device at high inlet particle concentration and high driving voltage. However, the device is suitable as a continuous flow-through concentrator only at low inlet particle concentration and moderate voltage levels. Finally, to further characterize the sound field inside the channel, energy density measurements were conducted. Acoustic and flow field analysis was performed by using the finite element models. Proper matching between the experimental and calculated energy densities was done and primary axial as well as lateral radiation forces inside the channel were estimated. Lateral radiation forces are found to be in comparable order of magnitude with viscous drag forces. Next, acoustic forces together with viscous drag forces were applied to the particle to predict particle trajectories. The general trends in separation heights from the particle trajectory model agree well with the experimental results. Moreover, numerical results show that particle trajectory came to a stop at high voltage and this could explain the particle trapping observed experimentally. _____________________________________________________________________ VI List of Figures _____________________________________________________________________ List of Figures Figure 3.1 Acoustic contrast factors at different compressibility and density ratios 27 Figure 3.2 Characteristics of one-dimensional standing wave . 28 Figure 3.3 Acoustic concentration in simple rectangular channel at MHz .30 Figure 3.4 Demonstration of acoustic concentration phenomena inside a simple rectangular channel . 35 Figure 3.5 One-dimensional layered resonator model 37 Figure 3.6 Two-layered resonator model with boundary conditions 41 Figure 3.7 Conditions for natural frequency of two-layered resonator 43 Figure 3.8 Boundary conditions and thicknesses matching them for an h-shaped channel 44 Figure 3.9 One dimensional three-layered model with adhesive layer……………… 46 Figure 3.10 Frequency response of pressure amplitude in the matching layer for different adhesive layer thicknesses………………………………………………… .47 Figure 4.1 Schematic on mode of operation of h-channel 50 Figure 4.2 h-shaped concentrator and its layers 51 Figure 4.3 Layout and dimensions of channel A 52 Figure 4.4 Layout and dimensions of channel B 53 Figure 4.5 Setup of experiment . 55 Figure 4.6 A snapshot of typical concentration experiment in an h-shaped channel . 56 Figure 4.7 Calibration chart for conversion between turbidity and approximate particles count per volume for 10-µm polystyrene particles in water 58 _____________________________________________________________________ VII List of Figures _____________________________________________________________________ Figure 5.1 Acoustic concentrations of 10-µm polystyrene beads in water at 2.1 MHz and 10 Vpp in channel A 60 Figure 5.2 Separation heights vs. inlet volume flow rate for different voltages for channel A 61 Figure 5.3 Snapshots of concentration of microbeads in channel B for various flow rates with 20 Vpp applied to piezo-actuator . 63 Figure 5.4 Snapshots of concentration of microbeads in channel B for various flow rates with 30 Vpp applied to piezo-actuator . 63 Figure 5.5 Separation heights vs. inlet volume flow rate for different voltages for channel B 64 Figure 5.6 Experimental setup for temperature measurement……………………… .66 Figure 5.7 Temperature versus operating time in channel B at 0.1 mL/min and different voltages…………………………………………………………………… 66 Figure 5.8 Temperature versus operating time in channel B at 0.2 mL/min and different voltages…………………………………………………………………… 66 Figure 5.9 Temperature versus operating time in channel B at 0.3 mL/min and different voltages…………………………………………………………………… 67 Figure 5.10 Temperature versus operating time in channel B at 0.4 mL/min and different voltages…………………………………………………………………… 67 Figure 5.11 Temperature versus operating time in Channel B at 40 Vpp………….…68 Figure 5.12 Experimental setup for particle concentration measurement…………….70 Figure 5.13 Result of particle concentrations in Channel B for different voltage levels …………………………………………………………………………71 Figure 5.14 Relative particle concentrations in upper outlet vs flow rate………… .74 Figure 5.15 Relative particle concentrations in lower outlet at different flow rates.…75 _____________________________________________________________________ VIII Chapter ___________________________________________________________________________ 7.3 Numerical Modeling and Analysis This study has also built the finite element models which can predict the ultrasound and flow fields inside the h-shaped concentrator. Results of acoustic energy densities from the finite element model were matched with the results from experimental measurements. A particle trajectory model together with acoustic radiation forces and hydrodynamic force from the finite element model was used to calculate particle trajectory in the h-shaped channel. The separation height results from the simulation model were in similar trend with the corresponding experimental ones. However, simulation model slightly overestimated the separation performance. This slight difference in separation heights between simulation and experimental results is attributed to the non-homogenous sound field observed in the experiments which are not replicated by the energy density matching between the experimental and numeral models. The simulation results, however, predicted the existence of lateral radiation forces well within comparable order of magnitude with the Stokes forces. This comparable magnitude of lateral forces is responsible for the significant particle trapping observed in the experiment. To conclude, this thesis (i) has provided a simple theoretical model for proper sizing the layer thicknesses of the millimeter-scale acoustic concentrator, (ii) proved that selection of layer thicknesses has profound effect on the performance of acoustic concentrator and proper sizing of the layer thicknesses can help improve concentration effectiveness, (iii) characterized the concentration effectiveness of hshaped concentrator and highlighted the existence and significance of particle trapping by lateral forces, and (iv) obtained a simulation model that can estimate the separation performance of the channels well within an acceptable order of magnitude. ___________________________________________________________________________ 130 Chapter ___________________________________________________________________________ 7.4 Directions of Future Work In this study, instead of cellular particles, polystyrene particles were used. It is reasonable since they are also commonly used in most studies as their material properties are quite comparable to biological cells [11, 88, 89]. Moreover, for designing the layer thicknesses, a simple one-dimensional piezoelectric model was used instead of three-dimensional analysis. However, as proved by the experimental results of separation heights on nominally designed channel and properly designed channel, although simple and only one-dimensional, this theoretical model can be used as a design tool for choosing proper layer thicknesses. However, the maximum achievable flow rate in this study is lower than that in the literature [19, 20]. Possible reasons are: (1) The electric displacement boundary condition used at the boundary of PZT layer in 1D model, which is used to design the channel layer thicknesses may not reflect actual electrical excitation. Therefore, the design of layer thicknesses used in this study may not be optimal yet although the improvement in separation performance was observed compared to the nominally designed channel. (2) Inability to increase the driving voltage due to rigorous temperature rise approximately 3°C/min at high voltage Therefore, the following future studies are implied to further optimize the performance of the acoustic concentrator (1) Designing the layer thickness by 1D model with electric potential boundary conditions, which may reflect actual experimental conditions since electric ___________________________________________________________________________ 131 Chapter ___________________________________________________________________________ potential difference is applied on the electrode surfaces of PZT actuator in an actual experiment. (2) Incorporate cooling system so that the concentrator can be tested at power input It should also be noted that although significant trapping by the lateral radiation forces was observed, they cannot be quantified accurately. This is because the turbidity measurement method could only provide approximate order of magnitude of particle concentrations at inlet and outlets. Therefore, further studies should address this by finding ways to quantitatively characterize particle trapping inside the channel, for example, by incorporating accurate online particle counting systems such as dielectrophoretic trapping devices at inlet, outlets and inside the channels. Such studies are necessary especially for concentration process at low inlet particle concentration. For the simulation model, instead of full three-dimensional analysis with channel walls and piezoelectric transducer, only two-dimensional FEM analysis of fluidic chamber was used for acoustic analysis. 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Acoustic pressure amplitude is the real or imaginary part of the complex pressure amplitude. ‫݌‬ሺ‫ݔ‬, ‫ݕ‬, ‫ݐ‬ሻ = ܴ݁൛ܲ෠ሺ‫ݔ‬, ‫ݕ‬ሻ݁ ௜ఠ௧ ൟ (A.1) The real part can be obtained by: p ( x, y , t ) = ˆ P ( x, y )e iωt + Pˆ * ( x, y )e −iωt { } (A.2) The square of the pressure term can also be derived: p ( x, y , t ) = ˆ P ( x, y )e iωt + Pˆ * ( x, y )e −iωt p ( x, y , t ) = ˆ2 P ( x, y )e i 2ωt + Pˆ ( x, y )Pˆ * ( x, y ) + Pˆ * ( x, y )e −2 iωt { } (A.3) { } (A.4) p ( x, y, t )dt , T ∫0 T Time-averaging the square of the pressure, p ( x, y, t ) = p ( x, y , t ) = ˆ P ( x, y )Pˆ * ( x, y ) (A.5) ___________________________________________________________________________ 142 Appendix ___________________________________________________________________________ The acoustic displacement velocity can be obtained from the following relationship. ‫ݒ‬Ԧ = − ௜ఠఘ ∇‫݌‬ ଵ (A.6) ೑ The velocity in the x-direction is  ∂Pˆ (x, y )eiωt  v x ( x, y, t ) = − Re   ∂x  iωρ f   ∂Pˆ ( x, y ) i  ωt + π2    = Re e  ωρ f  ∂x  1 = ωρ f  ∂Pˆ ( x, y ) i  ωt + π   ∂Pˆ ( x, y ) * −i  ωt + π    e   e   +     ∂x   ∂x  1 v ( x, y , t ) = ω ρ 2f x (A.7) *2 * π   ˆ  i 2 ωt + π   −i 2 ωt +  ∂Pˆ ∂Pˆ  ∂Pˆ   ∂P    2    +2 +   ( x, y )e   e  ∂x ∂x  ∂x   ∂x   (A.8) The time-averaged square of the x-velocity,  ∂Pˆ  1 ∂Pˆ  ( x , y ) v ( x, y , t ) = ( x , y ) 2   ω ρ f ∂x  ∂x  * x (A.9) Similarly, the time-averaged square of the velocity in the y-direction is 1 ∂Pˆ ( x, y )  ∂Pˆ ( x, y )    v ( x, y , t ) = ω ρ 2f ∂y  ∂y  * y (A.10) The square of the amplitude of the acoustic displacement velocity is v ( x, y, t ) = vx2 ( x, y, t ) + v 2y ( x, y, t ) (A.11) The time-averaged squared velocity is v ( x, y , t ) = v x2 ( x, y , t ) + v y2 ( x, y , t ) 1 v ( x, y , t ) = ω ρ 2f  ∂Pˆ ( x, y )  ∂Pˆ ( x, y ) * ∂Pˆ ( x, y )  ∂Pˆ ( x, y ) *    +       ∂y  ∂y    ∂x  ∂x   (A.12) (A.13) ___________________________________________________________________________ 143 Appendix ___________________________________________________________________________ The general expression for the acoustic energy density is: Eac p2  1   = ρf v + 2 ρ f c 2f    (A.14) For the two-dimensional field, Eac can be found by substituting (A.5) and (A.13) into (A.14), E 2D ac  ∂Pˆ ( x, y )  ∂Pˆ ( x, y ) * ∂Pˆ ( x, y )  ∂Pˆ ( x, y ) * ω  ˆ Pˆ *       + + P = ρ f ω  ∂x  ∂x  ∂y  ∂y  c 2f   (A.15) ___________________________________________________________________________ 144 Publications ___________________________________________________________________________ Publications M. H. Thein, K. M. Lim, Ultrasonic concentration and trapping of microsized particles in a fluid suspension, The Fourth International Symposium on Physics of Fluids (ISPF4), 13-16 June, 2011, Lijiang, China. ___________________________________________________________________________ 145 [...]... numerical results of Stokes force and radiation force in lateral direction for three matching cases at a flow rate of 0.1 ml/min and driving voltage of 30 Vpp ………………………….…………………………………….… 114 Figure 6.22 (a) x-component of hydrodynamic force against x-position in different channels (b) streamlines in h-shaped channel; the blue line shows the cross section along which hydrodynamic force in (a) are plotted………………….118... running in parallel was also reported by Y Liu [40] In this micro-device, particles in upper stream are transported to the lower stream by maintaining an offset between fluid- fluid interface and pressure nodal plane The effect of acoustic force on the interface deformation was also studied 2.3 Geometrical Design and Mode of Operation Standing wave acoustic resonator devices can also be classified into... Chapter 3 Theory Pertinent literature has been reviewed in Chapter 2 In this chapter, basic theory of acoustics in general is first discussed Next, theory of standing wave acoustics is discussed and radiation forces acting on the particle in the standing wave field are reviewed Finally, the theoretical model is developed to aid in selecting the proper thicknesses for layers in standing wave resonator... at inlet and outlets If the lateral force is found to exist and it is significant to trap the particles, the device operation mode could change from separation to trapping This would be a drawback or an advantage depending on the mode required Therefore, this may provide insights into the device’s operation and some operating guidelines for exploiting or avoiding the existence of particle trapping for. .. Literature Review In an ultrasonic field, suspended particles experience acoustic radiation force due to the nonlinear nature of the wave propagation in the medium Acoustic radiation force is a non-linear effect resulted by the change in the momentum of the incident wave and scattered wave over the surface of the particle [22] This acoustic radiation force for an object in an ideal fluid is a second-order... particularly standing acoustic waves at ultrasonic frequencies, have been very useful in separation science and microfluidic system Devices with different designs and different operating modes have been invented to exploit the ultrasonic standing wave for separating or concentrating particulate matters from the suspensions [11, 87-89] Acoustic wave is a pressure wave propagating in different media,... direction to fluid flow Standing wave was formed along the axis of the cylindrical tube by putting the transducer and reflector at either ends forming bands of particles By driving the device in a sweep mode at a certain nominal centre frequency, a drifting stationary wave was generated in the axial direction Then, particles in nodal lines are axially translated to and collected at the other end of the... _ the hindrance of operation due to the trapping even with the use of surfactant to reduce formation of particle aggregates [19, 20, 44] Based on all these studies, it can be noted that investigating particle trapping by lateral radiation forces in acoustic concentrators is important to determine the device’s mode of operation Although the existence of particle trapping was highlighted in the studies... chamber, i.e at millimetre- scale However, the method cannot work in a continuous mode since flushing is needed whenever the porous medium is saturated (e) Carrier Medium Exchange Another proven technology using acoustic radiation force is the so-called particle washing or carrier medium exchange [37-40] Petersson et al are the first to develop the micro-devices using this technology for blood wash application... device and to experimentally investigate the effect of layer thicknesses on separation performance of the device (3) To experimentally examine if particle trapping by lateral forces exists and how it affects the separation process by measuring particle concentrations at inlet and outlets (4) To estimate the radiation forces and separation performance by using a twodimensional finite element model and particle . MILLIMETRE-SCALE ULTRASONIC CONCENTRATOR FOR MICROPARTICLES IN FLUID THEIN MIN HTIKE NATIONAL UNIVERSITY OF SINGAPORE 2011 MILLIMETRE-SCALE ULTRASONIC. MILLIMETRE-SCALE ULTRASONIC CONCENTRATOR FOR MICROPARTICLES IN FLUID THEIN MIN HTIKE (M. ENG., Bandung Institute of Technology) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR. trapping. This would be a drawback or an advantage depending on the mode required. Therefore, this may provide insights into the device’s operation and some operating guidelines for exploiting