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Femtosecond-laser-nanomachining and the applications to nanofluidics

Femtosecond-laser-nanomachining and the applications to nanofluidics. Sanghyun Lee Ph.D. candidate Mechanical Engineering University of Michigan. BACK GROUND. Fast growing microfluidics markets “Microfluidics Technology”, BCC Research, 2006

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Femtosecond-laser-nanomachining and the applications to nanofluidics

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  1. Femtosecond-laser-nanomachining and the applications to nanofluidics Sanghyun Lee Ph.D. candidate Mechanical Engineering University of Michigan

  2. BACKGROUND • Fast growing microfluidics markets • “Microfluidics Technology”, BCC Research, 2006 • An average annual growth rate (AAGR) of 14.1% is expected over the next five years - $2.9B(2005), $3.2B(2006) $6.2B(2011) • Inkjet printing (largest market but slowly growing); chemical analysis, synthesis, and proteomics; healthcare-related applications (fastest growing); defense and public safety (smallest) • Microfluidic components -> complex systems (lab-on-a-chip, microTAS) • Needs for 3D geometries • Higher density, more complex functionality • Multi-layer stacking? • Needs for nanoscale geometries • Further miniaturization: faster analysis, smaller sample volumes, higher density, special functions (overlap of electrical double layer, nanoscale separation) • Nano-imprinting? Or, nanoscale lithography?

  3. BACKGROUND • Optical machining by femtosecond laser pulses • A true machining of 3D geometries • No intrinsic 2D limitation • Easily configure nanoscale geometries • Nano features under diffraction limit through a operation in the OCI regime

  4. AIMS of STUDY • To develop the fs-laser-nanomachining • A key method in 3D fabrication • A dominant method in nanoscale fabrication • Application • CE separation device using high AR nanoscale separation column • Suggesting future applications • Based on 3D geometries: micro-ELISA • Based on nanogeometries: nanoscale sensor, nanoscale fast mixer, pre-concentrator • Based on the increase capillary effects: surface measurements (contact angle, zeta-potential), diffusivity measurements, mole fractions of gas components in nanobubbles

  5. OVERVIEWS • Parametric researches of fs-laser-nanomachining • Pulse energy, pulse repetition rate, feed rate, etc. • Optimization of the parameters • Acoustics in nanoscale capillaries • Key phenomena for improving AR limitation • Degassed-water-assisted fs-laser-machining • nCE rapid separations • Most representative application requiring a long nanocapillary (AR>1000) • Faster speed (millisecond), higher resolution (Na>105), and less sample volume (femtoliter)

  6. Optical ablation & bubble generation Water Bubble expansion Glass substrate Femtosecond laser pulses 100x N.A.1.3 Objective Lens Ablation Feature size Energy Beam waist INTRODUCTION What is the water-assisted fs-laser-nanomachining? 1. True 3D fabrication 2. Frozen in time – minimal collateral damage - Photon-electron coupling : ns – ps - Heat transfer : µs - ns 3. Precise, deterministic and reproducible 4. Nano feature size, under diffraction limit: - Using tightly focused laser pulses - Operated in the OCI regime

  7. d L Dead end channel INTRODUCTION Major limitation in optical machining – Aspect ratio (L/d), mass productivity 3D high AR channels are requested for: • - Achieving complex micro/nanofluidic networks • Special applications such as separation ※Grigoropoulos, et al., Applied physics A, 2004

  8. INTRALASE fs pulsed laser - 700fs - 1054nm - 0.3 ~ 3kHz 100× N.A. 1.3 Objective lens Polarizer Second harmonic X-tal Green filter z y x INTRODUCTION Experimental description Nano-capillary (d=600nm) Water Glass (hg=170 µm) Nano stage Controller Dichroic mirror IR- mirror Green-block filter Shutter Attenuator CCD Camera IBM PC

  9. Water Debris extrusion Debris generation Inflow of water : surface tension and bubble collapse Glass substrate Outflow of water : bubble expansion (short L) and laser back scanning (long L) 100x N.A.1.3 Objective Lens : debris extrusion rate [m3/s] P: pressure by bubble generation [N/m2] Rcir: resistance of circulation [Ns/m5] INTRODUCTION Water circulation and debris extrusion process

  10. Water Debris extrusion Debris generation Glass substrate 100x N.A.1.3 Objective Lens INTRODUCTION Water circulation and debris extrusion process • Inflow of water • - Higher surface tension • - faster bubble collapse 2. Outflow of water - More bubble expansion - Laser back scanning can very effectively sweep the water plug entraining debris out of the capillary.

  11. CHAPTER 1. PARAMETRIC RESEARCHES OVERVIEWS 1. Improving machining efficiency : speed, power consumption – Major parameters: pulse energy, pulse repetition rate, feed rate, pulse duration, and pulse chopping – Single and multiple scanning methods 2. Improving machining effectiveness : aspect ratio -> Chapter 2 – Major contributors : pressure, temperature, and mole fraction of hydrogen (degassing) – Multiple scanning method – Acoustic nodes formation – Degassed-water-assisted fs laser nanomachining 3. Improving machining quality : reproducibility, dimensional accuracy, and surface roughness – Major parameters : stability of laser such as pulse energy and incident angle.

  12. Effect of pulse energy (PRR=1.5kHz) CHAPTER 1. PARAMETRIC RESEARCHES Single scanning • Higher pulse energy machines larger diameter capillary, wherein the internal pressure of bubbles decreases. • Based on the node equation (chapter2), the decreased pressure can improve AR, resulting in longer channel

  13. Effect of pulse repetition rate, PRR (pulse energy=14nJ) CHAPTER 1. PARAMETRIC RESEARCHES Single scanning • Excess energy input via higher PRR causes dehydration at the ablation site. • Higher PRR than optimum decreases the water circulation

  14. Effect of laser chopping (PRR=100kHz, PE=21nJ, DR=0.5) N: number of holes Blocking Opening γ: R.P.M Fast rotation τ τopen t Chopping wheel Slow rotation Duty ratio D.R. CHAPTER 1. PARAMETRIC RESEARCHES Laser pulse chopping – high PRR laser

  15. Time based machining performance (PRR=1.5kHz, PE=10nJ) CHAPTER 1. PARAMETRIC RESEARCHES Multiple scanning • Faster feed rates take less machining time. • Why? Machining with faster feed rates can have more numbers of laser back scanning, which increases the debris extrusion rate and water circulation.

  16. CHAPTER 1. PARAMETRIC RESEARCHES Conclusions • Parametric researches show that the optimum PRR and feed rate significantly affect the machining efficiency. • For very high PRR lasers, low frequency laser chopping of around 1kHz significantly improves the machining efficiency, although it decreases the average power input. • Higher machining efficiency of subsurface capillaries is mainly dependent upon better water circulation, which prevents dehydration at the ablation site and improves debris extrusion.

  17. Main channel Discontinuity Diffusion Secondary channel Internal pressure Pi Bubble collapse rate in nano-capillary Finally, united! Compression regime: > 214μm/s Bubble length(μm) WGW formation Diffusion regime: ≈ 1.8μm/s Water drop Time (s) 2μm Gas plug 1st water plug 2nd water plug CHAPTER 2. ACOUSTICS IN NANOSCALE CAPILLARIES OVERVIEWS – Applied Physics Letters 91 (2007) 1. Observation of the acoustic nodes formation – Abrupt termination of machining – Secondary channel becomes to machine under apparent discontinuity where is difficult region – Machining restoration after the difficult regions eventually connects the two channels 2. Node equation – Meta stable Water-Gas-Water structure – Resonance of the WGW structure and acoustic transmission in the gas plug – Pressure cancellation through the alternative pathway of the acoustic energy – Actual pressure and temperature of the gas plug – Temperature calculation during the machining – simple estimation and numerical calculation – Pressure measurements of nanobubbles – bubble compression in the nanocapillary 3. Strategies to improve AR – Pressure, temperature, and mole fraction of the gas plug – Lower pressure, higher temperature, and higher mole fraction of hydrogen – Degassing water increases the mole fraction of hydrogen – twice improvements of AR – Degassed-water-assisted fs-laser-nanomachining: AR bigger than 1000

  18. CHAPTER 3. NANO-CAPILLARY ELECTROPHORESIS OVERVIEWS 1. The first 3D nanoscale separation column – Separation column: d=650nm, L=804um -> AR>1000 2. Faster separations – millisecond order – Higher electric field via smaller capillary increases average migration speed of analytes – Same resolution with shorter overall length due to less band broadening 3. Higher resolution – number of plate bigger than 105 – Transient heat transfer characteristics in the fast separation reduces temperature rising to 1/10 – Diffusivity is a function of temperature – lower temperature less band broadening – With a same overall potential, faster separation can achieve higher resolution 4. Less sample volume – femtoliter order – Nanoscale capillary requires femtoliter sample volume to be analyzed (1-10fl) – 9fl volume with 40uM concentration is equivalent to about 500 zepto-mole molecules 5. Non-continuous-flow sample loading mechanism – Minimizes the wastes of samples – A continuous control of the volume of the sample plug -> significant for higher resolution

  19. CHAPTER 3. NANO-CAPILLARY ELECTROPHORESIS What is CE? • CE –Capillary Electrophoresis is a separation technique that separates species (analytes) by their charge and friction (mobility) in a capillary filled with an electrolyte.

  20. CHAPTER 3. NANO-CAPILLARY ELECTROPHORESIS Primary parameters measure the performances of CE separation 1. Number of plates: 2. Number of resolvable peaks: 3. Height of plate: 4. Resolution of two peaks:

  21. CHAPTER 3. NANO-CAPILLARY ELECTROPHORESIS Why smaller capillary better for speed? 1. Separation time: 2. Temperature rising due to Joule heating limits max. E-field 3. Smaller capillary increases the maximum E-field - Less heat generation and faster heat dissipation 4. The same resolution can be achieved with shorter length.

  22. CHAPTER 3. NANO-CAPILLARY ELECTROPHORESIS How does the fast separation improve resolution? 1. Maximum theoretical resolution: 2. Actual resolution due to temperature rising 3. Actual temperature rising of the fast separation (<1s) is about 1/10 times to the maximum due to the transient heat transfer characteristics; - Characteristic time constant is usually greater than 1s. - Assuming max. 30K temperature rising is allowed, the fast separation can improve resolution more than about 4.5% - Or, max. E-field can be increased about 3 times, which results in 3 times higher potential and 3 times higher resolution.

  23. - Slow separation : - Fast separation : CHAPTER 3. NANO-CAPILLARY ELECTROPHORESIS Heat transfer characteristics of fast separations : numerical analysis (FlexPDE)

  24. CHAPTER 3. NANO-CAPILLARY ELECTROPHORESIS Example of the fast separation based on microchip CE device • E=53kV/cm • V=8.6kV • Cross section: 7*26um2 • Separation time: < 10 ms J. Michael Ramsey Group

  25. CHAPTER 3. NANO-CAPILLARY ELECTROPHORESIS The effect of the length of sample plug on the resolution 1. Band broadening : - t> 1s (slow separation): - t< 1s (fast separation): 2. Optimizing linj is very important for improving resolution : 3. Ability to control linj is significantly required to keep high resolution

  26. Loading channel ~ 100μm Separation column Microchannels in PDMS header Detection area CHAPTER 3. NANO-CAPILLARY ELECTROPHORESIS Geometries of the nCE device : nanocapillaries at the center

  27. nCE nano-capillaries Solution reservoir Microchannels in PDMS header PDMS header (a) (b) Loading channel L=200μm Separation column L=804μm d=650±50nm Mask design of PDMS header Detection area L=48μm (c) (d) CHAPTER 3. NANO-CAPILLARY ELECTROPHORESIS Geometries of the nCE device: nanocapillaries + PDMS header

  28. Sample flow Electric field Sample loading Partial clogging CHAPTER 3. NANO-CAPILLARY ELECTROPHORESIS Non-continuous-flow sample loading mechanism • Minimum wastes of samples • Continuous control of linj by loading time

  29. CHAPTER 3. NANO-CAPILLARY ELECTROPHORESIS Fluorescent images of the separation

  30. CHAPTER 3. NANO-CAPILLARY ELECTROPHORESIS Electropherogram : a plot of fluorescence units over time

  31. CHAPTER 3. NANO-CAPILLARY ELECTROPHORESIS Electrokinetic flow characteristics Separation performances

  32. CONCLUSION • Parametric researches find that optimizing parameters is very important and effective for machining efficiency. Dehydration and debris extrusion via water circulation are mainly related to the machining efficiency. The researches give strategies to optimize laser parameters and machining parameters. • Acoustic nodes formation during subsurface capillary machining is found to be the major obstacles limiting AR, and the node equation gives strategies to overcome them. The developed degassed-water-assisted fs-laser-nanomachining achieves AR of bigger than 1000, which is huge improvement enough to fabricate nanoscale separation column. However, more improvements of AR are required to achieve highest resolution.

  33. CONCLUSION 3. The nCE rapid separation device developed based on the high AR subsurface nanocapillary machined by degassed-water-assisted fs-laser-nanomachining proves the advantages of nanoscale separation column on the separation performances. nCE separation is analyzed to be fast and high resolution separation with extremely small sample volume. 4. The optical machining by fs laser pulses constitutes a promising way to 3D and nanoscale micro/nanofluidics.

  34. ACKNOWLEDGEMENTS - Special thanks to Dr. E. F. (Charlie) Hasselbrink for his preliminary guidance and advices for this study. - This work was supported by a grant from IMRA corp. and NIH R21 EB006098-01. Thank you!

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