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Ge4000 report - Static Force Curve Activity in Nanofluidic Channels

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Undergraduate Research Project for Microsystems and Nanotechnology Minor

Undergraduate Research Project for Microsystems and Nanotechnology Minor

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  • 1. UNIVERSITY OF WISCONSIN-PLATTEVILLE GE4000 Project Report Static Force Activity in Nanofluidic Channels Jon Zickermann 12/14/2012This project hopes to contribute to the progress that has been made in the field of nanofluidics. Thisreport will take a look a set of nanofluidic channels. Surface topography will measured along with theforces and surface charges when the samples are submerged in water. Two methods of moving fluidswill be considered – flow by pressure gradients and electroosmotic flow. To minimize pressure dropthrough a nanofluidic system, surface treatments should avoided where flow rate is the primary focus.For electroosmotic flow, fluorine surface would be optimal despite the rougher surface. Both surfacetreatments had regressions that appeared to be a combination of both the gold and plain glass surfaces.
  • 2. Table of ContentsIntroduction .................................................................................................................................................. 2Background ................................................................................................................................................... 2 Atomic Force Microscopy ......................................................................................................................... 2 Equipment ............................................................................................................................................. 5 AFM Physics .......................................................................................................................................... 8 Nanofluidics .............................................................................................................................................. 8Project Goals ............................................................................................................................................... 11Results and Discussion ................................................................................................................................ 11 Surface Roughness .................................................................................................................................. 11 Force Curves - SiNi Tip ............................................................................................................................ 13 Plain Glass ........................................................................................................................................... 13 Br Treated ........................................................................................................................................... 13 Fluorine Treated.................................................................................................................................. 13 Force Curves – Spherical Tip ................................................................................................................... 14 Plain Glass ........................................................................................................................................... 14 Br Treated ........................................................................................................................................... 14 Fluorine Treated.................................................................................................................................. 14 Force Curve Data..................................................................................................................................... 15 MATLAB Results ...................................................................................................................................... 15 Charge Density Regressions .................................................................................................................... 17Conclusions ................................................................................................................................................. 18Acknowledgements..................................................................................................................................... 19Appendix ..................................................................................................................................................... 19 Plain Glass Surface Topography .............................................................................................................. 19 Br Surface Topography Images ............................................................................................................... 20 Fluorine Surface Topography Images ..................................................................................................... 21 Force Curve Calibration Data .................................................................................................................. 23 Charge Density Distributions .................................................................................................................. 23Works Cited ................................................................................................................................................. 24 Page 1 of 26
  • 3. IntroductionMicro and nanofluidics pose to greatly contribute to the fields of chemistry, physics and biology in theupcoming years. One example is in lab-on-a-chip systems, especially for mixing liquids at the nanoscale.As the field of nanofluidics matures, new ideas and concepts are being applied to lab-on-a-chip systems.The fundamental differences at the nanoscale physics in contrast with macroscale physics offeradditional advantages to those devices that use nanopores or nanochannels. However,nanoscale physicsand the effects on fluids have yet to be fully explored, especially in nanofluidics.This project hopes to contribute to the progress that has been made in the field of nanofluidics. Thisreport will take a look a set of nanofluidic channels fabricated and treated at The Ohio State University.Surface topography will measured along with the forces when the samples are submerged in water. Theresearch was coordinated by Dr. Yan Wu and carried out by Jon Zickermann.BackgroundAtomic Force MicroscopyAtomic Force Microscopes (AFMs) can allow imaging at the nanoscale which is beyond the limits ofoptical imaging, i.e., traditional optical microscopes. Conceptually, AFMs can be traced back to 1980s,specifically U.S. Patent 4,724,318by Gerd Binning (Seo & Jhe, 2007). AFMs use a microscopic techniqueimaging a surface topography by using attractive and repulsive interaction forces between a few atomsattached at a tip on a cantilever and a sample. In the case of attractive forces, there are three maincontributions causing AFM. These are short-range chemical forces, van der Waals forces andelectrostatic forces. As the effective ranges of these forces are different, one of them is dominantdepending on distance.van Der Waals interactions are based on the Coulomb interaction between electrically neutral atomswhich are locally charged by thermal and/or quantum fluctuations. van Der Waals interactions aregoverned by , where AH is the Hamaker constant (typically 1eV), R is the radius of thecantilever tip and z is the distance between the tip and the sample.Electrostatic forces are generatedbetween a charged or conductive tip and sample which have a potential difference (V). This force isgoverned by where εo is the dielectric constant. Additionally, ionic repulsion forces areencountered at close ranges. As an atom approaches another atom, the electronic wave function will beoverlapped and a very strong repulsion will be generated. This is also referred to as the Pauli exclusion.The last force of note is the capillary force, which is noticeable when a tip is close to the water layer, aliquid bridge called a meniscus is formed between the tip and the sample. This meniscus layer causes anattractive force (the capillary force) between the tip and the sample.In this project, three modes of AFM were used: contact, tapping and force modes. The first two wereused for surface roughness measurement, especially tapping mode. In contact mode, the probe(cantilever and tip) is scanned over the surface (or the sample is scanned under the probe) in an x-yraster pattern. The feedback loop maintains a constant cantilever deflection, and consequently a Page 2 of 26
  • 4. substantial, constant force on the sample. In contact mode, also referred to as AC mode, the probe alsomoves with a small vertical oscillation which is significantly faster than the raster scan rate. This leads tothe force on the sample is modulated such that the average force on the sample is equal to that incontact mode.When the probe is modulated with the tip in contact with a sample, the sample surfaceresists the oscillation and the cantilever bends. The variation in cantilever deflection amplitude at thefrequency of modulation is a measure of the relative stiffness of the surface. Figure 1: Basic AFM Conceptual Operation (Geisse) Figure 2: Basic Contact vs. Tapping Mode (Wu, 2011) Page 3 of 26
  • 5. Figure 3: Summary of Forces with AFMs (JPK Instruments) Page 4 of 26
  • 6. EquipmentAtomic Force MicroscopeThe AFM used for the project is the MFP-3D-BIO by Asylum Research. The unit offers a 90x90µm rangefor scanning in the x and y axis with a 0.5nm resolution and a 5 µm Z axis range with a 0.25nmresolution. Vibration reduction uses theHerzanAVI-200 unit, capable of responding to undesiredoscillations at 5-20ms(Asylum Research, 2009). Figure 4: The MFP-3D-BIO (Asylum Research, 2009) Figure 5: Active Vibration Filtering Unit(Herzan)Cantilever ProbesSurface RoughnessProbesSurface roughness was measured using the basic budget tips used by most students using for classes(such as Chemistry 4520Nanoscale Characterization and Fabrication at University of Wisconsin -Platteville). The tips are shaped like a polygon based pyramid. Tip radii are typically around 7nm andheight is 10-15μm. Page 5 of 26
  • 7. Figure 6: Dimensions of the Budget Tip UsediDrive SystemForce curve measurement utilized the iDrive system. The iDriveNbFeB magnet is fully enclosed andsealed within the cantilever holder which allows for unobstructed bottom view of samples and preventssample contamination. The iDrive system allows for probe actuation using electrical currents as showbelow: Figure 7: Schematic diagram showing the Lorentz Force exerted onto the cantilever(Asylum Research) Figure 8: iDrive Probe Holder Page 6 of 26
  • 8. SiNi Triangular TipsTwo shapes of cantilevered tips were used for force curve measurement. The SiNItips are softer than theeconomical probes and are compatible for the iDrive system. Lever Shape Triangular Lever Thickness 0.4µm Lever Width 13.4µm Lever Length 100µm Spring constant (N/m) 0.09 Resonant freq. (kHz) 32 Tip shape 4-sided pyramid Tip height 3µm Tip radius <40nm Tip angle <35° front <35° side Coating 40nm Au on tip side 50nm Au on reflex side Table 1: Values for the SiNi Tip(Asylum Research) Figure 9: SiNi ProbeSpherical TipsThe spherical tips, like the triangular SiNi tips, are softer than the standard probes. However, thespherical tip probes are gold-coated and offer a higher surface area than typical pyramid/cone shapetips. Figure 10: Example of Spherical Tip AFM Probe (Interaction between fine particles) Page 7 of 26
  • 9. AFM PhysicsTwo methods of determining the force measured by the probe are used by the software provide byAsylum Research: the thermal method and Sader method. Both methods differ by the method used tocalculate the spring rate, , used from the definition of Hooke’s Law . The thermal method isused primarily in the projectThe thermal method determines the spring rate as follows according to Asylum Research: Figure 11: Thermal Method Calculations (Asylum Research)The Sader method for a triangular tip determines the spring rate as:whereThe MATLAB scripts will calculate the electrical charge and Debye length. The Debye length is effectivelythe distance where electrical charges have an effect. The Debye length for this experiment is defined as:where is the permittivity of free space, is the dielectric constant, is the elementary charge, is theionic strength of the electrolyte, and is the Avogadro constant(Debye Length, 2007), (Attard, 1996).NanofluidicsNanophysicsNanofluidics is commonly defined as any liquid system where movement and control over liquids in oraround objects with one dimension at most 100 nm. Others limit dimensions to 10-50nm at most Page 8 of 26
  • 10. (Mukhopadhyay, 2006).Nanofluidicsapplies to fluids inside nanoscale channels, porous alumina andnanoscale conduits. Currently, the primary application of nanofluidics is in lab-on-a-chip applications,specifically separation and analysis of DNA strands.Nanofluidics can also be utilized in diodes or field-effect transistors. However, the application of nanofluidics could eventually extend tosuch nanoscalesystems like nanopumps, many of which are currently used at the larger microscale. Currently,nanophysics are still not fully understood. A table of the most common non-dimensional constants thatcan be used to characterize micro and nanoscale physics for fluids is as follows: Table 2: Common Nondimensional Constants (Oosterbroek, 1999), (Eijkel & van den Berg, 2005)The biggest difference between macroscale fluid dynamics and micro and nanoscale fluid mechanics isthe effects due to very low Reynolds numbers. At the micro and nanoscale, surface tension dominatesand the no-slip condition which is assumed at the macroscale does not apply.The greater amount of slipfavors more efficient flow.Figure 12: No-slip Assumption versus Slip Flow (Boundary slip and nanobubble study in micro/nanofluidics using atomic force microscopy, 2009)Two primary methods of fluid transport for micro and nanofluidics are utilized to move fluids: pressuregradients and voltage potentials (electroosmotic flow). For flow by pressure gradients, velocity can becalculated as follows: Page 9 of 26
  • 11. assuming the width of the channel is much greater than height. Here, is the fluid viscosity, b is thecritical unit of length and is the pressure gradient. It should be noted that the term is thecontribution due to the slip condition. For electroosmotic flow, the charactering equation is:where and are contributions due to the slip condition and is the contribution from theHelmholtz-Smoluchowsky velocity(Eijkel J. , 2007).Nanofluidics offer many advantages for some applications and disadvantages if used in the impropersystems. Scaling down microfluidic systems down to nanofluidic sizes offers the possibility to confinemolecules to very small spaces and subject them to controlled forces. Additionally, there is thepotential for precise control of liquid flow and molecular behavior at the nanoscale. However,nanofluidic systems are harder to fabricate compared to microfluidic counterparts. Additionally, there isa higher tendency for channels to get clogged and lower signal quality when trying to send voltages. Figure 13: Example Difference Between Nano and Micro Channels (Daiguji, 2009)NanofabricationSince the field of nanofluidics is years away from maturation, there is no standard method of fabricationfor nanofluidic devices. As with most micro and nanodevices, fabrication can be described by either top-down or bottom-up methods. Building a nanofluidic device using top-down methods is accomplishedfrom using photolithography methods on a substrate silicon wafer, which is how most Microelectromechanical systems (MEMS) devices are fabricated. From the top-down methods, nanofluidicdevices can be integrated on a MEMS chip on one wafer. Traditional top-down methods offer aneconomical method to nanofluidic device fabrication. For bottom-up techniques, self-assembledmonolayers (SAMs) can be used with biological materials to form a molecular monolayer on thesubstrate. Additionally, carbon nanotubes (CNTs) offers an alternative, however, this method is still in Page 10 of 26
  • 12. development and is years away from any nanofluidic applications. While not as economical, bottom-upmethods can precise shapes at the nanoscale.Project GoalsMultiple objectives were outlined at the start of the project. The first was to understand the operationprinciple of dynamic AFM imaging and static force curve measurements. The next objective is to learnthe impact of surface treatment of micro-nanofluidic channel wall on slip flow and electrokinectic flow.Another goal is to perform surface topography measurements and surface roughness measurementsusing AFM inside nanofluidic channels. These samples are nanochannels of depths of 80, 250 and450nm. One set of nanochannels were treated with bromine and another set treated with fluorine byShauryaPrakash at The Ohio StateUniversity. The next goal is to prepare an electrolyte solution withdifferent pH and concentration. Finally, static force curve activity at the nanofluidic channel wall inelectrolyte solutions will be measured. Basic adhesion forces can be calculated from the built-insoftware supplied by Asylum Research. The electrical charges and the level of charge versus distancefrom substrate surface will be calculated using a program written by Dr. Yan Wu. Figure 14: Example Graphs to be Created (Wu, 2011)If time does not allow, data will be calculated from only deionized water where the pH level is 6.0.Results and DiscussionSurface RoughnessThe values calculated from the AFM software were taken at three points. The design of thenanochannel resembled a “Y” shape when observed from the top. The three points were taken at each“leg” at approximately the same location for each sample. The AFM scans were ran at approximately0.20 to 0.40Hz for maximum accuracy and feedback precision. Scans that calculated surface RMS valuesthat appeared to be outliers were rejected and, if possible, rescanned with the probe recalibrated orrepositioned. Page 11 of 26
  • 13. 80nm 250nm 450nm Plain 0.950 1.314 0.851 Br 1.607 1.485 1.910 F 4.926 4.615 3.422 Table 3: Average Surface Roughness RMS Values in Nanometers Nanochannel ComparisonAverage Surface Roughness RMS 5.0 4.0 3.0 Plain (nm) 2.0 Br 1.0 F 0.0 80nm 250nm 450nm Channel Depth Figure 15: Visual Comparison of Surface RMS Values by Channel Depth Nanochannel Comparison Average Surface Roughness RMS (nm) 5.0 4.0 3.0 Plain 2.0 Br 1.0 F 0.0 Average Sample Values Figure 16: Average Sample Values Page 12 of 26
  • 14. Force Curves - SiNi TipPlain GlassBr TreatedFluorine Treated Page 13 of 26
  • 15. Force Curves – Spherical TipPlain GlassBr TreatedFluorine Treated Page 14 of 26
  • 16. Force Curve DataAs state before, average surface attraction forces can be calculated from the Asylum Research. Using acontinuous scan, multiple samples can be acquired with relative ease, allowing eliminating outliers.Additionally, the same data can be used for MATLAB calculations. µ (nN) σ (nN) Plain 6.30 0.078 Br 21.97 0.405 F 1.06 0.144 Table 4: Force data from SiNi Tip µ (nN) σ (nN) Plain 27.60 0.0249 Br 18.05 0.0019 F 15.33 2.7500 Table 5: Force data from Spherical TipMATLAB Results Debye length 1000 Length (nm) 100 10 1 Plain Br F Gold Page 15 of 26
  • 17. Charge Density 0.007Charge Density (C/m^2) 0.006 0.005 0.004 0.003 0.002 0.001 0.000 Plain Br F Gold Page 16 of 26
  • 18. Charge Density RegressionsPlain SurfaceBr SurfaceFluorine Surface Page 17 of 26
  • 19. Gold SurfaceConclusionsSurface topography scans reveal that treatments increase the roughness of the nanochannels, especiallyfluorine solutions, which on average had a 4nm increase in RMS value in surface roughness. Theseconclusions can bedetermined by visual inspection of the images generated from the AFM, where thebumps on the surface appear smoother on the untreated samples compared to the rough edgescommon to the surfaces of the fluorine treated samples.Therefore, to minimize pressure drop through ananofluidic system, surface treatments should avoided where flow rate is the primary focus andpressure drop needs to be minimized. Force measurement scans with the triangular tip reveal that bromine treatment produces a positivecharge buildup that strongly attracts electrical charges, whereas fluorine treatment produces a repulsiveforce that resisted the cantilever tip. The same scans ran with the spherical tip indicate that theattraction forces are stronger. These increases can be attributed to the larger surface area which allowsfor more charges to build on the tip surface. The plain surface sample attraction force is stronger thanany other force, spherical or triangular tip.Information from the MATLAB tells more about the force modulation from the AFM. Untreated, thecharge distribution is virtually identical to the typical models, as expected. This offers a template tocompare the other samples against. Inspection of the charts created by Excel show that the charges inthe bromine treated surface reach far from the substrate surface as indicated by the large Debyelengths. This is consistent to the force curves generated by the AFM software, where the cantileverprobe “jumped in” to the surface substrate at a faster rate than any other surface treatments. Thefluorine surface has a large concentration of charges near the surface, however, compared to the plainand bromine treated surfaces, the charges are repelling them. For electroosmotic flow, fluorine surfacewould be optimal despite the rougher surface.Both surface treatments had regressions that appeared tobe a combination of both the gold and plain glass surfaces. Page 18 of 26
  • 20. AcknowledgementsThe author wishes to thank Dr. Yan Wu for her patience and help on this project. Additionally, help frompeers doing research in the University of Wisconsin - Platteville cleanroom was very nice in helping theauthor start his research in the early days in this project. Finally, the author would like to recognize Dr.MichealMomot for allowing the author to share a cleanroom key for easy access to the University ofWisconsin - Platteville cleanroom.AppendixPlain Glass Surface Topography Page 19 of 26
  • 21. Br Surface Topography Images Page 20 of 26
  • 22. Fluorine Surface Topography Images Page 21 of 26
  • 23. Page 22 of 26
  • 24. Force Curve Calibration Data k (mN/m) Q Freq (kHz) Plain 83.32 15.2 31.267 Br 87.26 15.2 30.947 F 85.29 15.3 30.733 Table 6: Air Calibration Data for SiNi Tip k (mN/m) Q Freq (kHz) Plain 87.34 25.0 21.336 Br 84.82 25.1 21.319 F 89.85 24.9 21.568 Table 7: Air Calibration Data for Spherical TipCharge Density DistributionsPlain SurfaceBr Surface Page 23 of 26
  • 25. Fluorine SurfaceGold SurfaceWorks CitedDebye Length. (2007, January 22). Retrieved December 19, 2012, from Duke University: http://people.duke.edu/~ad159/files/p142/2.pdfBoundary slip and nanobubble study in micro/nanofluidics using atomic force microscopy. (2009, November 28). Soft Matter, pp. 29-66.Asylum Research. (2009, August 12). MFP-3D AFMs - Extensive Suite of System, Environmental, and Application Options Enabling Users to Broaden AFM Capabilities by Asylum Research. Retrieved December 16, 2012, from A to Z Nano: http://www.azonano.com/article.aspx?ArticleID=2343Asylum Research. (n.d.). iDrive™ Magnetic Actuated Cantilever . Retrieved from Asylum Research: http://www.asylumresearch.com/Products/iDrive/iDrive.shtmlAsylum Research. (n.d.). The Physics of Atomic Force Microscopy. Retrieved December 2012, from Asylum Research: http://www.asylumresearch.com/Applications/EquationCard.pdf Page 24 of 26
  • 26. Attard, P. (1996). Electrolytes and the Electric Double Layer. Adv. Chem. Phys.Daiguji, H. (2009, July 1). Ion transport in nanofluidic channels. Chemical Society Reviews, pp. 903-913.Eijkel, J. (2007). Liquid Slip in Micro-and Nanofluidic: Recent Research and its Possible Implications. Lab- on-a-Chip, pp. 299-301.Eijkel, J. C., & van den Berg, A. (2005, April 8). Nanofluidics: what is it and what can we expect from it? pp. 249-267.Geisse, N. (n.d.). AFM and Combined Optical Techniques. Retrieved December 16, 2012, from Asylum Research: http://tinyurl.com/asylumresearchafmHerzan. (n.d.). Active Vibration Control - TS Series . Retrieved December 17, 2012, from Herzan: http://www.herzan.com/products/active-vibration-control/ts-series.html#TS%20MODELSInteraction between fine particles. (n.d.). Retrieved December 2012, from http://www.mpip- mainz.mpg.de/documents/akbu/pages/particles.htmJPK Instruments. (n.d.). A Pratical Guide to AFM Force Spectroscopy and Data Analysis. JPK Instruments.Mukhopadhyay, R. (2006, November 1). WHAT DOES NANOFLUIDICS HAVE TO OFFER? PLENTY, SAY EXPERTS. Analytical Chemisty, pp. 7380-7382.Oosterbroek, E. (1999). Modeling, design and realization of microfluidic components.Seo, Y., & Jhe, W. (2007, December 17). Atomic force microscopy and spectroscopy. REPORTS ON PROGRESS IN PHYSICS, pp. 71-94.Wu, Y. (2011). Advanced AFM. Plattevile, WI. Page 25 of 26

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