Ge4000 report - Static Force Curve Activity in Nanofluidic Channels

UNIVERSITY OF WISCONSIN-PLATTEVILLE

GE4000 Project Report
Static Force Activity in Nanofluidic Channels
Jon Zickermann
12/14/2012

This project hopes to contribute to the progress that has been made in the field of nanofluidics. This
report will take a look a set of nanofluidic channels. Surface topography will measured along with the
forces and surface charges when the samples are submerged in water. Two methods of moving fluids
will be considered – flow by pressure gradients and electroosmotic flow. To minimize pressure drop
through 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 surface
treatments had regressions that appeared to be a combination of both the gold and plain glass surfaces.

Table of Contents
Introduction .................................................................................................................................................. 2
Background ................................................................................................................................................... 2
Atomic Force Microscopy ......................................................................................................................... 2
Equipment ............................................................................................................................................. 5
AFM Physics .......................................................................................................................................... 8
Nanofluidics .............................................................................................................................................. 8
Project Goals ............................................................................................................................................... 11
Results 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 .................................................................................................................... 17
Conclusions ................................................................................................................................................. 18
Acknowledgements..................................................................................................................................... 19
Appendix ..................................................................................................................................................... 19
Plain Glass Surface Topography .............................................................................................................. 19
Br Surface Topography Images ............................................................................................................... 20
Fluorine Surface Topography Images ..................................................................................................... 21
Force Curve Calibration Data .................................................................................................................. 23
Charge Density Distributions .................................................................................................................. 23
Works Cited ................................................................................................................................................. 24

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Introduction
Micro and nanofluidics pose to greatly contribute to the fields of chemistry, physics and biology in the
upcoming 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 offer
additional advantages to those devices that use nanopores or nanochannels. However,nanoscale physics
and 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. This
report 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. The
research was coordinated by Dr. Yan Wu and carried out by Jon Zickermann.

Background

Atomic Force Microscopy
Atomic Force Microscopes (AFMs) can allow imaging at the nanoscale which is beyond the limits of
optical 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 technique
imaging a surface topography by using attractive and repulsive interaction forces between a few atoms
attached at a tip on a cantilever and a sample. In the case of attractive forces, there are three main
contributions causing AFM. These are short-range chemical forces, van der Waals forces and
electrostatic forces. As the effective ranges of these forces are different, one of them is dominant
depending on distance.

van Der Waals interactions are based on the Coulomb interaction between electrically neutral atoms
which are locally charged by thermal and/or quantum fluctuations. van Der Waals interactions are
governed by , where AH is the Hamaker constant (typically 1eV), R is the radius of the
cantilever tip and z is the distance between the tip and the sample.Electrostatic forces are generated
between a charged or conductive tip and sample which have a potential difference (V). This force is
governed by where εo is the dielectric constant. Additionally, ionic repulsion forces are
encountered at close ranges. As an atom approaches another atom, the electronic wave function will be
overlapped 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, a
liquid bridge called a meniscus is formed between the tip and the sample. This meniscus layer causes an
attractive 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 were
used 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-y
raster pattern. The feedback loop maintains a constant cantilever deflection, and consequently a

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substantial, constant force on the sample. In contact mode, also referred to as AC mode, the probe also
moves with a small vertical oscillation which is significantly faster than the raster scan rate. This leads to
the force on the sample is modulated such that the average force on the sample is equal to that in
contact mode.When the probe is modulated with the tip in contact with a sample, the sample surface
resists the oscillation and the cantilever bends. The variation in cantilever deflection amplitude at the
frequency 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)

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Figure 3: Summary of Forces with AFMs (JPK Instruments)

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Equipment

Atomic Force Microscope
The AFM used for the project is the MFP-3D-BIO by Asylum Research. The unit offers a 90x90µm range
for scanning in the x and y axis with a 0.5nm resolution and a 5 µm Z axis range with a 0.25nm
resolution. Vibration reduction uses theHerzanAVI-200 unit, capable of responding to undesired
oscillations at 5-20ms(Asylum Research, 2009).

Figure 4: The MFP-3D-BIO (Asylum Research, 2009)

Figure 5: Active Vibration Filtering Unit(Herzan)

Cantilever Probes

Surface RoughnessProbes
Surface 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 and
height is 10-15μm.

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Figure 6: Dimensions of the Budget Tip Used

iDrive System
Force curve measurement utilized the iDrive system. The iDriveNbFeB magnet is fully enclosed and
sealed within the cantilever holder which allows for unobstructed bottom view of samples and prevents
sample contamination. The iDrive system allows for probe actuation using electrical currents as show
below:

Figure 7: Schematic diagram showing the Lorentz Force exerted onto the cantilever(Asylum Research)

Figure 8: iDrive Probe Holder

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SiNi Triangular Tips
Two shapes of cantilevered tips were used for force curve measurement. The SiNItips are softer than the
economical 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 Probe

Spherical Tips
The spherical tips, like the triangular SiNi tips, are softer than the standard probes. However, the
spherical tip probes are gold-coated and offer a higher surface area than typical pyramid/cone shape
tips.

Figure 10: Example of Spherical Tip AFM Probe (Interaction between fine particles)

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AFM Physics
Two methods of determining the force measured by the probe are used by the software provide by
Asylum Research: the thermal method and Sader method. Both methods differ by the method used to
calculate the spring rate, , used from the definition of Hooke’s Law . The thermal method is
used primarily in the project

The 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:

where

The MATLAB scripts will calculate the electrical charge and Debye length. The Debye length is effectively
the 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 the
ionic strength of the electrolyte, and is the Avogadro constant(Debye Length, 2007), (Attard, 1996).

Nanofluidics
Nanophysics
Nanofluidics is commonly defined as any liquid system where movement and control over liquids in or
around objects with one dimension at most 100 nm. Others limit dimensions to 10-50nm at most

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(Mukhopadhyay, 2006).Nanofluidicsapplies to fluids inside nanoscale channels, porous alumina and
nanoscale 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 nanoscale
systems 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 that
can 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 is
the effects due to very low Reynolds numbers. At the micro and nanoscale, surface tension dominates
and the no-slip condition which is assumed at the macroscale does not apply.The greater amount of slip
favors 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: pressure
gradients and voltage potentials (electroosmotic flow). For flow by pressure gradients, velocity can be
calculated as follows:

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assuming the width of the channel is much greater than height. Here, is the fluid viscosity, b is the
critical unit of length and is the pressure gradient. It should be noted that the term is the
contribution 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 the
Helmholtz-Smoluchowsky velocity(Eijkel J. , 2007).

Nanofluidics offer many advantages for some applications and disadvantages if used in the improper
systems. Scaling down microfluidic systems down to nanofluidic sizes offers the possibility to confine
molecules to very small spaces and subject them to controlled forces. Additionally, there is the
potential 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 is
a 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)

Nanofabrication
Since the field of nanofluidics is years away from maturation, there is no standard method of fabrication
for 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 accomplished
from using photolithography methods on a substrate silicon wafer, which is how most Micro
electromechanical systems (MEMS) devices are fabricated. From the top-down methods, nanofluidic
devices can be integrated on a MEMS chip on one wafer. Traditional top-down methods offer an
economical method to nanofluidic device fabrication. For bottom-up techniques, self-assembled
monolayers (SAMs) can be used with biological materials to form a molecular monolayer on the
substrate. Additionally, carbon nanotubes (CNTs) offers an alternative, however, this method is still in

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development and is years away from any nanofluidic applications. While not as economical, bottom-up
methods can precise shapes at the nanoscale.

Project Goals
Multiple objectives were outlined at the start of the project. The first was to understand the operation
principle of dynamic AFM imaging and static force curve measurements. The next objective is to learn
the 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 measurements
using AFM inside nanofluidic channels. These samples are nanochannels of depths of 80, 250 and
450nm. One set of nanochannels were treated with bromine and another set treated with fluorine by
ShauryaPrakash at The Ohio StateUniversity. The next goal is to prepare an electrolyte solution with
different pH and concentration. Finally, static force curve activity at the nanofluidic channel wall in
electrolyte solutions will be measured. Basic adhesion forces can be calculated from the built-in
software supplied by Asylum Research. The electrical charges and the level of charge versus distance
from 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 Discussion

Surface Roughness
The values calculated from the AFM software were taken at three points. The design of the
nanochannel 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 approximately
0.20 to 0.40Hz for maximum accuracy and feedback precision. Scans that calculated surface RMS values
that appeared to be outliers were rejected and, if possible, rescanned with the probe recalibrated or
repositioned.

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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 Comparison
Average 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

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Force Curves - SiNi Tip
Plain Glass

Br Treated

Fluorine Treated

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Force Curves – Spherical Tip
Plain Glass

Br Treated

Fluorine Treated

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Force Curve Data
As state before, average surface attraction forces can be calculated from the Asylum Research. Using a
continuous 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 Tip

MATLAB Results

Debye length

1000
Length (nm)

100

10

1
Plain Br F Gold

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Charge Density

0.007

Charge Density (C/m^2) 0.006
0.005
0.004
0.003
0.002
0.001
0.000
Plain Br F Gold

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Charge Density Regressions
Plain Surface

Br Surface

Fluorine Surface

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Gold Surface

Conclusions
Surface topography scans reveal that treatments increase the roughness of the nanochannels, especially
fluorine solutions, which on average had a 4nm increase in RMS value in surface roughness. These
conclusions can bedetermined by visual inspection of the images generated from the AFM, where the
bumps on the surface appear smoother on the untreated samples compared to the rough edges
common to the surfaces of the fluorine treated samples.Therefore, to minimize pressure drop through a
nanofluidic system, surface treatments should avoided where flow rate is the primary focus and
pressure drop needs to be minimized.

Force measurement scans with the triangular tip reveal that bromine treatment produces a positive
charge buildup that strongly attracts electrical charges, whereas fluorine treatment produces a repulsive
force that resisted the cantilever tip. The same scans ran with the spherical tip indicate that the
attraction forces are stronger. These increases can be attributed to the larger surface area which allows
for more charges to build on the tip surface. The plain surface sample attraction force is stronger than
any other force, spherical or triangular tip.

Information from the MATLAB tells more about the force modulation from the AFM. Untreated, the
charge distribution is virtually identical to the typical models, as expected. This offers a template to
compare the other samples against. Inspection of the charts created by Excel show that the charges in
the bromine treated surface reach far from the substrate surface as indicated by the large Debye
lengths. This is consistent to the force curves generated by the AFM software, where the cantilever
probe “jumped in” to the surface substrate at a faster rate than any other surface treatments. The
fluorine surface has a large concentration of charges near the surface, however, compared to the plain
and bromine treated surfaces, the charges are repelling them. For electroosmotic flow, fluorine surface
would be optimal despite the rougher surface.Both surface treatments had regressions that appeared to
be a combination of both the gold and plain glass surfaces.

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Acknowledgements
The author wishes to thank Dr. Yan Wu for her patience and help on this project. Additionally, help from
peers doing research in the University of Wisconsin - Platteville cleanroom was very nice in helping the
author 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 of
Wisconsin - Platteville cleanroom.

Appendix

Plain Glass Surface Topography

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Br Surface Topography Images

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Fluorine Surface Topography Images

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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 Tip

Charge Density Distributions
Plain Surface

Br Surface

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Fluorine Surface

Gold Surface

Works Cited
Debye Length. (2007, January 22). Retrieved December 19, 2012, from Duke University:
http://people.duke.edu/~ad159/files/p142/2.pdf

Boundary 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=2343

Asylum Research. (n.d.). iDrive™ Magnetic Actuated Cantilever . Retrieved from Asylum Research:
http://www.asylumresearch.com/Products/iDrive/iDrive.shtml

Asylum 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

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/asylumresearchafm

Herzan. (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%20MODELS

Interaction between fine particles. (n.d.). Retrieved December 2012, from http://www.mpip-
mainz.mpg.de/documents/akbu/pages/particles.htm

JPK 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.

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