Static Force Curve Activity in Nanofluidic Channels How various treatments effect the behavior of nanofluidic devices

Jon Zickermann University of Wisconsin-Platteville

Agenda

Background

AFM basics

Surface Topography

Force Measurement

Procedure

Goals of Project

Results

Surface Roughness

Force Curves

Analysis and Discussion

Conclusions

Background

Background

Research a part of the Microsystems and Nanotechnology Minor offered by UW-Platteville

GE4000 Research in Microsystems and Nanotechnology

The “capstone” for the minor

Worked with Dr. Yan Wu

Nanochannel samples were fabricated by Shaurya Prakash at Ohio State

Selected this project to do research on an Atomic Force Microscope and interest in micro/nanofluidics

Background

Project Members

Dr. Yan Wu

Ph.D., University of Illinois at Urbana-Champaign, M.S. The University of Alabama, B.E., Tsinghua University, China

Joined UW-Platteville staff in 2009

Shaurya Prakash

Assistant Professor, Mechanical & Aerospace Engineering at the Ohio State University

Jon Zickermann

B.S. Mechanical Engineering, Microsystems & Nanotechnology; University of Wisconsin-Platteville (Expected)

Transferred to UW-Platteville in Spring 2010 after receiving Associate’s in Arts and Science at UW-Washington County

Dr. Yan Wu

Jon Zickermann

Shaurya Prakash

Project Goals

Understand the operation principle of dynamic AFM imaging and static force curve measurements

Learn the impact of surface treatment of micro-nanofluidic channel wall on slip flow and electrokinectic flow

Perform surface topography measurements and surface roughness measurements using AFM inside nanofluidic channels

Calculate charge density distributions

Atomic Force Microscopy Basics

Surface Topography

Atomic Force Microscopes (AFMs) can allow imaging at the nanoscale – beyond limits of optical imaging

Analog to a finger feeling the surface

Two basic modes: Contact Mode and Tapping Mode

“In touch with atoms,” G. Binnig, and H. Rohrer, Reviews of Modern Physics, Vol. 71, No. 2, 1999http://www.tut.fi/en/units/departments/physics/research/computational-

physics/surfaces-and-interfaces-at-the-nanoscale/research/

How it works

“AFM and Combined Optical Techniques” Nicholas Geisse, Asylum Research

Basic Contact vs. Tapping Mode

“Advanced AFM,” Dr. Yan Wu, 2011

Basic Contact vs. Tapping Mode

“Fiber optic atomic force microscope,” http://physics-animations.com/Physics/English/afm_txt.htm

Detailed Contact vs. TappingCONTACT MODE TAPPING 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 substantial, constant force on the sample

The probe moves with a small vertical (z) oscillation (modulation) which is significantly faster than the raster scan rate.

This means the force on the sample is modulated such that the average force on the sample is equal to that in contact mode.

When the tip is 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

Interacting Forces

Summary of Interacting Forces

Force Calculation

Force Calculation

Determining spring rate from F = ks:

Sader Method:

where:

*All equations and constants courtesy of Asylum Research

http://www.asylumresearch.com/Applications/EquationCard.pdf

Equipment - iDrive

The iDrive NbFeB magnet is fully enclosed and sealed within the cantilever holder which allows for unobstructed bottom view of samples and prevents sample contamination.

iDrive allows auto tuning of the cantilever in fluid.

The cantilever tune with iDrive actuation closely resembles the thermal tune.

Clean cantilever tunes allow for the implementation of Q-control and other techniques in fluid.

iDrive cantilever holder (left) and schematic diagram of the cantilever which shows the

Lorentz Force exerted onto the cantilever (right).

Equipment - iDrive

Equipment - AFM

Asylum Research MFP-3D-BIO AFM

Specs:

Most accurate and sensitive AFM available with inverted optical microscope

Inverted microscope allows for fluorescence microscopy and many other types of optical investigation including Raman, ANSOM, and most other optical microscopy techniques (DIC, TIRF, etc.)

90 x 90 µm maximum window (0.5 nm resolution)

5 µm Z axis range (0.25 nm resolution)

Fully-enclosed in an acoustic chamber and placed on top of an active vibration-damping table

Voltage noise <70μV in a bandwidth of 1Hz to 10kHz.

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

Equipment – Tips for Force Curve

SiNi Triangle Tip Spherical Tip

0.05N/m Cantilever

5µm SiO2 Glass

Au surface

Nanofludics

Nanofluidics Basics

Definition: any liquid system where you have the movement and control over liquids in or around objects with one dimension at most 100 nm

Dimensions can be typically 10-50nm (Mukhopadhyay2006)

Applies to fluids inside nanoscale channels, porous alumina and nanoscale conduits

“As long as a hollow structure has a dimension on the nanoscale and can handle fluids, it qualifies for nanofluidics”

http://www.nano.org.uk/news/914/

Nanofluidics Applications

Primary applications: separation and analysis of DNA strands

Other uses:

Diodes

Field effect transistors

Lab-on-a-chip for nanoscale

Critical dimensionless parameters as specified in Oosterbroek (1999)

Bhushan, Wang (2010)

Nanofluidics

Nanofluidic Dynamics

Nanofluidic Dynamics

Fabrication

Top-down methods

Photolithography methods on a substrate silicon wafer

Can be integrated on a MEMS chip on one wafer

Traditional top-down methods offer an economical method to nanofluidic device fabrication

Bottom-up methods

Self-Assembled Monolayers can be used with biological materials to form a molecular monolayer on the substrate

Carbon Nanotubes offer a future option

Bottom-up methods can precise shapes at the nanoscale

Nanofluidics Advantages and Disadvantages

ADVANTAGES DISADVANTAGES

Offers the possibility to confine molecules to very small spaces and subject them to controlled forces.

Potential for precise control of liquid flow and molecular behavior at the nanoscale

Harder to fabricate

Higher tendency for channels to get clogged

Lower signal quality when trying to send voltages

Relatively unexplored area of nanotechnology

Procedure

Procedure

Surface topography of 3 samples using AC mode

Measured in the three segments

Force curve analysis in air and water

Using iDrive cantilever tips and

Electrolyte solution creation

Force curve analyses in electrolyte solutions of various pH levels

Results

Results – Surface RMS

Plain Glass

80nm: 950pm

250nm: 1.549nm, 1.421nm, 972pm

450nm: 589pm, 1.113nm

Bromine Treated

80nm: 2.11nm, 1.103nm

250nm: 1.549nm, 1.421nm

450nm: 1.91nm

Fluoride Treated

80nm: 4.926nm

250nm: 3.912nm, 5.318nm

450nm: 3.422nm

Fluoride Treated Sample from AFM

Force Curve Analysis

SiNi Tip

Calculated Results – Force Curves - SiNi

Plain Glass Sample:

k = 83.32mN/m

Q = 15.2

freq = 31.267kHz

Bromine Sample:

k = 87.26mN/m

Q = 15.2

freq = 30.947kHz

Fluorine Sample:

k = 85.29 mN/m

Q = 14.7

freq = 30.733kHz

Calibration

Calculated Results – Force Curves: SiNi Tip

Plain Glass Sample:

Adhesion Data:

µ = 6.30nN

σ = 0.078nN

Bromine Sample:

Adhesion Data:

µ = 21.97nN

σ = 0.405nN

Fluorine Sample:

Adhesion Data:

µ = 1.06nN

σ = 0.144nN

iDrive Cantilever Tip from Asylum Research’s website

In Water

Calculated Results – Force Curves

PLAIN GLASS FORCE CURVE

Calculated Results – Force Curves

BROMINE FORCE CURVE

Calculated Results – Force Curves

FLORO FORCE CURVE

Force Curve Results

Spherical Tip

Calculated Results – Force Curves: Spherical

Plain Glass Sample:

k = 87.34 mN/m

Q = 25.0

freq = 21.336kHz

Bromine Sample:

k = 84.82 mN/m

Q = 25.1

freq = 21.319kHz

Fluorine Sample:

k = 89.85 mN/m

Q = 24.9

freq = 21.568kHz

Calibration

Calculated Results – Force Curves: Spherical

Plain Glass Sample:

Adhesion Data:

µ = 27.60nN

σ = 0.024917nN

Bromine Sample:

Adhesion Data:

µ = 18.05nN

σ = 0.001897nN

Fluorine Sample:

Adhesion Data:

µ = 15.33nN

σ = 2.750`nN

Calculated Results – Force Curves

PLAIN FORCE CURVE

Calculated Results – Force Curves

BROMINE FORCE CURVE

Calculated Results – Force Curves

FLORO FORCE CURVE

Charge Density

Charge Density – Plain GlassCharge Distribution Charge Values

Charge Density – Br TreatedCharge Distribution Charge Values

Charge Density – Fluorine TreatedCharge Distribution Charge Values

Charge Density – Gold SurfaceCharge Distribution Charge Values

Analysis and Discussion

Surface Roughness

The untreated samples were the smoothest – around 1nm RMS – followed by the Bromine and Fluorine samples

• Untreated nanochannels favors flow by pressure gradients

• Fluorine nanochannels favors flow by electric differentials

Force Curves Data

Bromine treatment produces a positive charge buildup that strongly attracts electrical charges, whereas fluorine treatment produces a repulsive force that resisted the cantilever tip

Stronger attraction forces from spherical tip compared to triangular tips

Analysis and Discussion

The bromine treated surface reach far from the substrate surface as indicated by the large Debye lengths

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

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

Acknowledgements

Dr. Yan Wu for working with me and helping me out

Peers doing research in the cleanroom from helping me in the first week

You, the audience, for listening

Thank You!Any Questions?