1. The Effect of Different Geometries on Percolation in Two DimensionsBy
Allison Morgan
Dr. Alan Feinerman and Jared Weddell

2. Overview Introduction to Percolation Percolation Theory Project Goals
Experimental Design and Methods
Results
Conclusion

3. Percolation Theory
Percolation: the movement of a mass across a porous material [1].
Geologists study percolation to analyze the flow of fluid through micro-fractures in rocks [2].
Biologists investigate how drugs might diffuse through blood vessels [3].
[1] B. Last and D. Thouless: Physical Review Letters, 1971, 27, 1719 – 1722.
[2] B. Berkowitz and R. Ewing, Surveys in Geophysics 19, 23 (1998).
[3] J. Baish et. al., Role of Tumor Vascular Architecture in Nutrient and Drug Delivery: An Invasion Percolation-Based Network Model, Microvascular
Research 51, (1996).

4. Percolation Theory Continued
Percolation threshold: a critical fraction of area where a medium can still be conductive or fluid can still diffuse [4].
To treat solid cancerous tumors, doctors would like to observe how blood carries drugs: this process can be modeled with fractals and percolation clusters.
At the percolation threshold, qualities like the blood flow rate, oxygenation rate, and drug delivery rate approach zero [3].
[4] A. Hunt, Percolation Theory for Flow in Porous Media, (Springer, Berlin Heidelberg 2005).

5. Project Goals
Establish a better experimental set-up based on previous work.
Determine the percolation threshold for rectangle-shaped holes.
Examine how experimental rectangle data compares to previous experimental data for ellipses.

6. MatLab Output
The shapes are randomly distributed and oriented using MatLab. AutoCAD then visualizes the design.
Explain magenta lines
… this design will be cut onto aluminum mylar which we measure current across and will be described with more detail in the next slide
Percolated region
Effective area region

7. Current Experimental Setup
Aluminum baseplate*
Layer of double-sided adhesive
Brass rods
Acrylic fixtures and nylon screws*
Aluminum baseplate can fit two runs &
Two sheets of Mylar
found way to roll mylar out smoother
New nylon screws to avoid shorts
Layer of Mylar with conductive aluminum
* New items

8. Evaluation of Kerf
The error due to Weddell et. al.’s experimental setup was the result of underestimating the width of the laser’s cuts (kerf).
Previously, the width of the laser was estimated to be in a window of values.
We determined the kerf of the laser to be 114 microns.
Placing the rows a specified distance apart
Circles were chosen because it was consistent with our testing method
Kerf

9. Experimental Process
At the same time the laser is cutting, the current across both regions is measured.
As the number of holes increases across the left square, resistance increases.
A portion of the right square is eliminated every time the area on the percolated square is decreased by 1%.

10. Electrical Model Definition of resistance: ρ = resistivity
L = length over which the current is measured
τ = thickness of the conductive sheet
Initial resistance : Ro = (ρ * L) / (Ho * τ)
H0 is the initial height of the sheet
Final resistance: Rr = (ρ * L) / (Hr * τ)
Hr is the height remaining across
effective area square when the other
square has been fully percolated.
The percolation threshold is
Hr / H0 = R0 / Rr = Ir / I0 L H0 Hr

11. Example of Experimental Output
Raw data from 1500 circular cuts. The experimental percolation threshold was I0
Pc = Ir / I0 Ir
Percolated square
Effective area square
** Area removed is a non-linear function of time.

12. Validation of Testing
Averaged 5 runs of roughly 1500 circle shaped cuts.
Theoretical percolation threshold is 0.33 [5].
Previous experimental results yielded [6].
Experimental percolation threshold was ±
Because rectangle shaped holes have never been experimentally done before, we ran control tests with circles.
[5] B. Xia and M. Thorpe, Physical Review A 38, 2650 (1988).
[6] J. Weddell, A. Feinerman, Percolation Effects on Electrical Resistivity and Electron Mobility, Journal of Undergraduate Research, 5, 9 (2011).

13. Experimental Rectangle Results
The percolation threshold results below are an average for at least 4 runs of data at each aspect ratio rectangular cuts were made for each trial.
1.000
Aspect ratio
Percolation threshold for rectangles (± STD )
Percolation threshold for ellipses [6]
1.0000
0.496 ± 0.077
0.351
0.7000
0.533 ± 0.046
0.392
0.6000
0.492 ± 0.039
0.414
0.2500
0.563 ± 0.024
0.588
0.1000
0.697 ± 0.113
0.723
0.0125
0.969 ± 0.008
0.923
0.2500
Here is our data
First note the effects of changing aspect ratio
Overall trend in comparison to ellipses.
0.6 aspect ratio is low.
0.1000
Rectangles with aspect ratio 1, 0.25, and 0.1
[6] J. Weddell, A. Feinerman, Percolation Effects on Electrical Resistivity and Electron Mobility, Journal of Undergraduate Research, 5, 9 (2011).

14. Relationship Between Rectangles and Ellipses
Discuss overall trend
Describe leftmost point in agreement
Righter points not as much
The inconsistency is mostly due to randomness of these networks?
The percolation threshold is affected by geometry at high aspect ratios.

15. Conclusions
The new experimental set-up for measuring the percolation threshold is in agreement with prior published results.
For ellipses and rectangles, both follow the same trend: percolation threshold increases as the aspect ratio decreases

16. Conclusions
For aspect ratios less than or equal to 0.25, where both shapes are similarly stick-like, percolation thresholds cannot be differentiated.
For larger aspect ratios, where the shapes are more defined, the percolation thresholds for rectangles and ellipses begin to deviate.

17. Acknowledgments
The financial support from the National Science Foundation, EEC-NSF Grant # is gratefully acknowledged.
Dr. Alan Feinerman
Dr. Gregory Jursich
Dr. Christos Takoudis
Dr. Prateek Gupta
Jared Weddell
Ismail Mithaiwala

18. Appendix

19. Relationships Between Rectangles and Ellipses Continued
Professor Feinerman doesn’t get this slide. Maybe we can’t make this model with so few points between and -4.5.
The best-fit model shows percolation threshold as a function of log of aspect ratio.