Hydrophobic Materials and Their Effects on Drag Reduction

Hydrophobic Materials and Their Effects on
Drag Reduction

Mike Levine
Group 3: Mike Levine & P.J. Hognestad
Professor Gans, University Of Rochester Mechanical Engineering Dept.

4.29.2009

Abstract

The use of hydrophobic coatings has been increasingly used to reduce drag in a number of applications.
Experimental results have shown a substantial reduction in drag for turbulent and laminar flow ranges, with an
increased drag reduction at higher Reynolds numbers. [2-3] Our experiment confirms these trends for commercially

available hydrophobic coatings used as a marine hull coating.

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Introduction
A major source of energy loss in pipe systems is a direct result of drag, or the friction at the
solid-liquid boundary due to the no-slip condition. The use of hydrophobic materials at this
boundary introduces a reduced surface area of contact between the solid and fluid. Super
hydrophobic materials will trap a layer of air at the solid-liquid boundary, also called a plastron
layer. To be considered super hydrophobic, materials must exert a high droplet contact angle
typically greater than 150° [1]. The droplet contact angle is simply the angle made by a single
droplet of water and the surface in question. See figure 1 for an illustration of this.

Figure 1: water droplet contact angles image courtesy of Diamond Fusion international [4]

The introduction of this boundary has been shown to significantly reduce the drag of a fluid
flowing over it, up to 14% drag reduction for laminar flows [2] and 50% drag reduction for
turbulent flows [3]. Our experiments will be an attempt to further characterize the behavior of
hydrophobic coatings and materials with respect to drag reduction. Our experiment will examine
2 different hydrophobic coatings and a control set over a range of masses to determine drag
reduction potential over a range of Reynolds numbers.

Procedure

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In conducting our experiment, we first fashioned a cylindrical Plexiglas tank, approximately 1 m
tall, and 10 cm in diameter. Using two lasers and LED photo transistors with a USB-6009
interface and Lab view we were able to create a VI to act as a stopwatch. The system starts the
clock when the upper beam is interrupted, and stops the count when the lower beam is
interrupted. The virtual instrument can be seen in figure 3 below.

Figure 3: Labview Virtual instrument. Counts timed loop cycles to measure time.
The spheres used in the experiment were standard ping pong balls, of the single star quality
control tolerances. To utilize them in our experiment we created an equal sized hole (2-3 mm) in

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each ball with a soldering iron. This was done so that we were able to weight the balls, in
addition to providing a place to mount wire hooks during the coating process. To help reduce the
amount of lead shot added to each weight category the balls were filled with water. This also
eliminated the issue of trying to ensure the coated spheres were re-sealed. The coating process
was carried out by mixing the coating components according to the instructions listed
in appendix A, then by bending steel wire into hooks and inserting them into the tops of the balls
so that they could be dipped into their respective coatings. We were able to coat the ping pong
balls in 3 coats of Wearlon F-1M (Black) for the first group. The second group received 3 coats
of Wearlon Super F-6M primer and 4 coats of Wearlon Super F-6M finish (Blue), while the
control group used consisted of uncoated ping pong balls. Groups were allowed to dry between
coatings and allowed to cure for 48-72+ hours after either a primer or final finish was completed.

Once all coating had been completed, the wire hooks were removed so that the balls could be
weighed and then weighted accordingly to reach their target weight of 33, 38, 42, or 47 grams by
adding lead shot. To prevent the shot from falling out of the sphere a small dab of hot glue was
used to plug the hole made earlier on.

Once coated, the balls were dropped from a stationary release tube 5-10 cm above the
waterline (due to changing tank depth as spheres accumulate in tank), their times recorded. The
tank was emptied of all dropped balls, and refilled with water after each weight category was
dropped.

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Results

We were able to use formula 1 to calculate the drag force by subtracting our calculated buoyant
force (formula 2) from the known weight. This was used with formula 3 allowed us to calculate
and graph a measured Drag coefficient as a function of Reynolds number (see figure 4).

(1)

(2)

(3)

Figure 4 is graphing the average drag coefficient and average Reynolds numbers for each
weight category. Doing this helps reduce the noise in our data created across a wider range of
Reynolds numbers due to variations in speed for each combination of coatings and weights. In
doing this we can see that the Coefficient of drag for the F-6M coating is consistently lower than
that of the control group. With a drag coefficient ranging from 1.6-2.0 the F-6M outperformed
the control group which had drag coefficients ranging from 2.0 to 2.3. The data for F-1M shows
a similar range of drag coefficients (1.6-2.0) at the upper limit of Reynolds numbers that we
tested, around 20,000. At Reynolds numbers lower than about 15,000 the F-1M coating did not
show a significant reduction in drag coefficient

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Coefficient of Drag as a function of Reyolds Number (averaged)

2.4000

2.2000 y = -6E-09x2 + 0.0001x + 1.4192
2.0000 R2 = 0.5755

Drag Coefficient 1.8000

1.6000 y = 1E-12x3 - 5E-08x2 + 0.0008x - 1.9498 y = -2E-13x3 + 2E-09x2 + 9E-05x + 1.0646
1.4000 R2 = 1 R2 = 1

F-6M (Blue trendline) Uncoated (green trendline)

1.2000

1.0000 7000 9000 11000 13000 15000 17000 19000 21000 23000
5000

Reynolds Number

Uncoated Wearlon F-1M (Black) Wearlon Super F-6M (Blue)
Poly. (Uncoated) Poly. (Wearlon F-1M (Black)) Poly. (Wearlon Super F-6M (Blue))

Figure 4: Coefficient of Drag as a function of Reynolds Number (averaged for each weight
category)

Conclusions

The Wearlon F-6M coating showed a clear reduction in drag for the entire range of
Reynolds Numbers tested (roughly 10,000 – 20,000). There is an interesting change in the
data as the Reynolds number increase. Above a Reynolds number of about 15,000 the F-
6M increases its drag reduction relative to the control group. Additionally the data for
Reynolds numbers below 15,000 shows that the F-1M provides no discernable reduction in
drag, as it shares similar values for drag coefficients with the control group. This confirms
our expected results from other research mentioned earlier. The effects of drag reduction

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for this hydrophobic material are best seen in the upper half of Reynolds numbers, 10,000-
20,000.

We would recommend that further experimentation be designed to test drag
reduction for laminar flows, i.e. a wider and lower range of Reynolds numbers. In order to
eliminate potential avenues of error, such as wall effects, or variances in speed due to
object shape and weight tolerances we suggest the following for future work testing the
effectiveness of hydrophobic materials in drag reduction. A possible set up could be similar
to the labs used for marine testing. A floating “hull” of known geometry can be mounted to
measure the force of drag as water flows by in the channel. This would allow for consistent
and thorough data collection, greater control of the velocity, and therefore for more precise
Reynolds numbers over a larger range of values, including laminar flows.

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Works Cited

1. Wang, Shutao, and Lei Jiang. "Definition of Superhydrophobic States." Advanced
Materials 19 (2007): 3423-424.

2. Ou, Jia, Blair Perot, and Johnathan P. Rothstein. "Laminar drag reduction in
microchannels using ultrahydrophobic surfaces." PHYSICS OF FLUIDS 16 (2004):
4635-643.

3. Henoch, C., T. N. Krupenkin, P. Kolodner, J. A. Taylor, M. S. Hodes, A. M. Lyons, C.
Peguero, and K. Breuer. "Turbulent Drag Reduction Using Superhydrophobic Surfaces."
Proc. of 3rd AIAA Flow Control Conference, California, San Francisco. (2006)

4. LaCourse, Dr. William C. "Diamon-Fusion International: Test
Results." Nanotechnology, Glass Protection, Hydrophobic Coating and Protective
Coatings. 19 Feb. 2009
<http://www.diamonfusion.com/en/products/test%20results.html>.

5. Zimmermann, Jan, Felix A. Reifler, Giuseppino Fortunato, Lutz-Christian Gerhardt, and
Stefan Seeger. "A Simple, One-Step Approach to Durable and Robust Superhydrophobic
Textiles." Advanced Functional Materials 18 (2008): 3662-669.

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Appendices

Appendix A

Wearlon® Super F-1M - Anti-Fouling Marine Coating -

APPLICATION GUIDE AND TECHNICAL INFORMATION
Meets EPA, USDA, And FDA 21 CFR 175.300 Requirements

Surface Preparation

Wearlon® Super F-1M is a silicone-epoxy coating having unique release properties while
adhering to a broad range of substrates including wood, concrete, fiberglass, metals, and
most plastics including epoxies, polyurethanes, and alkyds.

For improved adhesion Wearlon® Super F-1M can be applied to treated surfaces as follows:
(Steel: SSPC 10. Aluminum: SSPC 7. Fiberglass: 80 grit sand. Concrete: 28 day minimum
cure and power wash using acid etch or shot blast.)

Mixing Instructions

Before mixing, shake or stir each component until homogenous. Wearlon® Super F-1M is a
2 component product packaged in 1 Gallon and 5 Gallon kit which contains the proper ratio
of ingredients. The entire contents of each container should be mixed together.

For quantities less than the pre-packaged kit, mix as follows: To 11 parts of the A
component, mix in 2 parts of the B component. NO INDUCTION PERIOD

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NECESSARY...Spray or Roll immediately.

Thinning and Clean up

It is not necessary to thin Wearlon® Super F-1M. Clean up by using soap and water while
Wearlon® Super F-1M is in an uncured state. For best results clean all equipment as soon
as the application is complete. After the material has cured you will need to use solvents to
remove the Wearlon® Super F-1M

Application and Recoating

For best results apply Wearlon on a hot sunny day or on a warm boat bottom to obtain the
maximum coating hardness. Allow to cure for at least 5 days before putting into the water.
For optimum speed and anti-fouling properties polish frequently with Speed Coat 49. Apply
Wearlon® Super F-1M with airless or air assist spray equipment. Improved speed through
water has also been obtained by applying Wearlon® Super F-1M using a 3/8” nap roller.
Wearlon® Super F-1M should not be sanded or abraded or cleaned with strong abrasives,
chemicals, or detergents. A light massage with plain water is adequate to remove any
fouling. For subsequent recoating the old Wearlon® Super F-1M should be sanded and
wiped down with Acetone prior to recoating. Shelf Life: 1 Year Storage: DO NOT FREEZE

Refer to Frequently Asked Questions (FAQs)
http://www.wearloncorp.com/index.php/faqs

Coating Type

Silicone/epoxy water based Color: Various (contact WEARLON® representative) Pot life: 60

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min. @ 68°F Induction Time: None Solids: by Weight 49% Coverage Rate: Approx. 200 sq.
ft. @ 3-4 mil DFT (85% Yield) Tensile Strength:>1750psi Elongation: ASTM
2370>5%Adhesion: ASTM D451>1000psi Abrasion: (CS 17/Kg/1000 cycles) <40 mg loss
Cure Time: Complete in 5 days. Dry to the touch in 2 hours. Force cure 300°F for 30 min.
Many applications can be returned to service the next day. VOC: ASTM 3960-1.0#/gl. Heat
Resistance: Do not exceed 275°F continuous service
CAUTION:
1. Wearlon® Super F-1M surfaces have a low coefficient of friction and therefore tend to be
slippery.
2. Wearlon® Super F-1M is not recommended as a material to prevent barnacle
attachment, but will assist in the ease of removing these attachments.
3. Wearlon® Super F-1M fades when exposed to UV light resulting in a slight yellowing
appearance. This does not change the performance properties. Always purchase pigmented
Wearlon® Super F-1 preferably dark colors to reduce the yellowing appearance.

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