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Ted Mooney, P.E. RET
Pine Beach, NJ
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for Metal Finishing since 1989
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Aluminum coating for good surface wetting with (bidistilled) water
Q. I'm looking for an aluminum coating which reduces the surface tension of water. The goal of the aluminum surface (in our case an air heat exchanger) is to bring water via capillarity to a sponge.
For this, I need a contact angle between (bidistilled) water and aluminum of less than 90°. The contact angle between water and non-coated aluminum is about 89 deg. I red about contact angles of 64 deg to even 5 deg.
Another constraint we have, is that the life-time of the coating should be above 10 years AND it should also be possible to be used in space (non flammable, non toxic in vacuum).
Ann Delahaye- Kruibeke, Belgium
What kind of environment would this coating be subjected to for the ten years?
Tom Pullizzi
Falls Township, Pennsylvania
A. I think the issue here is not to reduce the surface energy of water but to increase the surface tension of the aluminum. I would think a thorough anodizing should do the trick, as you should expect a critical surface tension greater than water, thus making the interaction of the surface & the water hydrophilic. Roughing the surface by mechanical means may help as well.
At the other end, a coating of titanium dioxide will increase the critical surface tension to 91, a ferric oxide coating to 107 or a tin oxide coating to 111. Wettability is primarily a matter of making sure the critical surface tension of the substrate is greater than the critical surface tension of the liquid. Dry aluminum has a critical surface tension of about 45, meaning it is not normally wetted by water.
Dale WoikaSurface Conversion Sciences - Bellefonte, Pennsylvania
A. Dear Delahaye:
We have a similar question as you showed. My project is to get the surface tension of aluminum/oil system at the room temperature. According to the Young Equation G(s,v)=G(s,l)+G(l,v)cos(angle),which G express the surface tension. s express the solid,v express the vapor, l express the liquid. the G(l,v),which is the surface tension of oil(liquid and Air) G(s,l) expressed the surface tension of aluminum(solid) and oil(liquid). The G(s,v), which express the surface tension of aluminum(solid) and air. From the equation, the G(l,v) can be obtained from the pendant drop method, Angle can be obtained from sessile method.In order to obtained the G(s,l),the G(s,v)have to first to be known.
So the question I have is what is the G(s,v) at room temperature. If I get it, I can further calculate the surface tension of aluminum/oil.
Stanley Tsaioptoelectronics - Hsin-Chu county, Taiwan, ROC
Q. Dear Dale,
Thank you for your answer on my question! We tried the TiO2 coating and the result is fabulous! But, it seems that the life time is not very long. After wetting with water on regular intervals during 2 month, the hydrophilic characteristics were already worse. Is there a method to keep this coating hydrophilic?
Ann Delahaye- Kruibeke, Belgium
A. Dear Ms.Ann Delahaye,
The Dehumidifying(or Wetting) method of aluminium coating that you have been seeking can be solved by the way of One layer or Two layer system of hydrophilic coating for aluminium fin stock because of life time (duration) to be short. 7 degree of contact angle is accepted. There are two methods, the one is pre-coating method (coil coating) and the other is post coating method(immerse coating).
Mr. Paul Haeng Shik, Choi- Seoul, Korea
Q. Dear Ms. Ann Delahaye,
I know this is a long shot. However, what procedure did you use to apply TiO2 to your parts? I too am looking to make an aluminum part hydrophilic and see that one can apply TiO2 by ion assisted deposition or by paint.
- Brook Park, Ohio
2006
A. You might look at lithographic printing where an aluminum substrate is made hydrophilic, and an overcoat of copper, or a polymer is made hydrophobic. I was using the search engine BING (Canadian), and searched "Lithography - is aluminum hydrophobic?". One item that showed up is "Hydrophylic Properties Overview - aluMATTER / Aluminum ...". The incomplete web address starts aluminum.matter.org.UK/Content/html/... Essentially, in its discussion it states that the aluminum area of lithographic printing plates are made hydrophilic by the technique of AC-electrograining or AC-graining. Nitric acid or hydrochloric acid in a bath attacks the aluminum plates as an alternating current goes through the bath to make fairly uniform size, depth and spacing pits that help to make the aluminum area of the printing plate more hydrophilic. If this is of interest, then try to find a printing plate manufacturer to see how they do it.
Colin Carnwath- Brantford, Ontario, Canada
November 30, 2014
Hi Miss Ann.
I am in HMX heat mass exchanger (cross-flow and counter-flow).
I have tried a roughening method on aluminum surface using orbital sander and sanding paper 40 Grit.
The result is not bad especially when I use a surfactant agent to promote the adhesion of water on treated aluminum surface and reduce the surface tension.
Once the surface is saturated with water film, the evaporation effect becomes superb but of course not as good as hydrophobic-phillic membrane.
Ciao,
- Doha qatar
February 8, 2015
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