Wednesday, September 07, 2011

Coffee Ring Formation and Rheology

I wasn't able to comment much last week about the recent discovery that had quite--a bit--of buzz (pun intended) on coffee ring formation (subscription required), but I think my perspective might make it worth the wait, as just like yesterday's discussion of a topic far removed from polymers (aspirin), it will definitely be related to polymers and rheology. As any of the links above will tell you, coffee ring formation has been a puzzle for a time - the question being why does the stain form as a ring at the outer perimeter and not as a uniform stain under the entire cup? The answer involves quite a bit of transport phenomena of the three most common types - mass, momentum and heat transfer, with evaporation being greatest at the staining location, encouraging the particles in the coffee to deposit their. While this has been known for some time, the researchers here found a new key variable - that particle shape has tremendous impact on ring formation, and that elongated particles do not form a ring pattern, but rather form a uniform staining pattern. The picture below from the article shows the particle shape above the corresponding stain pattern.

The capillary forces between the different particle shape is different and that difference is enough to totally change the outcome of the drying pattern. The authors suggest in the article that surface roughness may induce strong capillary effects and these results could be relevant to ink jet printing, which I can certainly see as being an issue (a more uniformly drying drop would result in a better image), but getting back to the idea of surface roughness had me thinking another thought: painting, as in painting the walls of your house.

If you ever undertake repainting the walls of your house, you'll find that flat (matte) paints are much easier to work with. As you move from section to section, you don't need to be overly concerned about keeping the edge of the paint wet - my wife and I have stopped for long periods of time in the middle of a wall and started right up again without any issues and trust me, she is plenty picky about these issues. In the same vein, touching up the paint, even months or years later is easy. But as you go to increasingly glossier finishes (eggshell to satin to gloss), these issues become much larger - we've tried glossy paint once and will never do it again, despite the fantastic scrubability. You could easily see how each section of the wall was divided up as we move along, and I suspect that it is probably for a reason related to this research - the particles in the glossy paint favor a ring formation type mechanism is seen in coffee.

So my question is this: Could the use of modified particle shapes in paint be useful in creating glossier paints that won't show how the wall was sectioned off? The challenge of course, will be changing particle shapes without monkeying with the other rheological aspects of the paint.
Yunker, P., Still, T., Lohr, M., & Yodh, A. (2011). Suppression of the coffee-ring effect by shape-dependent capillary interactions Nature, 476 (7360), 308-311 DOI: 10.1038/nature10344

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