When I have my driveway seal coated, it turns blue a few days after it is done and then turns back to black. I've heard it called "blue flash." What actually happens chemically to cause this?
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2 Answers
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It's not a chemical reaction, AFAIK, but due to the Tyndall effect or Rayleigh scattering from the fine particles of the asphalt-water emulsion as water evaporates and the size and/or dispersion of the asphalt particles change.
It is also possible that interference of light in an adhered water layer plays some part. This is the same effect that gives soap bubble their rainbow colors, which change as the bubbles dry -- note that just before it pops from drying, one area of a bubble may go colorless due to thinning of the film beyond the range of visible light interference.
answered Sep 7, 2016 at 23:42
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$\begingroup$ Thanks for your insight. Curious Fun question.. Why does it turn blue and not green or purple or pink? $\endgroup$– JennyCommented Sep 19, 2016 at 14:19
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$\begingroup$ Most likely because it is the scattering, rather than interference. The longer wavelengths pass through, and shorter are scattered more, but both scattered and transmitted rays are mixtures of various colors. Put a drop or two of milk in a glass of water, and shine a flashlight through. Looking into the beam looks orange-red, from the side, blue. $\endgroup$ Commented Sep 19, 2016 at 22:47
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$\begingroup$ Chemistry is so much fun. I will do the milk experiment with my son. Thanks :) $\endgroup$– JennyCommented Sep 21, 2016 at 15:51
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It's easy to see the effect with a white emulsion. Milk, especially skim milk when diluted with water will look blue to scattered light (and orange to transmitted light (looking thru milk into light source)). This is the Tyndall effect, aka Rayleigh Scattering. I have some doubt that this is what you're seeing. Without seeing it myself, or getting a better idea of what you mean by "blue" (and black), my first guess is that a BLACK asphalt emulsion will absorb too much light to ever turn blue. So, if the material is black initially, it should stay black (assuming the surface didn't get wet and that the coating didn't separate as it dried, squeezing the white-ish latext up to its surface - either of these situations could produce Tyndall effect reflection) So, if neither of those things happened then the only explanation I have for a black coating to change to blue then back to black would be interference colors. Interference colors (like many butterfly wing colors) are very sensitive to thickness so generally you get more than just one color - AND the color you get depends on the viewing angle compared to where the Sun is. This would be the same effect as that you would see if you put a drop of oil onto the surface of a pool or container of water -in general the colors aren't uniform. So, I have no 'good' answer to your question, but hope that the above helps YOU figure out what it is you saw.
I should add that it is barely conceivable that there were light stabilizers (UV stabilizers) in the mixture (although most asphalt emulsions won't contain such expensive ingredients) which might have separated out and turned the surface to a different color until they fully deteriorated or were washed away. This would be a real chemical reaction. I mention it to be thorough, I'd be quite surprised to ever encounter such a product...and frankly I'd worry about what I'd bought if I did see it.
