Subject: Re: Elliptical oil drops
Date: Fri, 12 Sep 2003 02:49:24 GMT
Benjamin Weiner writes:
>> Eureka! These are the ones and they are, in fact, everywhere where
>> there is slow traffic and automobile engines are stressed (hot) so
>> that they drip.
> Interesting, I never noticed these, although I tried to look for
> them while riding today.
>> I find them mostly on narrow mountain roads where engines get hot
>> and all sorts of residual oil drops off, ...
>> Why are rain drops no more than about 3mm in diameter (unless in a
>> strong downdraft)? How are they limited to this size? If you can
>> visualize this process with a more viscous fluid (oil, paint or hot
>> roofing tar), then you should come upon it. The three mentioned
>> substances produce these rings as well.
> This, at least, I think I understand: raindrops are limited in size
> by an instability of the underside (heavy fluid overlying lighter
> fluid.) Surface tension can stabilize the short length scales but
> not longer, so there is a maximum size dependent on the surface
> tension of the fluid.
It is also evident from the pressure profile of a sphere in an
airflow, that the highest pressure is at the stagnation point head-on,
and the lowest pressure is at about the 1/4 points. This will cause
the development shown in the raindrop URL below.
> I had to do a little research to find the mode of breakup, but here
> is a link with a picture:
Eureka again. We now have the source, cause and pictures of
elliptical oil drop rings on roads.
> I was surprised that an oil ring could be as big as the one Dave
> Salovesh photographed. The characteristic size must depend on the
> surface tension of the fluid and quite possibly on the relative
> timescale for drop breakup and freefall from car to road surface.
With higher viscosity and surface tension than water, the last stage
of the raindrop-blowup, the thick collar closes to make a "soap
bubble." Those who observed soap bubble landings probably noticed
that soap bubbles burst leaving a wet ring.
The rest is left as an exercise for the student.
Good work. "Cigars" all around for the researchers.