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Subject: Re: Elliptical oil drops
Message-ID: <Uua8b.20899$>
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.

Jobst Brandt

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