Re: Zero a or zero g?


Andy Resnick wrote:
> Michael Mcneil wrote:
>
> > Watching a fountain throw a jet of water an hundred feet or so, straight
> > up in the air, it struck me that the globules don't begin to fall apart
> > until after they have reached the zenith.
>
> Doubtful: your eye simply can't discern the individual droplets.
> There's a common demo in science museums of a stream of water and a
> strobe light which shows droplet formation very early in the stream.
>
> The Rayleigh stability of a liquid jet states that the longest stable
> fluid column is 2*pi*(jet radius), the source of the instability is
> surface tension. The wavelength is set by surface tension, for water in
> air the critical wavelength is about 1 cm. Viscosity sets the time
> scale for pinching, which is fast for water. Water is about 1 cSt = 1
> mm^2/s kinematic viscosity, so a 1 cm radius jet will pinch off in about
> 0.01 second.
>
> > This is true no matter how high the jet reaches. Bigger globules fall
> > apart soonest. I imagine this is due to the greater amount of air
> > resistance. So what holds them together all the way up?
>
> Surface tension. For water in air it's 72 erg/cm^2. The pressure
> difference across the surface of a fluid sphere is about s/r, where s is
> the interfacial energy and r the radius. Large spheres are easy to
> break up, small spheres will persist.
>
>
> > Is it something to do with the change of direction or is it the free
> > fall?
>
> No. The effect of hydrostatic pressure for a (small) drop will be small
> compared to the other forces, such as inertial.
>
>
> --
> Andrew Resnick, Ph.D.
> Department of Physiology and Biophysics
> Case Western Reserve University

Thanks, Doc., any chance of a translation?

I noticed that the stuff 1cm or smaller lasted all the way down.

Can you go over the bit about jet diameters once more.

.

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