What makes your knucles crack

Douglas A. East dougeast at voicenet.com
Tue Dec 9 09:22:16 EST 1997


The rate at which nitrogen will come out of solution is slow compared to
this phenomina.  I rather suspect that the water in the liquid
evaporates.  

Since evaporation only occures across a liquid-vapor boundry, and since
there is no such boundry at the start of the cracking procedure (no air
in the joint), the liquid pressure must drop well below the boiling
point for the 98.6F temperature of the liquid.  Once the bubble forms,
it grows quite rapidly.

In Fluid Dynamics, this is referred to as "cavitation".  Damage to the
metal parts in the fluid usually is traced to high stresses formed when
the bobble collapses again.  Can give pitting, for instance on ship
propellers or pump impellers.

The cavitation noise on submarine propellors was a significant source of
noise, and was initially used to search for and track other subs.  We
have figured out now how to avoid this source of noise by good propeller
design.

Do you damage your knuckles by cracking?  Dunno, but the possibility is
there over time.







Burki wrote:
> 
> Terry Brooks wrote:
> >
> > Can anyone tell me what makes your knuckles crack or where I might be able
> > to find this info.
> > Thank you
> 
> 1. What has this to do with microbiology?
> 
> 2. Our orthopedic surgeons *believe* in the following:
>    Nitrogen is dissolved in the synovial liquid. When you extend a
> joint    you lower the pressure inside, thus causing the N2 to go into
> the    gaseous phase (cf. a bottle of soda). This happens explosively,
> hence    the sound. Funny, isn't it?
> 
> --
> Didi Burki, Berne/Switzerland
> (to reply please replace amm by imm)





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