Ask A Scientist Chemistry Archive

name         Dave
status       other
grade        other
location     TX

Question -   I've seen small clear plastic packets of clear liquid
with a small metal clicker disc inside. The liquid crystallizes and
releases heat when you click the metal disc. Then you just boil them and
let cool to reset. They stay a liquid until the 'clicker' is clicked a
few times. How do they work? What is the 'trigger' mechanism? A shock
wave, a voltage, is the clicker a special metal?
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I think the shock wave from any toy metal clicker is quite enough to 
initiate crystallization,
releasing that latent heat.
Any shock which causes even a little cavitation causes high peak energies
where and when the cavity collapses.
These peak energies can provide the energy of crystal-nucleation.
I am wondering does it obviously crystallize from the center of the 
clicker outward,
or does the sound-wave somehow cause dispersed nuclei throughout the bag?
In an ultrasonic bath, reflections off the walls and top surface cause 
standing-wave peaks
and cavitation at multiple points in the bath.

Confirming tests:
     Try briefly touching the packet to the surface of the water bath in 
 an ultrasonic tank.
If shock is the mechanism, then that will trigger the packet too.
Laying a short segment of iron re-bar sideways against the packet and 
rapping the end with a hammer might do it too.
(Or any hefty piece of metal, even a thick aluminum plate).
However, the compressive elasticity of polyethylene plastic may tend to 
soften externally applied shocks.

A piezo-spark igniter, such as in a barbecue igniter or hand held 
stove-lighter ,
will provide a locally violent electrical impulse which also should be 
enough to start crystallization.

Take apart an empty hand held stove-lighter sometime.
You will find a little half-inch-long piezo-electric crystal rod,
mechanically somehow rapped by the trigger,
with a metal wire going to the igniter tip.
Your crystal-heat's clicker must conceal one of these piezo-crystals if it 
is electric.

A sonic clicker is cheaper than a piezo.  Does not need the special crystal.
So my first guess is that your clicker is sonic.
If the metal is sculpted to have a concave dimple in it somewhere,
that's even more likely to provide a focused shock wave in the liquid at 
the spherical center of that dimple.
If it clicks by two parts hitting each other, cavitation would probably 
occur at the point of impact as they first bounce apart.

Chemically special materials are not likely to be the cause.
Any metal has an sonic impedance and velocity higher than those of the 
water-based liquid;
these help make the shock-wave strong and spatially focusable.

Jim Swenson
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