# Homework Help: Sound when a droplet of water or other liquid falls on a hot surface

1. Jan 27, 2005

### Kikki

I was wondering whether anyone could give me a bit of a point in the right direction for a physics assignment I have been given. The Question is:

When a droplet of water or other liquid falls on a hot surface, it produces a sound. On what parameters does the sound depend?

Any thoughts?

Thanks, Kikki.

2. Jan 27, 2005

### maverick280857

Possibly a handwaiving argument but I'd say,

(A) mass of water droplet, impurities in water
(B) temperature difference between water droplet and hot surface
(C) emissivity of hot surface, rate of change of temperature with time
(D) height through which the droplet falls
(E) surface tension of water droplet and how viscous the water is...
(F) ambient characteristics -- more precisely ambient temperature, composition of environment, air resistance
(G) shape of metal surface (curved would be a better reflector of sound waves and so on...along the same lines as a satellite antenna...)
(H) measurement method...(and remember Heisenberg's Uncertainty Principle... )

Sorry for sounding ridiculous.....

Cheers
Vivek

3. Jan 27, 2005

### Andrew Mason

What causes the sound? It seems to me it must be the result of water vapour building up under the water droplet (as it spreads out across the surface) and then escaping rapidly when the drop is sufficiently spread out. So it is like the pop of a balloon. If that is what causes the pop sound, it seems to me that it would depend on the amount of pressure built up in the time it takes for the drop to flatten out.

Pressure is proportional to the quantity and temperature of the vapour. The amount of water turned to vapour and its temperature is proportional to the amount of heat that is transferred to the drop from the surface. This is a function of the heat conductivity of the surface material and its temperature. Heat capacity (specific heat) would be a factor as well as the temperature will drop due to the evaporation of the water.

Also the time it takes for the drop to flatten depends on the smoothness of the surface. The smoother the surface, the quicker it flattens and the lower the pressure build up.

So for maximum pop, use a rough surface with a high heat conductivity at a high temperature. It also has to have a heat capacity that is sufficient to ensure that the drop does not lower the surface temperature appreciably.

AM