Re: Why does cooking oil ripple when it is heated on a frying pan?

Well, convective energy transport has something to do with it - you are
looking at the tops of the convection cells in the oil. Convective energy
transport occurs because warm oil has a different density than cooler oil
and therefore rises - the convective liquid organises itself into cells
where there is upwelling in the middle and downwelling on the outside. Many
cells can form at the same time in a pan and the top of the oil surface
gets dimpled up where the oil is rising and dimpled down where the oil is
falling.

The really hard question is why you do not notice this phenomenon in other
heated liquids?

Thinking of hot water, it seems to me that the bubbles that form in boiling
water certainly spoil any chances you havce for seeing the dimples -
perhaps you can see the dimples before the water starts actually bubbling?
You will note that oil rarely gets so hot that it boils - this is because
the vapor pressure of oil is much less than that of water. You CAN get oil
to boil, but this is very dangerous to do as it probably flash ignites too
at that temperature - so do not try that one!

Another question is how many cells form? This must have to do with the
fluid properties of the oil - viscosity, density and surface tension - as
well as the temperature gradient from top to bottom of the pan. I should
think a dense viscous liquid will convect quite spectacularly!

If you wanted to experiment you could look at the surfaces of several hot
liquids and see which ones formed the surface patterns, and then see if
there is a connection between the liquid properties and the observations.

However, before you start heating gasoline or Napalm - take care and do
some thinking about which liquids to test: Do NOT ever heat acids or
flammable liquids! Nor things like mercury, ok? A good liquid to experiment
with is water - despite the problems of the bubbles noted above there is a
point just before boiling when convection sets in. Carefull observation of
the surface - perhaps at an angle with glancing light from the opposite
side will let you see the patterns, I think. You can then change the
density and viscocity of the water with various additives. Salt or sugar
will nicely dissolve in the water and at least change the density - perhaps
a chemist at your school can tell you about additives to water that will
change the viscosity without being flammable? Adding a little of passive
tracer such as aluminium powder, flour, or flower pollen will let you see
the motion of the liquid inside too.

Jearl Walker's "Flying Circus of Physics" book has several pages about the
convection phenomenon, as I recall.