Oil/water formations and forces

[oisalex]

Heat a saucepan of water and add uncooked rice. When the water is hot add a vegetable oil and stir. Soon the oil starts to form a perfect circle in the centre of the pan and floating grains of rice adhere to the rim of the oil circle. The rice grains rotate with the oil, apparently attached to the rim of the oil slick.

1) Why does the perfect circle of oil form and
2) why do the rice grains adhere to the rim and
3) why do the grains circulate with the oil, apparently fixed to the rim of the oil slick?

[mr_pointment]

1) Due to the rotation of the earth the boiling water will rotate in a clockwise direction contrary to the direction of bathwater down the old plughole as everybody knows.

2) Starch is a very strong adhesive (stick your balls down)

3) Dont understand this question.

Are you a young budding chef oisalex ?

[brachiopod]

For a moment there, mr_pointment, I thought you were being serious. Still, newly black and all that,,,

Anyway, to answer the question - it's all down to convection currents, density, and starchiness.

Due to the nature of the heating of your pan, the hottest water is at the outer edges. This less dense hot water rises, reaches the surface, and cools, and so wants to sink again. Meanwhile, newly heated water at the edges of the pan rises up to replace it, meaning the cooler water sinks at the centre of the pan. (think about a gas or electric ring - there is no heating in the centre).

This is called a 'convection current' or 'convection cell'.

Your oil, being less dense than water, floats on top and congregates at the convergence point in your sos'pan's convection current.

Your rice is denser than water, (it sinks after all). Although much rice circulates round in the currents, some will come into contact with the hottest parts of the pan (the outer base and sides), and just stick !!