They experimented with a thin film of oil coating a sapphire slice. This plate was suspended above another sapphire plate, with air in between, and the temperatures of the two plates were controlled by water baths. The team used sapphire because it is a good conductor of heat. When the sapphire plates were both at the same temperature, the oil dripped. When the lower one was hotter than the upper one by some critical amount, the oil stayed flat and drip-free.
Heat lowers the surface tension of the liquid film.
So, when the film starts to thicken, bringing the surface closer to the lower plate, it warms up and its surface tension decreases. Thin regions further away from the hot plate have a cooler surface and hence a higher surface tension. High surface tension tugs liquid away from regions of low surface tension. So the cool, thin parts of the film pull the thick parts flat again. For an air gap just over a quarter of a millimetre thick between the lower plate and an oil film an eighth of a millimetre thick, a difference of about 10 o C prevented drips, Burgess' team report in a forthcoming edition of the journal Physical Review Letters 1 .