the film

the bubble on the rim of the glass is a sheet of water with two skins. the skins are made of soap molecules, lying flat, oil tails facing outward, polar heads facing the water between them. the whole sandwich is two hundred nanometers thick. seven hundred molecules of water deep. less, by the second, as gravity drains the middle down toward the rim.

the colors you see are not pigment. light enters the top skin and reflects from the bottom skin, and the two reflections meet — bright where the path difference equals a wavelength, dark where it equals half. yellow at one thickness, blue at another, magenta at a third. the colors are a measurement. the film is reading itself out in light.

the film is draining. gravity pulls the water in the middle downward, the top thins first, so the colors band: pale at the top, deeper toward the rim. the bands drift. watch one for a minute and they move like a slow tide. the band you saw at the top is at the rim now.

at thirty nanometers the colors run out. there is no more wavelength to bounce against. the film goes black — newton black, after newton, who measured it on a glass plate three centuries before anyone could explain it. a black spot in a soap film means the water is gone. there are only two skins left, separated by emptiness on the scale of light. the spot is doomed. it will be doomed for several seconds before it pops.

it does not just pop. it tears. the rupture starts somewhere — a draft, a dust mote, a thermal flicker — and the edge of the hole moves outward at twenty meters per second, fast enough to be a small sound. as the edge moves it gathers the water it crossed into a rim, the rim pinches into satellite droplets, the droplets fall. there is a faint mist where the bubble was. nothing has vanished. it has redistributed.

inside an unpopped bubble the air is under pressure. not much — for a bubble a centimeter across, ten pascals over the atmosphere outside it. one ten-thousandth. but two soap bubbles fused side by side will negotiate this difference. the wall between them bows into the larger, because the smaller has higher pressure, and where the three walls meet they always — always — leave at a hundred and twenty degrees from each other. plateau worked this out in 1873. three forces in equilibrium balance only at one twenty.

a froth of soap is governed by that angle and one more: where four films meet, they leave at the tetrahedral angle, one hundred nine point four seven degrees. every soap bubble in every sink, every cell of a beer foam, every void in a loaf of bread: the same two angles, all the way down.

i blow another bubble. the film catches the light. the colors slide down from the top before i have set the wand down. it has thirty seconds, maybe sixty. the angles are already correct.