Mark Beckwith, age 12, of Dinuba, California, for his question:
Where is the seam in a bubble?
A bubble wears its seam on the outside but don't expect it to be a straight or curved line of neat stitching. Seams of this sort are used on a tennis ball to hold the sections of its topcoat in a spherical shape. Actually, a bubble's entire outer skin is a sort of seam. This may seem odd, but after all a seam is supposed to hold things together and this job is done by a bubble's outer limits.
The average bubble is a small ball of invisible gases governed by invisible energies and pressures. All this invisibility tends to shroud it in mystery. To picture what goes on, we must recall a few known facts about the nature of fluid gases and liquids. The common household bubble is a fascinating sphere of gas either surrounded by water or encased in a film of moisture.
The seam that holds it together is a sort of energy barrier of equal pressures between the gas and the liquid. Gases, of course, are composed of infinitesimal atoms and molecules which zoom around at fantastic speeds, bashing and bouncing off each other and changing directions millions of times a second. They get their peppy energy from heat and the warmer it gets the faster they go.
Gases always spread out to occupy all available space. When confined in a bottle, their speeding molecules bash their prison walls and this force creates pressure. If an air filled bottle is sealed and placed over a flame, the gas particles use the heat to speed up and expand. They may build up enough pressure to shatter their prison walls. When gaseous steam is chilled or compressed, its molecules slow down and cling together to form liquid water.
The birth of the common household bubble usually occurs under water, in a boiling pot. Water molecules near the stove get up enough speed to separate from the liquid and form a pocket of gaseous vapor. The denser water presses around on all sides. The warm vapor expands equally in all directions, which gives the gas pocket a spherical shape.
The ball of gas exerts an outward push against the inward push of the surrounding water. The two opposite pressures are equalized around the bubble. This zone of balanced pressure is the seam that holds the bubble together. But not for long. The lighter gas has buoyancy, which lifts it to the top of the water. There its seam bursts apart, the gaseous vapor mingles with the air and the bubble disappears.
Soap bubbles put on a more decorative show of longer duration. Particles of soapy water have more clinging power. When warm or even cool air is added, its gaseous pockets are seamed inside thin skins of clinging moisture. These bubbles hold together long enough to float up and away, showing blurred patches of the rainbow spectrum on their shiny skins. In the air, their surface seams can withstand a little unequal pressure. But eventually the energy barrier breaks down and even the soap bubbles burst apart.