Ever see a reverse bubble?

Question

If you pay attention when looking at water dripping on a smooth surface, you'll see a small percentage of the bubbles produced on impact are different; they are brilliant silver and shoot at high speed away from the impact point. You get more if the water is a little soapy so you might see them when washing the car. The 'bubbles' appear to have water inside them instead of air. I'm not sure of the mechanism of how they're formed. Any thoughts?

Answer 1

it works exactly the same as a normal bubble.

for a normal buble, the air rising to the surface does not have enough speed to break the waters surface tension. it therefor stays under the toplayer of water instead of merging with the air above.

when a water dropplet falls onto water at low speed, you occasionally get the same effect. the dropplet doesnt have enoughspeed to break the water tension, so it doesnt merge with the water, instead the droplet is suspend on the water surface in a sphere.

there reason you get more of these with soapy water is because soap actually strengthens the layer on the water, exactly why soapy water is usually mroe bubbly.

if you let the droplets run over the bulgy side of a spoon, you ll see you get even more.

Answer 2

Awesome question . . . are you sure they're anti-bubbles? I know exactly what you're talking about. I always attributed them to droplets held together by surface tension. I've mostly encountered them when making something hot and viscous--I assumed the rising steam gave them an extra cushion to float on--like air hockey--making them more plentiful in this situation. So you say they occur with soapy mixtures too? Is the concentration high, or low? Essentially the droplet has more cohesion than adhesion--perhaps the orientation of the soap in the droplet? You know how soap molecules have one aliphatic side and another polar side right? If all the polar join in the center, to minimize e-stat. that leaves the non-polar tails on the outside, meaning, at least for a short time, it behaves as oil dropped on water . . . This would be a stat. mech. problem from hell, but given the concentration of the soap and the radius of the bubble, you could find the outside concentration of soap, as well as the average alignment of the molecules.

Answer 3

One of my classmates studied these for her senior project when I was in college. I think these "reverse bubbles" are called boules. You are correct that they are filled with water and not air.

After so many years I don't remember all of the details, but I believe that they are small water droplets that stay separated from the main body on a thin layer of gas. At room temperature, this gas is air, but at elevated temperatures it can be steam. When these boules form on top of a water surface, you usually see them only traveling at high speed because only at such speeds will enough air be dragged underneath the boules to keep them separated from the main body of water. When the boules slow down, the film of air fails and they join with the main body of water.

You can also see these boules when you drop a small amount of water on a skillet heated well above the boiling point of water. (Use caution if you try this; you can also expect hot droplets of water to hit you if you happen to be too close or use too much!) The thin layer of steam keeps the boule off the skillet and provides insulation. Such a droplet will last a lot longer than the same amount of water that sticks to the skillet.

Answer 4

I voted for the first answer but the details are buried in a lot of verbiage. Water has surface tension. Under the right conditions, a perfect little sphere forms and the tension pulls so tight that any other forces, such as adhesion to a wet wall or another material are simply irrelevant and the sphere skitters off on its own. Friction is essentially nil as the point of contact is so small.