In the 1960s, when some were exploring the mysteries of outer space and quantum mechanics, one engineer noticed an extraordinary unexplained phenomenon in shampoo. The sudden, energetic, and seemingly spontaneous bursts of liquid that seemed to randomly squirt out from ordinary shampoo were a mystery for forty years. Here's why your shampoo, while being poured, sometimes leaps up and tries to get you.
Liquid soap has been around for a long time, and fluid dynamics has been very well studied. It's rare that new phenomena come up. So in 1963, physicists all must have gone running to their bathrooms to get hold of some shampoo. A new physical phenomenon was in town, and it was discovered by one Arthur Kaye. He was an engineer who happened, one day, to notice a weird physical phenomenon that no one had remarked on before. Pour shampoo, or liquid soap out onto a surface, and for a while it just coils up into a pile and then slowly oozes downward. Nothing special yet. But sometimes, seemingly at random, the falling soap seems to just hop right off the pile and squirt right up again. The squirt went in random directions, but it was incredibly energetic, as if that section of the falling soap gained the elasticity of a rubber ball. When he poured the soap onto a surface that was at an angle, he found could get a steady stream of soap rising up again.
Kaye noticed that this was a phenomenon common to all shear-thinning liquids. These are liquids that, when flowing or stressed, get more thin and less viscous. Soaps and shampoos were the most common of these that people had to hand. They were also used in industrial work, to lubricate machinery. Kaye believed that no one had remarked on it before because generally the spontaneous squirts were minor and quick. He didn't offer explanation for the odd phenomenon. He just put it forward, called it the Kaye Effect, and waited for people to figure out how it worked. It was only shampoo, right? How long could it take?
Forty years later, in 2006, people finally figured out how the Kaye Effect worked. There's a reason why it only works with shear-thinning liquids, and why it works most steadily on an angled surface. To begin with, on a flat surface, the liquid just piles up into a little hill. It's not flowing, so it gets a little thicker. Meanwhile, more liquid is coming down. When that liquid hits the pile at a certain angle, it's going to start to flow downwards. This creates a layer of liquid that is moving, and is a different thickness and texture from the liquid in the pile. This is kind of like greasing a runway, or the side of a hill. Anything that gets put on top of that layer will slide down quickly.
As the continuing stream of liquid hits this greased runway it moves fast. If it hits the bottom, or an indent in the pile, it gets hurled right back up into the air again. The squirt of liquid usually doesn't last long because the underlying pile changes shape. This is why pouring the soap onto an angled surface creates a more stable effect. You've already got the runway, all you need is enough grease sliding down it. In the video, there are several points when the stream of liquid actually bounces a couple of times, like a stone skipping across the water. As long as there is a layer of sliding liquid, acting as a lubricant, and a couple of divots in angled surface, the stream can jet up into the air again and again.
And for those of you wondering, yes, you can do this in your kitchen.
Image: Brocken In A Glory