Physics demonstrations: A short discussion of the Kaye effect

I’ve been gearing up for the second year of the UNC Charlotte Science and Technology Expo, which will be happening on campus on Sunday, April 21st.  I’ve been preparing a number of weird and unusual demos for the expo, and today I did my first run of the Kaye effect.  The video below shows the results of my experiments.

A thin stream of liquid soap is pouring into a plate below.  Though most of the time it just clusters in a pile, it occasionally fires off an arcing streamer!  These streamers of fluid can actually fly quite far — the ones in my initial experiment were traveling up to 8 inches, leaving the bowl entirely.  Here’s a snapshot of one of the streams, coming towards the camera:


The Kaye effect was first reported in 1963 by British engineer Alan Kaye, who noticed this unusual behavior when working with complex organic liquids.  A complete explanation for this phenomenon remained uncertain until 2006, when Dutch researchers did some clever work to elucidate the effect.

So how does this work?  It has been long known that liquid soap and shampoo are shear thinning fluids: this means that, under stress, the fluid flows better and becomes more “liquid-like.”

Most of the time, the soap just forms a heap on the bottom of the bowl:


Occasionally, however, a dimple forms at the top of the pile. Then a thin layer of shear-thinned fluid forms in the dimple, making a slippery barrier between the heap and the descending stream, preventing their merging.  The stream is deflected and launches from the dimple like a ski jumper:


Other shear thinning fluids include non-drip paint and ketchup, meaning that it might be possible to do a similar experiment with them!  Shear thinning fluids are sort of the converse of shear thickening fluids like oobleck, which acts like a solid when put under pressure.  It is even possible to walk on the surface of oobleck if one moves quickly enough!

The Kaye effect can be a great attention-getting demonstration for science expos, which is why I’m working on it.  Even more impressive is the observation (by the 2006 Dutch team) that the Kaye effect can be made stable if one makes it run downhill — essentially one ends up with a bouncing stream of liquid soap!

Now that I’ve got the basic phenomenon working, I’m going to tackle the downhill trick next.

The Kaye effect is really straightforward to achieve: I used a ring stand to hang a cake icing dispenser above a bowl.  With a 1 mm icing tip, I got a sufficiently thin stream of soap.

(I’m also hoping to make a more sophisticated video of the Kaye effect, but that will have to wait until I can figure out why my video editing software is constantly crashing.)

Here’s a pair of additional snapshots of streams from the video:




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