Perhaps you remember the video about how to ‘float’ coffee droplets on water posted on the Daily Grind a few weeks ago? The video featured an experiment that you could do at home in which droplets of water (or coffee, or even, if you were feeling adventurous, tea) could be made to stay as spherical droplets on the surface of a shallow dish of water for minutes at a time. Of course there were a few tricks: The water had soap added to it (10ml of soap to 100ml of water) and the shallow dish was on a loudspeaker which was playing music at the time. The whole experiment was very pretty. But hopefully as well as appreciating the aesthetics, you were asking ‘how’ and ‘why’? Why does the addition of soap mean that these globules of liquid appear to float on the liquid surface? And is the rumour you have heard about a connection with quantum physics true?
Well it turns out that people have known about these floating droplets for over a hundred years but why they behave as they do is still being investigated. It is another case of cutting-edge science appearing in your coffee cup*. So it’s worth taking a look at what is going on and why we needed to add soap and vibration for the droplets to remain stable on the water surface.
It seems to appeal to common sense and to everyday experience that if we drop a droplet onto a bath of water, the droplet will merge with the water and become part of the bath. After all, when we bring two drops that we have dripped on a table close to each other, at a certain distance between the two drops, they appear to touch and then rapidly merge into one big droplet (try it). And when it rains onto a pond, we don’t see lots of spherical droplets hovering over the surface of the pond! We know that it is the attractive van der Waals forces that bring the two drops together and then the effects of surface tension that minimise the surface area of the drops so that they become one big drop. So how is it that we can get a droplet to remain, as a droplet, on the surface of a bath of water?
How to bounce water droplets on a water surface
It could be said that the answer can be pulled out of thin air: Before the drops can merge, the air that separates them has to escape from the area between the droplet and the water bath. If the droplet can somehow be made to bounce back upwards before the air separating the droplet from the bath becomes thin enough for the two liquids to combine, the air could be made into a cushion to keep pushing the droplet upwards. This is why the experiment needs to be done with a vibrating dish of water, each time the surface vibrates upwards it is providing the drop with an acceleration upwards that overcomes gravity, like a miniature trampoline: The droplet is not floating, it is bouncing.
So why soap? We all know that the addition of soap decreases the surface tension of the water. But that is not why the addition of soap helps to stabilise the drops in this instance. No, soap has another effect and that is to increase the surface viscosity (and surface elasticity) of the water. Think about the air between the droplet and the dish. As the droplet bounces down (ie. the distance between droplet and water becomes a minimum), the air gets squeezed out of the layer between the droplet and the bath. On the other hand, as the droplet reaches its peak height, air will rush into the gap between the drop and the bath. If the liquid is not very viscous (eg. water), as the air rushes in (or gets squeezed out), it will combine with the liquid and form a turbulent layer on the surface of the droplet. If the viscosity is increased, the air cannot ‘entrain’ the liquid as the droplet bounces and so the drop keeps its shape more easily and is more stable. Soap increases the surface viscosity of the droplet and so helps with this effect. However soap also increases the surface elasticity and makes it harder for the air to flow out of the layer separating the drop from the bath. It is because soap does multiple things to the water (or coffee) that more recent studies have focussed on liquids with controllable viscosity but minimal surfactant effects, i.e. silicone oils. It is just that if you want it to work with coffee, it is easier to add the soap to get the experiment to work.
An “un-cut” video of coffee on water shows how tricky it can be to actually get these drops to be stable on the surface of the water.
Which leaves the quantum link. The experiment shown in the videos show single droplets (or droplet patterns) stabilised by vibrations caused by music. If instead of music you use fixed frequencies to excite resonances through the speakers, it is possible to get the droplet to resonate in a controlled way and, at a certain point, it will move. As the droplet moves, it appears to be guided by the vibrations of the liquid underneath the drop, it is a particle guided by a ‘pilot wave’. It turns out that such walking droplets show behaviour reminiscent of the ‘wave particle duality‘ found in quantum physics where particles (such as electrons and other sub-atomic particles) can be described both as particles and as waves. You can find a video describing the similarities between these bouncing droplets and quantum effects here.
* Ok, so you may not want to add soap to your coffee to see this effect but actually I first observed it in a milky tea. Adding milk to the coffee/tea would increase its viscosity which makes the observation of the bouncing droplets more likely. The ‘milk’ used in the video was actually soya milk which did not appear to increase the viscosity sufficiently to allow the droplets to bounce on the surface without soap.