viscosity

Making a splash

You spilled your coffee, a terrible accident or an opportunity to start noticing?

Why do some droplets splash  while others stay, well, drop like? It turns out that there is some surprising physics at play here. When a drop of water, or coffee, falls from a height and onto a flat surface (such as glass), we are accustomed to seeing the droplet fracture into a type of crown of smaller droplets that form a mess over the surface. Visually spectacular, these splashing droplets have even been made into an art form (here).

Fast frame-rate photography reveals how each micro-droplet breaks away from the splashing drop:

Video taken from Vimeo – “Drop impact on a solid surface”, a review by Josserand and Thoroddsen.

 

So it perhaps surprising to discover that there are many things about this process that we do not yet understand. Firstly, if you reduce the gas pressure that surrounds the drop as it falls, it does not make a splash. In the extreme, this means that if you were to spill your coffee in a vacuum, you would not see the crown-like splashing behaviour that we have come to expect of falling liquids. Rather than splash, a droplet falling in low pressure spreads out on impact as a flattening droplet. This counterintuitive result was first described in a 2005 study (here) that compared the effect on splashing of droplets with different viscosities (methanol, ethanol, 2-propanol) falling through different gasses.

cortado, Brunswick House, everyday physics, coffee cup science

Don’t spill it!
But would a latte splash more or less than a long black?

The authors of the study ruled out the effect of air entrapment surrounding the droplet as it falls as high speed photography had not indicated any air bubbles in the droplet just before impact. Instead they considered that whether a drop splashes on impact – or not – depended on the balance between the surface tension of the falling liquid and the stress on the drop created by the restraining pressure of the surrounding gas. Calculating these stresses led to a second surprising result. Whether a drop splashes on impact or not depends on its viscosity (as well as the gas pressure and the speed of impact). But the surprising bit is that the more viscous the liquid, the greater the splash.

From a common-sense perspective (that may or may not have any bearing on the reality of the situation), an extremely viscous liquid like honey should not splash as much as a less viscous liquid like coffee. This suggests that there is an upper-limit in viscosity to the relation predicted in the 2005 study. After all, although the authors did change the viscosity of the liquids, the range of viscosity they studied was not as great as the difference between coffee and honey. This sounds like a perfect experiment for some kitchen-top science and so if any reader can share the results of their experiments on the relative splashes formed by coffee and honey, I would love to hear of them.

 

Something brewing in my V60

kettle, V60, spout, pourover, v60 preparation

The new V60 “power kettle”

It was my birthday a short while ago and someone who knows me well got me a perfect present: a kettle specially adapted for making pour-over V60 style coffees. Until this point I had been struggling with a normal kettle with it’s large spout but now, I can dream that I pour like a barista. Of course, it is important to try out your birthday present as soon as you receive it. And then try it again, and again, just to make sure that it does really make a difference to your coffee. So it is fair to say, that recently I have been enjoying some very good coffees prepared with a variety of lovely beans from Roasting House and my new V60/V60 kettle combination.

Spending the time to prepare a good coffee seems to make it even more enjoyable (though it turns out that whether you agree with this partly depends on why you are drinking coffee). Grinding the beans, rinsing the filter, warming the pot, the whole process taken slowly adds to the experience. But then, while watching the coffee drip through the filter one day, I saw a coffee drop dance over the surface of the coffee. Then another one, and another, a whole load of dancing droplets (video below). Perhaps some readers of Bean Thinking may remember a few months back a similar story of bouncing droplets on soapy water. In that case, fairly large drops of water (up to about 1cm wide) were made to ‘float’ on the surface of a dish of water that had been placed on a loudspeaker.

Sadly, for that initial experiment the coffee had been made undrinkable by adding soap to it. The soap had the effect of increasing the surface viscosity of the droplets which meant that the drop could bounce back from the vibrating water surface before it recombined with the liquid. Adding soap to the coffee meant that these liquid drops could ‘float’ (they actually bounce) on the water for many minutes or even longer (for more of the physics behind this click here).

science in a V60

A still from the video above showing three drops of coffee on the surface.

On the face of it, there are some similarities between the drops dancing on the coffee in my V60 and these bouncing droplets. As each drop falls from the filter, it creates a vibration on the surface of the coffee. The vibration wave is then reflected back at the edges of the V60 and when the next drop falls from the filter it is ‘bounced’ back up by the vibration of the coffee.

But there are also significant differences. Firstly, as mentioned, there was no soap added to this coffee (I was brewing it to drink it!). This means that the viscosity of the drops should be similar to that of ordinary water. Although water drops can be made to bounce, the reduced viscosity means that this is less likely. Secondly, the water is hot. This acts to reduce the viscosity still further (think of honey on hot toast). Perhaps other effects (such as an evaporation flux or similar) could be having an effect, but it is noticeable that although the drops “live” long enough to be caught on camera, they are not very stable. Could it be that the vibrations caused by the droplets hitting the coffee are indeed enough to bounce the incoming droplets back up but that, unlike the soapy-water, these “anti-bubbles” do not survive for very long? Or is something deeper at play? I admit that I do not know. So, over to you out there. If you are taking time to make coffee in a V60, why not keep an eye out for these bouncing droplets and then do some experiments with them. Do you think that the bounce vibration is enough to sustain the bouncing droplet – does the speed of pour make a difference? Is it associated with the heat of the coffee? I’d be interested to hear what you think.

(The original soapy-coffee bouncing droplet video).

If you see anything interesting or odd in your coffee, why not let me know, either here in the comments section below, e-mail, or over on Twitter or on Facebook.

Bouncing Coffee

floating, bouncing drops

Water droplets ‘floating’ on a bath of water (actually they bounce rather than float).

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.

lilies on water, rain on a pond, droplets

When it rains, the rain drops don’t float on the pond

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.

Coffee baubles

resonating coffee

Not the best image of a resonating coffee but you hopefully get the idea

Most people, at some point in their lives, must have pushed a take-away coffee cup across a table and watched as patterns form on the liquid surface. Sometimes these patterns seem to stand still, we’d say that they form ‘resonances’. On even rarer occasions, on dragging your cup across the surface, you may have seen coffee droplets jump out of the coffee and then dance on the coffee surface for a couple of seconds as the liquid vibrates.

Today’s Daily Grind investigates these ‘floating droplets’ with an experiment in time for Christmas: Decorate your coffee with coffee baubles.

To make these droplets form on your coffee in a controllable way you will need a few bits of equipment:

  1. A couple of loud-speakers with the woofers exposed
  2. Some sort of liquid soap (washing up liquid, hand soap, soap for hand washing clothes etc)
  3. Some water (or coffee but you will do horrible things to it)
  4. A shallow dish (I used the bottom of an old yoghurt pot)
  5. A “dropper”, a pipette or syringe would be ideal, a straw will probably work.

You can do this completely systematically, in which case you’ll also need a signal generator to provide a fixed frequency output to the speakers (I used “ScorpionZZZ’s Lab, Signal Generator Lite for iPhone). Or you can just go straight to the fun bit which is to make these droplets dance to music. It’s Christmas so it’s entirely up to you!

floating drops, resonances, speakers, kitchen top science

Balance a shallow dish on the woofer of a speaker. A roll of sellotape can be used to couple the vibrations of the speaker to the dish if necessary.

Balance your speakers on a flat surface and put the shallow dish so that it sits in good contact with the woofer. Because my dish was ever so slightly larger than the vibrating bit of the speaker, I ‘coupled’ the speaker to the dish with a roll of sellotape. Mix 10ml of soap with 100ml of water (this does not have to be exact but you may want to investigate just how much/little soap you can get away with). If you are using coffee rather than water, you will need to mix 10ml soap with 100ml coffee.

Pour about half the soapy-water into the dish and then turn the speakers on. If you are using a signal generator, watch what happens as you sweep the frequency from 10-200 Hz. Now, either choose a frequency which shows a nice resonance pattern on the water, or start playing the music through the speakers. Music with a good beat will work well (I watched drops dance to Tiesto, Blondie, and Josh Woodward’s “coffee”).

Drip a drop of the remaining soapy-water onto the resonating surface. A video of my playing with these droplets can be seen above. Although not all the drops will float, it is fairly easy to start to form patterns of flowers or rows of droplets and then it’s worth just playing.  How big a droplet can be made to float without collapsing? How many minutes can you get a drop to last before it sinks? What happens if you combine a drop of black (soapy) coffee with a drop of milky (soapy) coffee?

Have fun, and please do share your videos and photos of your experiments with me on Facebook or Twitter.

Disclaimers & Credits:

No coffee was wasted in the making of this video. A very good coffee from Roasting House was thoroughly enjoyed before the remnants were diluted and mixed with soap.

Inspiration & experimental details taken from Jearl Walker’s great article “The Amateur Scientist” in Scientific American, p. 151 (1978).