After seeing this story on the BBC I thought I’d have a look at mercury. Element Hg and number 80 on the periodic table, its shiny mirror-like surface and upside down meniscus are probably two of its most identifiable traits. But a new one for me is how dense it is - you can float a whole lot more on top of mercury than water, including people.
Although inadvisable due to its poisonous nature, this picture from National Geographic Magazine October 1972 shows a man floating on top of a pool of mercury.
Google mercury and see a whole list of amazing uses and images.
Lasers are not quite so easy to get your hands on but they can be used for pretty much anything. Here a laser sheet and some smoke allows the visualisation of the tip vortices of a large rotating model helicopter blade. These vortices occur due to the difference in pressure between the bottom and top surface of the blade. At the tip this difference causes the flow to ‘curl up’ around the end of the blade.
Although I don’t only show things you can try at home, I try to provide a large proportion of posts that you can - and here is another easy experiment to try at home.
First take a cup of water and cool it in the fridge or freezer. Second, take another cup of water and add a small amount of food colouring to it. Heat this second cup (be careful) in a microwave or a saucepan. Once this is done carefully pour the heated, coloured liquid onto the colder water. Hopefully two layers will form due to the different densities of the liquids. As the cool water warms up and the coloured liquid cools down the densities change until eventually the coloured water is denser than the layer below.
This situation triggers the Rayleigh-Taylor instability and will hopefully form similar fantastic images to those above.
Photo credit: Physics central
It’s been a long time since last posting but hopefully I’ll be back up and
running for the foreseeable future and what a great post to start with.
We’ve all experienced (or participated) in the classic prank of tapping someone’s beer bottle with another beer bottle and watching the ensuing comedy as the beer froths over. Little did I know there was so much to why this happens as is explained fantastically in this video.
One thing immediately noticeable in slow motion is what look like small ‘whorls’ as the bubble plume moves upward which is a manifestation of the Rayleigh-Taylor instability which occurs at the interface between two fluids of different density.
Posted Aug 29th: Finally my favourite post so far - what a flame looks like in space. It doesn’t seem that spectacular but when I came across it, it seemed strange at first but makes perfect sense once you think about it.
Fire! Fire! That is one thing you don’t want to hear aboard the International Space Station, unless, that is, you’re conducting experiments to see how fire differs in space from on earth. Today’s photo shows the difference between earthbound (left) flames and those occurring in space (right).
A flame will heat the surrounding air and cause it to expand and become less dense. On Earth gravity pulls the colder, more dense, air around the flame downwards while the hotter, less dense, air rises up and away. This process - otherwise known as convection - allows oxygen to feed into the bottom of the flame with the sinking cold air and fuels the flame further. This gives rise to the classic tear-drop shape of a candle.
In space however gravity’s effects are not felt and so Earth-like convection does not occur. Instead, the heated air simply radiates outwards in a sphere (as seen right). This has implications for the ferocity of the flame: in space oxygen is not fed into the flame via convection but must be left to diffuse into the flame a much slower process.
Source: NASA and video, Smithsonian
Posted June 3rd: Rayleigh-Taylor in the bathroom
An absolutely brilliant example of the Rayleigh-Taylor instability of two fluids occurring by accident in the home. This instability occurs when a denser fluid is placed on top of a lighter fluid in a gravitational field. (It can also occur under other circumstances) The heavier fluid begins to sink into the lighter fluid below and often forms the characteristic mushroom-like shapes seen here.
The redditor who posted this stated “Soap dispenser was half empty, filled it with a different soap. Looked like this the next day”
What’s interesting here is that in this case the densities of the liquids were such that the movement of the liquids was very slow as compared for example to this.
Posted June 11th: Tubercles
Flow Viz hot off the press! There’s a lot here but stick with it. The picture on the left is from a flow visualization experiment we ran last week as a test-of-method but we got rather good results so I had to show it.
What you can see is the front end of a flat, metal plate, with a wavy front end (leading edge) in one of our wind tunnels where the flow is from right to left. We used a method of flow visualization which involves spraying china clay and oil of wintergreen solution onto the surface. Turbulent flow experiences higher shear stresses than laminar flow and this causes the oil to evaporate faster in turbulent areas of flow. This causes the china clay to show through and so these patches turn white. Ignoring the red box - where the method did not work in this case - we can see a fantastically clear pattern of alternating turbulent and laminar streaks on the leading edge (the blue box). The turbulent streaks (white) are created by vortices being shed from the wavy leading edge.
These experiments were inspired by nature, in particular the humpback whale. These whales have so-called tubercles on the front of their fins. As you can see their fins effectively have a wavy front end. These tubercles and the streaks they create have a large effect on the efficiency of their movement and their agility and have already been used for turbines and other systems.
Posted July 23rd: The storm after the sun
What a difference a day makes! England has been basking in the sunshine of one of the hottest summers in recent memory but that all broke yesterday with thunder storms and torrential rain. What better way to see fluid effects than stomp in a puddle?
In the second of this week’s ‘Effects you can see at home’ we see a rather larger version of a water impact than the often seen droplet impacts. Despite the large size of the wellington we still see the same fluid effects: The ‘coronet' normally seen is still here and can be seen in the pinched off droplets at the top of the splash. This pinching off is driven by the Plateau-Rayleigh Instability a surface tension effect. You can also see that in one place the film has become too thin and once again surface tension has caused a hole to form. If we could watch the next few moments of this interaction many more holes would form as the film stretches and thins.
Posted Sept 18th: Amazing weather
Apocalypse! That’s what facing up to a sandstorm (or Haboob) can look like. These amazing pictures were taken off the coast of Onslow Australia and show the classic wall of dust seen at the front of many dust storms.
Particles of dust can be pulled into the air by strong winds via a process known as saltation and the electric charge created from particles rubbing together can also have an effect. There is a fantastic explanation here of the various mechanisms which can form these phenomena but one thing that’s really interesting is they are often formed as the death throws of a thunderstorm, kind of like an exhalation after breathing in.
Posted 22nd July: Still not sure what’s going on here!
It’s all in the reading of the tea leaves! As I’ve mentioned before one of the things I love about the physics of fluids is how it is everywhere we go, even in the most unexpected of places. This week I’m going to show some fluid dynamic effects you can see without specialist equipment, hopefully in your own home.
I noticed this one while having some tea over the weekend. In the video you can clearly see that, despite the bottom of the mug being almost vertical, the traces of tea scum are moving upwards. (It’s clearer in the second half of the video.) What’s going on here? Well I’ll be honest and say I’m not 100% sure. My guess is to say that it is an example of the Marangoni effect. This is effectively the flow of liquid from a region with low surface tension towards a region with high surface tension, and the classic example is that of ‘Tears of wine’. In the case of the Tears of wine, alcohol evaporates faster from the thin film around the glass than the water. This increases the surface tension of the film and draws liquid up from the bulk of fluid in the glass. Eventually gravity overcomes surface tension and the liquid falls back down the side of the glass known as ‘Tears’ or sometimes ‘Legs’. Although this is not an alcoholic tea it could well be a similar process, and the traces of tea scum are simply following the direction of flow.
Having said this I am skeptical because it looks like the fluid on the right is moving downwards. So there could well be some other physics at work here and I’d be interested to hear if anyone out there has any thoughts.