Friday 3rd July started with breakfast in the cafeteria of the University of New England, in Maine, USA. I had a very interesting chat, over breakfast, with the previous night’s seminar speaker. We were at the University of New England for the Crystal Growth and Assembly scientific conference.
This was at the end of the conference so were then bussed back from Maine to the nearest international airport, at Boston about 2 hours drive south. A good friend and I had a few hours at the airport. We caught up and talked science.In particular about what is called the Marangoni effect.
The Marangoni effect is so simple to see you can see it in your kitchen in 2 minutes, if you just have water, washing-up liquid, a bowl and ground pepper. A YouTube clip showing the experiment is here. The Marangoni effect is also so complex and subtle, we basically have no ability to calculate it from what we know of water, soap, and small particles (the ground pepper).
The Marangoni effect is just a consequence of the fact that liquid surfaces exert forces. These forces pull on liquid surfaces, making them contract. These forces are what cause liquid droplets to pull themselves into spheres, and what opposes bubble formation. This force is reduced when a soap (surfactant) is added, which is why bubbles are much easier to form if you add washing-up liquid to water.
None of this is particularly subtle, but the Marangoni effect occurs when on the same surface you have lots of soap in one part of a liquid surface, and little or no soap in another part of the same surface. Then in the bit with little soap the force pulling inwards to contract that part of the liquid surface is weak but in the bit without the soap the contracting force is still high. Then the part of the liquid surface with no soap pulls the other part of the surface towards itself.
This is like a tug-of-war between a really strong guy and a weakling, the strong guy wins and pulls the weak guy towards him. So the surface of the liquid flows towards the area with no soap. This creates flow both at the surface and underneath. This soap-induced flow is the Marangoni effect, which can be demonstrated by a five-year old in a kitchen, but it turns out, it is very hard to understand how we can go from soap molecules to this flow.
I really enjoyed thinking and talking about this science problem. It was time to board my flight. After the six-and-a-half flight from Boston to Heathrow, one of the first things I did at about 7am in Heathrow, was to do a welcome tweet to the visitors to Saturday’s Open Day for prospective students and their parents. If they were impressed by someone getting out of bed that early on a Saturday, they should not have been. I did not go to bed at all. I had also finished reviewing a paper on the flight, so I used Heathrow’s wifi to submit the review while waiting for my bus.
I was on duty at the open day in the afternoon. As this was 24 hours after I got out of bed the Starbucks venti iced latte I got on the way in was a real life saver. I could have not helped inform our 400 plus prospective students and parents without stupidly high caffeine levels.
Open days are important. I had to tell a kid who was not doing A-level maths, and her father, that it would be difficult for her to start a physics degree in 2016. Most questions you get are easy to answer, and the answers you give are good news, but sometimes, although you want to give good news, you can’t. Pretty much all physics departments require A-level maths as well as physics, as we need students to know some calculus, trig etc, without which they will struggle. I went through the options available to her. I was glad of the caffeine here, you really need to get these conservations right, and give accurate information, as the prospective student will make potentially life-changing decisions based in part on them.
Then it was time to stagger home before the last of the caffeine left my system and I collapsed. It was fun day, and I packed a fair amount into the approximately 31 hours I was awake. It was good to talk science with my colleague and friend, and to talk physics degrees with the prospective students and their parents.
Chance & Necessity 