Soapsuds and sand in a traffic jam
A stream of moving cars can become stuck in a traffic jam. A similar process occurs with materials such as sand, toothpaste or shaving foam, which can also transition from flowing behaviour to a fixed, solid state. A Vici grant will enable Professor Martin van Hecke to investigate just what is happening to the material during such a transition.
Disorderly
Martin van Hecke
If you tamp a pile of sand down firmly you will be able to walk over it. However, with powdery sand, such as on the beach, you will sink in. In short, sand can be fixed and solid but also loose and ‘flexible’. This is not only true for sand but for many other ‘disorderly materials’ too. Disorderly means that the particles in these materials do not have a clear hierarchy, just as in sand but also, for example, in soapsuds, mayonnaise or toothpaste.
Unpredictable
This disorder is the territory of Martin van Hecke, Professor of the Organisation of Disordered Matter. He is fascinated by the complex, unpredictable forms that such materials take despite the natural laws for the behaviour of single particles being so simple. No single lab has more pots of sand, marbles or soapsuds than his.
Traffic jam
Van Hecke makes use of computer simulations to examine the forces between particles in different materials, such as, in this case, soapsuds.
Van Hecke has received a Vici grant of EUR 1.5 million to investigate what happens at the point at which such disorderly matter transforms from a flowing to a fixed state. This is known in the literature as ‘jamming’, in reference to a traffic jam. Van Hecke explains: ‘The circumstances of a traffic jam are comparable: you have cars that are flowing that then become stuck.’
Shock wave
He suspects that a substance that is close to the transition from flowing to fixed becomes extremely fragile and thus sensitive to disruptions. Van Hecke says, ‘If I hit a block of steel, a sound wave will pass through it. If I hit it very hard you will get a shock wave. If I induce this fragile state in sand – loosely adhering sand, that is – I would expect this shock wave to occur even when I hit it gently. We call this extreme mechanics.’
Mayonnaise
So how do you induce this fragile state in disorderly matter? By ensuring that the transition of the particles is small. Van Hecke says, ‘To give an example: if you make mayonnaise, you mix oil droplets with vinegar. If there are enough oil droplets, they push against each other and start to change shape. The system becomes ‘jammed’, which means it has become . If you make the mayonnaise a bit slacker, the oil droplets will then be almost round and you will be in the regime of these extreme mechanics.’
Growth
Van Hecke has been awarded the Vici grant as an endorsement of the growth that this field of research, generally known as ‘soft matter’, is experiencing. He is obviously extremely pleased with the funding. ‘This is really fundamental research and it is difficult to find funding for it. This is the biggest grant I have ever received,’ was his happy response.
A system of soft balls that are packed in increasingly small volume, and become 'jammed' in the system. It is precisely at the transition point that the ballse just touch in response to the system. Close to the transition, the response is less regular and more extreme.
Links
See also
- Understanding the game of marbles (8 September 2009)
- Swirling sticks: shape does matter (5 July 2007)
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