Swarming particles


Some of you may be aware that an electrolyte gradient can move small particles around. This is called diffusiophoresis. The first time I heard about it was when I came across this paper in Angewandte Chemie a few days ago. Michael Ibele, Thomas Mallouk and Ayusman Sen demonstrate how the phenomenon causes micrometre-sized AgCl particles to self-organise in strange ways when UV radiation falls on them. There are creepy quicktime movies attached to the paper. It all looks so alive.

Of course it is just a physical effect. In a gradient the cations and anions of the electrolyte diffuse at different rates, depending on the direction. The result is an electrical field in the solution that can influence particles. Additionally, in a gradient the thickness of the ionic double layer at any wall varies in thickness, resulting in a pressure difference.

Under UV radiation AgCl decomposes into Silver. The reaction requires the presence of water and releases chloride, protons and oxygen gas:

4 AgCl + 2 H2O –> 4 Ag + 4 H+ + 4 Cl + O2

Since this decomposition is usually asymmetric this causes a gradient around the particle, in which the particle itself is moved around.

Edit: Apparently embedding quicktime videos doesn’t work. The movies are freely available in the supporting information of the paper.

If the particle is alone, this movement is completely random, but when particles are close to a wall and meet each other, strange things happen. First, all particles move randomly, pushed along by their self-induced gradient. But if two particles meet they tend to stay close to each other, without ever clumping together. Stable swarms start to form, in which all individuals seem to respect each others privacy: There are very regular gaps in the mob.

What brings the particles together is the electroosmotic flow that dominates over long distances. But when the partners are too close the flow along the electric double layer, which is directed away from the electrolyte souce, pushes them apart. The researchers added silica to the suspension, which have a similar zeta potential, and within minutes every AgCl particle was surrounded by a flock of silica acolytes.

Sen, Mallouk and Ibele did a few other strange tricks with AgCl under UV radiation, among them directed travel. Take a look at the rest of their movies. This degree of self-organisation is fairly impressive, even if it stops after a while because the particles are completely coated with silver.

So where does this lead? The authors claim, unsurprisingly, that they discovered a new design principle for intelligent micromachine swarms that act as an autonomous collective. Personally I don’t think so. The principle only works under rather special conditions, for example there needs to be a wall with a specific zeta potential close by and the like. However, this could be a very useful development when it comes to creating materials with a defined micro- or nanostructure.

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