Tuesday, 23 June 2015

Dissipative self-assembly

I'm currently at a conference on Engineering of Chemical Complexity hosted by the TUM in Munich. Today we had an interesting talk from Thomas Hermans on ``dissipative self-assembly". To understand this concept, we need to think about steady states.

At a first glance, many of the things around us don't change over time -- they are in steady states. For example, the hotel I'm currently sitting in looks pretty much the same as it did five minutes ago. A fundamental principle of physics is that isolated systems tend to relax towards an "equilibrium state" (this is essentially the famous second law of thermodynamics). Equilibrium is an example of a steady state, because when a system reaches equilibrium, it has nowhere else to go. It isn't, however, the only example. In fact, most objects that we see in steady states are actually stuck in "kinetic traps", including the hotel. Really, the equilibrium state of the materials that make up this hotel wouldn't look very welcoming! For a start, everything would be much closer to the ground. If we wait long enough, of course, the hotel would fall down. but the rate at which this happens is very slow and it seems apparently "trapped" in a hotel-like steady-state to the casual observer.

Dr. Hermans wants us to consider a third, fundamentally distinct type of "dissipative" steady state. In kinetically trapped or equilibrium steady states, the system maintains itself - you don't need to supply anything to keep it in the steady state. However, think about a human body which is, roughly speaking, in a steady state; it needs to be constantly supplied with food to stay this way. This situation is typical of many biological systems - they are out of equilibrium, but not kinetically trapped, and rapidly relax towards equilibrium unless they are fed with fuel in some form. Feeding them with  fuel keeps them in a "dissipative" steady state, which is called dissipative because fuel gets used up in the process.

Dr. Hermans is looking to design artificial biochemical assemblies that exist in such dissipative steady states. Why might this be worthwhile? Hallmarks of biological systems include their flexibility, repairability and adaptability, features that are probably much more natural in dissipative assemblies in which there is a constant turn-over of material. As yet, the results seem preliminary and I can't find any publications - although a discussion of the principle of dissipative self-assembly can be found in this article, "Droplets out of equilibrium".

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