Active matter as an engine

Last night as I was trimming my presentation in view of my talk today at this conference, I thought it would be a good idea to include as many references and connections to what the people here do, being from a very different community. As I was going round in circles I realized that the Janus particles, the archetype of active matter, would make for perfect thermodynamic engines!

Janus particles are nanoparticles with two hemispheres having different physical properties, e.g. on the one side they catalyze some chemical reaction (typically hydrogen peroxide into oxygen plus water), or warms up easier when lighted, etc. This makes so that they have a typically asymmetric behavior and that when they are put together they start displaying collective motion. Their dynamics is broadly studied. But what I’m interested here is not their dynamics, but rather their thermodynamics. The thermodynamician has to make sure that the resources (hydrogen peroxide) are constantly replenished from the environment, and therefore needs to ask “how much it will cost? And how can we benefit from it?”.

Since a Janus particle transforms energy from one form to another, for example by transforming chemical energy into directed motion, they work as a thermodynamic engine and one can start asking eminently thermodynamic questions such as the efficiency of energy conversion. For example, one could somehow extract the kinetic energy of the flow of Janus particles and rectify it to make work on something. What is more interesting, here, is not that we have yet another engine, whose efficiency is anyway likely to be low. But rather, that these systems behave collectively, hence one can start asking questions about how the efficiency improves/decreases in terms of the collective behavior.

Furthermore, one could engineer funny particles whose hemispheres both catalyze different kinds of reactions, and start playing with the chemical potentials of the reactants. One might see the arousal of competition for resources.

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