Thursday, May 9, 2013

On the dynamics of Janus colloids (#JChemPhys)

Janus colloids are particles that like the Roman god, Janus, have two faces. Chemically, the two faces in a Janus colloid correspond to different interactions based on what is coating the surface of each of the two halves. (In principle, they don't have to be spherical. There's some recent work on cylinders emerging as well.) We, and others, have been curious about whether a container of Janus particles would have different structure than spherically attractive particles. We showed in earlier work that, at equilibrium, such containers can indeed be quite similar. This was disappointing, in part, because it meant that you couldn't make new equilibrium structures by carefully making Janus particles. It may not be surprising to you, though, because it simply means that if you are in a room filled with people, then most of the time, you can turn your head so that you are facing someone you like. As such, you can ignore the direction you don't like. Similarly we can, for the most part, ignore the unhappy contacts in a container of Janus colloids (in the fluid regime.)

But what about the motion of this system? It turns out that this is indeed highly dependent on the Janus particle interaction strengths. Equally interesting is the fact that if you try to remove degrees of freedom from this system, you get some crazy results. First of all, if you get rid of them without doing anything else, then the particles move too quickly. (This is a known problem in coarse-graining.) If you trick the system to get the rates of particle diffusion right (as is often done), then other stuff breaks down. We found in the Janus colloids, that the association of the particles (that is which are neighbors are next to which) is highly dependent on the Janus particle interactions strengths. This means that performing coarse-grained molecular dynamics simulations to model the motion of large assemblies of molecules is more difficult to do correctly(!) than we had anticipated...

These latter findings are available, in far too much detail, in our second article on Janus Colloids which just appeared on line in J. Chem. Phys. (doi:10.1063/1.4803864).


  1. Very interesting!

    Any comment about the equilibrium structure of Janus cylinders compared to one-component cylinders? I was working with someone who made, for lack of a better word, Janus Peanut particles (imagine a sphere of material 2 growing on the face of a sphere of material 1 so they resemble peanuts). I assumed the particles where randomly oriented when choosing how to average over incident angles/polarizations to simulate their experimental absorption spectra, but I wonder if this was the most proper choice?

    1. Thanks!

      To your question.... As an initial condition, that should be ok. But then one would need to equilibrate the system under the influence of the orienting field. (We have done some work on needles in which we had to do as such.) Meanwhile for your peanuts, you would need a smooth potential so that you could do dynamics. It was the development of such a potential for our system that enabled us to look at the dynamics of these Janus particles for the first time.