Crazily enough, the summer’s over. I’ve been bitten by the research bug, which means that I’m not as enthusiastic about taking classes as I once was, although they all look very interesting and useful. In particular, balancing nocturnal data-taking sessions with going to class/taking tests has never been my forte; and it’s tougher now, what with graduate school applications and pesky standardized tests.
Anyway, the past month’s pretty much consisted of finishing up taking/analyzing data for this paper I’ve been working on, as well as actually writing it. When I started out, I thought my data was somewhat interesting. Thanks to an excellent theorist collaborator, we have a good sense of what’s going, and it’s more interesting than I thought (which is always nice).
I did take a little vacation and went to the Princeton Center for Complex Materials (PCCM) summer school on condensed matter physics (unfortunately they don’t have their talks online yet). This is the second year that I’ve gone; hopefully there will be more. Two talks in particular resonated with me – Philip Kim’s talk on electronic transport in graphene, and Paul Chaikin’s talk on some aspects of colloidal physics. The graphene talk was of particular interest from a technical point of view. While it was very cool, given the nature of my work it dealt with things that I’m very familiar with and/or think about regularly, so I’m not going to describe it here.
On the other hand, Chaikin’s talk was interesting to me from a more conceptual point of view. I’ve encountered some notions of soft matter physics before, but his talk really drove home how exciting some of the things going on in that field are. He started off discussing his recent highly-publicized work on packing hard particles (interesting not only for studying granular materials or phase transitions, but also for designing three-dimensional colloidal photonic crystals) – the point being that random ellipsoid packings (like those formed using M&M’s) can pack denser than ‘conventional’ random jammed packings, even potentially approaching the FCC packing fraction of 0.74, because of their added rotational degrees of freedom. This, in turn, may help understand how glasses form. For me it wasn’t the story but how they fleshed it out (experimentally and using simulations) that was the exciting part.
Unfortunately I had an experiment to finish up and had to miss Chaikin’s second talk, on replication and self-assembly using colloids; his third talk on ‘random organization’ describing some work by David Pine (at NYU) and Jerry Gollub (at Haverford, but also affiliated with Penn) was equally good. He started out discussing reversibility in viscous liquids – that is, the fact that low Reynolds number shear flows are time reversible, a notion I first came across in an excellent article by Brewer and Hahn describing NMR spin echos. There’s a classic demonstration of this phenomenon by G. I. Taylor using a cylindrical Couette cell, although I couldn’t find a nice movie online. Anyway, similar experiments have been performed involving tracking small dyed spheres placed in the liquid while shearing them. Interestingly, while the system remains reversible for low enough strain amplitude, for whatever reason (e.g. collisions/chaos), hydrodynamic irreversibility sets in very quickly as the strain amplitude is increased. Really striking stuff. At the end of the day, colloids really are great systems to work with: they’re easy to make, specific and controllable (e.g. using DNA sticky ends or electric fields), and exhibit all kinds of interesting condensed matter phenomena.
Now that I think of it, the PCCM summer school was so enjoyable partly because it filled in the void left by the lack of regular talks and seminars over the summer. Thankfully, now that the new semester has started, that void has been filled again. Most recently, Jack Harris (from Yale) gave a talk on coupling light and MEMS using radiation pressure, something I don’t know much about. Part of the experimental challenge is in using the right structures; interestingly, a route his group is taking is to use silicon nitride membranes with holes drilled in them. I find this particularly amusing because our group has used (indeed, I made some my first year of research) similar porous membranes to image carbon nanotubes or nanotube-derived structures (like ‘peapods’) using TEM – it never occurred to me that they could be used for such a different purpose.
Anyway, writing this post has been very relaxing; now it’s time to get back to work…
