Kinetically Constrained A place to keep the longer thoughts of Doug Ashton.

Moving a Blog

Toward the end of my PhD, in 2008, I started a blog called Kinetically Constrained to talk about science and, as I was into it at the time, bad science. The name came from my thesis topic about kinetically constrained models of glass formers. It also played to that bigger feeling that, despite being such complex unique creatures, our choices are relatively constrained sometimes, and as a group we’re much more predictable than we might think. I used a Google service, called Blogger, which did everything I needed. Eventually I got a nice domain name and off I went. I stopped blogging in 2014 but recently want somewhere to put longer thoughts.

Journals for e-readers

One thing that makes me cross is that despite the terrifying amount of money our library pays to buy back our research in the form of journals, they’re still not terribly easy to read. I’ve got an e-reader now and I’d like to read things on that, just the sort of value-added that the publishers could do. Unfortunately everything is still just a pdf file only to be printed on A4. There are some utilities for coping with this but it’s not really ideal. I wanted to see how tough it is. So I tried to convert my last paper into something that would look nicer on an e-reader (in my case a kindle).

The Renormalisation Group

A new video which more or less completes the critical phenomena series. Jump straight to it if you want to skip the background. One of my favourite topics is the critical point. I’ve posted many times on it, so to keep this short you can go back here for a summary. In brief, we’re looking at a small point on the phase diagram where two phases begin to look the same. The correlation length diverges and all hell breaks loose. Well, lots of things diverge. At the critical point all length scales are equivalent and, perhaps most remarkably, microscopic details become almost irrelevant.

The thermodynamic limit

This post has been at the back of mind for a while and written in small, most likely disjoint pieces. I wanted to think about connecting some of the more formal side of statistical mechanics to our everyday intuitions. It’s probably a bit half baked but this is a blog not a journal so I’ll just write a follow-up if I think of anything. I’m often accused of living in a rather idealised world called the thermodynamic limit. This is completely true. To see why this is a good thing or a bad thing I should probably say something about what I think it is.

Just hurry up and sit down!

As a semi frequent flyer, and incredibly impatient stand-behinderer I couldn’t resist linking to this - Time needed to board an airplane: A power law and the structure behind it from a Norwegian group, Vidar Frette and Per Hemmer. Boarding strategy is of great importance to airlines, where the turn around time of planes – especially short haul – can make a real dent in profits. For the authors of this paper, however, it seems they just think it’s a neat model to test out 1D problems where the particles are distinguishable, rather than the more common indistinguishable particles. In a traffic model the cars are usually identical, whereas here the passengers have a specific seat booking.

A phase diagram in a jar

One of the things I love about colloids is just how visual they are. Be it watching them jiggling around under a confocal microscope, or the beautiful TEM images of crystal structures, I always find them quite inspirational, or at least instructional, for better understanding statistical mechanics. Sedimentation Just to prove I’m on the cutting edge of science, I recently discovered another neat example from 1993. At the liquid matter conference in Vienna Roberto Piazza gave a talk titled “The unbearable heaviness of colloids”. As a side note there was a distinct lack of playful titles, maybe people were too nervous at such a big meeting.

Back from the dead

Can’t remember the number of times I’ve said I’ve been away because I’ve been busy, but this time it’ll be different. Well it probably won’t be different, it looks like I’m destined to be an inconsistent blogger! It’s now been three months since I arrived in the Netherlands for my new job and I’m enjoying it a lot here. The pace is much faster in the group than I’m used to but I’m enjoying the buzz of lots of interesting things getting done. Now I’m more settled I’m hoping for a spectacular return to blogging - there’s certainly enough to talk about here!

Universality at the critical point

Time for more critical phenomena. Another critical intro I’ve talked about this a lot before so I will only very quickly go back over it. The phase transitions you’re probably used to are water boiling to steam or freezing to ice. Now water is, symmetrically, very different from ice. So to go from one to the other you need to start building an interface and then slowly grow your new phase (crystal growth). This is called a first order phase transition and it’s the only way to make ice. Now water and steam are, symmetrically, the same. At most pressures the transition still goes the same way – build an interface and grow.

An early look at simulation

While I was putting together the post on 2D disks I came across a lovely paper from 1962 on 2D melting by Alder and Wainwright. From there I found this paper from 1959: Studies in Molecular Dynamics. I. General Method by the same authors. They describe the “event driven” molecular dynamics (MD) algorithm. Normally, with MD you calculate forces, and thus accelerations, and update this way. Hard disks or spheres behave more like snooker balls, the forces are more or less instantaneous impulses that conserve momentum so it’s better to deal with collision events and leave out the acceleration part.

Lipid membranes on the arXiv

A while ago I discussed lipid membranes and how they could exhibit critical behaviour. There were some lovely pictures on criticality on giant unilamellar vesicles (GUVs) which are sort of model cell walls. That work was done by Sarah Keller and friends in Seattle. This morning on the arXiv I saw this new paper, also by Sarah: Dynamic critical exponent in a 2D lipid membrane with conserved order parameter They look at the critical dynamics of the GUV’s surface. Being embedded in a 3D fluid does have its consequences so they’ve attempted to account for the effect of hydrodynamic interactions. I haven’t poured over their model but the paper looks really nice.