Research paper: The effects of polydispersity and metastability on crystal growth

I’ve just uploaded a preprint of a new paper me and my supervisor are writing to arXiv. It’s a freely-available repository research in loads of different areas which people use to make research available before and while its in peer review for a journal.

This one is to do with crystal growth in soft condensed matter. That includes colloidal crystals and closely related things such as proteins, which must be crystallised in order to study their structure in biological/medical research. The broad question of ‘What’s the best way to grow a crystal?’ is relevant in a lot of scenarios, especially given that one is often quite free to vary the conditions in the system to optimise growth; for instance the interactions in a e.g. colloidal suspension can be easily tuned by adding other species such as polymer coils into the solution.

The dynamics of phase transitions, i.e. how systems do or do not actually reach their true equilibrium state, is an important consideration in applying thermodynamics to soft matter. In this paper, we simulate crystal growth (as shown in the video here) in the presence of metastable gas-liquid separation, which may be encouraged or avoided by tuning the interaction potential in a system, and polydispersity, which usually cannot be avoided in soft matter. There’s a variety of nice visualisations showing the effects of these two factors on the crystal growth dynamics, and we find that they can interact in a complex and previously unknown way. The simulation findings are related to existing experimental data and to theoretical considerations. Here’s the link:

The effect of metastability and polydispersity on crystal growth kinetics

This work, in early form, was the subject of a recent internal seminar in the Soft Matter Group at Leeds. I’ve uploaded the slides and an audio recording from the seminar.

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Philosophy of science

Something that I used to study at undergraduate but haven’t managed to for a while is the philosophy of science. As a subject it doesn’t get that much public airtime, but as science progresses and brings into the public consciousness theories which clash more and more dissonantly with our intuition, it’s more important than ever.

We (I include myself and in fact every other scientist in this, as well as the public in general) need to understand i) What questions can and does science ask about the world? ii) How are answers sought? and iii) What is the philosophical status of those answers once they are arrived at? Confusion in this area makes space for the context and content of scientific results to be manipulated to almost any end, and can also reduce the aesthetic impact of scientific theories; as Nagel points out in an excellent and tangentially related essay, the revelatory statement that ‘All mass is just energy’ is surprisingly lacking in punch on its own because, without some idea of the supporting conceptual background, we can’t really tell what it means for this to be true.

Philosophy of science discusses and formalises questions that all good scientists and interested observers consider at some point. I’ll hopefully get back to reading more of it and writing a bit more on here. For now, as a kind of introduction, I’ve uploaded two final-year essays from my philosophy of science course. Like most things written by a student for an assessor who knows much more about the subject than they do, they tend to be a strange mix of endearing simplicity and specialists-only balls-out complexity — I hope that the essential ideas still come through and maybe prompt anyone who is interested to research further:

What is a gene? 

concerns the often-neglected field of the philosophy of biology. The ‘gene’, ingrained in the modern consciousness and underpinning huge swathes of new research, is a surprisingly hazy concept. I describe its heritage and development and discuss ways that people have sought to clarify its meaning.

Critically examining structural realism

looks at a big question: How and to what extent do scientific theories describe how the world really is? I introduce the problems of various apparently appealing viewpoints and outline structural realism, which attempts to get round those problems.

How to stop the cable on 9-volt adaptors from breaking

Here’s a common problem for people who own small electronic devices, especially power adaptors for effects boxes.

This is an adaptor from my compressor; it outputs a special voltage, has a special plug on the end, and is expensive to replace. I found that out when, as with every other adaptor like this, the extremely thin cable eventually broke at the point where it joins the body of the adaptor. Even if you’re really careful, a few months or years of wear and tear is usually enough to break it because whenever tension is applied to the cable, it is applied to the same place — a join with only very weak stress relief. The cable bends and flexes in all directions, weakens, and after a while either the coating or the wire itself breaks.

When I got the replacement I came up with a nice way of preventing the same thing happening again. I took a cable tie, wrapped it around the body of the adaptor and loosely threaded the power cable in and out of the tie, following it once around the body. Then I tightened the cable tie and snipped it off, as shown in the picture below.

Threading the cable loosely through a tie wrapped round the body to relieve tension at the join.

This means that the join between the cable and body (the bit that always breaks) is never subject to tension and never moves, so it doesn’t break. Instead, pulling on the cable just smoothly induces a little bit of tension and only a slight bending at all the points where it crosses over the cable tie. The stress in any one part of the cable is never enough to break it, so it doesn’t break even if you grab the cable by the end and swing the adaptor around the place. And that’s magic.