I am writing a review at the moment for Annual Reviews in Materials Research on inkjet printing. I have almost finished and I was just downloading a couple of articles on coffee stains when though some morphic resonance related phenomenon, I knocked over my mug of coffee. So an excuse for a blog post on coffee stains.

And just to prove this is really science, here is a link to Deegan’s classic paper on coffee stains as published in Nature (apologies if you don’t have access).

Why is this of interest to anyone? Coffee staining occurs when a liquid containing a concentrated suspension of particles evaporates. If the liquid is a drop then the contact line (where the drop edge is) is where the liquid deposit is thinnest. Evaporation from the drop is driven primarily by local chemical potential and thus the curvature. This means that the first precipitation of the particles occurs at the contact line and prevents the liquid drop from retracting as it evaporates image b below).

If the contact line is pinned, then liquid has to flow from the interior of the drop to replenish that which is evaporating from the drop edge. This flow drags particles (or coffee) with it and increases deposition at the drop edge. In extreme cases practically all the solute or suspension is deposited at the edge of the drop forming a nice (or ugly as beauty is in the eye of the beholder) ring.

Are you still not clear why this is interesting or possibly useful? If we use inkjet printes the image is made up of lots of dots that are deposited as a liquid and then dry to form coloured patches. We can also use this technology to print lines (bar codes) or even electronic circuits. Evaporation from these tiny drops is particularly sensitive to coffee staining (images below).

So that s why you can find research into coffee staining funded by your favourite funding agencies.

Dunham Massey Hall

One of the advantages in living on the southern rim of Greater Manchester is the proximity of fantastic National Trust properties such as Dunham Massey (above). I also have Tatton Park a few kms further south but Dunham is within walking distance.

I was out for an early morning run today in Dunham Massey park when I heard a strange rattling or clicking sound. I looked to my right and there was a bunch of fallow deer sparring with their antlers as part of the rutting season. Unfortunately I did not have my camera on me so here is an image from the web:

The stags in this image appear to be fully mature, the ones I saw seemed to be a group of younger, more juvenile stags who were practising antler wrestling. Still an interesting perspective as I had been musing whether i had been getiing into a bit of a rut with my work.

Apologhies but I couldn’t resist that one.

It has been several months since I last posted. With a busy summer, entertaining kids when there is no school and other distractions, I have omitted to sit down and type about things. No matter.

Earlier this week I went to the Euromat Conference in Glasgow. This is organised by the federation of European Materials Societies and is one of those multiple parallel session occasions with the interesting talks distributed 100s of metres apart so you cannot easily flit between rooms to hear them. In many ways these inconveniences do not matter because the real reason to come to scientific meetings is to meet people and talk about your work. And you never know who you might meet.

At the conference dinner (in the Kelvingrove Art Gallery – one of those 19th Century monuments to municipal civic pride that grace British Cities) I was talking to an amiable senior American academic about some recent work in my group, when he said “Yes I wrote a paper on that in 1980”.
“Oh you’re that Davis”, I blurted out.
We then had an interesting chat and I promised (still not done it though) to send him a copy of a recent paper I wrote with a student of mine and also to send him an important paper he had missed that had been published 5 years ago by a Dutch group.

I had had my arm twisted to organise a session on printing materials and devices. It is always difficult to persuade people to come to a session in a meeting that is frankly not in the Premier League but it was heartening to find the room fairly full for the talks. There had been rumours going round that some sessions had audiences in single figures.

European meetings do not seem to have the buzz of the larger or similar size US organised ones (at least in Materials Science). This is partly because European meetings cycle round different countries and tend to lack any continuity of organisation. In addition, US meetings seem to serve a community gathering function with academics almost feeling obliged to attend to confirm their existence. In which case the meetings can act as a snapshot of what is going on. Again European meetings do not serve this purpose, although arguably that is where they could be most effective in fostering communication/collaboration across national boundaries.

There has been a flurry of activity on Nature Network in recent weeks concerning the reaction of practitioners in CAM to criticism in the printed media. This of course includes the libel action taken by the British Chiropractic Association against Simon Singh. This libel action is on the content of an article that was originally published in the Guardian and the BCA have chosen to sue Simon and not the Guardian. With that background out of the way, I would like to point readers towards the following Mortarboard Blogpost in the Education Guardian, which asks the very appropriate question considering the teaching of CAM in UK universities. What I find interesting is not so much the blogpost/article but that the comments on the post are uniformly against CAM having any academic credentials. So maybe there is hope for science based medicine if the Guardian readers (an allegedly gullible bunch) think this way.

On the radio (Today Programme, BBC Radio 4) this morning was a piece on spider silk from tarantulas. Spiders and their silk are a perennial favourite with those among my discipline who wish to poularise it and make it accessible to the public. We are regularly told (and told in this article) that spider silk is “stronger even than than steel” and “very, very stretchy” and that “researchers envisage a huge host of commercial applications”. Unfortunately, researchers have been saying the same for the good old orb web spider and the host of commercial applications have yet to materialsise.

Orb Web Spider in its Web

I will return to the hype of things being “stronger than steel” in a bit but I will first comment on the commercial possibilities of spider silk. Silk is a protein based fibre and we know the sequence of amino acids in a large range of natural silks from arachnids and from insects. The bit of nature we find hardest to mimic is the action within the spinnerets of spiders (or silkworms/silkmoths) that converts the protein suspension into the fibres. This has prevented us from making artificial (synthesised) spider or silkworm silks and has also prevented us from processing natural silks into artificial fibres with the mechanical properties that rival those of natural silks. Before we get too fixated on spider silk, we should also consider why there is a major international textile industry that uses silkworm silk and not one that uses spider silk, despite man’s familiarity with both animals for millenia. The difference between these two sources of silk is that we have domesticated the silkworm/silkmoth and that it provides gramme quantities of silk in each cocoon/chrysalis. Spiders are, as yet undomesticated and we cannot produce commercial quantities of their silk. Hence most commercial applicatiopns of silks use insect derived material. However, spiders are still very good marketing material; e.g. the Oxford Biomaterials web site with its extensive use of spider derived product names. However, to the best of my knowledge all current commercial silk derived products from all manufacturers use materials (fibroin proteins) derived from silkworm silk. These are still wonderful products but spiders sound better from a marketing perspective.

I will now put my sceptical materials scientist hat on and talk about the strength claims of spider silk. Is spider silk stronger than steel? Well like all good scientific questions, the answer depends on how you frame the question. If you ask the question “Is spider silk stronger than the sort of steel you use to make car body panels?”, then the answer is yes. But if you ask “Is spider silk stronger than the sort of steel you use to make ultra-high strength steel wire?” the answer is no. You could also ask whether spider silk is stronger than silkworm silk? and the answer is probably no (Shao and Vollrath, Nature, 2002). Spider silk is strong (typically about 1 GPa tensile strength) but so are many other materials when they are manufactured as fibres with dimensions smaller than a few microns. Spider silk is about the same strength as carbon fibre, and about the same strength as humble polyethylene when it is drawn into a thin fibre. Steel, by the way, can have a strength in excess of 5 GPa when in highly drawn wire form.

This discussion returns to the essential core of the science of materials when applied to mechanical properties. Strength in particular is an extrinsic property, in that we can control its value by manipulating the structure of a material at the supramolecular scale or the scale of the microstructure. Spiders obtain the properties of their silk fibres through molecular manipulations that occur within the spinnerets induced by fluid shear. High strength steel wires are formed through plastic deformation, work hardening and crystal rotations, also induced by shear flow but now a completely different set of mechanisms are exploited to achieve high strengths. There is equal wonder in how we manipulate materials to achieve high strengths for engineering applications as there is wonder at the exploits of spiders.