Apr 17 2010

Lasers go oooooommmmmmm

In a previous post I mentioned, in passing, surface quasi-elastic light scattering (SQELS). SQELS is a fancy way of measuring the surface tension of a liquid using light, it has some advantages over the alternative method: sticking something into the surface of the liquid and measuring the pull but it is technically challenging to do.

The basic idea of SQELS is this: if you take a liquid surface, even in the absence of breezes or shakes, it is perturbed by tiny waves the properties of which tell you about the surface properties of the liquid. These waves have frequencies of 10kHz, wavelengths of 0.1mm and amplitudes of only a few angstroms. They are driven by the thermal motion that means everything, on a small enough scale, is jiggling away incessantly. To measure these waves laser light is shone on the surface, most of the light is scattered elastically that’s to say it stays exactly the same colour. However, some of the light is scattered inelastically (or quasi-elastically since the effect is small) – it changes colour slightly, the power spectrum of the surface waves is imprinted onto the laser light in terms of shifts in its colour. So all we need to do is measure the power spectrum of the light reflected from the surface to find out about the surface properties of the liquid.

It turns out I don’t have any photos of the SQELS apparatus in all its glory, so I shall describe it in words. The whole thing is found on an 8 foot by 4 foot by 1 foot thick optical table. A fine, very solid table whose top surface is a sheet of brushed steel, pierced by a grid of threaded holes 25mm apart. The apparatus is in the form of a large U covering two long sides and one short side of the table. A laser is bolted at the start of the U; light heads from the laser along the table through a set of lenses, polarisers, a diffraction grating, then upwards through a periscope before being directed down onto the liquid in Langmuir trough. The Langmuir trough is protected by a cardboard box, decorated in the style of a Freisian cow with holes cut roughly in the sides to allow light in and out. Captured after reflection from the surface of the liquid, the laser light is directed back down to the table surface by a second periscope from where it passes back along the long side of the table into a photomultiplier tube – the detector.

The cardboard box is there to stop air currents disturbing the surface of the liquid, vibration is the enemy for this experiment because the liquid in the Langmuir trough picks up the slightest disturbance and wobbles around. Sitting on an optical table weighing a large fraction of a tonne isn’t enough – it needs to be on the ground floor too because buildings wobble and in this instance the Langmuir trough sat on an active anti-vibration table – a bit like noise cancellation headphones but the size of a small coffee table. You can manage without active anti-vibration if you’re willing to do your experiments in the dead of night.

The cardboard box is emblematic of a piece of research apparatus: much of it is constructed from pre-fabricated components, some of it is custom-made in the departmental workshop but then there are the finishing touches that depend on your ingenuity and black masking tape. I did have plans to get the cardboard box remade in perspex but the box was just the right size and if I wanted more holes in it I could easily cut them with a knife so it was never worth the effort. I seem to remember a bit of drainpipe being involved too. As an experimental scientist you get your eye tuned in to spot things just right to add to your apparatus.

The laser is a single mode solid-state laser producing light of 532nm wavelength – a brilliant green colour. Three things are important about lasers: firstly, they are fantastically bright; secondly, they produce light of a very pure colour – a single wavelength. Thirdly, lasers go “oooooooommmmmmmmm”, whilst conventional light-sources go “pip-pip—-pip-pip—pip”. Technically this is described as “coherence”, we’re using a laser in part because we want something to compare against and a conventional light-source isn’t going to work for this. If you’re measuring a small change, it’s very handy to have a “ruler” close at hand, and in this case the elastically scattered light is that ruler.

You’ll notice that I’ve not said anything about the results we obtained using the SQELS; truth be told, despite all the hours spent building the apparatus, doing the experiments and analysing the data the results we obtained told us little more than that which we could get by easier and simpler means that I described in my earlier post. I also had the sneaking suspicion that it would have helped if I knew more about optical engineering.

(I got distracted in the middle of this post, browsing through the Newport optical components catalogue site!)

Reference
Cicuta, P., and I. Hopkinson. “Studies of a weak polyampholyte at the air-buffer interface: The effect of varying pH and ionic strength.” Journal of Chemical Physics 114(19), 2001, 8659-8670. (pdf)

Apr 12 2010

Why I’m voting Liberal Democrat

In a change from usual service I’m writing a political blog post, to cut to the chase: Vote Liberal Democrat! This post tries to explain why.

I’ve been a member of the Liberal Democrats since I was an undergraduate at Bristol University, 20 years ago. As a student I attended a party conference, did a bit of canvassing and I was also a “teller” a couple of times. Since then I’ve been in cover, very deep cover, I pay my monthly membership and a bit extra at election time and that’s pretty much the limit of my activism.

As a casual Liberal Democrat I don’t keep a close eye on party policy, essentially I rely on them being my sort of people and doing the right thing, it’s with some relief I can report that I agree entirely with Nick Clegg in his interview with The Observer this week. The whole interview is well worth a read, but I’ll pick up on one point: there’s a real value in a hung (or balanced) parliament with no party in overall control. Reducing the national debt will be a priority for any incoming government, this is likely to be at least a bit painful and I think it’s very obvious this is better done with a government that holds representatives from more than the 40% or less of the popular vote that a majority government is likely to get.  Many other issues will be with us for years to come: care for the elderly, climate change, pensions, the shape of our democratic systems. The solutions that politicians come up with should be robust, and have cross-party support, on recent evidence they need to work on this cooperative aspect of politics. Across the world and the UK, in devolved government and European elections we use a form of proportional representation, the sky has not fallen in. I’m fed up with the “smack of firm government” that first-past-the-post gives us.

The expenses scandal has had a big impact on politics in the last year, the Liberal Democrats came out well on this with relatively few outrageous claims and a very definite plan on how to address the problem which unfortunately was not picked up by the other parties. MPs had an expenses system which begged to be abused, I’m sure that with the same system at the place I work we would see a similar range of behaviour.

I’ve written in the past about the science policies of the Liberal Democrat, Labour and Conservative parties based around a debate organised by the Campaign for Science and Engineering in the UK. In summary, science has done fairly well by Labour over the last 13 years, with a noticeable wobble at the end over the science advisers, particularly on drugs advice. The Tories seem rather uncommitted to science, and look like they would do no better over science advisers. In the Liberal Democrats we have a champion in the form of Dr Evan Harris, who I really wish was my MP. He has done sterling work on the Science and Technology Select Committee, as well as campaigning on libel reform.

I vote in the City of Chester constituency, pragmatically there is absolutely no point in me turning out to vote. It will have no effect on the outcome, come May 7th not one particle of an MP in parliament after will have my electoral support.

Often party X will tell Liberal Democrats to vote for them to prevent party Y getting in, my response is in the form of an analogy: if I want chocolate cake for pudding the offer of an apple or a plate of cheese and biscuits will not satisfy, and may cause offence and derision. Vote for chocolate cake, vote Liberal Democrat!

Apr 10 2010

Experiments for obsessive compulsives

It feels like I’ve not really been writing about science very much recently, so I thought I’d return to some work on which I spent a few years, with my former PhD student, Pietro Cicuta.

We looked at the properties of a protein from milk (β-casein) spread on the surface of water: principally the effect that it had on the surface tension as a function of amount of protein. Experimental variables were the acidity and saltiness of the water. We did this using a Langmuir trough which I’ll describe below. b-casein is what’s known as a random coil protein: in contrast to many proteins, which curl up into a well-defined, unique shape, β-casein flops around like a piece of string. This work is directly relevant to people working in the food industry, and more generally interesting to people who work with polymers (chemists) and proteins (biologists).

β-casein acts as a surfactant which helps stabilise fat globules in milk. Surfactants are common type of molecule, the name is a contraction of “surface”, “active” and “agent”, unsurprisingly they are found at surfaces: typically between one liquids, like oil and water, but also at interfaces between liquid and air. Surfaces are important, they keep the inside in, and the outside out. Surfactant molecules help with this important process by stabilising surfaces (the natural tendency of liquids is to form big blobs, surfactants stop this process). Some examples: the cells in your body are surrounded by surfactants, mayonnaise contains surfactants from egg which keep the oil suspended in the water, all manner of cleaning products for clothes, hair, work by using surfactants to stabilise dirt in water, and foams are formed using surfactants.

To achieve this magic surfactants share a common feature: part of the molecule likes oil and part of the molecule likes water, so to keep both parts happy they hang around at interfaces. Most surfactants are like little tadpoles with water-loving heads and water-hating tails. β-casein is a bit different, parts of the string like water, so they try to stick into the water and parts don’t like water so they head for the air. However changing the acidity and saltiness of the water changes the strength of the love for water.

In most cases substances love water because they have an electrical charge, and this is why salinity and aciditiy are important in this experiment: if you change the acidity of the water the electrical charge on the protein changes because of the chemistry of the protein, if you change the salinity then how well the water can see the charges changes. It’s a bit like fog, when there is no salt in the water it’s as if the electric charges are seen through clear air and their influence spreads far and wide, adding salt is like a mist reducing the visibility of charges until ultimately the electric charges can’t be seen at all.

A Langmuir trough is a way of probing surfactant properties. It comprises a shallow trough made of Teflon, a barrier made of Teflon (which can be swept across the surface of liquid in the trough) and a surface tension sensor. Teflon is used because water sits on top of it forming a proud meniscus rather than spreading out, damply. The sensor is nothing more than a bit of filter paper attached to a force measuring device, dip it into the water and feel the pull – that’s surface tension. The idea with the barrier is that you place the barrier at one end of the trough, drop your molecules on the surface and then slide the barrier along, the molecules on the surface have few places to go so the decreasing the area amounts to increasing the concentration of the molecules at the surface. It’s really the 2D version of compressing a gas with a piston. I tried to find a picture of a trough with a single barrier, but couldn’t – the principle of the two-barrier trough shown here is the same, the black tower in the middle is the surface tension sensor.

Langmuir trough experiments are ideal for obsessive-compulsives: before you start your actual experiment you have to get the surface of the liquid you’re using absolutely clean. To do this you clean your trough, add in the ultrapure water, compress the surface, hoover (with a glass pipette connected to a vacuum pump) contaminants off the surface if there was an upturn in the surface tension, then go back to compressing the surface, hoovering the surface etc. Some times it just doesn’t work and you spend a morning trying to get your trough clean. Doing this for an oil/water interface is difficult, much more difficult actually I never succeeded. The core of the problem is that you don’t need much material to make a surface dirty, imagine painting a ball – the amount of paint required to cover the surface is much smaller than the volume of the ball.

The Langmuir trough was developed by Irving Langmuir, building on work by Agnes Pockels done towards the end of the 19th century. You’ll often see references to the Langmuir-Blodgett trough, the two terms seem to be used interchangeably but my understanding is that the Langmuir-Blodgett device is used to deposit surface active molecules onto a surface (which is not what we were doing). The Blodgett is Katherine Blodgett, who was the first woman to be awarded a PhD in Physics from Cambridge University.

From the surface tension data we extracted two things: firstly, how the protein molecules interact with each other – this comes from the early part of the compression data when the molecules are just starting to touch each other. Secondly, we get some idea of the innards of the protein from what happens when we squeeze the molecule harder and it starts to deform. Think of it like a bunch of eggs if you’re bouncing them around in a basket you find out about how bouncy they are, if you grab hold of one and squeeze it really hard you first discover that it has a tough outer shell, then you discover it has a soft squishy inside, then you discover you have egg all over your hands and you forgot to get the kitchen towel out before you started.

We find out this information about interactions and internal properties as a function of acidity and salinity, which we can then compare with theories of charged polymers. This comparison turned out to work quite nicely, and Pietro came up with a neat way of illustrating how bits of the molecule appeared to plunge into the water as the surface layer was compressed.

This is pretty much my most cited piece of work, with just less than 30 citations. So there you go, several years in the lab condensed into just over a 1000 words, although I didn’t mention the Surface Quasi-elastic Light Scattering (SQELS).

Reference
Cicuta, P., and I. Hopkinson. “Studies of a weak polyampholyte at the air-buffer interface: The effect of varying pH and ionic strength.” Journal of Chemical Physics 114(19), 2001, 8659-8670. (pdf)

Apr 04 2010

Caerwys – a breezy spring walk

Off to Afon-wen and Caerwys today for a brisk walk featuring steep walking through woods, spring flowers, llamas, highland cows and a newt. You can see the route here:


View Caerwys in a larger map

It’s a walk from “Walking in the Clwydian Range” by Carl Rogers, real men make up their own walks from OS maps and scouting missions but I’m lazy. A cool spring day today, in the distance we could still see the odd patch of snow on the hillsides. The trees were bare but the flowers had started to come out:

Clockwise from top left: Primrose, violet, wood anemone, and celandine
There were many birds out, singing away enthusiastically. I was going to show a picture of a woodpecker which we heard tapping away very close by, but we didn’t see it so it would really have been a conceptual piece so I’d like you take that conceptuality one step further and imagine a picture of a tree with an unseen woodpecker in it. Caerwys is the home of llamas, which I also treat with a degree of respect, since they spit if you offend them:

Although it’s a very rural location there are both signs of modern industrialisation in the from of quarries and sandpits and also the remnants of older work, including this lime kiln:

And also this rather creepy corrugated iron building which put me in mind of “Jeepers Creepers” or “Deliverance“:

With a vivid imagination, a walk in the countryside is never boring! We walked through the village of Caerwys, which is quite pretty – many of the houses seem to be well-made from the local limestone. No photos though, mainly because the streets were full of parked cars which are an aesthetic abomination and I don’t like photographing people’s houses in close up – it seems rude. However, I had no qualms about photographing this fine house across Ysceifiog Lake which was created for fishing by the Earl of Denbigh in 1904:

We next passed through a small nature reserve: Y Ddol Uchaf, which is a designated Site of Special Scientific Interest (SSSI) whose information board promised newts, and lo – I saw that there were newts and it was good:

On the final stretch back to the car we were treated to these handsome chickens:

And some Highland cows, we first sighted some of these in the distance across a field then we passed close by to another group behind a photography unfriendly fence then eventually coming to some easily accessible ones in photogenic mud:

This one, is the bull in the group, I’m a country lad and was easily able to identify him as such by dint of his sizeable testicles (which I refrained from photographing):

So there you go, a memorial of our Easter Sunday walk immortalised for when I am old and incapable of leaving the house.

Apr 03 2010

A brief history of gadgets

This is a post about gadgets and my relationship with them, spurred by my purchase of the latest gadget: an HTC Desire smartphone aka “Shiny”.

I have a suspicious relationship with telephones, basically I consider talking to people at a distance a form of devilry and if you expect me to type messages in a 26 letter alphabet using a 12 key keyboard you’ve got another thing coming. Telephone use at SomeBeans Towers is simple: most nights Mrs SomeBeans rings her dad, once a week on Sunday my mum rings me, roughly once a month my dad rings me with a list of computer problems for solution and once every three weeks I ring Majestic to arrange wine delivery : “Simples”. All phone calls beyond this are a cause for chaos, consternation, confusion etc. I appreciate this makes us “anomalous” but I’m too old to care.

Mobile phone use is even more occasional, whilst skiing we sometimes arrange slope side meet-ups via mobile phone. Mrs SomeBeans was reduced to hysterics watching me typing a text message, moving my lips as I did it and eventually giving up because of cramp. When buying a house use of the devil’s mobile speech-horn is inevitable. At the start of the last house purchase I sat on the train with my mobile phone ringing from my pocket thinking: “Why isn’t that person answering their phone? It sounds an awful lot like mine (I think)”, on the previous house buying occasion I attempted to recharge my phone via it’s headphone socket, it died.

My last trip to the phone shop was rather embarrassing for all concerned, I bought a Samsung E250 slider phone. I completely missed the point of the “slider” bit and asked the phone-geek whether it was a touch screen phone: “No, sir”. I expressed a desire for a phone embedded in an SLR camera lens, at which point the phone-geek claimed that the picture from some phone was as good as an SLR, so I like to think I wasn’t the only one to come away from the experience looking like a complete idiot. The Shiny was bought over the internet to avoid embarrassment.

I did wonder about the touch screen aspects of the Shiny, I believe there a two types of people in the world: those that are happy to smear their horrible, greasy fingerprints across displays, and those that wish to kill them. I fall into the second group, so there was some risk I would have a touch-screen phone which I was psychologically incapable of touching. Fortunately this has turned out not to be the case, whilst transferring over numbers from the old Samsung I repeatedly tried to use the screen as a touch-screen, to the chagrin of all involved.

Why the HTC Desire? I do a bit of programming and the Android operating system on which it runs is relatively straightforward for me to program on (the iPhone requires you to use a Mac). Android has such magic software as Google Goggles which carries out picture based searching – it works too: it successfully recognised “Luncheon of the Boating Party” by Renoir on our wall, as well as a rather more obscure photo, and Tasty by Kelis (front or back cover). It has a radio too (you notice phone functionality is pretty much the last thing on my mind here). Chatting to friends of a more phone-friendly nature I got the impression that the Desire was the way to go. Blackberries looked a bit serious and business-like (and don’t have radios).

The predecessors of the Shiny are my Psion 5mx, a fabulous PDA with a lovely almost proper keyboard and built-in software which did not need to be supplemented, I gave it up after many years because the connector between screen an keyboard started to break regularly, I followed this with a Dell Axim x51v which I never really loved. My phone history is completely unmemorable, my first mobile phone looked like a toy bone for a dog, and made young people laugh. I’ve had two other phones since then but I scarcely used them. In other gadgets I got a bluetooth GPS to talk to my PDA so I could geotag my photos, then I got a Garmin GPS60 which did it rather more robustly.

I joined the digital camera revolution rather early with a Kodak DC210, this was a revelation to me since previously my photography experience was limited to taking a few shots on film in a weeks holiday, finally filling a film after 18 months having completely forgotten where I was or what technically I had done to achieve the effect I had. After the Kodak I got an Olympus C750UZ, in parallel I also had a Casio Exilim S20 for it’s extreme compactness, then I went SLR picking up a Canon 300D from my father-in-law on which I became completely hooked, upgrading to a 400D shortly thereafter. The reason for going to the SLR was that even with a reasonably good point-and-shoot I was finding there were photos I knew I just couldn’t take because I couldn’t control the camera. An SLR gets round this problem by having a decent set of real controls, like a focus ring on the lenses, rather than some buried menu options and octopus-friendly button pressing. The thing with an SLR is that the camera body serves the function of a gateway drug, your dealer makes the real money on the lenses*. There’s probably a business model in giving away SLR camera bodies and making your profit solely on lenses and accessories. I’ve subsequently got a second Casio Exilim S10 to fill my dinky camera needs, this camera will take photos when people smile and claims that it can recognise different people and prioritise snapping according to your preference. This seems like a new way of offending friends and family, is making your wife anything other than top smiling priority grounds for divorce?

Returning to the HTC Desire: it’s fantastic! It can replace phone, pda, GPS and dinky camera: all in a beautiful package. The interface is a joy to use, wave your finger around on it and you go skittering off to different parts as if you’re skimming across the surface of a large desk. My old PDA felt like you were peering into a tiny fixed porthole on it’s innards. Tapping buttons on the screen gives you a touch of vibration feedback. Even the internet is pretty usable, as is the 200 page PDF manual.

I have a bit of a mixed attitude to gadgets: things that are nice to use like the Psion, Canon 400D and Shiny, I really like. Things with crap interfaces, I can’t abide: programming video recorders and central heating systems I hate for their horribly kludgey 20 random keystrokes with no user feedback nastiness.

And now, if you don’t mind, I will return to fondling Shiny.

*For the camera fans I have the 10-22mm, 50mm f/1.8, 18-55mm kit, 28-135mm, 100mm macro, EF 70-300mm lenses.