Small-angle Scattering: More than nice curves. [Guest post by M. Gallagher-Jones]

Coherent waves emanating from a raindrop impact on water. CC0-licenced image from: http://pixabay.com/en/rain-drops-raindrops-water-drops-71481/

[ed: Marcus Gallagher-Jones just finished his Ph.D. project on VUV and X-ray lasers for imaging of biological macromolecules]

I was happy to receive an invitation from Brian to write a post for his blog. I can honestly say that I owe a good deal of my knowledge of SAXS from helpful discussions with Brian and from reading LaN. Over the years we’ve shared a country, a workplace, and one more important thing: a scattering geometry. So without further ado I’m delighted to introduce to you the technique which has occupied my time for much of the past four years, Coherent Diffractive Imaging (CDI). Read more »

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The Dark Side of Science.

Panel from Dresden Codak. Reproduced with permission, Copyright 2011 Aaron Diaz. Source: http://dresdencodak.com/2011/06/16/dark-science-10/

A little step sideways from small-angle scattering for this week’s post. As you are probably aware by now, I sometimes use the LaN weblog to crystallize ideas into something resembling a coherent story. This is needs to be done now, as I am preparing another presentation (due late January), one that covers a tangential topic from my usual repertoire: a little overview of bad tidings in science. Read more »

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Error estimation using Fourier Transforms

left: Fourier components and low- and high-pass window. Right: original data, low-pass filtered data, and high-pass filtered data.

Estimating uncertainties on data values has always been an important and under-emphasized part of small-angle scattering. Uncertainties are critical to your data: they tell you what is most likely a real difference, and what is probably just measurement noise. Fortunately, many datasets come complete with data uncertainties, but there are still quite a few cases where this is not the case, or where the provided uncertainty estimates are unrealistic. So what can we do?

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Moving LaN update day to Tuesday

Today is another holiday in Japan. There is a tendency here for the national holidays to be on Mondays or Fridays, and there is typically about one per month of those. So I hope you do not mind, but I would like to move the weekly update date to Tuesdays instead.

Mind you, this will be Tuesday in Japan, so if you are reading this in America, you can still read the updates on Monday.

I will not take this opportunity to relax this week, there will be an update tomorrow. Yesterday, a funny thought struck me with respect to error estimation, and I have been playing around a bit to test it. So tomorrow you will have all kinds of error estimating goodness!

 

See you then!

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Looking at Nothing, Seeing a Lot.

STM image of "stripy nanoparticles" and structure retrieved from SANS data. Reproduced from DOI: 10.1039/C3SC52595C with permission requested from The Royal Society of Chemistry on November 15, 2014.

Today is a day of relief for Dr. Julian Stirling and his eight co-authors (with many looking forward to the response, including Raphaël Lévy). The paper released today opposes ten years of prolific work from a group claiming to have made and observed stripes on the surface of nanoparticles (c.f. Figure 0, Figure 1 in this post). While most of the work revolves around scanning probe microscopy (SPM), small-angle scattering also played a minor role (c.f. Figure 2 and this paper). This, coupled with modern approaches to publication, led to my inclusion in the (otherwise amazing) list of authors. Here is how this came to be.

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Spending money on Bonse Hart Bling-Bling

Not a lot of money.

[ed1: Progress on the Everything SAXS book has been good: a framework is now in place with chapters, the revision ID on the title page and even a Makefile and readme. The time approaches to enter some content!

ed2: Something cool approaches (next week).]

Most of the times we are scrounging for crumbs on the lab floor, sticking parts together with chewing gum and cardboard, but every now and then a small wad of crumpled dollar bills is pushed into ones hand with a hushed whisper: “spend it now!”. Read more »

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Spheres, rods, discs, which can fit what?

Table showing which model can be used to fit scattering patterns from a variety of polydisperse shapes.

A remark in a recent paper by Dr. Yojiro Oba (currently at KURRI) caught my attention. It discusses which shape assumption can be appropriate to fit a scattering pattern of polydisperse systems. In the paper, the shape assumption is spherical (based on TEM evidence), and a further remark goes as follows:

Since no qa (a < 4) behaviour is observed, the possibility of anisotropic shapes such as a rod, disc, and ellipsoid of revolution is denied.

That is an elegant way of putting it (note that it is an unidirectional exclusion and does not work the other way), and it sounds about right. So let’s test this with some simulations. These, of course, cannot prove that the statement is true, but can only disprove the statement. Read more »

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