Week of 10/10/08

In attempt to make these posts more regular I will be trying a different approach of simply posting links to various papers of interest, without much graphics or commentary.

• V. A. Martinez, G. Bryant and W. van Megen, “Slow Dynamics and Aging of a Colloidal Hard Sphere Glass,” Physical Review Letters 101, 135702-4 (2008)
• J. M. Brader, M. E. Cates and M. Fuchs, “First-Principles Constitutive Equation for Suspension Rheology,” Physical Review Letters 101, 138301-4 (2008)  also reviewed by
  N. Wagner, “How colloidal dispersions relax under stress,” Physics 1, 22 (2008)
• D. K. Satapathy, O. Bunk, K. Jefimovs et al., “Colloidal Monolayer Trapped near a Charged Wall: A Synchrotron X-Ray Diffraction Study,” Physical Review Letters 101, 136103-4 (2008)

Dynamical phase transition in periodically sheared colloids

This weeks item is Nature Physics paper Random organization in periodically driven systems by Corte, Golub, Chaikin and Pine.

When you consider a dilute suspension of colloidal particles undergoing a periodic driving force – such as shear, at low density and low strain, particles far from their near neighbours will be undergoing periodic motion around the same point. However, if the two

particles are near each other, they are likely to collide during shearing, changing their relative position. These particles can collide until they become sufficiently separated and settle into reversible fluctuations around their positions – hence self-organizing behaviour.

At high values of maximum strain, or at high densities, however, the particles never cease colliding, leading to irreversible dynamics – hence dynamic phase transition above a well-defined strain threshold.

Nanoparticle Self-assembly with DNA

Image on the left is the cover of Jan. 31 issue of Nature.

Anyone who took high school chemistry has played with “sticks and balls” models of molecules or crystalline atoms. There are magnetic toy sets which allow kids to assemble their own version of crystals.

A similar feat, but on nanoscale, was accomplished by two groups – one at Brookhaven (Nykypanchuk et. al  Nature 451, 549-552 (2008)) and another at Northwestern (Park et. al, Nature 451, 553-556 (2008)).

The basic idea behind these two experiments is to graft different strands of DNA molecules onto particles, creating two “species” of particles with complimentary pairs of DNA strands. These two different types of strands can merge together into a double helix at high temperatures, making a strong connection between particles of the opposite species. The result is a cubic structure, similar to ion salts.

The nanoparticle crystals held together by DNA molecules are rather fragile, and most of volume is occupied by water. The typical size of the unit cell is on the order of 35-50 nm, with particle size just 10 nm in size. The nanoparticles therefore occupy only a tiny fraction of the total crystal volume, with density of such “fluffy” crystals less than milk foam in your latte.

Near-Field X-ray Speckle

There is a brand new paper that appeared today in Nature Physics by Cerbino and co-workers that describes a new x-ray coherent technique based on observations of Near-Field Speckle pattern.

Typically x-ray speckles (or visible light speckles) are observed in Fraunhoffer, or far-field diffraction regime, in which parallel beam approximation can be applied.

The other extreme regime is the near-field (aka Fresnel) geometry, where the detector is placed in the relative vicinity of the sample. This regime is often ignored by scientists because of the complicated scattering patterns caused by interference between scattered and transmitted beams.

However, recently it was shown that Fresnel geometry has some advantages – both in terms of performing lensless imaging microscopy: curved wavefronts, resulting from for example focusing Fresnel Zone Plate optics, result in faster convergence of lensless imaging algorythms – see this paper by Williams et al. Phys. Rev. Lett. 97, 025506 (2006) - and now in terms of using near-field x-ray speckle for X-ray Photon Correlation Spectroscopy.

The Near-Field Speckle setup is limited to relatively small Q-range – the example used in the featured paper by Cerbino et. al is covering ultra-small angle scattering range of Q<0.001 inverse nanometers, corresponding to lengthscales on the order of 10 microns. While such low angles are difficult to access with far-field hard x-ray speckle, the same lengthscales can in principle be reached with visible light (laser) speckle – dynamic light scattering techniques. However, one big advantage of x-rays here is their penetrating
ability and no complications due to multiple scattering effects. Therefore, Near Field X-ray Speckle has a lot of potential for use in non-transparent materials and thick specimens where multiple light scattering effects make laser-based measurements difficult.

Quasi-Forbidden Bragg peaks from soft matter

This week’s item is a rather technical Nature Materials paper by Forster et al., “Order causes secondary Bragg peaks in soft materials”[Nature Materials 6, 888 - 893 (2007)].

Atomic crystals can often be well-ordered, meaning that the correlation length on which “perfect” atomic order exist can extend over many thousands (or even millions) unit cells. Grain boundaries, dislocations and other defects are a common cause of breaking the perfectly ordered chain of atoms.

Soft materials – liquid crystals, colloids, mesoporous materials etc. – typically consist of fairly large unit cells and it is more difficult to get these materials as well-ordered as atomic crystals. All atoms are identical, but colloidal solutions, for example, are often fairly polydisperse, and therefore crystallize with some difficulty – if at all. It is no surprise that the correlation lengths – especially when expressed in unit cells – is far shorter in soft matter, compared to atomic crystals, such as Si or Pb.

Correlation lengthscales can be determined via Debye-Scherrer formalism that relates width of the x-ray or neutron scattering peak to the typical coherent domain size within the sample.

Forster et al. address the issue of finite correlation lengths by analysis of secondary “forbidden” (or quasi-forbidden) Bragg reflections. For example, for a perfect body-centered cubic lattice 001 reflection does not exist – only (002), (011) and other indices that add up to an even number. But once you introduce some disorder, these forbidden peaks become “alive”, since destructive interference responsible for precisely canceling out contributions to these forbidden reflections becomes somewhat faulty.

Surprisingly enough, people haven’t dealt much with ordered, but only over short-range distances materials, at least not to the extent of coming up with sophisticated treatment of intensities of these secondary Bragg peaks that can answer questions like: is the material truly homogeneous but has a lot of disorder, or is it “patchy”? Forster’s paper represents a key step in dealing with these important issues.

Two PRLs on Colloidal Glasses

Confocal laser 3D microscopy studies of colloidal dynamics (typical size of particles on the order of a micron) have been producing a lot of results in the literature lately. Here are two PRLs, pulled almost by random out of selection of several other papers on similar topics just this month alone. Both have Eric Weeks of Emory as an author, who also happens to have a nice writeup about confocal microscopy technique.