A shortlived SPPS facility is still producing papers – this week it’s the PRL paper by Aaron Lindenberg and some 28 co-authors ” X-Ray Diffuse Scattering Measurements of Nucleation Dynamics at Femtosecond Resolution” Phys. Rev. Lett. 100, 135502 (2008).
This is yet another pump-probe experiment, where pump is a femtosecond laser which ablates/melts a crystal, and a probe is a sub-picosecond x-ray pulse from SPPS. X-ray probe pulse length is still a limiting parameter in overall time resolution of such pump-probe setups. This experiment had a time resolution of 700 fs, but in the near future at XFEL facilities such as LCLS the time resolution will approach tens of femtosecond.
Lindenberg and coworkers were able to look at both high-angle and small-angle diffuse scattering resulting in ablation process in this time-resolved mode. Their data indicates presence of short-lived nanoscale voids (shown in green in the figure on the right) in the liquid state caused by the laser pulse, and these voids merge together to form larger voids over the timescale of 20 ps or so – claims supported by molecular dynamics simulations. While their data was taken in reciprocal space, by recording ensemble-averaged structure factor S(q) at various time delays from the laser pulse, in the future one could envision fully inverting the speckle patterns shown in the figure above, to obtain a real-space images of the nanoscale voids.
Nature Materials has a News and Views article by Ian Robinson titled “Coherent diffraction: Giant molecules or tiny crystals?”, which reviews recent coherent electron diffraction results by Huang et al. featured here earlier. One of the interesting points made in this mini-review is the phase diagram on the left showing a transition from bulk cubic crystal to decahedral and icosahedral structures, including quasi-molten and liquid phases.
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.
Two recent PRLs are addressing the issue of how two droplets merge into one.
Kamel Fezzaa and Yujie Wang from Argonne use ultrafast x-ray phase-contrast imaging to take sub-microsecond exposures of droplet coalescence, which is complete in just under a milisecond.
The studied liquid droplets are ~1mm (bar size in the image on the left) in size, and can be seen with <5 micron resolution using phase contrast (as opposed to adsorption contrast) using high energy 13keV x-rays. Fezzaa and Wang cleverly used the hybrid filling pattern of Advanced Photon Source, where each electron bunch produces a short x-ray pulse 472 nanoseconds long used for imaging, with each pulse separated from the next one by 3.6 microseconds. The result is a sub-microsecond “shutter time” defined by the length of each pulse, with consecutive images taken 3.6 microseconds apart.
Of particular interest in this study is the stability of torroidal air bubble formed due to air trapped by the two rapidly coalescing droplet menisci. Fezzaa and Wang show for the first time that the torroidal bubble remains trapped until some 400 microsecond after the droplets start merging.
The second recent paper on this topic of droplet coalescence is by Sara Case and Sid Nagel at University of Chicago. PRL 100, 084503 (2008)
Case and Nagel abandon the visual approach to studies of ultrafast coalescence process, and instead adopt a technique which measures the changes in conductivity across the connection between the two droplets as a function of time. When the droplets begin to coalesce, the effective resistivity is high, since it is defined primarily by the width of the narrow region where the two droplets touch each other. As they coalesce, this resistivity will drop. This technique proves to be especially useful in the timescale range from sub-microsecond to hundreds of microsecond. Case and Nagel observe a cross-over in power-law behavior for R(t) from 1/t for small t to 1/√t at large t, but do not see time-dependent fluctuations in R(t) which would be the signature of the connected menisci repeatedly disconnecting and reconnecting again.
Observations of ball-lightning – long-lived (2 to 50 seconds) bright fireballs size of baseball to beach ball – have been observed for centuries. Previous theoretical work ascribes the longevity of the ball lightnings to the slow oxidation process of silicon, forming nanoparticle networks. Now, Mitchell et al. Phys. Rev. Lett. 100, 065001 (2008) have created artificial ball-lightning using localized microwaves and have studied them in-situ with synchrotron radiation (using small-angle x-ray scattering), proving that the fireballs do indeed contain nanoparticles with sizes of the order 50 nm.
Small Angle X-ray Scattering, or SAXS, has been used to all kinds of samples – liquid, vapor or solid, but this may be the first time this technique was applied to plasma.
Meanwhile, Wang et al. (Nature Physics, advanced publication) performed an ultra-fast time resolved study of morphology of optically dense jets from fuel injector nozzle. The achieved microsecond temportal resolution is due to application of time-resolved full-field phase contrast imaging. Unlike typical radiography (such as used at the dentist’s office) that is sensitive to the mass density (adsorption) of material through intensity measurements, phase contrast measurements rely on phase changes.
Techniques using visible light scattering are suffering from problems due to multiple scattering from various interfaces of jet droplets – ironically, these interfaces are precisely what serves as a contrast mechanism in the x-ray phase contrast imaging technique used by Wang et. al in this study.
In an article Phys. Rev. Lett. 100, 024501 (2008) Nicolas Bremond, Abdou Thiam, and Jerome Bibette are studying coalescence of the emulsion droplets (sizes on the order of tens of microns) in microfluidic channels. What they find is that coalescence of the droplets happens when the dropets begin to separate, and proceeds in a cascading “chain reaction” of sort, with an entire train of particles coalescing together, once the first pair of particles start merging.
Some flat, sheet-like shaped fish, like rays and skates, can glide along the ocean bottom almost effortlessly, and the authors of this paper try to explore the parameter space and find the best conditions under which elastic sheets (or foils) have the easiest time to propagate in a Newtonian or non-Newtonian fluids.
The basic summary of the result is that thin (unlike the partially wetting macroscopic droplet shown on the left) liquid films apeear to be heavily pinned (at least at longer wavelengths) – and as a result do not show the capillary fluctuations that exist at free surfaces of liquids. Dynamically these liquid films appear very similar to solids.
By using Small Angle Neutron Scatterig (SANS) they are able to study water confined in mesoporous silica channels and find a density minimum at -63 deg C.
The density *maximum* of water at 4 deg C is a well known anomaly – one of many, many anomalies of water. The 4 deg maximum has some interesting consequences for living organisms – the denser 4 C water “sinks” to the bottom and makes it possible for fish and other water organisms to survive the winters, since bottoms of lakes and ponds don’t freeze out even in the harshest winters and remain at “balmy” 4 C.
Now SANS measurements find the opposite phenomena – a density *minimum*, at -63 C. But since bulk water in equilibrium freezes at 0 C, the researchers had to play some tricks to produce metastable, “supercooled” water. In absence of nucleation centers one could supercool bulk water by some 20-30 degrees – in fact this is the state in which water can exist in atmospheric clouds, in a form of supercooled droplets. But to go further than that one has to confine water to nanoscopic cylindrical channels, as was done in this work.
Several other previous studies on this topic by Chen group at MIT:
This week’s journal club items (delayed by a few days due to travel) are two recent papers by IBM group on Vapor-Liquid-Solid growth of silicon and germanium nanowires, using properties of AuSi and AuGe low-temperature eutectic alloys.
This week’s Journal Club update is inspired by yesterday’s colloquium by Helmut Dosch, one of the directors of Max Planck institute. His excellent talk was on solid-liquid interfaces, one of the most puzzling and inaccessible types of interfaces. Solids and liquids have very different properties, and what happens (on atomic scale) when the two of them meet is currently not known. Little of what is known can be attributed to Max Planck/Stuttgart group lead by Dosch – it could be argued his group made more progress than the rest of the world combined. Here’s a sampler of some of the work Helmut described, with majority already featured in our shared liquids topic news selection.
This is my first post since recent move to wordpress.
This week we have four items:
1. An excellent review in this week’s Science by Simon Billinge and Igor Levin titled “The Problem with Determining Atomic Structure at the Nanoscale”. The fundamental problem of determining atomic structure of non-periodic materials is an old one, but is becoming especially critical in context of nano-structures, where finite correlation lengths on the order of nanometers, even in case of crystalline structures, makes it impossible to apply standard crystallography methods applicable for materials with long-range periodicity. Authors focus on PDF, TEM, EXAFS and other techniques, but also briefly mention newly available coherent x-ray scattering techniques based on phase-retrieval algorithms.
A shortlived SPPS facility is still producing papers – this week it’s the PRL paper by Aaron Lindenberg and some 28 co-authors ” X-Ray Diffuse Scattering Measurements of Nucleation Dynamics at Femtosecond Resolution” Phys. Rev. Lett. 100, 135502 (2008).
Lindenberg and coworkers were able to look at both high-angle and small-angle diffuse scattering resulting in ablation process in this time-resolved mode. Their data indicates presence of short-lived nanoscale voids (shown in green in the figure on the right) in the liquid state caused by the laser pulse, and these voids merge together to form larger voids over the timescale of 20 ps or so – claims supported by molecular dynamics simulations. While their data was taken in reciprocal space, by recording ensemble-averaged structure factor S(q) at various time delays from the laser pulse, in the future one could envision fully inverting the speckle patterns shown in the figure above, to obtain a real-space images of the nanoscale voids.
at University of Chicago.
Observations of ball-lightning – long-lived (2 to 50 seconds) bright fireballs size of baseball to beach ball – have been observed for centuries. 
and have studied them in-situ with synchrotron radiation (using small-angle x-ray scattering), proving that the fireballs do indeed contain nanoparticles with sizes of the order 50 nm.
achieved microsecond temportal resolution is due to application of time-resolved full-field phase contrast imaging. Unlike typical radiography (such as used at the dentist’s office) that is sensitive to the mass density (adsorption) of material through intensity measurements, phase contrast measurements rely on phase changes.
A recent paper by 
inclined) 
This week’s journal club items (delayed by a few days due to travel) are two recent papers by IBM group on Vapor-Liquid-Solid growth of silicon and germanium nanowires, using properties of AuSi and AuGe low-temperature eutectic alloys.
