Foam bubbles finally brought to order

In 1994, Denis Weaire and Robert Phelan of Trinity College Dublin’s School of Physics made a landmark discovery in foam physics, and created a new ideal structure of foam.  It is the most efficient way to partition space into equal volume cells while minimising surface area – something soap bubbles strive to do in nature.   Their geometry of soap bubbles improved on a previous principle devised by the physicist, Lord Kelvin a century ago.

The Weaire-Phelan structure consists of two kinds of polyhedral bubbles with twelve and fourteen sides respectively. The structure can be cut along planes, showing the existence of layers of bubbles. Since its introduction in 1994 it has played an important role in theory and simulation of foams, for example in the study of elastic properties.

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The structure went on to inspire the design of the 2008 Olympic Games' iconic building, the Water Cube in the National Aquatic Center in Beijing.  Many millions have admired its elegant framework of steel beams, which follow the pattern of the ideal foam.

The physicists have now gone a step further and this month succeeded in turning the mathematical concept into real foam.

Now it exists in reality, thanks to the work of a team led by Dr. Ruggero Gabbrielli, from the University of Trento, in an SFI-funded visit to Trinity College.  Back in 1994 while the concept was computed, with the help of the software by Kenneth Brakke, they were unable to fabricate the new foam.

Acknowledging that the previous failures could be put down to the shape of the containers used, Gabbrielli along with Brakke designed a receptacle whose walls had an intricate form that would encourage and accommodate the Weaire-Phelan bubbles.  It was made in Trinity’s nanoscience institute, CRANN  and proved an instant success when bubbles of the right size were introduced into it.

“Wonderful!” says Weaire, now an Emeritus Professor in the School of Physics. “We shall call this the Italian Job.  It opens up a lot of further possibilities.”

In response to whether the new foam could be of any practical use: “Not immediately”, says Professor Stefan Hutzler, Head of the Foams and Complex Systems Research group in the School of Physics. “Let’s just admire its extraordinary beauty first.  But in solidified form and on various scales, such exotic ordered foams could find applications as chemical filters, heat exchangers and optical components.”

“It’d be interesting to come up with a proof of optimality,” Ruggero says.  “Scientists have been looking at this problem for quite a while, but a rigorous result is still missing.”

The paper reporting the fabrication of the Weaire-Phelan structure was accepted for publication in the time-honored science journal Philosophical Magazine Letters on the 25th of November 2011.  This is the same journal in which both Kelvin (in 1887) and Weaire and Phelan (in 1993) published their work on the structure of ideal foam.

Provided by Trinity College Dublin (news : web)

Glowing beacons reveal hidden order in dynamical systems: Experimental confirmation of a fundamental physical theorem

 A dynamical system in which repeated measurements on a single particle yield the same mean result as a single measurement of the whole ensemble is said to be ergodic. The ergodic theorem expresses a fundamental physical principle, and its validity for diffusive processes has now been demonstrated.

The so-called ergodic theorem formulates a fundamental physical principle relating to the behavior of dynamical systems. Essentially the theorem states that in a multiparticle system each individual particle behaves just as "chaotically" as does the system as a whole. In other words, one can extrapolate from the behavior of a single element to that of the whole system. Strangely enough, in spite of its wide-ranging implications, the theorem has not been rigorously tested experimentally. A collaborative effort mounted by Professor Christoph Bräuchle's team in the Department of Chemistry at LMU Munich and Professor Jörg Kärger's group at Leipzig University has now confirmed the validity of the theorem by measuring the diffusive behavior of ensembles of particles and the trajectories of single molecules in the same system. Using fluorescent molecules as tracers and high-resolution imaging methods, the LMU investigators were able to track the paths of individual molecules, while the Leipzig group studied the collective behavior of the whole ensemble. "It will be very interesting to take a closer look at systems that do not conform to the tenets of the ergodic theorem and to determine the reasons for their aberrant behavior," says Bräuchle.

The term "diffusion" refers to the random motion of particles, such as atoms and molecules, under the influence of thermal energy. This physical process is an essential component of innumerable phenomena in nature, and also plays a crucial role in many technological procedures. For instance, in virtually all chemical reactions, diffusion is responsible for bringing reactants sufficiently close together to enable them to react at all. It is generally accepted that the ergodic theorem is applicable to the dynamics of diffusive processes. The theory basically states that repeated measurements of a given variable -- such as the distance covered by a particle in a given time interval -- should yield the same average value as a single measurement of the same variable on a collection of particles -- provided the system considered is in a state of equilibrium. However, as Kärger points out, "although diffusive processes have been investigated for the past 150 years, the principle of ergodicity has not yet been experimentally verified."

This is because it has so far been possible to quantify diffusive processes only by means of ensemble measurements -- i.e. measurements of many particles simultaneously. One of the most informative methods for this purpose is pulsed-field gradient nuclear magnetic resonance (PFG-NMR), a technique for which Kärger and his group are well known. The actual trajectory of a single particle, on the other hand, could not be observed directly. "With the development of single-molecule spectroscopy and single-molecule microscopy, we can now follow the trajectories -- and therefore monitor the diffusion behavior -- of single molecules," Bräuchle explains. Optical tracking methods visualize molecules on the basis of their fluorescence, making it possible for their positions to be localized and monitored with a precision of a few nanometers.

This still leaves one problem to be solved -- successful application of the two methods requires very different, indeed apparently conflicting, conditions. NMR measurements need high concentrations of molecules with large diffusion coefficients, while single-molecule spectroscopy works best with extremely dilute solutions of species with small diffusion coefficients. By using particular organic dyes with high fluorescence yields in combination with porous silicate glasses containing networks of nanometer-sized channels in which the dye molecules can diffuse, the researchers were able to create conditions that were compatible with both methods. This experimental set-up allowed them to perform single-molecule and ensemble measurements on the same system.

When the two teams compared their data, they found that the diffusion coefficients (the parameter that describes diffusive motion) obtained by the two techniques agreed with each other -- providing the first experimental confirmation of the ergodic theorem in this context. The next step will be to examine systems in which the theory does not apply. "The diffusion of nanoparticles in cells looks like an interesting example," says Bräuchle, "and for us the important thing is to find out why the ergodic theorem doesn't hold in this case."

The project in Munich was carried out under the support of the Cluster of Excellence "Nanosystems Initiative Munich" (NIM) and DFG Priority Program 749 (Dynamics and Intermediate Molecular Transformations), while the work in Leipzig was supported by the DFG as part of Research Unit 877 (From Local Constraints to Macroscopic Transport).

The above story is reprinted from materials provided by Ludwig-Maximilians-Universitaet Muenchen (LMU).

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

Florian Feil, Sergej Naumov, Jens Michaelis, Rustem Valiullin, Dirk Enke, Jörg Kärger, Christoph Bräuchle. Single-Particle and Ensemble Diffusivities-Test of Ergodicity. Angewandte Chemie, 2011; DOI: 10.1002/ange.201105388

A hidden order unraveled: Microscopic views on quantum fluctuations

 Fluctuations are fundamental to many physical phenomena in our everyday life, such as the phase transitions from a liquid into a gas or from a solid into a liquid. But even at absolute zero temperature, where all motion in the classical world is frozen out, special quantum mechanical fluctuations prevail that can drive the transition between two quantum phases.

Now a team around Immanuel Bloch and Stefan Kuhr at Ludwig-Maximilians University (LMU) and the Max Planck Institute of Quantum Optics (MPQ) has succeeded in directly observing such quantum fluctuations. Using a high resolution microscope, they were able to image quantum-correlated particle-hole pairs in a gas of ultracold atoms. This allowed the physicists to unravel a hidden order in the crystal and to characterize the different phases of the quantum gas. The work was performed together with scientists from the Theory Division at the MPQ and ETH Zurich. These measurements open new ways to characterize novel quantum phases of matter.

The scientists start by cooling a small cloud of rubidium atoms down to a temperature near absolute zero, about minus 273 degree Celsius. The ensemble is then subjected to a light field that severely restricts the motion of the particles along one-dimensional tubes of light aligned in parallel. An additional standing laser wave along the tubes creates a one-dimensional optical lattice that holds the atoms in a periodic array of bright and dark regions of light.

The atoms move in the periodic light field like electrons in solids. As these can be electric conductors or insulators, also the one-dimensional quantum gases can behave like a superfluid or like an insulator at low temperatures. In particular, the height of the optical lattice potential plays an important role: it determines whether the atom is fixed on a particular lattice site or whether is able to move to a neighbouring site. At very large lattice depths, each lattice site is occupied by exactly one atom. This highly ordered state is called a "Mott insulator," after the British physicist and Nobel laureate Sir Neville Mott. When the lattice depth is decreased slightly, the atoms have enough energy to reach a neighbouring site by quantum mechanical tunneling. In this way, pairs of empty and doubly occupied sites emerge, so-called particle-hole pairs. Intriguingly, these quantum fluctuations also occur at absolute zero temperature, when all movement in the classical world is frozen out. The position of the quantum-correlated particle-hole pairs in the crystal is completely undetermined and is fixed only by the measurement process.

In recent experiments, the physicists around Stefan Kuhr and Immanuel Bloch had already developed a method, which allowed to image single atoms lattice site by lattice site. The atoms are cooled using laser beams, and the fluorescence photons emitted in this process are used to observe the atoms with a high resolution microscope. Holes naturally show up as dark spots, but so do doubly occupied sites as the two particles kick each other out of the lattice in the experiment. Therefore particle-hole pairs appear as two neighbouring dark lattice sites. "With our technique, we can directly observe this fundamental quantum phenomenon for the first time," describes doctoral student Manuel Endres enthusiastically.

The physicists measure the number of neighbouring particle-hole pairs through a correlation function. With increasing kinetic energy, more and more particles tunnel to neighbouring sites and the pair correlations increase. However, when the number of particle-hole pairs is very large, it becomes difficult to unambiguously identify them. Hence the correlation function takes on smaller values. Finally, the ordered state of a Mott insulator vanishes completely und the quantum gas becomes a superfluid again. Here fluctuations of holes and particles occur independently. The correlation function measured in the experiment is very well reproduced by model calculations, which were performed by scientists from the Theory Division at the MPQ and the ETH Zurich. Interestingly, the same investigations on two-dimensional quantum-gases clearly showed that quantum fluctuations are not as prominent as in one-dimensional systems.

The scientists extended their analysis to correlations between several lattice sites along a string. Such non-local correlation functions contain important information about the underlying many-body system and can be used as an order parameter to characterize different quantum phases. In the experiment described here, such non-local order parameters have been measured for the first time. In the future, the scientists plan to use these measurements for the detection of topological quantum phases. These can be useful for robust quantum computers and could help to understand superconductivity at high temperatures. (MPQ)

Story Source:

The above story is reprinted (with editorial adaptations ) from materials provided by Ludwig-Maximilians-Universität München.

Journal Reference:

M. Endres, M. Cheneau, T. Fukuhara, C. Weitenberg, P. Schauss, C. Gross, L. Mazza, M. C. Banuls, L. Pollet, I. Bloch, S. Kuhr. Observation of Correlated Particle-Hole Pairs and String Order in Low-Dimensional Mott Insulators. Science, 2011; 334 (6053): 200 DOI: 10.1126/science.1209284

Making materials to order: Fine-tuning mechanical, electrical, thermal, other properties of composites

 A team of researchers at MIT has found a way to make complex composite materials whose attributes can be fine-tuned to give various desirable combinations of properties such as stiffness, strength, resistance to impacts and energy dissipation.

The key feature of the new composites is a “co-continuous” structure of two different materials with very different properties, creating a material combining aspects of both. The co-continuous structure means that the two interleaved materials each form a kind of three-dimensional lattice whose pieces are fully connected to each other from side to side, front to back, and top to bottom.

The research — by postdoc Lifeng Wang, who worked with undergraduate Jacky Lau and professors Mary Boyce and Edwin Thomas — was published in April in the journal Advanced Materials. The research was funded by the U.S. Army through MIT’s Institute for Soldier Nanotechnologies.

The initial objective of the research was to “try to design a material that can absorb energy under extreme loading situations,” Wang explains. Such a material could be used as shielding for trucks or aircraft, he says: “It could be lightweight and efficient, flexible, not just a solid mantle” like most present-day armor.

In most conventional materials — even modern advanced composites — once cracks start to form they tend to propagate through the material, Wang says. But in the new co-continuous materials, crack propagation is limited within the microstructure, he says, making them highly “damage tolerant” even when subjected to many crack-producing events.

Some existing composite materials, such as carbon-carbon composites that use fibers embedded in another material, can have great strength in the direction parallel to the fibers, but not much strength in other directions. Because of the continuous 3-D structure of the new composites, their strength is nearly equal in all dimensions, Wang says.

Thomas, the Morris Cohen Professor of Materials Science and Engineering and head of MIT’s Department of Materials Science and Engineering, says that in most existing composite materials, the fibers form disordered mass with “zero continuity,” while the other material — typically a resin that fills the space and then hardens — is continuous and connected in three dimensions. The material that forms the continuous structure “tends to dominate the properties” of the composite, he says. “But when both materials are continuous, you can get benefits that are surprisingly synergistic, not just additive.”

In their experiments, the MIT researchers combined two polymer materials with quite different properties: one that is glass-like, strong but brittle, and another that is rubber-like, not so strong, but tough and resilient. The result, Thomas says, was a material “that is stiff, strong and tough.”

In the quest for new materials with specific combinations of properties, Thomas says, “we’ve pretty much exhausted the natural homogeneous materials,” but the new fabrication techniques developed in this research can “take to another level” the material development process.

The researchers designed the new materials through computer simulations, then made samples that were tested under laboratory conditions. The simulations and the experimental data “agree nicely,” Thomas says. While this initial research focused on tuning the material’s mechanical properties, the same principles could be applied to controlling a material’s electrical, thermal, optical or other properties, the researchers say.

The process could even be used to make materials with "tunable" properties: for example, to allow certain frequencies of phonons — waves of heat or sound — to pass through while blocking others, with the selection of frequencies tuned through changes in mechanical pressure. It could also be used to make materials with shape-memory properties, which could be compressed and then spring back to a specific form.

Richard Vaia, acting chief of the Nanostructured and Biological Materials Branch at Wright-Patterson Air Force Base in Ohio, says this work is “an exciting demonstration of the crucial importance of architecture in materials-by-design concepts.”

Vaia says this work “provides an example of the future of composite and hybrid materialstechnology where direct-write fabrication, printing technologies and complex fiber-weaving techniques are not simply manufacturing tools, but an integral part of a robust, implementable digital design and manufacturing paradigm.”

The next step in the research, Thomas says, is to make co-continuous composites out of pairs of materials whose properties are even more drastically different than those used in the initial experiments, such as metal with ceramic, or polymer with metal. Such composites could be very different from any materials made before, he says.
This story is republished courtesy of MIT News (http://web.mit.edu/newsoffice/), a popular site that covers news about MIT research, innovation and teaching.

Provided by Massachusetts Institute of Technology (news : web)

'Tall order' sunlight-to-hydrogen system works, neutron analysis confirms

 Researchers at the Department of Energy's Oak Ridge National Laboratory have developed a biohybrid photoconversion system -- based on the interaction of photosynthetic plant proteins with synthetic polymers -- that can convert visible light into hydrogen fuel.

Photosynthesis, the natural process carried out by plants, algae and some bacterial species, converts sunlight energy into chemical energy and sustains much of the life on earth. Researchers have long sought inspiration from photosynthesis to develop new materials to harness the sun's energy for electricity and fuel production.

In a step toward synthetic solar conversion systems, the ORNL researchers have demonstrated and confirmed with small-angle neutron scattering analysis that light harvesting complex II (LHC-II) proteins can self-assemble with polymers into a synthetic membrane structure and produce hydrogen.

The researchers envision energy-producing photoconversion systems similar to photovoltaic cells that generate hydrogen fuel, comparable to the way plants and other photosynthetic organisms convert light to energy.

"Making a, self-repairing synthetic photoconversion system is a pretty tall order. The ability to control structure and order in these materials for self-repair is of interest because, as the system degrades, it loses its effectiveness," ORNL researcher Hugh O'Neill, of the lab's Center for Structural Molecular Biology, said.

"This is the first example of a protein altering the phase behavior of a synthetic polymer that we have found in the literature. This finding could be exploited for the introduction of self-repair mechanisms in future solar conversion systems," he said.

Small angle neutron scattering analysis performed at ORNL's High Flux Isotope Reactor (HFIR) showed that the LHC-II, when introduced into a liquid environment that contained polymers, interacted with polymers to form lamellar sheets similar to those found in natural photosynthetic membranes.

The ability of LHC-II to force the assembly of structural polymers into an ordered, layered state -- instead of languishing in an ineffectual mush -- could make possible the development of biohybrid photoconversion systems. These systems would consist of high surface area, light-collecting panes that use the proteins combined with a catalyst such as platinum to convert the sunlight into hydrogen, which could be used for fuel.

The research builds on previous ORNL investigations into the energy-conversion capabilities of platinized photosystem I complexes -- and how synthetic systems based on plant biochemistry can become part of the solution to the global energy challenge.

"We're building on the photosynthesis research to explore the development of self-assembly in biohybrid systems. The neutron studies give us direct evidence that this is occurring," O'Neill said.

The researchers confirmed the proteins' structural behavior through analysis with HFIR's Bio-SANS, a small-angle neutron scattering instrument specifically designed for analysis of biomolecular materials.

"Cold source" neutrons, in which energy is removed by passing them through cryogenically chilled hydrogen, are ideal for studying the molecular structures of biological tissue and polymers.

The LHC-II protein for the experiment was derived from a simple source: spinach procured from a local produce section, then processed to separate the LHC-II proteins from other cellular components. Eventually, the protein could be synthetically produced and optimized to respond to light.

O'Neill said the primary role of the LHC-II protein is as a solar collector, absorbing sunlight and transferring it to the photosynthetic reaction centers, maximizing their output. "However, this study shows that LHC-II can also carry out electron transfer reactions, a role not known to occur in vivo," he said.

The research team, which came from various laboratory organizations including its Chemical Sciences Division, Neutron Scattering Sciences Division, the Center for Structural Molecular Biology and the Center for Nanophase Materials Sciences, consisted of O'Neill, William T. Heller, and Kunlun Hong, all of ORNL; Dimitry Smolensky of the University of Tennessee; and Mateus Cardoso, a former postdoctoral researcher at ORNL now of the Laboratio Nacional de Luz Sincrotron in Brazil.

"That's one of the nice things about working at a national laboratory. Expertise is available from a variety of organizations," O'Neill said.

The work, published in the journal Energy & Environmental Science, was supported with Laboratory-Directed Research and Development funding. HFIR is supported by the DOE Office of Science.

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by DOE/Oak Ridge National Laboratory.

Journal Reference:

Mateus B. Cardoso, Dmitriy Smolensky, William T. Heller, Kunlun Hong, Hugh O'Neill. Supramolecular assembly of biohybrid photoconversion systems. Energy & Environmental Science, 2011; 4 (1): 181 DOI: 10.1039/C0EE00369G

Note: If no author is given, the source is cited instead.

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