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Researchers make green chemistry advance with new catalyst for reduction of carbon dioxide

Researchers at Oregon State University have made a key advance in the green chemistry pursuit of converting the greenhouse gas carbon dioxide into reusable forms of carbon via electrochemical reduction.

Published in Nature Energy, the study led by Zhenxing Feng of the OSU College of Engineering and colleagues at Southern University of Science and Technology in China and Stanford University describes a new type of electrocatalyst.

The catalyst can selectively promote a CO2 reduction reaction resulting in a desired product — carbon monoxide was the choice in this research. A catalyst is anything that speeds the rate of a chemical reaction without being consumed by the reaction.

“The reduction of carbon dioxide is beneficial for a clean environment and sustainable development,” said Feng, assistant professor of chemical engineering. “In contrast to traditional CO2 reduction that uses chemical methods at high temperatures with a high demand of extra energy, electrochemical CO2 reduction reactions can be performed at room temperature using liquid solution. And the electricity required for electrochemical CO2 reduction can be obtained from renewable energy sources such as solar power, thus enabling completely green processes.”

A reduction reaction means one of the atoms involved gains one or more electrons. In the electrochemical reduction of carbon dioxide, metal nanocatalysts have shown the potential to selectively reduce CO2 to a particular carbon product. Controlling the nanostructure is critical for understanding the reaction mechanism and for optimizing the performance of the nanocatalyst in the pursuit of specific products, such as carbon monoxide, formic acid or methane, that are important for other chemical processes and products.

“However, due to many possible reaction pathways for different products, carbon dioxide reduction reactions have historically had low selectivity and efficiency,” Feng said. “The electrocatalysts need to promote the reaction with high selectivity to get one certain product, carbon monoxide in our case. Despite many efforts in this field, there had been little progress.”

Feng and his research co-leaders tried a new strategy. They made nickel phthalocyanine as a molecularly engineered electrocatalyst and found it showed superior efficiency at high current densities for converting CO2 to carbon monoxide in a gas-diffusion electrode device, with stable operation for 40 hours.

“To understand the reaction mechanism of our catalyst, my group at OSU used X-ray absorption spectroscopy to monitor the catalyst’s change during the reaction processes, confirming the role of the catalyst in the reaction,” Feng said. “This collaborative work demonstrates a high-performance catalyst for green processes of electrochemical CO2 reduction reactions. It also sheds light on the reaction mechanism of our catalyst, which can guide the future development of energy conversion devices as we work toward a negative-carbon economy.”

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Materials provided by Oregon State University. Original written by Steve Lundeberg. Note: Content may be edited for style and length.

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New method to determine the origin of stardust in meteorites

Scientists have made a key discovery thanks to stardust found in meteorites, shedding light on the origin of crucial chemical elements.

Meteorites are critical to understanding the beginning of our solar system and how it has evolved over time. However, some meteorites contain grains of stardust that predate the formation of our solar system and are now providing important information about how the elements in the universe formed.

In a study published by Physical Review Letters, researchers from the University of Surrey detail how they made a key discovery connected to the “pre-solar grains” found in primitive meteorites. This discovery has provided new insights into the nature of stellar explosions and the origin of the chemical elements. It has also provided a new method for astronomical research.

Dr Gavin Lotay, Nuclear Astrophysicist and Director of Learning and Teaching at the University of Surrey, said: “Tiny pre-solar grains, about one micron in size, are the residuals of stellar explosions that occurred in the distant past, long before our solar system existed. Stellar debris eventually became wedged into meteorites that, in turn, crashed into the Earth.”

One of the most frequent stellar explosions to occur in our galaxy is called a nova, which involves a binary star system consisting of a main sequence star orbiting a white dwarf star — an extremely dense star that can be the size of Earth but has the mass of our Sun. Matter from the main star is continually pulled away by the white dwarf because of its intense gravitational field. This deposited material initiates a thermonuclear explosion every 1,000 to 100,000 years and the white dwarf ejects the equivalent of the mass of more than thirty Earths into interstellar space. In contrast, a supernova involves a single collapsing star and, when it explodes, it ejects almost all of its mass.

As novae continually enrich our galaxy with chemical elements, they have been the subject of intense astronomical investigations for decades. Much has been learned from them about the origin of the heavier elements, for example. However, a number of key puzzles remain.

Dr Lotay continues: “A new way of studying these phenomena is by analysing the chemical and isotopic composition of the pre-solar grains in meteorites. Of particular importance to our research is a specific nuclear reaction that occurs in novae and supernovae — proton capture on an isotope of chlorine — which we can only indirectly study in the laboratory.”

In conducting their experiment, the team, led by Dr Lotay and Surrey PhD student Adam Kennington (also a former Surrey undergraduate), pioneered a new research approach. It involves the use of the Gamma-Ray Energy Tracking In-beam Array (GRETINA) coupled to the Fragment Mass Analyzer at the Argonne Tandem Linac Accelerator System (ATLAS), USA. GRETINA is a state-of-the-art detection system able to trace the path of gamma rays (g-ray) emitted from nuclear reactions. It is one of only two such systems in the world that utilise this novel technology.

Using GRETINA, the team completed the first detailed g-ray spectroscopy study of an astronomically important nucleus, argon-34, and were able to calculate the expected abundance of sulfur isotopes produced in nova explosions.

Adam Kennington said: “It’s extremely exciting to think that, by studying the microscopic nuclear properties of argon-34, it may now be possible to determine whether a particular grain of stardust comes from a nova or a supernova.”

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Aquatic robots can remove contaminant particles from water

Corals in the Ocean are made up of coral polyps, a small soft creature with a stem and tentacles, they are responsible for nourishing the corals, and aid the coral’s survival by generating self-made currents through motion of their soft bodies.

Scientists from WMG at the University of Warwick, led by Eindhoven University of Technology in the Netherlands, developed a 1cm by 1cm wireless artificial aquatic polyp, which can remove contaminants from water. Apart from cleaning, this soft robot could be also used in medical diagnostic devices by aiding in picking up and transporting specific cells for analysis.

In the paper, ‘An artificial aquatic polyp that wirelessly attracts, grasps, and releases objects’ researchers demonstrate how their artificial aquatic polyp moves under the influence of a magnetic field, while the tentacles are triggered by light. A rotating magnetic field under the device drives a rotating motion of the artificial polyp’s stem. This motion results in the generation of an attractive flow which can guide suspended targets, such as oil droplets, towards the artificial polyp.

Once the targets are within reach, UV light can be used to activate the polyp’s tentacles, composed of photo-active liquid crystal polymers, which then bend towards the light enclosing the passing target in the polyp’s grasp. Target release is then possible through illumination with blue light.

Dr Harkamaljot Kandail, from WMG, University of Warwick was responsible for creating state of the art 3D simulations of the artificial aquatic polyps. The simulations are important to help understand and elucidate the stem and tentacles generate the flow fields that can attract the particles in the water.

The simulations were then used to optimise the shape of the tentacles so that the floating particles could be grabbed quickly and efficiently.

Dr Harkamaljot Kandail, from WMG, University of Warwick comments:

“Corals are such a valuable ecosystem in our oceans, I hope that the artificial aquatic polyps can be further developed to collect contaminant particles in real applications. The next stage for us to overcome before being able to do this is to successfully scale up the technology from laboratory to pilot scale. To do so we need to design an array of polyps which work harmoniously together where one polyp can capture the particle and pass it along for removal.”

Marina Pilz Da Cunha, from the Eindhoven University of Technology, Netherlands adds:

“The artificial aquatic polyp serves as a proof of concept to demonstrate the potential of actuator assemblies and serves as an inspiration for future devices. It exemplifies how motion of different stimuli-responsive polymers can be harnessed to perform wirelessly controlled tasks in an aquatic environment.”

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A titanate nanowire mask that can eliminate pathogens

As part of attempts to curtail the Covid-19 pandemic, paper masks are increasingly being made mandatory. Their relative effectiveness is no longer in question, but their widespread use has a number of drawbacks. These include the environmental impact of disposable masks made from layers of non-woven polypropylene plastic microfibres. Moreover, they merely trap pathogens instead of destroying them. “In a hospital setting, these masks are placed in special bins and handled appropriately,” says László Forró, head of EPFL’s Laboratory of Physics of Complex Matter. “However, their use in the wider world — where they are tossed into open waste bins and even left on the street — can turn them into new sources of contamination.”

Researchers in Forró’s lab are working on a promising solution to this problem: a membrane made of titanium oxide nanowires, similar in appearance to filter paper but with antibacterial and antiviral properties.

Their material works by using the photocatalytic properties of titanium dioxide. When exposed to ultraviolet radiation, the fibers convert resident moisture into oxidizing agents such as hydrogen peroxide, which have the ability to destroy pathogens. “Since our filter is exceptionally good at absorbing moisture, it can trap droplets that carry viruses and bacteria,” says Forró. “This creates a favorable environment for the oxidation process, which is triggered by light.”

The researchers’ work appears today in Advanced Functional Materials, and includes experiments that demonstrate the membrane’s ability to destroy E. coli, the reference bacterium in biomedical research, and DNA strands in a matter of seconds. Based on these results, the researchers assert — although this remains to be demonstrated experimentally — that the process would be equally successful on a wide range of viruses, including SARS-CoV-2.

Their article also states that manufacturing such membranes would be feasible on a large scale: the laboratory’s equipment alone is capable of producing up to 200 m2 of filter paper per week, or enough for up to 80,000 masks per month. Moreover, the masks could be sterilized and reused up a thousand times. This would alleviate shortages and substantially reduce the amount of waste created by disposable surgical masks. Finally, the manufacturing process, which involves calcining the titanite nanowires, makes them stable and prevents the risk of nanoparticles being inhaled by the user.

A start-up named Swoxid is already preparing to move the technology out of the lab. “The membranes could also be used in air treatment applications such as ventilation and air conditioning systems as well as in personal protective equipment,” says Endre Horváth, the article’s lead author and co-founder of Swoxid.

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Why shaving dulls even the sharpest of razors

Razors, scalpels, and knives are commonly made from stainless steel, honed to a razor-sharp edge and coated with even harder materials such as diamond-like carbon. However, knives require regular sharpening, while razors are routinely replaced after cutting materials far softer than the blades themselves.

Now engineers at MIT have studied the simple act of shaving up close, observing how a razor blade can be damaged as it cuts human hair — a material that is 50 times softer than the blade itself. They found that hair shaving deforms a blade in a way that is more complex than simply wearing down the edge over time. In fact, a single strand of hair can cause the edge of a blade to chip under specific conditions. Once an initial crack forms, the blade is vulnerable to further chipping. As more cracks accumulate around the initial chip, the razor’s edge can quickly dull.

The blade’s microscopic structure plays a key role, the team found. The blade is more prone to chipping if the microstructure of the steel is not uniform. The blade’s approaching angle to a strand of hair and the presence of defects in the steel’s microscopic structure also play a role in initiating cracks.

The team’s findings may also offer clues on how to preserve a blade’s sharpness. For instance, in slicing vegetables, a chef might consider cutting straight down, rather than at an angle. And in designing longer-lasting, more chip-resistant blades, manufacturers might consider making knives from more homogenous materials.

“Our main goal was to understand a problem that more or less everyone is aware of: why blades become useless when they interact with much softer material,” says C. Cem Tasan, the Thomas B. King Associate Professor of Metallurgy at MIT. “We found the main ingredients of failure, which enabled us to determine a new processing path to make blades that can last longer.”

Tasan and his colleagues have published their results in the journal Science. His co-authors are Gianluca Roscioli, lead author and MIT graduate student, and Seyedeh Mohadeseh Taheri Mousavi, MIT postdoc.

A metallurgy mystery

Tasan’s group in MIT’s Department of Materials Science and Engineering explores the microstructure of metals in order to design new materials with exceptional damage-resistance.

“We are metallurgists and want to learn what governs the deformation of metals, so that we can make better metals,” Tasan says. “In this case, it was intriguing that, if you cut something very soft, like human hair, with something very hard, like steel, the hard material would fail.”

To identify the mechanisms by which razor blades fail when shaving human hair, Roscioli first carried out some preliminary experiments, using disposable razors to shave his own facial hair. After every shave, he took images of the razor’s edge with a scanning electron microscope (SEM) to track how the blade wore down over time.

Surprisingly, the experiments revealed very little wear, or rounding out of the sharp edge over time. Instead, he noticed chips forming along certain regions of the razor’s edge.

“This created another mystery: We saw chipping, but didn’t see chipping everywhere, only in certain locations,” Tasan says. “And we wanted to understand, under what conditions does this chipping take place, and what are the ingredients of failure?”

A chip off the new blade

To answer this question, Roscioli built a small, micromechanical apparatus to carry out more controlled shaving experiments. The apparatus consists of a movable stage, with two clamps on either side, one to hold a razor blade and the other to anchor strands of hair. He used blades from commercial razors, which he set at various angles and cutting depths to mimic the act of shaving.

The apparatus is designed to fit inside a scanning electron microscope, where Roscioli was able to take high-resolution images of both the hair and the blade as he carried out multiple cutting experiments. He used his own hair, as well as hair sampled from several of his labmates, overall representing a wide range of hair diameters.

Regardless of a hair’s thickness, Roscioli observed the same mechanism by which hair damaged a blade. Just as in his initial shaving experiments, Roscioli found that hair caused the blade’s edge to chip, but only in certain spots.

When he analyzed the SEM images and movies taken during the cutting experiments, he found that chips did not occur when the hair was cut perpendicular to the blade. When the hair was free to bend, however, chips were more likely to occur. These chips most commonly formed in places where the blade edge met the sides of the hair strands.

To see what conditions were likely causing these chips to form, the team ran computational simulations in which they modeled a steel blade cutting through a single hair. As they simulated each hair shave, they altered certain conditions, such as the cutting angle, the direction of the force applied in cutting, and most importantly, the composition of the blade’s steel.

They found that the simulations predicted failure under three conditions: when the blade approached the hair at an angle, when the blade’s steel was heterogenous in composition, and when the edge of a hair strand met the blade at a weak point in its heterogenous structure.

Tasan says these conditions illustrate a mechanism known as stress intensification, in which the effect of a stress applied to a material is intensified if the material’s structure has microcracks. Once an initial microcrack forms, the material’s heterogeneous structure enabled these cracks to easily grow to chips.

“Our simulations explain how heterogeneity in a material can increase the stress on that material, so that a crack can grow, even though the stress is imposed by a soft material like hair,” Tasan says.

The researchers have filed a provisional patent on a process to manipulate steel into a more homogenous form, in order to make longer-lasting, more chip-resistant blades.

“The basic idea is to reduce this heterogeneity, while we keep the high hardness,” Roscioli says. “We’ve learned how to make better blades, and now we want to do it.”

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How a crystalline sponge sheds water molecules

In a new study, scientists answer this question in detail for a porous, crystalline material made from metal and organic building blocks — specifically, cobalt(II) sulfate heptahydrate, 5-aminoisophthalic acid and 4,4′-bipyridine.

Using advanced techniques, researchers studied how this crystalline sponge changed shape as it went from a hydrated state to a dehydrated state. The observations were elaborate, allowing the team to “see” when and how three individual water molecules left the material as it dried out.

Crystalline sponges of this kind belong to a class of materials called metal-organic frameworks (MOFs), which hold potential for applications such as trapping pollutants or storing fuel at low pressures.

“This was a really nice, detailed example of using dynamic in-situ x-ray diffraction to study the transformation of a MOF crystal,” says Jason Benedict, PhD, associate professor of chemistry in the University at Buffalo College of Arts and Sciences. “We initiate a reaction — a dehydration. Then we monitor it with x-rays, solving crystal structures, and we can actually watch how this material transforms from the fully hydrated phase to the fully dehydrated phase.

“In this case, the hydrated crystal holds three independent water molecules, and the question was basically, how do you go from three to zero? Do these water molecules leave one at time? Do they all leave at once?

“And we discovered that what happens is that one water molecule leaves really quickly, which causes the crystal lattice to compress and twist, and the other two molecules wind up leaving together. They leak out at the same time, and that causes the lattice to untwist but stay compressed. All of that motion that I’m describing — you wouldn’t have any insight into that kind of motion in the absence of these sort of experiments that we are performing.”

The research was published online on June 23 in the journal Structural Dynamics. Benedict led the study with first authors Ian M. Walton and Jordan M. Cox, UB chemistry PhD graduates. Other scientists from UB and the University of Chicago also contributed to the project.

Understanding how the structures of MOFs morph — step by step — during processes like dehydration is interesting from the standpoint of basic science, Benedict says. But such knowledge could also aid efforts to design new crystalline sponges. As Benedict explains, the more researchers can learn about the properties of such materials, the easier it will be to tailor-make novel MOFs geared toward specific tasks.

The technique the team developed and employed to study the crystal’s transformation provides scientists with a powerful tool to advance research of this kind.

“Scientists often study dynamic crystals in an environment that is static,” says co-author Travis Mitchell, a chemistry PhD student in Benedict’s lab. “This greatly limits the scope of their observations to before and after a particular process takes place. Our findings show that observing dynamic crystals in an environment that is also dynamic allows scientists to make observations while a particular process is taking place. Our group developed a device that allows us to control the environment relative to the crystal: We are able to continuously flow fluid around the crystal as we are collecting data, which provides us with information about how and why these dynamic crystals transform.”

The study was supported by the National Science Foundation (NSF) and U.S. Department of Energy, including through the NSF’s ChemMatCARS facility, where much of the experimental work took place.

“These types of experiments often take days to perform on a laboratory diffractometer,” Mitchell says. “Fortunately, our group was able to perform these experiments using synchrotron radiation at NSF’s ChemMatCARS. With synchrotron radiation, we were able to make measurements in a matter of hours.”

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Transforming e-waste into a strong, protective coating for metal

A typical recycling process converts large quantities of items made of a single material into more of the same. However, this approach isn’t feasible for old electronic devices, or “e-waste,” because they contain small amounts of many different materials that cannot be readily separated. Now, in ACS Omega, researchers report a selective, small-scale microrecycling strategy, which they use to convert old printed circuit boards and monitor components into a new type of strong metal coating.

In spite of the difficulty, there’s plenty of reason to recycle e-waste: It contains many potentially valuable substances that can be used to modify the performance of other materials or to manufacture new, valuable materials. Previous research has shown that carefully calibrated high temperature-based processing can selectively break and reform chemical bonds in waste to form new, environmentally friendly materials. In this way, researchers have already turned a mix of glass and plastic into valuable, silica-containing ceramics. They’ve also used this process to recover copper, which is widely used in electronics and elsewhere, from circuit boards. Based on the properties of copper and silica compounds, Veena Sahajwalla and Rumana Hossain suspected that, after extracting them from e-waste, they could combine them to create a durable new hybrid material ideal for protecting metal surfaces.

To do so, the researchers first heated glass and plastic powder from old computer monitors to 2,732 F, generating silicon carbide nanowires. They then combined the nanowires with ground-up circuit boards, put the mix on a steel substrate then heated it up again. This time the thermal transformation temperature selected was 1,832 F, melting the copper to form a silicon-carbide enriched hybrid layer atop the steel. Microscope images revealed that, when struck with a nanoscale indenter, the hybrid layer remained firmly affixed to the steel, without cracking or chipping. It also increased the steel’s hardness by 125%. The team refers to this targeted, selective microrecycling process as “material microsurgery,” and say that it has the potential to transform e-waste into advanced new surface coatings without the use of expensive raw materials.

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Water molecules are gold for nanocatalysis

Nanocatalysts made of gold nanoparticles dispersed on metal oxides are very promising for the industrial, selective oxidation of compounds, including alcohols, into valuable chemicals. They show high catalytic activity, particularly in aqueous solution. A team of researchers from Ruhr-Universität Bochum (RUB) has been able to explain why: Water molecules play an active role in facilitating the oxygen dissociation needed for the oxidation reaction. The team of Professor Dominik Marx, Chair of Theoretical Chemistry, reports in the high-impact journal ACS Catalysis on 14 July 2020.

Rushing for gold

Most industrial oxidation processes involve the use of agents, such as chlorine or organic peroxides, that produce toxic or useless by-products. Instead, using molecular oxygen, O2, and splitting it to obtain the oxygen atoms needed to produce specific products would be a greener and more attractive solution. A promising medium for this approach is the gold/metal oxide (Au/TiO2) system, where the metal oxide titania (TiO2) supports nanoparticles of gold. These nanocatalysts can catalyse the selective oxidation of molecular hydrogen, carbon monoxide and especially alcohols, among others. A crucial step behind all reactions is the dissociation of O2, which comprises a usually high energy barrier. And a crucial unknown in the process is the role of water, since the reactions take place in aqueous solutions.

In a 2018 study, the RUB group of Dominik Marx, Chair of Theoretical Chemistry and Research Area coordinator in the Cluster of Excellence Ruhr Explores Solvation (Resolv), already hinted that water molecules actively participate in the oxidative reaction: They enable a stepwise charge-transfer process that leads to oxygen dissociation in the aqueous phase. Now, the same team reveals that solvation facilitates the activation of molecular oxygen (O2) at the gold/metal oxide (Au/TiO2) nanocatalyst: In fact, water molecules help to decrease the energy barrier for the O2 dissociation. The researchers quantified that the solvent curbs the energy costs by 25 per cent compared to the gas phase. “For the first time, it has been possible to gain insights into the quantitative impact of water on the critical O2 activation reaction for this nanocatalyst — and we also understood why,” says Dominik Marx.

Mind the water molecules

The RUB researchers applied computer simulations, the so-called ab initio molecular dynamics simulations, which explicitly included not only the catalyst but also as many as 80 surrounding water molecules. This was key to gain deep insights into the liquid-phase scenario, which contains water, in direct comparison to the gas phase conditions, where water is absent. “Previous computational work employed significant simplifications or approximations that didn’t account for the true complexity of such a difficult solvent, water,” adds Dr. Niklas Siemer who recently earned his PhD at RUB based on this research.

Scientists simulated the experimental conditions with high temperature and pressure to obtain the free energy profile of O2 in both liquid and gas phase. Finally, they could trace back the mechanistic reason for the solvation effect: Water molecules induce an increase of local electron charge towards oxygen that is anchored at the nanocatalyst perimeter; this in turn leads to the less energetic costs for the dissociation. In the end, say the researchers, it’s all about the unique properties of water: “We found that the polarizability of water and its ability to donate hydrogen bonds are behind oxygen activation,” says Dr. Munoz-Santiburcio. According to the authors, the new computational strategy will help to understand and improve direct oxidation catalysis in water and alcohols.

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Plato was right. Earth is made, on average, of cubes

Plato, the Greek philosopher who lived in the 5th century B.C.E., believed that the universe was made of five types of matter: earth, air, fire, water, and cosmos. Each was described with a particular geometry, a platonic shape. For earth, that shape was the cube.

Science has steadily moved beyond Plato’s conjectures, looking instead to the atom as the building block of the universe. Yet Plato seems to have been onto something, researchers have found.

In a new paper in the Proceedings of the National Academy of Sciences, a team from the University of Pennsylvania, Budapest University of Technology and Economics, and University of Debrecen uses math, geology, and physics to demonstrate that the average shape of rocks on Earth is a cube.

“Plato is widely recognized as the first person to develop the concept of an atom, the idea that matter is composed of some indivisible component at the smallest scale,” says Douglas Jerolmack, a geophysicist in Penn’s School of Arts & Sciences’ Department of Earth and Environmental Science and the School of Engineering and Applied Science’s Department of Mechanical Engineering and Applied Mechanics. “But that understanding was only conceptual; nothing about our modern understanding of atoms derives from what Plato told us.

“The interesting thing here is that what we find with rock, or earth, is that there is more than a conceptual lineage back to Plato. It turns out that Plato’s conception about the element earth being made up of cubes is, literally, the statistical average model for real earth. And that is just mind-blowing.”

The group’s finding began with geometric models developed by mathematician Gábor Domokos of the Budapest University of Technology and Economics, whose work predicted that natural rocks would fragment into cubic shapes.

“This paper is the result of three years of serious thinking and work, but it comes back to one core idea,” says Domokos. “If you take a three-dimensional polyhedral shape, slice it randomly into two fragments and then slice these fragments again and again, you get a vast number of different polyhedral shapes. But in an average sense, the resulting shape of the fragments is a cube.”

Domokos pulled two Hungarian theoretical physicists into the loop: Ferenc Kun, an expert on fragmentation, and János Török, an expert on statistical and computational models. After discussing the potential of the discovery, Jerolmack says, the Hungarian researchers took their finding to Jerolmack to work together on the geophysical questions; in other words, “How does nature let this happen?”

“When we took this to Doug, he said, ‘This is either a mistake, or this is big,'” Domokos recalls. “We worked backward to understand the physics that results in these shapes.”

Fundamentally, the question they answered is what shapes are created when rocks break into pieces. Remarkably, they found that the core mathematical conjecture unites geological processes not only on Earth but around the solar system as well.

“Fragmentation is this ubiquitous process that is grinding down planetary materials,” Jerolmack says. “The solar system is littered with ice and rocks that are ceaselessly smashing apart. This work gives us a signature of that process that we’ve never seen before.”

Part of this understanding is that the components that break out of a formerly solid object must fit together without any gaps, like a dropped dish on the verge of breaking. As it turns out, the only one of the so-called platonic forms — polyhedra with sides of equal length — that fit together without gaps are cubes.

“One thing we’ve speculated in our group is that, quite possibly Plato looked at a rock outcrop and after processing or analyzing the image subconsciously in his mind, he conjectured that the average shape is something like a cube,” Jerolmack says.

“Plato was very sensitive to geometry,” Domokos adds. According to lore, the phrase “Let no one ignorant of geometry enter” was engraved at the door to Plato’s Academy. “His intuitions, backed by his broad thinking about science, may have led him to this idea about cubes,” says Domokos.

To test whether their mathematical models held true in nature, the team measured a wide variety of rocks, hundreds that they collected and thousands more from previously collected datasets. No matter whether the rocks had naturally weathered from a large outcropping or been dynamited out by humans, the team found a good fit to the cubic average.

However, special rock formations exist that appear to break the cubic “rule.” The Giant’s Causeway in Northern Ireland, with its soaring vertical columns, is one example, formed by the unusual process of cooling basalt. These formations, though rare, are still encompassed by the team’s mathematical conception of fragmentation; they are just explained by out-of-the-ordinary processes at work.

“The world is a messy place,” says Jerolmack. “Nine times out of 10, if a rock gets pulled apart or squeezed or sheared — and usually these forces are happening together — you end up with fragments which are, on average, cubic shapes. It’s only if you have a very special stress condition that you get something else. The earth just doesn’t do this often.”

The researchers also explored fragmentation in two dimensions, or on thin surfaces that function as two-dimensional shapes, with a depth that is significantly smaller than the width and length. There, the fracture patterns are different, though the central concept of splitting polygons and arriving at predictable average shapes still holds.

“It turns out in two dimensions you’re about equally likely to get either a rectangle or a hexagon in nature,” Jerolmack says. “They’re not true hexagons, but they’re the statistical equivalent in a geometric sense. You can think of it like paint cracking; a force is acting to pull the paint apart equally from different sides, creating a hexagonal shape when it cracks.”

In nature, examples of these two-dimensional fracture patterns can be found in ice sheets, drying mud, or even the earth’s crust, the depth of which is far outstripped by its lateral extent, allowing it to function as a de facto two-dimensional material. It was previously known that the earth’s crust fractured in this way, but the group’s observations support the idea that the fragmentation pattern results from plate tectonics.

Identifying these patterns in rock may help in predicting phenomenon such as rock fall hazards or the likelihood and location of fluid flows, such as oil or water, in rocks.

For the researchers, finding what appears to be a fundamental rule of nature emerging from millennia-old insights has been an intense but satisfying experience.

“There are a lot of sand grains, pebbles, and asteroids out there, and all of them evolve by chipping in a universal manner,” says Domokos, who is also co-inventor of the Gömböc, the first known convex shape with the minimal number — just two — of static balance points. Chipping by collisions gradually eliminates balance points, but shapes stop short of becoming a Gömböc; the latter appears as an unattainable end point of this natural process.

The current result shows that the starting point may be a similarly iconic geometric shape: the cube with its 26 balance points. “The fact that pure geometry provides these brackets for a ubiquitous natural process, gives me happiness,” he says.

“When you pick up a rock in nature, it’s not a perfect cube, but each one is a kind of statistical shadow of a cube,” adds Jerolmack. “It calls to mind Plato’s allegory of the cave. He posited an idealized form that was essential for understanding the universe, but all we see are distorted shadows of that perfect form.”

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Wireless aquatic robot could clean water and transport cells

Researchers at Eindhoven University of Technology developed a tiny plastic robot, made of responsive polymers, which moves under the influence of light and magnetism. In the future this ‘wireless aquatic polyp’ should be able to attract and capture contaminant particles from the surrounding liquid or pick up and transport cells for analysis in diagnostic devices. The researchers published their results in the journal PNAS.

The mini robot is inspired by a coral polyp; a small soft creature with tentacles, which makes up the corals in the ocean. Doctoral candidate Marina Pilz Da Cunha: “I was inspired by the motion of these coral polyps, especially their ability to interact with the environment through self-made currents.” The stem of the living polyps makes a specific movement that creates a current which attracts food particles. Subsequently, the tentacles grab the food particles floating by.

The developed wireless artificial polyp is 1 by 1 cm, has a stem that reacts to magnetism, and light steered tentacles. “Combining two different stimuli is rare since it requires delicate material preparation and assembly, but it is interesting for creating untethered robots because it allows for complex shape changes and tasks to be performed,” explains Pilz Da Cunha. The tentacles move by shining light on them. Different wavelengths lead to different results. For example, the tentacles ‘grab’ under the influence of UV light, while they ‘release’ with blue light.

FROM LAND TO WATER

The device now presented can grab and release objects underwater, which is a new feature of the light-guided package delivery mini robot the researchers presented earlier this year. This land-based robot couldn’t work underwater, because the polymers making up that robot act through photothermal effects. The heat generated by the light fueled the robot, instead of the light itself. Pilz Da Cunha: “Heat dissipates in water, which makes it impossible to steer the robot under water.” She therefore developed a photomechanical polymer material that moves under the influence of light only. Not heat.

And that is not its only advantage. Next to operating underwater, this new material can hold its deformation after being activated by light. While the photothermal material immediately returns to its original shape after the stimuli has been removed, the molecules in the photomechanical material actually take on a new state. This allows different stable shapes, to be maintained for a longer period of time. “That helps to control the gripper arm; once something has been captured, the robot can keep holding it until it is addressed by light once again to release it,” says Pilz Da Cunha.

FLOW ATTRACTS PARTICLES

By placing a rotating magnet underneath the robot, the stem circles around its axis. Pilz Da Cunha: “It was therefore possible to actually move floating objects in the water towards the polyp, in our case oil droplets.”

The position of the tentacles (open, closed or something in between), turned out to have an influence on the fluid flow. “Computer simulations, with different tentacle positions, eventually helped us to understand and get the movement of the stem exactly right. And to ‘attract’ the oil droplets towards the tentacles,” explains Pilz Da Cunha.

OPERATION INDEPENDENT OF THE WATER COMPOSITION

An added advantage is that the robot operates independently from the composition of the surrounding liquid. This is unique, because the dominant stimuli-responsive material used for underwater applications nowadays, hydrogels, are sensitive for their environment. Hydrogels therefore behave differently in contaminated water. Pilz Da Cunha: “Our robot also works in the same way in salt water, or water with contaminants. In fact, in the future the polyp may be able to filter contaminants out of the water by catching them with its tentacles.”

NEXT STEP: SWIMMING ROBOT

PhD student Pilz Da Cunha is now working on the next step: an array of polyps that can work together. She hopes to realize transport of particles, in which one polyp passes on a package to the other. A swimming robot is also on her wish list. Here, she thinks of biomedical applications such as capturing specific cells.

To achieve this, the researchers still have to work on the wavelengths to which the material responds. “UV light affects cells and the depth of penetration in the human body is limited. In addition, UV light might damage the robot itself, making it less durable. Therefore we want to create a robot that doesn’t need UV light as a stimuli,” concludes Pilz Da Cunha.

Video: https://www.youtube.com/watch?v=QYklipdzesI&feature=emb_logo

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