Tech

After graduating or leaving college, many students face a difficult choice: Try to pay off their student loans as fast as possible to save on interest, or enroll in an income-based repayment plan, which offers affordable payments based on their income and forgives any balance remaining after 20 or 25 years.

There are pros and cons to each option, and trying to discern the better path can be daunting. That's why University of Colorado Boulder's Yu-Jui Huang and Saeed Khalili, a former graduate student in financial mathematics, along with Dublin City University's Paolo Guasoni, decided to throw a little mathematical muscle at the problem.

The researchers developed a novel mathematical model for determining the optimal student loan repayment strategy, based on an individual borrower's specific circumstances. In April, they published a paper outlining their approach in the SIAM Journal on Financial Mathematics.

Instead of choosing one of these distinct options and sticking with it, some borrowers should consider combining the two to create their own hybrid repayment strategy, the researchers found.

"The rule of thumb is that if your balance is really small, just pay it as quickly as possible, and if your balance is large, then enroll in an income-based scheme right away," said Huang, a CU Boulder assistant professor of applied mathematics who specializes in mathematical finance and applied probability.

"We find that, between these two extremes, there's actually a third strategy, which is, you should pay as much as possible over the first several years. And after that, switch to an income-based repayment scheme."

The model incorporates basic, fundamental mathematics, Huang said, but is likely the first of its kind for student loans. Past studies were mostly empirical, estimating the actual effects of student loans on the economy and on individual borrowers. Very little research has been conducted through the lens of mathematics on the best strategy a student borrower should employ, he said.

The researchers saw an opportunity to contribute to the academic literature while at the same time helping borrowers make savvy repayment decisions. Student loans now total roughly $1.7 trillion and affect nearly 45 million borrowers in the United States, hampering their ability to buy homes, start businesses and attend graduate school.

"We made the model as simple as possible," Huang said. "For many students, this can save them money."

The model takes into account the fact that borrowers have to pay income tax on any loan amount that's forgiven under an income-based repayment plan, as well as the compounding interest rates of various student loans. It helps borrowers determine when they should stop making regular payments and switch to an income-based repayment scheme, a point in time called the critical horizon.

"The critical horizon is the time at which the benefits of forgiveness match the costs of compounding," the researchers write.

Already, the researchers are considering ways to improve their model. For one, they hope to incorporate more randomness into the model, which right now asks borrowers to take their best guess at their future income level, tax rate and living expenses. They also want to consider lifestyle changes that may affect borrowers' motivation for paying off student loans, such as getting married, buying a house and having children.

"In practice, what people say is, 'Yes, I'm going to be a dentist. Looking at past data, I know my starting salary should be this and, after a few years, my salary should grow to this particular stage and so on,'" Huang said. "The purpose of introducing the randomness here is because some dentists become really rich in five or 10 years, and some others are not so rich. Even if you look at the data, you can't be quite sure which category you will eventually fall into."

Though the researchers have no plans themselves to turn their formula into some sort of widely accessible calculator, they're open to existing student loan repayment calculators adopting their model so that I can help as many borrowers as possible.

"Right now, students don't really have any kind of concrete or rigorous guidelines--they may just have these general impressions but there's no math to justify those," Huang said. "We have created a simple model, but one that's undergone a very rigorous mathematical treatment."

New findings published this week in Physical Review Letters suggest that carbon, oxygen, and hydrogen cosmic rays travel through the galaxy toward Earth in a similar way, but, surprisingly, that iron arrives at Earth differently. Learning more about how cosmic rays move through the galaxy helps address a fundamental, lingering question in astrophysics: How is matter generated and distributed across the universe?

"So what does this finding mean?" asks John Krizmanic, a senior scientist with UMBC's Center for Space Science and Technology (CSST). "These are indicators of something interesting happening. And what that something interesting is we're going to have to see."

Cosmic rays are atomic nuclei--atoms stripped of their electrons--that are constantly whizzing through space at nearly the speed of light. They enter Earth's atmosphere at extremely high energies. Information about these cosmic rays can give scientists clues about where they came from in the galaxy and what kind of event generated them.

An instrument on the International Space Station (ISS) called the Calorimetric Electron Telescope (CALET) has been collecting data about cosmic rays since 2015. The data include details such as how many and what kinds of atoms are arriving, and how much energy they're arriving with. The American, Italian, and Japanese teams that manage CALET, including UMBC's Krizmanic and postdoc Nick Cannady, collaborated on the new research.

Iron on the move

Cosmic rays arrive at Earth from elsewhere in the galaxy at a huge range of energies--anywhere from 1 billion volts to 100 billion billion volts. The CALET instrument is one of extremely few in space that is able to deliver fine detail about the cosmic rays it detects. A graph called a cosmic ray spectrum shows how many cosmic rays are arriving at the detector at each energy level. The spectra for carbon, oxygen, and hydrogen cosmic rays are very similar, but the key finding from the new paper is that the spectrum for iron is significantly different.

There are several possibilities to explain the differences between iron and the three lighter elements. The cosmic rays could accelerate and travel through the galaxy differently, although scientists generally believe they understand the latter, Krizmanic says.

"Something that needs to be emphasized is that the way the elements get from the sources to us is different, but it may be that the sources are different as well," adds Michael Cherry, physics professor emeritus at Louisiana State University (LSU) and a co-author on the new paper. Scientists generally believe that cosmic rays originate from exploding stars (supernovae), but neutron stars or very massive stars could be other potential sources.

Next-level precision

An instrument like CALET is important for answering questions about how cosmic rays accelerate and travel, and where they come from. Instruments on the ground or balloons flown high in Earth's atmosphere were the main source of cosmic ray data in the past. But by the time cosmic rays reach those instruments, they have already interacted with Earth's atmosphere and broken down into secondary particles. With Earth-based instruments, it is nearly impossible to identify precisely how many primary cosmic rays and which elements are arriving, plus their energies. But CALET, being on the ISS above the atmosphere, can measure the particles directly and distinguish individual elements precisely.

Iron is a particularly useful element to analyze, explains Cannady, a postdoc with CSST and a former Ph.D. student with Cherry at LSU. On their way to Earth, cosmic rays can break down into secondary particles, and it can be hard to distinguish between original particles ejected from a source (like a supernova) and secondary particles. That complicates deductions about where the particles originally came from.

"As things interact on their way to us, then you'll get essentially conversions from one element to another," Cannady says. "Iron is unique, in that being one of the heaviest things that can be synthesized in regular stellar evolution, we're pretty certain that it is pretty much all primary cosmic rays. It's the only pure primary cosmic ray, where with others you'll have some secondary components feeding into that as well."

"Made of stardust"

Measuring cosmic rays gives scientists a unique view into high-energy processes happening far, far away. The cosmic rays arriving at CALET represent "the stuff we're made of. We are made of stardust," Cherry says. "And energetic sources, things like supernovas, eject that material from their interiors, out into the galaxy, where it's distributed, forms new planets, solar systems, and... us."

"The study of cosmic rays is the study of how the universe generates and distributes matter, and how that affects the evolution of the galaxy," Krizmanic adds. "So really it's studying the astrophysics of this engine we call the Milky Way that's throwing all these elements around."

A global effort

The Japanese space agency launched CALET and today leads the mission in collaboration with the U.S. and Italian teams. In the U.S., the CALET team includes researchers from LSU; NASA Goddard Space Flight Center; UMBC; University of Maryland, College Park; University of Denver; and Washington University.The new paper is the fifth from this highly successful international collaboration published in PRL, one of the most prestigious physics journals.

CALET was optimized to detect cosmic ray electrons, because their spectrum can contain information about their sources. That's especially true for sources that are relatively close to Earth in galactic terms: within less than one-thirtieth the distance across the Milky Way. But CALET also detects the atomic nuclei of cosmic rays very precisely. Now those nuclei are offering important insights about the sources of cosmic rays and how they got to Earth.

"We didn't expect that the nuclei - the carbon, oxygen, protons, iron - would really start showing some of these detailed differences that are clearly pointing at things we don't know," Cherry says.

The latest finding creates more questions than it answers, emphasizing that there is still more to learn about how matter is generated and moves around the galaxy. "That's a fundamental question: How do you make matter?" Krizmanic says. But, he adds, "That's the whole point of why we went in this business, to try to understand more about how the universe works."

The most accurate distance measurement yet of ultra-diffuse galaxy (UDG) NGC1052-DF2 (DF2) confirms beyond any shadow of a doubt that it is lacking in dark matter. The newly measured distance of 22.1 +/-1.2 megaparsecs was obtained by an international team of researchers led by Zili Shen and Pieter van Dokkum of Yale University and Shany Danieli, a NASA Hubble Fellow at the Institute for Advanced Study.

"Determining an accurate distance to DF2 has been key in supporting our earlier results," stated Danieli. "The new measurement reported in this study has crucial implications for estimating the physical properties of the galaxy, thus confirming its lack of dark matter."

The results, published in Astrophysical Journal Letters on June 9, 2021, are based on 40 orbits of NASA's Hubble Space Telescope, with imaging by the Advanced Camera for Surveys and a "tip of the red giant branch" (TRGB) analysis, the gold standard for such refined measurements. In 2019, the team published results measuring the distance to neighboring UDG NGC1052-DF4 (DF4) based on 12 Hubble orbits and TRGB analysis, which provided compelling evidence of missing dark matter. This preferred method expands on the team's 2018 studies that relied on "surface brightness fluctuations" to gauge distance. Both galaxies were discovered with the Dragonfly Telephoto Array at the New Mexico Skies observatory.

"We went out on a limb with our initial Hubble observations of this galaxy in 2018," van Dokkum said. "I think people were right to question it because it's such an unusual result. It would be nice if there were a simple explanation, like a wrong distance. But I think it's more fun and more interesting if it actually is a weird galaxy."

In addition to confirming earlier distance findings, the Hubble results indicated that the galaxies were located slightly farther away than previously thought, strengthening the case that they contain little to no dark matter. If DF2 were closer to Earth, as some astronomers claim, it would be intrinsically fainter and less massive, and the galaxy would need dark matter to account for the observed effects of the total mass.

Dark matter is widely considered to be an essential ingredient of galaxies, but this study lends further evidence that its presence may not be inevitable. While dark matter has yet to be directly observed, its gravitational influence is like a glue that holds galaxies together and governs the motion of visible matter. In the case of DF2 and DF4, researchers were able to account for the motion of stars based on stellar mass alone, suggesting a lack or absence of dark matter. Ironically, the detection of galaxies deficient in dark matter will likely help to reveal its puzzling nature and provide new insights into galactic evolution.

While DF2 and DF4 are both comparable in size to the Milky Way galaxy, their total masses are only about one percent of the Milky Way's mass. These ultra-diffuse galaxies were also found to have a large population of especially luminous globular clusters.

This research has generated a great deal of scholarly interest, as well as energetic debate among proponents of alternative theories to dark matter, such as Modified Newtonian dynamics (MOND). However, with the team's most recent findings--including the relative distances of the two UDGs to NGC1052--such alternative theories seem less likely. Additionally, there is now little uncertainty in the team's distance measurements given the use of the TRGB method. Based on fundamental physics, this method depends on the observation of red giant stars that emit a flash after burning through their helium supply that always happens at the same brightness.

"There's a saying that extraordinary claims require extraordinary evidence, and the new distance measurement strongly supports our previous finding that DF2 is missing dark matter," stated Shen. "Now it's time to move beyond the distance debate and focus on how such galaxies came to exist."

Moving forward, researchers will continue to hunt for more of these oddball galaxies, while considering a number of questions such as: How are UDGs formed? What do they tell us about standard cosmological models? How common are these galaxies, and what other unique properties do they have? It will take uncovering many more dark matter-less galaxies to resolve these mysteries and the ultimate question of what dark matter really is.

One of the main reasons for the hold-ups in the mass vaccination campaigns against COVID-19 are the precautions that must be taken regarding the handling of the vaccines and their administration. They have to be transported under very specific conditions and the syringes used for their administration must be prepared at the same vaccination site. Healthcare workers must ensure they avoid any sudden movements of the vaccines so as not to affect the vector they use, messenger RNA molecules in the case of the Pfizer BioNTech and Moderna vaccines.

But research led by the Hospital del Mar Pharmacy Department and Neuropharmacology-Neurophar Research Group at Pompeu Fabra University (UPF), may lead to changes in these standard procedures. The study, which has just been published in the medical journal Clinical Microbiology and Infection, demonstrates that these reconstituted vaccines can be transported following just minor precautions. In fact, the study reveals that the messenger RNA remains stable for at least three hours, under movement conditions similar to those caused by road transport and at room temperature.

Vaccines subjected to stress testing

The study arose from the Hospital del Mar's experience distributing vaccines administered to healthcare professionals at the outset of the vaccination campaign, in January this year. The SubDirectorate-General for Health Services of the Generalitat, headed at the time by Dr. Carmen Cabezas, was asked for authorisation for the doses to be prepared in horizontal laminar flow chambers (a workspace set up to stop the entry of potentially contaminating microorganisms) in the Pharmacy Department, under the condition that the syringes were not transported outside of the centre's vaccination sites. The process was led by Dr. Santi Grau, director of the Hospital del Mar's Pharmaceutical Department, and Dr. Olivia Ferrández, head of the Pharmacy Department, both authors of the paper, who decided to carry out tests to see how the stability of these vaccines was affected after transportation. To do this, they collaborated with Dr. Rafael Maldonado, also an author of the study and coordinator of the Neuropharmacology-Neurophar Research Group at UPF, and Dr. Elena Martín-García, member of the same group, who analysed the response of the vaccines to various tests.

To do this, vials were used that had been returned to the Hospital del Mar Pharmacy Department, which could not be administered as they had lost their microbiological traceability. According to current protocols, the messenger RNA vaccines, kept at a temperature of between 2 and 25ºC, cannot be used six hours after the first dosis has been withdrawn from the vial. In the study, the vaccines, prepared at the Pharmacy Department facilities, were divided into three groups. One was left, unmoved and at room temperature (21ºC), for three hours. A second group, also kept at room temperature for the same amount of time, was subjected to gentle movements, similar to the movement caused by road transport. And a third, under the same temperature and time conditions, was subjected to a strong intermittent shaking movement. The results of the tests were compared with a final group of recently thawed vaccines.

Negligible degradation of the messenger RNA

As Dr. Grau explains, the results showed that the main vector in both vaccines, the RNA messenger, showed practically no degradation under any of the conditions. According to Dr. Maldonado, the analysis of the data reveals that "the degradation of the RNA messenger was negligible, less than 1%, both for the fresh sample and the one subjected to movement. For the samples subjected to shaking, the degradation was higher, but still not excessive, around 5% for the two analysed vaccines". He goes on to assert that, "during road transport and at room temperature (between 21 +/- 1ºC) for three hours, there is no kind of alteration in the stability of the messenger, which maintains the integrity of a freshly prepared sample. Therefore, under these conditions, by imitating road transport, there is no degradation of either the reconstituted Pfizer-BioNTech or Moderna vaccines".

Dr. Elena Martín-García, also an author of the paper and researcher at UPF, highlighted the soundness of the results."The data are conclusive and very clear. The RNA messenger shows impressive stability in both of the analysed COVID-19 vaccines", she concluded. This can be of great help in the vaccination process, according to Dr. Olivia Ferrández. "The vaccination process, led by nursing professionals, is not limited to the administration of the vaccine, but also includes the registration of the vaccine batch being administered. Our results, therefore, contribute to streamlining the vaccination process and facilitates the work at vaccination sites," she added.

The research may help to bring about a change in strategy in how the vaccines are handled, helping in their delivery throughout the population. As Dr. Santi Grau comments, "our data can help boost the mass vaccination campaigns in rural areas or countries with precarious transport networks and healthcare infrastructure, where doses could be prepared in a health centre and transported by road to rural or remote areas, limiting the likelihood of errors being made during preparation at vaccination sites".

Researchers at the University of Gothenburg have observed the absorption of a single electron by a levitated droplet with such a magnification that it is visible with the naked eye and can even be measured with a normal millimeter scaled ruler.

Matter in the universe is composed of elementary particles like electrons, protons, and neutrons. They are everywhere, but they are so small that the human eye cannot discern them. In the last century, physicists have proven the existence of these particles through different experiments, but in most cases the observation of the particles have been indirect.

- Electrons are one of these fundamental particles. In 1909, Robert Millikan proved that the charge of the electron is quantized. In other words, there exists a minimum, indivisible amount of charge. He demonstrated that the electron´s charge is quantized by letting hundreds of charged droplets fall in an electric field and then perform a statistical analysis of their motion.

An experiment with a single levitated drop

-Now we have created a modern version of this classical experiment by levitating a single droplet in air using a laser, says Javier Marmolejo, Ph.D. at the Department of physics at the University of Gothenburg.

In this experiment, the quantization of the electric charge is directly visible for the first time without advanced equipment or a complex statistical analysis.

- We trapped a drop using a laser inside a strong electric field and added individual electrons by exposing it to alfa radiation. The drop performed quantized jumps every time it absorbed one or a few electrons. By magnifying the image of the droplet using a single lens, we were able to see the effect of a single electron absorption and to measure the jumps with a ruler. The bright spot moved about one millimeter for every absorbed electron (see video).

The drop had a diameter of 29 micrometers, which roughly corresponds to the thickness of a thin human hair. Despite this, it contains around 3.7 x1015 negatively charged electrons.

- The feat is incredible when one considers that the effect of adding single electron to a droplet that already has 3 700 000 000 000 000 is visible with the naked eye.

Now that it is possible to "see the effect of a single electron", a new opportunity emerges to better communicate science regarding elementary particles to the general public, the researchers comment.

Researchers from Skoltech and Saratov State University have designed a simple and easily reproducible labeling system for individual cells that enables researchers to track single cell behavior and migration for tasks requiring extreme precision. The paper was published in the journal ACS Applied Materials and Interfaces.

Modern biomedical science and developmental biology often require scientists to track and trace individual cells, whether it is to establish the best purified cells from various types of cell lines, in particular to select mesenchymal stem/stromal cells best suited for tissue regeneration or even to observe a parasite within a host. One way to do this is with a rainbow of various fluorescent proteins, shining in all kinds of colors under ultraviolet or blue light. However, this requires creating new transgenic lines capable of expressing these proteins and necessarily shifting away from the original cell cultures. Some of the proteins are also toxic and degrade quickly.

Instead of marker proteins, Gleb Sukhorukov of Skoltech and Queen Mary University of London and his colleagues decided to use polymer multilayer microcapsules, which many cells can impregnate and keep inside for days. The team built a hybrid polymeric microcapsule with carbon nanodots (CNDs) loaded with rhodamine B, a fluorescent dye; these components in combination makes the capsule photo-convertible.

The researchers tested their system on individual cell marking on six cell lines, including the famously immortal HeLa line and healthy human embryonic fibroblasts. The system worked quite well, with no tags decomposing even after 48 hours of observation and no traceable damage to the marked cells. And since a single cell can be marked with several clearly distinguishable microcapsules, this tool can be used for combinational coding of individual cells within populations.

"Although there were other approaches for single cell optical labelling, they were all based on genetic modification of the cells and encoding of photo-convertible proteins, which limits the application, especially on stem cells and other cells where genetic modification is not possible or not appropriate. Our approach gives a simple tool, applicable for various cell lines as photoconvertible capsules are readily internalized within cells. Another advantage is the possibilities for combinatorial labelling: in other words, each labelled cells could have a unique combination of switched capsules that gives a "code" to single cells and facilitates the monitoring of cell moves within population," Sukhorukov says.

He notes that the team expects biologists and biomedical researchers interested in solving problems of single cell movement and cell-to-cell communication in the populations to reach out and discuss possible solutions.

"Apart from that, we do not know much about how cells of the same type are communicating in the population, why some cell become "leaders" as they are the first to respond to stimuli and others just follow. We believe that once we could select and demonstrate the biological or biochemical differences in individual cells in their population, our method should become more in demand and widespread," Sukhorukov concludes.

The production of chemicals is a cumbersome business. Often, only a small part of what is actually wanted is produced in the factory. The large remainder is unusable - or even worse. Examples? The defoliant "Agent Orange" used by the US army in the Vietnam War was produced in great hurry. It contained dioxin as an impurity. As a result, not only did trees in the combat zone lose their foliage, but US soldiers and Vietnamese civilians also fell ill with cancer years later.

There are also examples from agriculture: In the production of the insecticide lindane, a hexachlorocyclohexane (HCH), only less than 15 percent of the desired substance is produced; 85 percent of the reaction broth is hazardous waste. In the 1950s, this toxic mixture was still sprayed in its entirety on fields and orchards. Later on the effective lindane was separated and sold pure, the rest being dumped in landfills. There the chemicals often still lie today. Lindane has been banned in the EU since 2007, and it has not been used in Switzerland for some time.

The flame retardant hexabromocyclodecane (HBCD) is also a mixture of several substances. It was invented in the 1970s, produced on a scale of several 10,000 tons per year and used in polystyrene insulation boards for house facades, in textiles and in plastics for electrical appliances. It has been banned worldwide since 2014. In Switzerland, plastic containing HBCD is not recycled, but must be destroyed in waste incineration.

Internationally outlawed

Since 2004, the Stockholm Convention on Persistent Organic Pollutants has regulated the handling of such long-lived environmental toxins. Switzerland ratified the agreement in 2003, but all these substances are already in the environment - and finely distributed. HBCD is found in sewage sludge, in fish, in air, water and soil. In 2004, the World Wildlife Fund (WWF) took blood samples from eleven European environment ministers and three health ministers and detected HBCD and lindane in the blood of every single one of them.

Bacteria, the rescuers from the soil

It begs the question: Can we recapture or detoxify the chemical waste of past generations? Fortunately, scientists aren't shying away from icky places in their search for solutions. In 1991, they discovered three strains of bacteria that could consume lindane and its useless chemical siblings in chemical waste sites in France, Japan and India almost simultaneously: Sphingobium francense, Sphingobium japonicum and Sphingobium indicum. Could these bio-cleaners perhaps also digest the flame retardant HBCD and other toxins?

Empa chemist Norbert Heeb and Eawag microbiologist Hans-Peter Kohler, together with researchers from the Zurich University of Applied Sciences (ZHAW) and two Indian institutes, put them to the test. They modified the genes of the Indian bacteria and produced HCH-degrading enzymes in pure form. An enzyme is a protein molecule, a bio-catalyst so to speak, with which bacteria, but also other living cells, can build up or break down chemical substances. The pollutant molecule HCH inserts itself into the enzyme like a key into a lock. Then part of the molecule is split off. The now harmless fragments are released again, and the enzyme is ready to take up the next pollutant molecule.

Mutations open up opportunities

Together with undergraduate student Jasmin Hubeli, Heeb investigated not only the enzyme variants found in landfills, but also an enzyme obtained from a genetically modified bacterial strain. Here, the researchers had deliberately enlarged the "keyhole" so that the larger HBCD molecules could be broken down more easily. The result: The genetic modification influenced the rate, at which the pollutant was broken down.
Empa researcher Heeb is hopeful about their results: "This means that we now actually have a chance to use biological methods to render harmless these long-lived toxins produced by mankind and distributed over large areas." There is still a long way to go, however. The lock-and-key principle of helpful enzymes still needs to be figured out in more detail before tailor-made enzymes for chemical toxins are available in the future.

As a new member of photovoltaic family, antimony trisulfide (Sb2S3) has the satisfactory bandgap of 1.7eV, benefiting the fabrication of the top absorber layer of tandem solar cells. Due to special quasi-one-dimensional structure, it shows advantages of less dangling bonds. Based on these advantages, the vacancy defects upon the surface causing the recombination of the carriers could be reduced sharply, which helps to solve the photovoltaic problems in solar cells.

In the previous studies, the relationships between conformation, chemical composition and structure of deep-level defects on Sb2S3 films are unclear from the aspect of experiment.

In a study published in Nature Communications, a research team led by CHEN Tao from University of Science and Technology of China (USTC) of the Chinese Academy of Sciences discovered the unique defect properties of low-dimensional materials particularly Sb2S3 through building the bridge between the deep-level defects of Sb2S3 and anion/cation ratio.

The researchers prepared both Sb-rich and sulphur-rich Sb2S3 films by using the method of thermal evaporation deposition. Based on the excellent performance of the devices, the deep-level transient spectroscopy (DLTS) was applied to detect the characterizations of defects.

The sulphur-rich Sb2S3 films showed an excellent performance compared with Sb-rich Sb2S3 films as the lower density of defect and less detrimental to carrier transport were achieved, which matches with the improvement in photovoltaic performance. Based on theoretical calculations, it seems that the defects are trend to appear in Sb-rich Sb2S3 films.

Notably, the sulphur-rich Sb2S3 devices fabricated by thermal evaporation showed the highest record power conversion efficiency, which means that the material is capable of being more tolerant to vacancy defects, and indicates that the addictive introduce to the vacancy will not lower the lifetime of carriers.

This study provides a new solution to regulate the photovoltaic properties of Sb2S3.

Scientists have developed a new method that improves dispensing of viscoelastic fluids - a vital process for circuit board production, 3D printing and other industrial applications

Viscoelastic fluids are difficult to dispense as liquid bridges that form between the substrate and nozzle must be broken

New research has found that twisting these liquid bridges breaks them in a quicker and cleaner way than the conventional method of stretching them

Researchers used high speed imaging to observe that when twisted, a crack forms at the edge of the liquid bridge and propagates towards the center

The underlying mechanism that breaks the liquid bridge was found to be "edge fracture" and is the first time that scientists have found a useful application for this phenomenon

If you've ever tried to lift a pizza slice covered in hot, melted cheese, you've no doubt encountered the long, cheesy strings that bridge one pizza slice from the next. Keep lifting the pizza slice and these cheese bridges eventually break, covering the plate, table (or even your lap) in long, thin strands of cheese. While this is just a minor inconvenience with pizza, it is a longstanding problem in industry, where liquids with similar properties to melted cheese - dubbed viscoelastic fluids - need to be cleanly and speedily dispensed.

Now, scientists have developed a new technique that uses rotation to break these liquid bridges. Their findings, published 11 June 2021 in PNAS, could improve the speed and precision of dispensing viscoelastic fluids, in applications ranging from circuit board production and food processing to live tissue engineering and 3D printing.

"Viscoelastic fluids, like ketchup, silly putty and toothpaste, have very strange properties - when squeezed slowly, they flow like a fluid, but at faster speeds, they act like an elastic solid," said co-first author, San To Chan, who is a PhD student and JSPS DC2 Fellow in the Micro/Bio/Nanofluidics Unit at the Okinawa Institute of Science and Technology Graduate University (OIST). "These unique properties make dispensing these fluids quite difficult."

Currently, the standard method in industry involves lifting the nozzle away from the surface on which the liquid has been deposited. Although this effectively breaks the bridge, it draws the deposited liquid up into a long, thin peak, known as a capillary tail. If the liquid bridge breaks in multiple places, small droplets of fluid, called satellite droplets, also form. Capillary tails and satellite droplets can contaminate products or short-circuit electronic chips.

"The higher the nozzle is retracted, the longer the capillary tail, so the greater the chance for contamination," Chan explained. "Since the nozzle can't be lifted too high, the liquid bridge is thicker and takes longer to break, which slows down the whole dispensing process."

To overcome these challenges, Chan and his colleagues devised a simple solution: instead of stretching the liquid bridge, it could be destabilized through twisting.

In the study, the research team tested this idea on viscoelastic silicone oil, which is 60,000 times more viscous than water. The scientists placed a droplet of silicone oil between an upper and lower plate. Using high-speed imaging, they found that when the liquid bridge was twisted by rotating the upper plate, it caused a crack halfway between the ends of the liquid bridge. The crack then spread inwards from the edge towards the center, cutting the bridge cleanly in two without forming capillary tails or satellite droplets.

Importantly, this process took about a second, compared to the ten seconds typically needed to dispense the same fluid using the conventional retraction method.

For their next step, the scientists uncovered the underlying mechanism that causes the liquid bridge to break when placed under torsion. They teamed up with a research lab from Eindhoven University of Technology, who simulated what Chan and his colleagues had observed experimentally. The simulations provided concrete information about how the liquid bridge reacted, validating what the scientists had suspected: the crack was caused by "edge fracture".

"This is particularly striking as edge fracture has been characterized as a really undesirable phenomenon that scientists try to stop from occurring," said Dr. Simon Haward, who is the group leader for Micro/Bio/Nanofluidics Unit. "This is the first time that edge fracture has been found to have a beneficial application."

In the next phase of their research, the team plans to experiment with different viscoelastic fluids to confirm that the same effect applies. They also plan to increase the speed of the dispensing process further, potentially by combining both rotating and retracting the upper plate.

"For many industries, swapping from a nozzle that retracts to one that spins is relatively easy, but it has far-reaching ramifications," said senior author, Professor Amy Shen. "Faster and more precise liquid dispensing could lower energy consumption, and fewer contaminated products could mean that less raw material used."

During flock encounters, a single vocal interaction seems to be sufficient for making the decision of whether to recruit an individual or flock. Parrots are known for their splendid ability to imitate, including the contact calls of other individuals during vocal interactions. Such rapid vocal matching is hypothesised to precede and mediate the formation of new flocks. But how are such interactions perceived by others?

Heidi M. Thomsen, first author and PhD student at the Department of Biology, University of Copenhagen explains:

-"By using a novel experimental design, we were able to gain valurable insights in the flock decisions of a social parrot, the orange-fronted conure (Eupsittula canicularis). We conducted a field experiment in which flocks of wild orange-fronted conures were attracted to two loudspeakers simulating two orange-fronted conures engaged in a contact call interaction. During the interaction, one simulated individual would act as the leader by calling first, followed by the other individual. We specifically tailored the playback so the contact calls of the follower would imitate those of the leader. The listening wild flocks could now decide to fly after either the leader or the follower to fuse with it".

The results showed that flocks primarily chose to fuse with the leaders of the simulated vocal interactions. Furthermore, flocks responded with higher contact call rates and with contact calls that were more similar to the playback when they chose to fuse with a leader compared to trials where they chose a follower. Especially if the leader was a male.

-"Although we do not yet know why leaders are preferred, our findings suggest that orange-fronted conure flocks rely on eavesdropping on vocal interactions to infer the relative quality of unfamiliar individuals. While orange-fronted conures frequently engage in contact call imitation during vocal interactions with conspecifics, we here illustrate that vocal imitation also has implications for potential third party listeners": says Heidi M. Thomsen.

These birds respond selectively to interactants, indicating that contact call imitation plays an essential role in facilitating the formation and maintenance of affiliative social interactions - and not only with the individuals they directly interact with. This is a missing key to understanding the function of vocal imitation in parrots.

Seiichi Shirai (1905-1983) was an influential architect whose work has affected the designs of significant architects of the 20th century. Associate Professor Kosuke Hato of the Department of Architecture, Faculty of Engineering, Shinshu University has studied the work of Shirai and examined why the architect worked extensively on calligraphy. Hato's strategy is to clarify the relationship between the architect and his activity of calligraphy through Shirai's Theory of Tradition.

The 1950s in Japan is known as a time when architects actively discussed traditions, and Shirai is a representative example. Hato, in his past article, clarified not only the 1950s but every aspect of the Theory of Tradition in the written works of Shirai. This time Hato clarifies the meaning of practicing calligraphy. Hato states that Shirai's practice of calligraphy can be summarized as the four characteristics: interest in links with the primitive aspects of kanji; emphasis on "Yo (utility)"; "Gyo (ascetic practice)" which is nature as sustained conflicts between philosophy and the physical self; and a rapid writing style that aimed for something beyond one's consciousness without the constraint of accuracy of form.

Verifying the above content against the characteristics of Shirai's Theory of Tradition, Hato writes, "both were established by a similar consciousness of purpose, and that they both share an emphasis on "Yo," but apart from this, the idea of grasping universal potential of object without being misled by its external form and regional origin in the Theory of Tradition was put into practice as the "Gyo" of seeking to create something beyond form and consciousness in the practice of calligraphy."

Shirai's calligraphy influenced his architecture, as did Le Corbusier's painting on his designs and urban planning. Associate Professor Hato will continue to elucidate the relationship Shirai's Theory of Tradition had to his practice of calligraphy and architectural works.

Is it possible to drive nanoparticles to orbit below the light diffraction limit using a Gaussian beam? A recent joint research project reported in Nature Communications says yes.

It is well known that light possesses not only energy but also momentum. When light irradiates an object, momentum is transferred to the object, thus generating light pressure on the object. At the microscopic scale, microparticles and nanoparticles (such as biocells and macromolecules) can be manipulated by the light force. Atoms can be cooled by light pressure to achieve atomic clocks, Bose-Einstein condensation, and so on.

In addition to the linear momentum of light being transferable, the angular momentum of light can also be transferred to an object, thus causing object rotation. Since the conversion of momentum is usually derived from the linear interaction between light and objects, the orbital rotation speed and orbital radius have so far been limited to no more than 100 Hz in water and no less than one micrometer, respectively.

Recently, however, a team led by Prof. JIANG Yuqiang from the Institute of Genetics and Developmental Biology of the Chinese Academy of Sciences, in collaboration with Prof. QIU Chengwei from the National University of Singapore, Prof. YANG Yuanjie from the University of Electronic Science and Technology of China, and Prof. XIAO Liantuan from Shanxi University, has overcome these limits.

Based on the nonlinear optical effect, the researchers have achieved an ultra-fast orbital rotation rate for nanoparticles at the subdiffraction scale.

The researchers trapped gold nanoparticles using a circularly polarized NIR femtosecond laser beam with Gaussian mode. In the linear interaction regime, the trapped particles only spin in the beam center. In the nonlinear regime, however, an annular potential well can be formed by the effect of the "trap split," and the tangential optical force enhanced by the nonlinear polarization between the femtosecond laser and gold nanoparticles causes the particles to orbit at an ultra-fast speed in the annular trap well.

As a result, the spin angular momentum of light is converted into the orbital angular momentum of particles with super high efficiency.

In this work, the minimum radius of rotation was about 70 nm, which is far below the diffraction limit; and the highest orbital rotation speed exceeded 1000 r/s, one order faster than previously reported speeds.

The study reveals a new mechanism of spin angular momentum conversion to orbital angular momentum, and provides a new method of light manipulation.

Since the orbital radius and orbital rotation speed can be controlled by adjusting the power of the femtosecond laser, the NA of the objective lens, and the material of the nanoparticles, it can be widely applied in various fields, such as optical micromachines, nanorheology, laser microfabrication, and so on.

The energy density of traditional lithium-ion batteries is approaching a saturation point that cannot meet the demands of the future - for example in electric vehicles. Lithium metal batteries can provide double the energy per unit weight when compared to lithium-ion batteries. The biggest challenge, hindering its application, is the formation of lithium dendrites, small, needle-like structures, similar to stalagmites in a dripstone cave, over the lithium metal anode. These dendrites often continue to grow until they pierce the separator membrane, causing the battery to short-circuit and ultimately destroying it.

For many years now, experts worldwide have been searching for a solution to this problem. Scientists at Friedrich Schiller University in Jena, together with colleagues from Boston University (BU) and Wayne State University (WSU), have now succeeded in preventing dendrite formation and thus at least doubling the lifetime of a lithium metal battery. The researchers report on their method in the renowned journal "Advanced Energy Materials".

Two-dimensional membrane prevents dendrite nucleation

During the charge transfer process, lithium ions move back and forth between the anode and the cathode. Whenever they pick up an electron, they deposit a lithium atom and these atoms accumulate on the anode. A crystalline surface is formed, which grows three-dimensionally where the atoms accumulate, creating the dendrites. The pores of the separator membrane influences the nucleation of dendrites. If ion transport is more homogeneous, dendrite nucleation can be avoided.

"That's why we applied an extremely thin, two-dimensional membrane made of carbon to the separator, with the pores having a diameter of less than one nanometer," explains Professor Andrey Turchanin from the University of Jena. "These tiny openings are smaller than the critical nucleus size and thus prevent the nucleation that leads to the formation of dendrites. Instead of forming dendritic structures, the lithium is deposited on the anode as a smooth film." There is no risk of the separator membrane being damaged by this and the functionality of the battery is not affected.

"To test our method, we recharged test batteries fitted with our Hybrid Separator Membrane over and over again," says Dr Antony George from the University of Jena. "Even after hundreds of charging and discharging cycles, we couldn't detect any dendritic growth."

"The key innovation here is stabilizing electrode/electrolyte interface with an ultra-thin membrane that does not alter current battery manufacturing process," says Associate Professor Leela Mohana Reddy Arava from the WSU. "Interface stability holds key in enhancing the performance and safety of an electrochemical system."

Applied for a patent

High energy density batteries extend the driving range of electric vehicle (EVs) for the same weight/volume of the battery that a modern EV possesses and make portable electronic devices last longer in a single charge. "The separator gets the least amount of attention when compared to the other components of the battery," says Sathish Rajendran, a graduate student at WSU. "The extent to which a nanometer thick two-dimensional membrane on the separator could make a difference in the lifetime of a battery is fascinating."

As a result, the research team is confident that their findings have the potential to bring about a new generation of lithium batteries. They have therefore applied for a patent for their method. The next step is to see how the application of the two-dimensional membrane can be integrated into the manufacturing process. The researchers also want to apply the idea to other types of batteries.

Nitrogen from agriculture, vehicle emissions and industry is endangering butterflies in Switzerland. The element is deposited in the soil via the air and has an impact on vegetation - to the detriment of the butterflies, as researchers at the University of Basel have discovered.

More than half of butterfly species in Switzerland are considered to be at risk or potentially at risk. Usually, the search for causes focuses on intensive agriculture, pesticide use and climate change. A research team led by Professor Valentin Amrhein from the University of Basel, however, has been investigating another factor - the depositing of nitrogen from agriculture and exhaust fumes from industry and traffic in soils via the air. In the journal Conservation Biology, the research team reports a connection between this unintentional fertilization and the low diversity of butterflies in Switzerland.

It was already known from previous studies that too much nitrogen leads to denser vegetation, but with a smaller selection of plant species. Nitrogen stimulates the growth of less demanding plants in particular, with more specialized species being displaced. "We wanted to find out whether a nitrogen surplus also indirectly affects the diversity of butterflies via this change in vegetation," explains Dr. Tobias Roth, lead author of the study.

The team analyzed data from Biodiversity Monitoring Switzerland on the diversity and prevalence of plants and butterflies on 383 plots throughout Switzerland. The result was clear: the more nitrogen introduced via the air to the areas studied, the less diverse the vegetation and hence the butterfly species.

"As caterpillars, some butterfly species need certain plant species as food, or are dependent on a certain microclimate," Roth explains. Over-fertilization results in open, warm and dry places becoming cooler, shadier and damper due to stronger plant growth.

The nitrogen surplus impacts the prevalence of a large number of butterfly species in Switzerland, such as those that prefer open and dry sites. The researchers saw the clearest effect in rare and endangered species. "Nitrogen from the air is likely to be an important factor in the reason why these species are endangered," Roth remarks.

Existing literature on the diversity of butterflies explains the presence or absence of species primarily in terms of habitat quality or climate. A literature review by the research team revealed that plant diversity and vegetation density have so far received less attention. "We believe that the impact of nitrogen enrichment on butterflies has been underestimated," says Amrhein. Nitrogen appears to play a similarly extensive role as climate change when it comes to butterfly diversity.

While the researchers do not see a simple approach for improving the situation, technical improvements continue to offer a certain potential. "In the past, slurry was sprayed on farmland, for example, and some of this was transferred to other areas of land by the wind," Roth explains. Today, he says, drag hoses are used increasingly to apply the slurry directly to the soil. This reduces nitrogen input via the air to other areas where it is not wanted.

In addition, buffer zones and adapted landscape management can also help to partially mitigate the negative impact on sensitive habitats: this includes measures to prevent scrub encroachment, such as grazing or more frequent mowing. This is beneficial not only for demanding plant species, but also for butterflies. According to the researchers, however, there is ultimately no way around environmentally friendly consumer behavior when it comes to reducing unwanted nitrogen input, for example through the reduction of vehicle emissions and livestock farming. Around two thirds of nitrogen input into sensitive ecosystems in Switzerland today originate from ammonia emissions from livestock farming.

The 2018 Nobel Prize in Physics was shared by researchers who pioneered a technique to create ultrashort, yet extremely high-energy laser pulses at the University of Rochester.

Now researchers at the University's Institute of Optics have produced those same high-powered pulses--known as chirped pulses--in a way that works even with relatively low-quality, inexpensive equipment. The new work could pave the way for:

Better high-capacity telecommunication systems

Improved astrophysical calibrations used to find exoplanets

Even more accurate atomic clocks

Precise devices for measuring chemical contaminants in the atmosphere

In a paper in Optica, the researchers describe the first demonstration of highly chirped pulses created by a using a spectral filter in a Kerr resonator--a type of simple optical cavity that operates without amplification. These cavities have stirred wide interest among researchers because they can support "a wealth of complicated behaviors including useful broadband bursts of light," says coauthor William Renninger, assistant professor of optics.

By adding the spectral filter, the researchers can manipulate a laser pulse in the resonator to widen its wavefront by separating the beam's colors.

The new method is advantageous because "as you widen the pulse, you're reducing the peak of the pulse, and that means you can then put more overall energy into it before it reaches a high peak power that causes problems," Renninger says.

The new work is related to the approach used by Nobel laureates Donna Strickland '89 (PhD) and Gerard Mourou, who helped usher in a revolution in the use of laser technology when they pioneered chirped pulse amplification while doing research at the University's Laboratory for Laser Energetics.

The work takes advantage of the way light is dispersed as it passes through optical cavities. Most prior cavities require rare "anomalous" dispersion, which means that the blue light travels faster than red light.

However, the chirped pulses live in "normal" dispersion cavities in which red light travels faster. The dispersion is called "normal" because it is the much more common case, which will greatly increase the number of cavities that can generate pulses.

Prior cavities are also designed to have less than one percent loss, whereas the chirped pulses can survive in the cavity despite very high energy loss. "We're showing chirped pulses that remain stable even with more than 90 percent energy loss, which really challenges the conventional wisdom," Renninger says.

"With a simple spectral filter, we are now using loss to generate pulses in lossy and normal dispersion systems. So, in addition to improved energy performance, it really opens up what kinds of systems can be used."

Other collaborators include lead author Christopher Spiess, Qiang Yang, and Xue Dong, all current and former graduate research assistants in Renninger's lab, and Victor Bucklew, a former postdoctoral associate in the lab.

"We're very proud of this paper," Renninger says. "It has been a long time coming."