̽��ֱ�� of Cambridge - materials

Study measures effectiveness of different face mask materials when coughing

sc604 — Thu, 29 Oct 2020 02:31:56 +0000

̽��ֱ��researchers, from the ̽��ֱ�� of Cambridge and Northwestern ̽��ֱ��, tested the effectiveness of different fabrics at filtering particles between 0.02 and 0.1 micrometres – about the size of most viruses – at high speeds, comparable to coughing or heavy breathing. They also tested N95 and surgical masks, which are more commonly used in healthcare settings.

Previous studies have only looked at a small selection of fabrics when the wearer is breathing normally, when particles are expelled at lower speed. Studying more fabrics and testing them at higher speeds provides a more robust evidence base for the effectiveness of fabric masks.

̽��ֱ��results, reported in the journal BMJ Open, show that most of the fabrics commonly used for non-clinical face masks are effective at filtering ultrafine particles. N95 masks were highly effective, although a reusable HEPA vacuum bag actually exceeded the N95 performance in some respects.

As for homemade masks, those made of multiple layers of fabric were more effective, and those which also incorporated interfacing, which is normally used to stiffen collars, showed a significant improvement in performance. However, this improvement in performance also made them more difficult to breathe through than an N95 mask.

̽��ֱ��researchers also studied the performance of different fabrics when damp, and after they had gone through a normal washing and drying cycle. They found that the fabrics worked well while damp and worked sufficiently after one laundry cycle, however previous studies have shown that repeated washing degrades the fabrics, and the researchers caution that masks should not be reused indefinitely.

“Fabric masks have become a new necessity for many of us since the start of the COVID-19 pandemic,” said first author Eugenia O’Kelly from Cambridge’s Department of Engineering. “In the early stages of the pandemic, when N95 masks were in extremely short supply, many sewers and makers started making their own fabric masks, meeting the demands that couldn’t be met by supply chains, or to provide a more affordable option.”

While there are numerous online resources which help people make their own masks, there is little scientific evidence on what the most suitable materials are.

“There was an initial panic around PPE and other types of face masks, and how effective they were,” said O’Kelly. “As an engineer, I wanted to learn more about them, how well different materials worked under different conditions, and what made for the most effective fit.”

For the current study, O’Kelly and her colleagues built an apparatus consisting of sections of tubing, with a fabric sample in the middle. Aerosolised particles were generated at one end of the apparatus, and their levels were measured before and after they passed through the fabric sample at a speed similar to coughing.

̽��ֱ��researchers also tested how well each fabric performed in terms of breathing resistance, based on qualitative feedback from users. “A mask which blocks particles really well but restricts your breathing isn’t an effective mask,” said O’Kelly. “Denim, for example, was quite effective at blocking particles, but it’s difficult to breathe through, so it’s probably not a good idea to make a mask out of an old pair of jeans. N95 masks are much easier to breathe through than any fabric combinations with similar levels of filtration.”

In preparation for the study, the researchers consulted with online sewing communities to find out what types of fabric they were using to make masks. Due to the severe shortage of N95 masks at the time, several of the sewers reported that they were experimenting with inserting vacuum bags with HEPA filters into masks.

̽��ֱ��researchers found that single-use and reusable vacuum bags were effective at blocking particles, but caution that the single-use bags should not be used in face masks, as they fall apart when cut, and may contain component materials which are unsafe to inhale.

“It’s a matter of finding the right balance – we want the materials to be effective at filtering particles, but we also need to know they don’t put users at risk of inhaling fibres or lint, which can be harmful,” said O’Kelly.

̽��ֱ��researchers caution that their study has several limitations: namely, that they did not look at the role which fit plays in filtering particles. In a related project, O’Kelly has been studying how the fit of masks in healthcare settings can be improved. In addition, many viruses are carried on droplets which are larger than those looked at in the current study.

However, O’Kelly says the results may be useful for sewers and makers when choosing which fabric to use for making masks. “We’ve shown that in an emergency situation where N95 masks are not available, such as in the early days of this pandemic, fabric masks are surprisingly effective at filtering particles which may contain viruses, even at high speeds.”

Further information about the research can be found at: www.facemaskresearch.com

Reference:
Eugenia O’Kelly et al. ‘Ability of fabric face mask materials to filter ultrafine particles at coughing velocity.’ BMJ Open (2020). DOI: 10.1136/bmjopen-2020-039424

A team of researchers has tested everything from t-shirts and socks to jeans and vacuum bags to determine what type of mask material is most effective at trapping the ultrafine particles that may contain viruses such as SARS-CoV-2, the virus which causes COVID-19.

It’s a matter of finding the right balance – we want the materials to be effective at filtering particles, but we also need to know they don’t put users at risk of inhaling fibres or lint, which can be harmful

Eugenia O'Kelly

Kate Trifo via Unsplash

Woman in face mask walking down the street during a coronavirus lockdown

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. Images, including our videos, are Copyright © ̽��ֱ�� of Cambridge and licensors/contributors as identified. All rights reserved. We make our image and video content available in a number of ways – as here, on our main website under its Terms and conditions, and on a range of channels including social media that permit your use and sharing of our content under their respective Terms.

Yes

Easy-to-make, ultra-low-power electronics could charge out of thin air

Anonymous — Tue, 13 Oct 2020 11:25:02 +0000

Electronics that consume tiny amounts of power are key for the development of the Internet of Things, in which everyday objects are connected to the internet. Many emerging technologies, from wearables to healthcare devices to smart homes and smart cities, need cost-effective transistors and electronic circuits that can function with minimal energy use.

Printed electronics are a simple and inexpensive way to manufacture electronics that could pave the way for low-cost electronic devices on unconventional substrates – such as clothes, plastic wrap or paper – and provide everyday objects with ‘intelligence’.

However, these devices need to operate with low energy and power consumption to be useful for real-world applications. Although printing techniques have advanced considerably, power consumption has remained a challenge – the different solutions available were too complex for commercial production.

Now, researchers from the ̽��ֱ�� of Cambridge, working with collaborators from China and Saudi Arabia, have developed an approach for printed electronics that could be used to make low-cost devices that recharge out of thin air. Even the ambient radio signals that surround us would be enough to power them. Their results are published in the journal ACS Nano.

Since the commercial batteries which power many devices have limited lifetimes and negative environmental impacts, researchers are developing electronics that can operate autonomously with ultra-low levels of energy.

̽��ֱ��technology developed by the researchers delivers high-performance electronic circuits based on thin-film transistors which are ‘ambipolar’ as they use only one semiconducting material to transport both negative and positive electric charges in their channels, in a region of operation called ‘deep subthreshold’ – a phrase that essentially means that the transistors are operated in a region that is conventionally regarded as their ‘off’ state. ̽��ֱ��team coined the phrase ‘deep-subthreshold ambipolar’ to refer to unprecedented ultra-low operating voltages and power consumption levels.

If electronic circuits made of these devices were to be powered by a standard AA battery, the researchers say it would be possible that they could run for millions of years uninterrupted.

̽��ֱ��team, which included researchers from Soochow ̽��ֱ��, the Chinese Academy of Sciences, ShanghaiTech ̽��ֱ��, and King Abdullah ̽��ֱ�� of Science and Technology (KAUST), used printed carbon nanotubes – ultra-thin cylinders of carbon – as an ambipolar semiconductor to achieve the result.

“Thanks to deep-subthreshold ambipolar approach, we created printed electronics that meet the power and voltage requirements of real-world applications, and opened up opportunities for remote sensing and ‘place-and-forget’ devices that can operate without batteries for their entire lifetime,” said co-lead author Luigi Occhipinti from Cambridge’s Department of Engineering. “Crucially, our ultra-low-power printed electronics are simple and cost-effective to manufacture and overcome long-standing hurdles in the field.”

“Our approach to printed electronics could be scaled up to make inexpensive battery-less devices that could harvest energy from the environment, such as sunlight or omnipresent ambient electromagnetic waves, like those created by our mobile phones and wifi stations,” said co-lead author Professor Vincenzo Pecunia from Soochow ̽��ֱ��. Pecunia is a former PhD student and postdoctoral researcher at Cambridge’s Cavendish Laboratory.

̽��ֱ��work paves the way for a new generation of self-powered electronics for biomedical applications, smart homes, infrastructure monitoring, and the exponentially-growing Internet of Things device ecosystem.

̽��ֱ��research was funded in part by the Engineering and Physical Sciences Research Council (EPSRC).

Reference:
L. Portilla et al. ‘Ambipolar Deep-Subthreshold Printed-Carbon-Nanotube Transistors for Ultralow-Voltage and Ultralow-Power Electronics.’ ACS Nano (2020). DOI: 10.1021/acsnano.0c06619

Researchers have developed a new approach to printed electronics that allows ultra-low-power electronic devices which could recharge from ambient light or radiofrequency noise. ̽��ֱ��approach paves the way for low-cost printed electronics that could be seamlessly embedded in everyday objects and environments.

Luis Portilla

Artist's impression of a hybrid-nanodielectric-based printed-CNT transistor

Yes

Scientists find upper limit for the speed of sound

Anonymous — Mon, 12 Oct 2020 09:17:26 +0000

̽��ֱ��result - about 36km per second - is around twice as fast as the speed of sound in diamond, the hardest known material in the world.

Waves, such as sound or light waves, are disturbances that move energy from one place to another. Sound waves can travel through different mediums, such as air or water, and move at different speeds depending on what they’re travelling through. For example, they move through solids much faster than they would through liquids or gases, which is why you’re able to hear an approaching train much faster if you listen to the sound propagating in the rail track rather than through the air.

Einstein’s theory of special relativity sets the absolute speed limit at which a wave can travel which is the speed of light and is equal to about 300,000km per second. However, until now it was not known whether sound waves also have an upper speed limit when travelling through solids or liquids.

̽��ֱ��study, published in the journal Science Advances, shows that predicting the upper limit of the speed of sound is dependent on two dimensionless fundamental constants: the fine structure constant and the proton-to-electron mass ratio.

These two numbers are already known to play an important role in understanding our Universe. Their finely-tuned values govern nuclear reactions such as proton decay and nuclear synthesis in stars and the balance between the two numbers provides a narrow ‘habitable zone’ where stars and planets can form and life-supporting molecular structures can emerge. However, the new findings suggest that these two fundamental constants can also influence other scientific fields, such as materials science and condensed matter physics, by setting limits to specific material properties such as the speed of sound.

̽��ֱ��scientists tested their theoretical prediction on a wide range of materials and addressed one specific prediction of their theory that the speed of sound should decrease with the mass of the atom. This prediction implies that the sound is the fastest in solid atomic hydrogen. However, hydrogen is an atomic solid at very high pressure above 1 million atmospheres only, pressure comparable to those in the core of gas giants like Jupiter. At those pressures, hydrogen becomes a fascinating metallic solid conducting electricity just like copper and is predicted to be a room-temperature superconductor. Therefore, researchers performed state-of-the-art quantum mechanical calculations to test this prediction and found that the speed of sound in solid atomic hydrogen is close to the theoretical fundamental limit.

Professor Chris Pickard, from Cambridge's Department of Materials Science and Metallurgy, said: “Soundwaves in solids are already hugely important across many scientific fields. For example, seismologists use sound waves initiated by earthquakes deep in the Earth's interior to understand the nature of seismic events and the properties of Earth's composition. They’re also of interest to materials scientists because sound waves are related to important elastic properties including the ability to resist stress.”

Professor Kostya Trachenko, Professor of Physics at Queen Mary, said: “We believe the findings of this study could have further scientific applications by helping us to find and understand limits of different properties such as viscosity and thermal conductivity relevant for high-temperature superconductivity, quark-gluon plasma and even black hole physics.”

Reference:
K. Trachenko et al. ‘Speed of sound from fundamental physical constants.’ Science Advances (2020). DOI: 10.1126/sciadv.abc8662

Adapted from a Queen Mary ̽��ֱ�� of London press release.

A research collaboration between the ̽��ֱ�� of Cambridge, Queen Mary ̽��ֱ�� of London and the Institute for High Pressure Physics in Troitsk has discovered the fastest possible speed of sound.

PublicDomainPictures from Pixabay

Soundwave

Yes

Cambridge researchers awarded European Research Council funding

sc604 — Wed, 01 Apr 2020 13:19:46 +0000

One hundred and eighty-five senior scientists from across Europe were awarded grants in today’s announcement, representing a total of €450 million in research funding. ̽��ֱ��UK has 34 grantees in this year’s funding round, the second-most of any ERC participating country.

ERC grants are awarded through open competition to projects headed by starting and established researchers, irrespective of their origins, who are working or moving to work in Europe. ̽��ֱ��sole criterion for selection is scientific excellence.

ERC Advanced Grants are designed to support excellent scientists in any field with a recognised track record of research achievements in the last ten years.

Professors Mete Atatüre and Jeremy Baumberg, both based at Cambridge’s Cavendish Laboratory, work on diverse ways to create new and strange interactions of light with matter that is built from tiny nano-sized building blocks.

Baumberg’s PICOFORCE project traps light down to the size of individual atoms which will allow him to invent new ways of tugging them, levitating them, and putting them together. Such work uncovers the mysteries of how molecules and metals interact, crucial for creating energy sustainably, storing it, and developing electronics that can switch with thousands of times less power need than currently.

"This funding recognises the huge need for fundamental science to advance our knowledge of the world – only the most imaginative and game-changing science gets such funding," said Baumberg.

Atatüre’s project, PEDESTAL, investigates diamond as a material platform for quantum networks. What gives gems their colour also turns out to be interesting candidates for quantum computing and communication technologies. By developing large-scale diamond-semiconductor hybrid quantum devices, the project aims to demonstrate high-rate and high-fidelity remote entanglement generation, a building block for a quantum internet.

" ̽��ֱ��impact of ERC funding on my group’s research had been incredible in the last 12 years, through Starting and Consolidator grants. I am very happy that with this new grant we as UK scientists can continue to play an important part in the vibrant research culture of Europe," said Atatüre.

Professor Judith Driscoll from Cambridge’s Department of Materials Science & Metallurgy was also awarded ERC funding for her work on nanostructured electronic materials. She is also spearheading joint work of her team, as well as those of Baumberg and Atatüre, on low-energy IT devices.

"My approach uses a different way of designing and creating oxide nano-scale film structures with different materials to both create new electronic device functions as well as much more reliable and uniform existing functions," she said. "Cambridge is a fantastic place that enables all our approaches to come together, driven by cohorts of inspirational young researchers in our UK-funded Centre for Doctoral Training in Nanoscience and Nanotechnology – the NanoDTC."

Professor John Robb from Cambridge’s Department of Archaeology was awarded an ERC grant for the ANCESTORS project on the politics of death in prehistoric Europe. ̽��ֱ��project takes the methods developed in the ‘After the Plague’ project and the taphonomy methods developed in the Scaloria Cave project and apply them to a major theoretical problem in European prehistory - the nature of community and the rise of inequality.

"This project is really exciting and I’ll be working with wonderful colleagues Dr Christiana ‘Freddi’ Scheib at the ̽��ֱ�� of Tartu and Dr Mary Anne Tafuri at Sapienza ̽��ֱ�� of Rome," said Robb. " ̽��ֱ��results will allow us to evaluate for the first time how inequality affected lives in prehistoric Europe and what role ancestors played in it."

Four researchers at the ̽��ֱ�� of Cambridge have won advanced grants from the European Research Council (ERC), Europe’s premier research funding body.

Yes

̽��ֱ��'P' word

lw355 — Thu, 16 Jan 2020 08:00:00 +0000

How do we shift our 'take, make, throw-away' plastic world towards 'recycle, recover, re-use'? It's time for blue-sky thinking plus practical measures in the battle to reduce plastic waste.

‘Magnetic graphene’ switches between insulator and conductor

sc604 — Fri, 01 Feb 2019 09:40:47 +0000

̽��ֱ��international team of researchers, led by the ̽��ֱ�� of Cambridge, say that their results, reported in the journal Physical Review Letters, will aid in understanding the dynamic relationship between the electronic and structural properties of the material, sometimes referred to as ‘magnetic graphene’, and may represent a new way to produce two-dimensional materials.

Magnetic graphene, or iron trithiohypophosphate (FePS₃), is from a family of materials known as van der Waals materials, and was first synthesised in the 1960s. In the past decade however, researchers have started looking at FePS₃ with fresh eyes. Similar to graphene – a two-dimensional form of carbon – FePS₃can be ‘exfoliated’ into ultra-thin layers. Unlike graphene however, FePS₃ is magnetic.

̽��ֱ��expression for electrons’ intrinsic source of magnetism is known as ‘spin’. Spin makes electrons behave a bit like tiny bar magnets and point a certain way. Magnetism from the arrangement of electron spins is used in most memory devices, and is important for developing new technologies such as spintronics, which could transform the way in which computers process information.

Despite graphene’s extraordinary strength and conductivity, the fact that it is not magnetic limits its application in areas such as magnetic storage and spintronics, and so researchers have been searching for magnetic materials which could be incorporated with graphene-based devices.

For their study, the Cambridge researchers squashed layers of FePS₃ together under high pressure (about 10 Gigapascals), they found that it switched between an insulator and conductor, a phenomenon known as a Mott transition. ̽��ֱ��conductivity could also be tuned by changing the pressure.

These materials are characterised by weak mechanical forces between the planes of their crystal structure. Under pressure, the planes are pressed together, gradually and controllable pushing the system from three to two dimensions, and from insulator to metal.

̽��ֱ��researchers also found that even in two dimensions, the material retained its magnetism. “Magnetism in two dimensions is almost against the laws of physics due to the destabilising effect of fluctuations, but in this material, it seems to be true,” said Dr Sebastian Haines from Cambridge’s Department of Earth Sciences and Department of Physics, and the paper’s first author.

̽��ֱ��materials are inexpensive, non-toxic and easy to synthesise, and with further research, could be incorporated into graphene-based devices.

“We are continuing to study these materials in order to build a solid theoretical understanding of their properties,” said Haines. “This understanding will eventually underpin the engineering of devices, but we need good experimental clues in order to give the theory a good starting point. Our work points to an exciting direction for producing two-dimensional materials with tuneable and conjoined electrical, magnetic and electronic properties.”

̽��ֱ��research was funded by the Engineering and Physical Sciences Research Council (EPSRC).

Reference:
C.R.S. Haines et al. ‘Pressure-Induced Electronic and Structural Phase Evolution in the van der Waals Compound FePS₃.’ Physical Review Letters (2018). DOI: 10.1103/PhysRevLett.121.266801

Researchers have found that certain ultra-thin magnetic materials can switch from insulator to conductor under high pressure, a phenomenon that could be used in the development of next-generation electronics and memory storage devices.

Magnetism in two dimensions is almost against the laws of physics, but in this material, it seems to be true

Seb Haines

Yes

Living in a material world: why 'things' matter

lw355 — Wed, 18 Oct 2017 08:00:59 +0000

From the tools we work with to the eyeglasses and dental implants that improve us, our bodies are shaped by the things we use. We express and understand our identities through clothing, cars and hobbies. We create daily routines and relate to each other through houses and workplaces. We imagine place, history and political regimens through sculptures and paintings.

Even when we think we are dealing with abstract information, the form it takes makes a huge difference. When printing liberated the written word from the limited circulation of handwritten manuscripts, the book and the newspaper became fundamental to religious and political changes, and helped create the modern world.

Studies of material culture focus upon things not just as material objects, but also on how they reflect our meanings and uses. Throughout the humanities and social sciences, there is a long tradition of thinking principally about meaning and human intention, but scholars are now realising the immense importance of material things in social life.

At the core of material culture studies is the question of how people and things interact. This is a simple, sweeping question, but one long overlooked, thanks to historically dominant philosophical traditions that focus narrowly on human intention. In fact, it’s only in the past decade that scholars have posed the question of material agency – how things structure human lives and action.

Material culture studies have emerged as central in many disciplines across the ̽��ֱ�� of Cambridge. In archaeology and history, scholars see material objects as fundamental sources for the human past, counterbalancing the discourse-oriented view that written texts give us. Should we use historical sources to see what people think they ate, or count their rubbish to find out what they really consumed? Combining the two gives us answers of unprecedented scope.

Geographers ask why it makes a difference whether workplaces are organised into separate offices or open-plan cubicles. Literary scholars draw attention to how experience and meaning are built around things, like Marcel Proust’s remembering of things long past as a madeleine cake is dipped in tea; even books themselves are artefacts of a singular and powerful kind. Likewise, studying anatomical models and astronomical instruments empowers an understanding of the history of science as a practical activity. And anthropologists explore the capacity of art to cross cultures and express the claims of indigenous peoples.

Material things are also at the heart of new fields such as heritage studies. Memory itself is material, as we’ve seen recently in the USA, where whether to keep or tear down statues of historic figures such as Confederate generals can polarise people.

Unlike most newly emerging fields in the sciences, material culture studies are grounded in a sprawling panoply of related approaches rather than in a tightly focused paradigm. They come from a convergence of archaeology, anthropology, history, geography, literary studies, economics and many other disciplines, each with its own methods for approaching human–thing interactions.

̽��ֱ��reasons for this interest are not hard to find. ̽��ֱ�� ̽��ֱ�� offers a rare combination of three essential foundations for the field. One is world-class strength in the humanities and social sciences, sustained by institutions like the Centre for Research in the Arts, Social Sciences and Humanities (CRASSH), an essential venue for interdisciplinary collaboration as shown by its 'Things' seminar series (see panel).

Most human dilemmas are material dilemmas in some way

̽��ֱ��second is the capacity for a huge range of scientific analyses of materials. ̽��ֱ��third is our immensely varied museum collections: the Fitzwilliam Museum’s treasures; the Museum of Classical Archaeology’s 19th-century cast gallery; the Museum of Archaeology and Anthropology’s worldwide prehistoric, historic and ethnographic collections; and many others. Where else can scholars interested in the material aspect of Victorian collecting study Darwin’s original finches or Sedgwick’s and Scilla’s original fossils, boxes, labels, archives and all?

Whether it’s work on historic costume, craft production, religion or books, the study of material culture offers unparalleled insights into how humans form their identities, use their skills and create a sense of place and history.

But it is not only a descriptive and historical field. Most human dilemmas are material dilemmas in some way. Where did our desire for things come from and how did the economics of consumerism develop? How can we organise our daily lives to reduce our dependence on cars? Should we care where the objects we buy come from before they reach the supermarket shelves? How do repatriation claims grow out of the entangled histories of museum objects?

̽��ֱ��shape of this new field is still emerging, but Cambridge research will be at the heart of it.

Professor John Robb is at the Department of Archaeology, Professor Simon Goldhill is at the Faculty of Classics, Professor Ulinka Rublack is at the Faculty of History and Professor Nicholas Thomas is at the Museum of Archaeology and Anthropology.

Things structure our lives. They enrich us, embellish us and express our hopes and fears. Here, to introduce a month-long focus on research on material culture, four academics from different disciplines explain why understanding how we interact with our material world can reveal unparalleled insights into what it is to be human.

Studies of material culture focus upon things not just as material objects, but also on how they reflect our meanings and uses.

John Robb, Simon Goldhill, Ulinka Rublack, Nicholas Thomas

Harlow Heslop

All the things

Curious objects and CRASSH courses

You’ve had a difficult time lately. You’re thinking that all this bad luck might be more than coincidence. You trim your nails, snip some hair and bend a couple of pins. You put them in a bottle with a dash of urine, heat it up and put it in a wall. That’ll cure the bewitchment, you say to yourself.

Making a ‘witch bottle’ like this would be an entirely reasonable thing to do 400 years ago. It would also be reasonable to swallow a stone from a goat’s stomach to counteract poisoning and hide an old shoe in a chimney breast to increase the chance of conceiving.

“All of these objects took on layers of meaning for their owners, and the fact these strong connections existed at all gives us glimpses of people’s beliefs, hopes and lives,” says Annie Thwaite, a PhD student in the Department of History and Philosophy of Science. She is also one of the convenors of a seminar series on ‘Things’ at the Centre for Research in the Arts, Social Sciences and Humanities (CRASSH).

“Material culture was a crucial part of medicine in the 17th century. Objects like witch bottles are often dismissed as ‘folkish’. But by investigating the bottles’ architectural and geographical situation, their material properties and processes, you start to look through the eyes of their owners. Fearful of supernatural intrusion into their homes and bodies, people would go to great efforts to use something they regarded as a legitimate element of early modern medical practice.”

Charms and amulets, votives and potions, myths and magic will be discussed as this year’s ‘Things’ seminars begins a new focus on imaginative objects.

“Like material culture studies, the seminar series is broad and varied,” she explains. “We might just as easily examine the skills required to craft objects as the power of objects to become politicised.

“Things matter greatly to humans. We have short lives and our stuff outlives us. While we can’t tell our own story, maybe they can.”

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. For image use please see separate credits above.

Yes

Licence type:

Attribution-Noncommercial-ShareAlike

Study reveals mysterious equality with which grains pack it in

tdk25 — Mon, 26 Jun 2017 15:00:36 +0000

At the moment they come together, the individual grains in materials like sand and snow appear to have exactly the same probability of combining into any one of their many billions of possible arrangements, researchers have shown.

̽��ֱ��finding, by an international team of academics at the ̽��ֱ�� of Cambridge, UK, and Brandeis ̽��ֱ�� in the US, appears to confirm a decades-old mathematical theory which has never been proven, but provides the basis for better understanding granular materials – one of the most industrially significant classes of material on the planet.

A granular material is anything that comprises solid particles that can be seen individually with the naked eye. Examples include sand, gravel, snow, coal, coffee, and rice.

If correct, the theory demonstrated in the new study points to a fact of remarkable – and rather mysterious – mathematical symmetry. It means, for example, that every single possible arrangement of the grains of sand within a sand dune is exactly as probable as any other.

̽��ֱ��study was led by Stefano Martiniani, who is based at New York ̽��ֱ�� but undertook the research while completing his PhD at St John’s College, ̽��ֱ�� of Cambridge.

“Granular materials are so widely-used that understanding their physics is very important,” Martiniani said. “This theory gives us a very simple and elegant way to describe their behaviour. Clearly, something very special is happening in their physics at the moment when grains pack together in this way.”

̽��ֱ��conjecture that Martiniani tested was first proposed in 1989 by the Cambridge physicist Sir Sam F. Edwards, in an effort to better understand the physical properties of granular materials.

Globally, these are the second-most processed type of material in industry (after water) and staples of sectors such as energy, food and pharmaceuticals. In the natural world, vast granular assemblies, such as sand dunes, interact directly with wind, water and vegetation. Yet the physical laws that determine how they behave in different conditions are still poorly understood. Sand, for example, behaves like a solid when jammed together, but flows like a liquid when loose.

Understanding more about the mechanics of granular materials is of huge practical importance. When they jam during industrial processing, for example, it can cause significant disruption and damage. Equally, the potential for granular materials to “unjam” can be disastrous, such as when soil or snow suddenly loosens, causing a landslide or avalanche.

At the heart of Edwards’ proposal was a simple hypothesis: If one does not explicitly add a bias when preparing a jammed packing of granular materials – for example by pouring sand into a container – then any possible arrangement of the grains within a certain volume will occur with the same probability.

This is the analogue of the assumption that is at the heart of equilibrium statistical mechanics – that all states with the same energy occur with equal probability. As a result the Edwards hypothesis offered a way for researchers to develop a statistical mechanics framework for granular materials, which has been an area of intense activity in the last couple of decades.

But the hypothesis was impossible to test – not least because above a handful of grains, the number of possible arrangements becomes unfathomably huge. Edwards himself died in 2015, with his theory still the subject of heated scientific debate.

Now, Martiniani and colleagues have been able to put his conjecture to a direct test, and to their surprise they found that it broadly holds true. Provided that the grains are at the point where they have just jammed together (or are just about to separate), all possible configurations are indeed equally likely.

Helpfully, this critical point – known as the jamming transition – is also the point of practical significance for many of the granular materials used in industry. Although Martiniani modelled a system comprising soft spheres, a bit like sponge tennis balls, many granular materials are hard grains that cannot be compressed further once in a packed state.

“Apart from being a very beautiful theory, this study gives us the confidence that Edwards’ framework was correct,” Martiniani said. “That means that we can use it as a lens through which to look at a whole range of related problems.”

Aside from informing existing processes that involve granular materials, there is a wider significance to better understanding their mechanics. In physics, a “system” is anything that involves discrete particles operating as part of a wider network. Although bigger in scale, the way in which icebergs function as part of an ice floe, or the way that individual vehicles move within a flow of traffic (and indeed sometimes jam), can be studied using a similar theoretical basis.

Martiniani’s study was undertaken during his PhD, while he was a Gates Scholar, under the supervision of Professor Daan Frenkel from the Department of Chemistry. It built on earlier research in which he developed new methods for calculating the probability of granular systems packing into different configurations, despite the vast numbers involved. In work published last year, for example, he and colleagues used computer modelling to work out how many ways a system containing 128 tennis balls could potentially be arranged. ̽��ֱ��answer turned out to be ten unquadragintilliard – a number so huge that it vastly exceeds the total number of particles in the universe.

In the new study, the researchers employed a sampling technique which attempts to compute the probability of different arrangements of grains without actually looking at the frequency with which these arrangements occur. Rather than taking an average from random samples, the method involves calculating the limits of the possibility of specific arrangements, and then calculates the overall probability from this.

̽��ֱ��team applied this to a computer model of 64 soft spheres - an imaginary system which could therefore be “over-compressed” after reaching the jamming transition point. In an over-compressed state, the different arrangements were found to have different probabilities of occurrence. But as the system decompressed to the point of the jamming transition, at which the grains were effectively just touching, the researchers found that all probabilities became equal – exactly as Edwards predicted.

“In 1989, we didn’t really have the means of studying whether Edwards was right or not,” Martiniani added. “Now that we do, we can understand more about how granular materials work; how they flow, why they get stuck, and how we can use and manage them better in a whole range of different situations.”

̽��ֱ��study, Numerical test of the Edwards conjecture shows that all packings become equally probable at jamming is published in the journal Nature Physics. DOI: 10.1038/nphys4168.

For the first time, researchers have been able to test a theory explaining the physics of how substances like sand and gravel pack together, helping them to understand more about some of the most industrially-processed materials on the planet.

Granular materials are so widely-used that understanding their physics is very important. Clearly, something very special is happening at the moment when grains pack together in this way.

Stefano Martiniani

Wikimedia Commons

A huge range of materials are classified as granular – including sand, gravel, snow, nuts, coal, rice, barley, coffee and cereals. Globally, they are the second-most processed type of material in industry, after water.

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