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Soft matter and liquids

Soft matter and liquids

Flash Physics: Drawing water from dry air, material glows under stress, physicist bags economics award

18 Apr 2017 Sarah Tesh

Flash Physics is our daily pick of the latest need-to-know developments from the global physics community selected by Physics World's team of editors and reporters

Drip dry: the water extractor works in arid regions

Using the Sun to extract water from dry air

A new solar-powered system that can extract water from air in arid regions of the world has been unveiled by researchers in the US and Saudi Arabia. Led by Omar Yaghi of the University of California, Berkeley, and Evelyn Wang of the Massachusetts Institute of Technology, the team created the device using a metal-organic framework (MOF). The device is powered by heat from sunlight. It can harvest 2.8 litres of liquid water per kilogram of MOF per day at relative humidity levels of 20–30% – which are common in arid regions of the world. MOFs combine metals with organic molecules to create rigid, porous structures that are ideal for storing gases and liquids. The system comprises a kilogram of compressed MOF crystals that sits below a solar absorber and above a condenser plate. Ambient air diffuses through the porous MOF, where water molecules preferentially attach to the interior surfaces. Sunlight heats up the MOF and drives the bound water toward the condenser, where the vapour condenses and drips into a collector. “This work offers a new way to harvest water from air that does not require high relative humidity conditions and is much more energy efficient than other existing technologies,” says Wang. Yaghi adds: “There is a lot of potential for scaling up the amount of water that is being harvested. It is just a matter of further engineering now.” The system is described in Science.

Material glows in response to stress

A material that repeatedly lights up in response to mechanical forces has been developed by researchers at Okinawa Institute of Science and Technology Graduate University in Japan. To create the material, Georgy Filonenko and Julia Khusnutdinova incorporated stress-sensing molecules called photoluminescent mechanophores into the common polymer, polyurethane. While mechanophores are not new, they are typically one-use only. They emit light when a strong force breaks a specific chemical bond between atoms or pulls apart two molecular patterns. The radical change in structure causes a shift in the wavelength of light emitted (the glow), but it is difficult to return the molecule to its original, “off” state. Therefore, Filonenko and Khusnutdinova developed a molecule that relies upon dynamic rather than structural changes. Their phosphorescent copper complexes move rapidly when the host polymer is in a relaxed state, and the motion suppresses light emission. Yet when a mechanical force is applied, the movement of the polymer chains, and hence the mechanophores, slows, and consequently the complexes are able to luminesce. The light emitted is visible to the naked eye when the material is bathed in UV light and becomes brighter with increasing force. However, unlike previous stress-reacting materials, Filonenko and Khusnutdinova’s can revert to its original, non-luminescent state as no chemical bonds have been broken. The new mechanophores are described in Advanced Materials and could be used to assess stress and dynamics in soft materials.

Physicist bags prestigious economics award

The physicist-turned-economist Dave Donaldson has won the John Bates Clark Medal of the American Economic Association (AEA). The medal is given to “economist under the age of 40 who is judged to have made the most significant contribution to economic thought and knowledge”. Described by the AEA as “the most exciting economist in the area of empirical trade,” Donaldson has studied topics as diverse as the economic impact of railways in 19th century India and the consequences of climate change on agricultural markets. Donaldson, 38, is associate professor of Economics at Stanford University in California and is a dual citizen of Canada and the UK. He studied physics at the University of Oxford before doing an MSc and PhD in economics at the London School of Economics.

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