Droplets map out tension in stretched films

Liquid droplets sprayed onto a stretched film reveal asymmetries in the tension within the film – according to physicists in Canada and France. Rafael Schulman, Kari Dalnoki-Veress and colleagues at McMaster University and ESPCI Paris found that glycerol on an elastic polymer film formed circular droplets when the film is stretched with a tension that is uniform in all directions. However, when the tension is greater in one direction, the droplets form with elliptical shapes. What is more, the long axes of the elliptical droplets point along the direction of highest tension. By measuring the 3D shape of a droplet, the team was also able calculate the local tension of the film. By studying droplets distributed across a film, the researchers were able to measure the stress vector at different points in the material – mapping how shear and boundaries affect stress, for example. The technique is described in Physical Review Letters and could lead to a new non-destructive way of measuring stress.

3D-printing sensitive robot skin

A 3D-printed electronic fabric could allow robots to feel. The "bionic skin" has been developed by Michael McAlpine of the University of Minnesota in the US and colleagues, and is a step towards wearable electronics for human skin. To create the sensing fabric, the team built a customized 3D printer and used specialized "inks" to build the layers of the skin. The resulting structure has a base layer of silicone topped with electrodes and a coil-shaped pressure sensor, all made of conductive silver-silicone ink. A sacrificial layer holds the layers in place while the ink sets and is then washed away in the final manufacturing stage. Unlike conventional 3D-printing materials, the inks used set at room temperature and stretch up to three times their original size. "This is a completely new way to approach 3D printing of electronics," says McAlpine, "We have a multifunctional printer that can print several layers to make these flexible sensory devices. This could take us into so many directions from health monitoring to energy harvesting to chemical sensing." The bionic skin, presented in Advanced Materials, could also be applied to surgical robots, giving surgeons a sense of touch while working remotely. The discovery could even lead to printing electronics onto human skin. "While we haven't printed on human skin yet, we were able to print on the curved surface of a model hand using our technique," McAlpine says: "We also interfaced a printed device with the skin and were surprised that the device was so sensitive that it could detect your pulse in real time." The next step for the research is to develop semiconductor inks and print on a human body.

Accelerator institute spins out beam-monitor technology

A new commercial device for monitoring beam loss in accelerators has been developed by D-Beam, which is a spin-out company from the Cockcroft Institute accelerator centre at the Daresbury Laboratory in the UK. The company was co-founded by Carsten Welsch and Alexandra Alexandrova who are both at the University of Liverpool, which is one of five partners that operate the Cockcroft Institute. The company's first product is new type of sensor that can monitor the "halo" of particles lost by a beam of particles as it moves through an accelerator. In some cases this loss introduces unwanted noise into experiments, and in some extreme situations beam loss can damage accelerators. The system uses optical fibres fitted with advanced light detectors. Whenever a stray particle crosses a fibre, it creates a light pulse that is recorded with extreme precision – revealing both the time and place in the accelerator where the particle was detected. "Another product we are considering for commercialization is a gas-jet-based monitor that can characterize the profile of the beam, another key feature that needs constant surveillance," says Welsch. The monitor – which will be deployed in the next upgrade of the Large Hadron Collider at CERN – fires a cold supersonic gas jet shaped across the path of the beam. When the beam particles hit the atoms of the gas, light is generated, which creates a "photograph" of the beam's profile.


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