On an apparently normal cube a pattern of hollows and bulges appears when the cube is compressed. Physicists from Leiden University and FOM Institute AMOLF together with colleagues from Tel Aviv University have developed a method to design such three-dimensional More info
structures and to construct these using simple building blocks. This paves the way for the use of 'machine materials' in, for example, prostheses and wearable technology. The researchers will publish their findings on 28 July in Nature.
Normally, atoms and molecules determine the properties of the materials they form. However that is different for 'metamaterials' designed by humans. "In the case of metamaterials, the spatial structure determines the material's behavior," explains group leader Martin van Hecke. "For example, a pattern of holes in a sheet of material gives rise to a mechanical response that is completely different than in the same material without holes. We also wanted to investigate this phenomenon for a three-dimensional pattern of holes."
Van Hecke and his colleagues designed a cube-shaped, flexible building block with a hole in it. If pressure is applied to such a block then some of the sides cave in, whereas others bulge out. By stacking several of these building blocks researchers could make three-dimensional structures. Van Hecke: "The orientation of the blocks in the metamaterial is important. Under pressure, all of the hollow and bulging sides must fit exactly together. Most of the stacks are 'frustrated': somewhere within two hollows or bulges meet. However a large number of fitting solutions for this three-dimensional puzzle were found."
Van Hecke's colleagues at Tel Aviv University calculated the number of possible, non-frustrated stacks for different cubes of building blocks. "For one cube of 14x14x14 building blocks that is a number with no less than 65 figures," says Van Hecke. "For each possible stack the deformation within the cube results in a specific pattern on the sides of the cube. By smartly combining the building blocks we can program the material such that every desired pattern appears on the sides of a compressed cube. Surprisingly such a cube can also be used to analyse patterns. If we press it against a pattern of hollows and bulges then we measure a force that is dependent on the pattern."
Although Van Hecke's research is fundamental in nature there are applications on the horizon. "This type of programmable 'machine materials' could be ideal for prostheses or wearable technology in which a close fit with the body is important," says Van Hecke. "If we can make the building blocks more complex or produce these from other materials then the possibilities really are endless."
To demonstrate that any pattern can be produced on a cube's surface, the researchers developed a cube of 10x10x10 blocks on which a smiley appears when the cube is compressed.
Credits: Corentin Coulais
Physicists have studied the astrophysical neutrino signal as reported by the IceCube collaboration from a different angle with their ANTARES detector. The Milky Way centre was an obvious prime suspect to be a source, but this hypothesis is now only More info
Gotta catch ‘em all! Physicists are always in the hunt for any kind of particle raining down from the sky. Amongst them are neutrinos—one of the hardest to catch. These ultralight particles are so difficult to detect because they penetrate through anything, including detectors. This also means that they are extremely interesting for scientists, because they travel from the inside of space objects directly to Earth, without getting deflected along the way. And with that, they keep a bunch of information safely stored inside them.
To catch them, scientists need massive detectors made of several cubic kilometers of ice or water, like IceCube on Antarctica or ANTARES in the Mediterranean Sea. IceCube has recently reported many detected neutrinos, with a higher number coming from the Southern sky. The centre of the Mily Way is located there, so our Galaxy’s core was an obvious prime suspect to be responsible for a good part of this neutrino influx. However, the signal events in IceCube have a limited resolution, so it remained unclear where the mysterious signal comes from.
Now an international team of physicists, including Leiden University’s Dorothea Samtleben, have used the ANTARES detector to look at the signal at high resolution from a better angle. They show that under certain plausible assumptions on the neutrino flux properties only two of the events detected by IceCube could originate from the Milky Way. ANTARES will now continue with a newly developed reconstruction method to also probe even higher energetic neutrino fluxes from the Milky Way as cause.
‘We are assuming that the so far detected astrophysical neutrinos come from sources with violent “explosions”,’ says Samtleben. ‘We don't know whether the detected neutrinos come from our own galaxy or from outside, and so far also no significant correlation of the neutrino directions could be found with any other known astrophysical source’.
The ANTARES team publishes their results in Physics Letters B on 10 September, but the article is already accessible online.
A new NMR microscope gives researchers an improved instrument to study fundamental physical processes. It also offers new possibilities for medical science, for example to better study proteins in Alzheimer patients’ brains. Publication as Editors' Suggestion in Physical More info
If you get a knee injury, physicians use an MRI machine to look right through the skin and see what exactly is the problem. For this trick, doctors make use of the fact that our body’s atomic nuclei are electrically charged and spin around their axis. Just like small electromagnets they induce their own magnetic field. By placing the knee in a uniform magnetic field, the nuclei line up with their axis pointing in the same direction. The MRI machine then sends a specific type of radio waves through the knee, causing some axes to flip. After turning off this signal, those nuclei flip back after some time, under excitation of a small radio wave. Those waves give away the atoms’ location, and provide physicians with an accurate image of the knee.
MRI is the medical application of Nuclear Magnetic Resonance (NMR), which is based on the same principle and was invented by physicists to conduct fundamental research on materials. One of the things they study with NMR is the so-called relaxation time. This is the time scale at which the nuclei flip back and it gives a lot of information about a material’s properties.
To study materials on the smallest of scales as well, physicists go one step further and develop NMR microscopes, with which they study the mechanics behind physical processes at the level of a group of atoms. Now Leiden PhD students Jelmer Wagenaar and Arthur de Haan have built an NMR microscope, together with principal investigator Tjerk Oosterkamp, that operates at a record temperature of 42 milliKelvin—close to absolute zero. In their article in Physical Review Applied they prove it works by measuring the relaxation time of copper. They achieved a thousand times higher sensitivity than existing NMR microscopes—also a world record.
With their microscope, they give physicists an instrument to conduct fundamental research on many physical phenomena, like systems displaying strange behavior in extreme cold. And like NMR eventually led to MRI machines in hospitals, NMR microscopes have great potential too. Wagenaar: ‘One example is that you might be able to use our technique to study Alzheimer patients’ brains at the molecular level, in order to find out how iron is locked up in proteins.’
NMR microscope, consisting of a thin wire and a small magnetic ball (fake colour purple). The purple ball induces a uniform magnetic field, so that the surrounding atomic nuclei all line up with their axis pointing in the same direction. The researchers send radio waves through their sample, causing some nuclei to flip the other way, and measure how long it takes before they flip back again.
Physicist Scott Waitukaitis receives an NWO Veni grant to research the Leidenfrost effect for squishy materials. This effect is well-known for dancing water droplets in a frying pan.
Have you ever spilled water on a hot frying pan? You will see More info
the water droplets do a funny dance, floating around on a thin layer of vapor. This is known as the Leidenfrost effect. And as silly as this effect may seem, it is used in serious applications. It is for example essential in nuclear reactors, where the Leidenfrost effect makes sure that the cooling water doesn’t directly touch the smoldering hot nuclear fuel rod. In case of direct contact, the water would immediately vaporize in a giant explosion.
Leiden physicist Scott Waitukaitis is interested in the fundamental science behind this effect, especially for squishy materials. NWO has awarded him a Veni grant to perform experimental research on the subject. 'We all know that water floats on a hot surface, and the same goes for stiff solids like dry ice,’ he says. ‘But squishy materials bounce up and down (see video below, edit.). No-one has studied this phenomenon and with this Veni grant I’m going to conduct experiments using high-speed cameras to look at the height of the bounces, the temperature dependence, the sounds, the amount of water is vaporized per bounce, etcetera.’
In the video below you see hydrogel spheres which are fully soaked with water. Waitukaitis is going to develop formulas to describe the exact behavior of the balls.
Scott Waitukaitis and Martin van Hecke (2016) Origami building blocks: Generic and special four-vertices, Physical Review E, 93, 023003. [DOI][pdf]
A. Vinante, M. Bahrami, A. Bassi, O. Usenko, G. Wijts, T.H. Oosterkamp (2016) Upper bounds on spontaneous wave-function collapse models using millikelvin-cooled nanocantilevers, Phys. Rev. Lett., 3, 116, 090402. [DOI][pdf]
6 Sept, 13:45, Academy building, Rapenburg 73
Thesis Defense Ke Liu - Instituut-Lorentz: Gauge Theory and Nematic Order. The rich landscape of orientational phase transition. Prof.dr. J. Zaanen