Linking Nanoparticles with DNA "Velcro"

LU14111 - We have discovered a new type of interaction between micro/nanoscale particles that results from the entanglement of DNA strands attached to their surfaces. Self-complementary DNA single strands on a particle can be induced to form loops. When loops are formed on adjacent particles, they can form mechanical links similar to the ones macroscopic hook-and-loop materials like Velcro rely on. The interactions can be created and broken by a combination of forces, temperature, light sensitive crosslinking and enzymatic unwinding of the topological links. This novel topological interaction may lead to new materials and phenomena such as particles strung on nano-necklaces.

The Physics of Paper Cuts and Guillotines: Slicing Softly with Shear

The top two frames show how dicing deforms a material before cutting it. A slicing motion (bottom frames) allows the cutting wire to pass through easily without damaging surrounding material.

LS12904 - A soft solid is more easily sliced using a combination of pushing a blade down and slicing rather than diced by simply pushing down on it with the same knife. To explain why this is so, we experimentally probe the slicing and dicing of a soft agar gel with a wire, along with a combination of theory and numerical simulations of cutting of a highly deformable solid. We find that purely dicing leads to deformations of the soft solid, so that the blade has to penetrate deeply into the sample in order to cut it, often damaging portions that are not in the path of the cutter. In contrast, a slicing motion cuts more cleanly without damaging the surrounding material, which explains the mechanics of painful paper cuts and design of slanted guillotine blades.

Generating something from nothing: how changing boundaries generate particles

LU13319 -It is a remarkable prediction of quantum field theory that the vacuum can generate real particles (pairs of photons) when a reflecting surface suddenly accelerates. We have created an acoustic analog to this effect using a Bose-Einstein condensate. Instead of creating photons, we generate pairs of phonons by rapidly changing the speed of sound in a condensate. This process was first predicted for electromagnetism in 1970 and is known as the dynamical Casmir effect. The effect is related to Hawking radiation, a process in which particles are spontaneously produced at the horizon of a black hole. An extension of our measurement technique may permit the observation of "sonic Hawking radiation", in an appropriately tailored Bose-Einstein condensate.

Impact of subsurface water flow on shaping landscapes

ES10795 - We show that the splitting of channel heads and the evolution of surface topography is strongly influenced by water flow in the subsurface. When ground water emerges at the surface, producing a spring, the flow removes grains from the surface by erosion, progressively digging a deeper channel, which in turn can draw more water, inducing the growth of a river. Seepage erosion is said to shape many examples of valleys, canyons, and river networks and assumed to produce amphitheater-headed valleys. However, the mechanism by which seepage channels grow and form networks is far from clear.

By performing model laboratory experiments and analysis of the ground water flow, we find significant differences between the case where the groundwater comes primarily through a boundary from a far away source and the case where it is fed by uniform local rain. Our study supports the notion that a channel network can develop in a homogeneous landscape whereby groundwater flow splits as the channels grow leading the channels to split in turn. Our observations have important implications for the interpretation of field data because numerous perturbations present in nature that could influence channel dynamics. Perturbation of the erosion front due to random avalanching events is shown to not lead to channel splittings unless supported by underlying changes in groundwater flow.

Taking Pictures of Quantum Sound Waves

LV12662 - In quantum physics, all objects have a wavelike nature.  The more energy the object possesses, the faster its wave oscillates.  In this work, we study the particles of sound which exist in an ultra-cold gas called a Bose-Einstein condensate.  Previous works measured the energy of the sound particles, but did not observe the corresponding oscillations.  We look directly at these sound particles, and see that they indeed oscillate.  We are able to see the sound particles very clearly with the help of the surrounding Bose-Einstein condensate.  This occurs because the Bose-Einstein condensate is also governed by the laws of quantum physics, so it also acts like a wave.  The large wave of the Bose-Einstein condensate magnifies the small wave of the sound particle.  Upon studying the oscillations of the sound particles, we discovered a surprise.  The oscillation rate is smaller than expected.  This is because the long, narrow Bose-Einstein condensate acts as a flow channel, which only allows certain oscillation rates.  The smaller-than-expected rate implies that the speed of sound is slower than previously thought.  This implies that the Bose-Einstein condensate is less stable than was believed previously.

Tracing the explosion of a molecule

AU10815 - Intense laser can destroy molecular bonds and cause the explosion of molecule on the ultrafast time scale. Shooting movies of such processes allows understanding how the matter responds to the strong laser light and may help to develop tools to control radiation damage. In our work we have traced the explosion of an iodine molecule exposed to intense laser pulse by an ultra-short soft-X-ray pulse from free-electron laser. We were able to probe the temporal development of the electron configuration in the disintegrating molecule and could detect when the molecule gets ionized, how long the separation of atoms takes and what happens during this process: In a molecule the electrons surrounding the atomic core form the molecular bond by sharing electrons between the individual atoms. By breaking the chemical bond, the distributed electron being described by a quantum mechanical wave function has to become localized at an atomic core after a certain time in the exploding molecule. We could observe the time it takes before the electrons localize and that the probability of losing further electrons depends on the separation of the atomic fragments in the molecule.