In urban areas, buildings and concrete surfaces create a heat effect that can make temperatures in the city substantially higher than in nearby rural areas. That heat isn’t just above ground, either. It seeps into the subsurface, measurably increasing groundwater temperatures. In a recent study, authors suggest this excess subsurface heat could be reclaimed and recycled (via heat pumps and other heat exchangers) in urban areas to offset peoples’ needs and to help groundwater return to its normal temperature. They found that even focusing on heat stored in the top meter of the subsurface could provide green heating for much of the world’s urban populations. (Image credit: J. Dylag; research credit: S. Benz et al.)
Category: Research
Eroding Grains
When a spacecraft comes in for a landing (or a tag similar to what OSIRIS-REx did), there’s a turbulent jet that points straight into a bed of particles. How those particles react — how they erode and the crater that forms — depends on many factors, including the cohesion between particles. In these experiments, researchers investigated such a jet (in air) and its impact on particles with differing amounts of cohesion.
When there is little cohesion between particles, erosion takes place a single particle at a time (Image 1). Once there’s some cohesion, the jet’s velocity has to be higher to trigger erosion (Image 2). Once erosion does begin, it includes both singular and clumped particles. In highly cohesive beds, velocities must be even higher to create erosion, which takes place with large clusters of particles flying off together (Image 3). (Image and research credit: R. Sharma et al.)
Hydrophobic Ice
Water is an endlessly peculiar substance, eager to adopt many configurations. Each molecule can form up to four, highly-directional bonds. In this study, researchers found an unexpected configuration, a 2D type of ice known as bilayer hexagonal ice, on a corrugated gold surface. Bilayer hexagonal ice has been known since the late 1990s, but it was thought to be comparatively rare. In this form, water molecules assemble in an ice only two molecular layers thick, with hydrogen bonds between neighboring molecules taking up nearly all possible binding sites. With nowhere to bind, additional water cannot add to the ice’s thickness, making the ice as a whole hydrophobic or “water-fearing”.
This illustration shows a type of 2D ice, known as bilayer hexagonal ice, as it forms on a corrugated gold surface. From above (top half), the water molecules align to the surface with some molecules (red) in the troughs and others (pink) along the ridges. Viewed from the side (lower half), most of the molecules bind with their neighbors, leaving few H-bond sites available where more water layers of water could attach. This inability to add more vertical layers is why the ice appears hydrophobic. Previously, this type of ice had only been found on hydrophobic, flat surfaces. In the latest research, though, researchers found that surface corrugations allowed the ice to form, even on a surface that was only slightly hydrophobic. Observations like these help theorists modeling water and its interactions with surface. (Image credit: top – E. McKenna, illustration – APS/A. Stonebraker; research credit: P. Yang et. al.; via APS Physics; submitted by Kam-Yung Soh)
When Rivers Jump
Avulsions — sudden changes in the course of a river — are a river’s equivalent of an earthquake, and they can be similarly devastating for those in the river’s path. In a recent study, authors combed through 50 years’ worth of satellite data to catalog over 100 avulsions and categorize them into three regimes. About a quarter of the observed avulsions took place in the river delta’s fan, where the river spreads out once it exits a canyon or valley. These avulsions, they found, occur when rivers lose confinement and sediment can build up.
This animation of satellite images shows the sudden avulsion — a dramatic change in the river’s course — that took place on the Kosi River in 2008. Among the other observations, the team linked avulsion location to the river’s flow properties. Most of these remaining avulsions took place in the river’s backwater region, where the river begins to slow down before its outlet. The last category of avulsion took place far upstream of the backwater region on rivers with high sediment flows. During flood conditions, erosion can travel far upstream on these rivers, causing avulsions in unexpected places. Changes in sediment load due to human activities, like deforestation, could even cause rivers to change from the backwater regime to the high-sediment load one. (Image credit: top – R. Simmon/USGS, bottom – S. Brooke et al.; research credit: S. Brooke et al.; via AGU Eos; submitted by Kam-Yung Soh)
Liquid-in-Liquid Printing
With 3D printing and other recent technologies, manufacturing options are always in flux. Here, researchers explore a method for printing a liquid inside of a liquid. Their materials are specially chosen such that the injected liquid forms an emulsion at its interface with the surrounding fluid. Once injection ends, the interface forms a wrinkly, viscoelastic skin that acts like a tube. As shown below, the tube is robust enough that it can be pumped full of yellow-dyed water without any loss of structure. (Image and research credit: P. Bazazi et al.)
Inhibiting Marine Lightning
Thunderstorms over the ocean have substantially less lightning than a similar storm over land. Scientists wondered whether this difference could be due to lower cloud bases over the ocean or differences in the cloud droplets’ nuclei. But a new study instead implicates coarse sea spray as the deciding factor. By tracking the full lifetime of storm systems through remote sensing, the team found that fine aerosols can increase lightning activity over both land and ocean. But adding coarse sea salt from sea spray reduced lightning by 90% regardless of fine aerosols. With sea salt in the mix, clouds seem to develop fewer but larger condensation droplets, providing less opportunity for the electrification necessary to generate lightning. (Image credit: Z. Tasi; research credit: Z. Pan et al.)
Rain-Driven Prey Capture
Pitcher plants often entice their insect victims with sweet nectar before trapping them in inescapable viscoelastic goo. But some species go even further. Nepenthes gracilis, a species native to Southeast Asia uses its leafy springboard to lure its prey. Once an ant crawls to the underside of the leaf, a falling rain drop will spell its doom. When drops hit the leaf, it deflects down and jerks up, thanks to its shape and stiffness. The motion catapults insects into the pitcher, where digestive fluids await. While we’ve seen some fast-moving plants before, this is a rare example of a plant with an externally-driven speed mechanism. With it, the pitcher plant doesn’t have to wait or expend any metabolic effort to reset for the next insect. (Image credit: GFC Collection/Alamy; research credit: A. Lenz and U. Bauer; via New Scientist)
Absorbing Sound with Moth Wings
Manmade soundproofing tends to be porous and bulky or very limited in the range of frequencies it can handle. In contrast, moths are natural absorbers of ultrasound, having evolved to avoid reflecting those frequencies back to the bats hunting them. Researchers took the structures from a moth wing and applied them to an aluminum disk to see how the coating performed. They found that the moth wing’s structures reduced sound reflection by as much as 87% at the lowest frequency tested (20kHz, still beyond human hearing.) As researchers explore how the individual structures of the wing perform, they hope to adapt the moth’s prowess to soundproof within the human range of hearing. (Image and research credit: T. Neil et al.; via Physics World)
Aligning by Bubble Array
Assembling structures from small components is often difficult. Techniques like optical tweezers are limited to very small objects, and magnetic techniques only work with certain materials. Here, researchers use acoustical forces on bubbles to move and align centimeter-sized objects.
When a single bubble oscillates in an ultrasonic field, its changing size creates pressure variations around it. When an acoustic wave scatters off one bubble and impacts another, it sets up a small attractive force between the bubbles, known as the secondary Bjerknes force. For individual bubble pairs, this force is extremely small and unable to affect much. But using arrays of bubbles — one array on a fixed object and another on a floating object — researchers amplified the attraction and showed that the resulting forces could manipulate and align their components. (Image credit: top – J. Thomas, others – R. Goyal et al.; research credit: R. Goyal et al.; via APS Physics)
Microscale Kelvin-Helmholtz
When we think of cavitation in a flow, we often think of it occurring at a relatively large scale — on the propeller of a boat, for example. But cavitation takes place on microscales, too, including around fuel-injection nozzles. In this study, researchers investigated submillimeter-scale cavitation using a flow through a tiny Venturi tube. What they found was something we usually associate with larger scale flows: the Kelvin-Helmholtz instability.
The wavy shape of a Kelvin-Helmholtz instability forms when two layers of fluid move past one another at different speeds and the interface where they meet becomes unstable. Here, that happens along a cavitation bubble, where the bubble and the flow meet. Interestingly, at these scales, the Kelvin-Helmholtz instability seems to be the primary method of break-up, instead of shock wave interactions.
For those keeping track, we’ve now seen the Kelvin-Helmholtz instability from the quantum scale up to 160 thousand light-years. It’s hard to achieve a much wider range than that! (Image and research credit: D. Podbevšek et al.; submitted by M. Dular)
