Interest in micro-aerial vehicles (MAVs) has proliferated in the last decade. But making these aircraft fly is more complicated than simply shrinking airplane designs. At smaller sizes and lower speeds, an airplane’s Reynolds number is smaller, too, and it behaves aerodynamically differently. The photo above shows the upper surface of a low Reynolds number airfoil that’s been treated with oil for flow visualization. The flow in the photo is from left to right. On the left side, the air has flowed in a smooth and laminar fashion over the first 35% of the wing, as seen from the long streaks of oil. In the middle, though, the oil is speckled, which indicates that air hasn’t been flowing over it–the flow has separated from the surface, leaving a bubble of slowly recirculating air next to the airfoil. Further to the right, about 65% of the way down the wing, the flow has reattached to the airfoil, driving the oil to either side and creating the dark line seen in the image. Such flow separation and reattachment is common for airfoils at these scales, and the loss of lift (and of control) this sudden change can cause is a major challenge for MAV designers. (Image credit: M. Selig et al.)
Droplets containing two liquids with different properties can be made into fluid machines. Engineers in the lab of Stanford University’s Manu Prakash (creator of the 50-cent paper microscope) found that mixtures of simple ingredients like water and propylene glycol-based food coloring could propel themselves in intricate patterns and move other droplets around a standard glass slide.
“We demonstrate experimentally and analytically that these droplets are stabilized by evaporation-induced surface tension gradients and that they move in response to the vapour emitted by neighbouring droplets,” the authors write in a paper published yesterday in the journal Nature. “Our fundamental understanding of this robust system enabled us to construct a wide variety of autonomous fluidic machines out of everyday materials.”
The work trying to understand the dynamics of these different fluids began with an unexpected observation by coauthor and bioengineering graduate student Nate Cira in 2009. While completing an unrelated experiment as an undergraduate, Cira noticed that drops of food coloring began to move of their own accord on a glass slide. See video below.
What makes drops of food coloring able to dance, chase, sort themselves, or align with one another? This unexpected behavior is a consequence of food coloring consisting of two mixed liquids: water and propylene glycol. Both have their own surface tension properties and evaporation rates, which ultimately drives the behavior you see in the animations above. Both long-range and short-range interactions are observed. The former are due to vapor from each droplet adsorbing onto the glass around the droplet, thereby changing the local surface tension and causing nearby drops to feel an attractive force. The short-range effects are also surface-tension-driven. Droplets with lower surface tension will naturally try to flow toward areas of higher surface tension, which causes them to “chase” dissimilar adjacent drops. You can learn more about the research in the videos linked below (especially the last two), or you can read about the work in this article or the original research paper. (Image credit: N. Cira et al., source videos 1, 2, 3, 4; GIFs via freshphotons; submitted by entropy-perturbation)
One species of bacteria forms dynamic, living crystals, says new research from Rockefeller University. Biophysicists have revealed that fast-swimming, sulfur-eating microbes known as Thiovulum majus can organize themselves into a two-dimensional lattice composed of rotating cells, the first known example of bacteria spontaneously forming such a pattern. The regular, repeated arrangement of the microbial cells shares the geometry of atoms within a mineral crystal, but the dynamics are fundamentally different; the bacterial crystals constantly move and reorganize as a result of the power generated by individual cells within them,
Scientists at Georgia Tech measured eyelashes from 22 mammals, and found that average eyelash length hovers around 1/3 of eye width. Why? That’s the ideal length to keep junk out of your eye and keep your tears from evaporating. Eyelashes accomplish this necessary function by altering the air flow around the eye - not just by physically blocking falling dust.
“If women use false eyelashes they could actually dry out their eyes a little faster and have to blink more frequently.” - Guillermo Amador, lead author
Our friend Destin from Smarter Every Day recently hung out with some American Idol contestants. Destin wasn’t singing (who knows, maybe he has a voice like an angel), but rather he introduced them to the amazing physics of acoustic levitation. Apparently that very cool thing wasn’t cool enough, so he destroyed the droplets and shot the whole thing at super slo-mo. It’s gonna blow your mind.
Watching rain drops hit a puddle or lake is remarkably fascinating. Each drop creates a little cavity in the water surface when it impacts. Large, energetic drops will create a crown-shaped splash, like the ones in the upper animation. When the cavity below the surface collapses, the water rebounds into a pillar known as a Worthington jet. Look carefully and you’ll see some of those jets are energetic enough to produce a little satellite droplet that falls back and coalesces. Altogether it’s a beautifully complex process to watch happen over and over again. (Image credit: K. Weiner, source)