Microfluidic channels are excellent at creating a steady supply of droplets. But depending on the characteristics of the two viscous fluids being used, as well as factors like flow rate and channel geometry, the results can be anything from well-defined and separated drops to steady jets to wild instabilities. The image above shows a series of different outcomes, including waves that break on the edges of drops and ligaments that stretch around the central fluid. (Image and research credit: X. Hu and T. Cubaud)
Month: February 2020
To Beat Surface Tension, Tadpoles Make Bubbles
For tiny creatures, surface tension is a formidable barrier. Newborn tadpoles are much too small and weak to breach the air-water surface in order to breathe. Researchers found that, instead, the 3 millimeter creatures place their mouths against the surface, expand their mouth to generate suction, and swallow a bubble consisting largely of fresh air.
When they’re especially small, some of these species are essentially transparent (Image 1), allowing researchers to see the bubble directly. But even as the tadpoles aged (Images 2 and 3) and grew strong enough to breach the surface, they observed many instances in which the tadpoles continued this bubble-sucking method to breathe. (Image and research credit: K. Schwenk and J. Phillips; via Cosmos; submitted by Kam-Yung Soh)
Surface Jets in Coalescing Droplets
What goes on when droplets merge is tough to observe, even with a high-speed camera. There are many factors at play: any momentum in the droplets, surface tension, gravity, and Marangoni forces, to name a few. A new study that simultaneously records multiple views of coalescence is shedding some light on these dynamics.
The results are particularly interesting for droplets that are somewhat physically separated so that they only coalesce after one drop impacts near the other. In this situation, with droplets of equal surface tension, researchers observed a jet that forms after impact (Image 1) and runs along the top surface of the coalescing drops (Image 2). That location is a strong indication that the jet is created by surface tension and not other forces.
To test that further, the researchers repeated the experiment but with droplets of unequal surface tension. They found that when the undyed droplet’s surface tension was higher (Image 3), Marangoni forces enhanced the surface jet, as one would expect for a surface-tension-driven phenomenon. But if the dyed droplet had the higher surface tension (Image 4), it was possible to completely suppress the jet’s formation. (Image, research, and submission credit: T. Sykes et al., arXiv)
Paint Versus Hydrogel
In this bizarre short film, we get to see a battle between dissolution and absorption. I think the Chemical Bouillon team has coated hydrogel beads in a layer of paint and then immersed them in water. As the beads absorb water, they expand and grow, tearing their fragile outer layer of paint to smithereens.
One thing that struck me when watching several of the sequences is just how regular the hole spacing in the paint is for the round hydrogels. That hints at an orderly breakdown in the solid paint layer while the interior hydrogel polymer symmetrically expands. It’s a little like watching holes grow in a splash curtain. (Video and image credit: Chemical Bouillon)
Using Electric Fields to Avoid Dripping
Anyone who’s painted a room at home is familiar with the frustration of drips. At certain inclinations, practically every viscous liquid develops these gravity-driven instabilities. They’re troublesome in manufacturing as well, where viscous films are often used to coat components and unexpected drips can ruin the process.
To avoid this, researchers are adding electric fields into the mix. For dielectric fluids — liquids sensitive to electric fields — this addition acts like extra surface tension, stabilizing the film and preventing drips from forming. The researchers’ mathematical models predict the electric field strength necessary for a given fluid layer depending on its inclination. (Image credit: stux; research credit: R. Tomlin et al.; via APS Physics)
Ice Patterns
Periods of freezing and thawing can leave complicated patterns in ice, as seen in this aerial photo of Binnewater Lake in New York. Ice rarely forms evenly on large bodies like this, so there are always underlying weaknesses. A hard freeze may have caused the ice to contract, forming the initial radial pattern. Then warmer periods of melting allowed water to rise into the cracks and expand them. As the process repeats, the visible pattern emerges.
Also note the star-like crack patterns near the shore. These may have formed in spots where something like a stick protruding from the water’s surface allowed warmer water up onto the ice to melt the snow sitting atop it. (Image credit: D. Spitzer; via EPOD; submitted by Kam-Yung Soh)
Inferring Flows with Neural Networks
Fluid dynamicists have long used flow visualization methods to get a qualitative sense for flows, but it’s rare to derive much quantitative data from this imagery. But that may soon change thanks to a new computational technique, called Hidden Fluid Mechanics, that uses data from flow visualizations combined with physics-informed neural networks to derive the underlying velocities and pressures in a flow.
The technique relies on two important ideas. One is that the dye, smoke, or other method of visualizing the flow does not alter the underlying flow; it’s just something carried along by the fluid. In other words, the flow behaves exactly the same whether or not you inserted dye or smoke.
The second key idea is that the Navier-Stokes equations — which are derived from conservation of mass, momentum, and energy — accurately describe the physics of a flow. That assumption is critical to the technique since it uses those equations to constrain the flow fields the algorithm reconstructs.
So here, roughly speaking, is what the algorithm actually does: researchers feed it concentration data from a flow visualization — essentially how much smoke or dye is present at every point in space and time — and the neural network reconstructs, based on the Navier-Stokes equations, what velocity and pressure field would produce that concentration data.
The researchers demonstrate the capabilities of their algorithm by comparing its results to flows where all the information is known. The first image in the gallery above shows concentration data for the flow in an aneurysm. The full flow field is known already from a numerical simulation, but the researchers gave their new algorithm only the concentration data. From that, it reconstructed the streamlines for the aneurysm’s flow, shown in the second image as “Learned”. The “Exact” streamlines on the left are taken from the original numerical simulation data. As you can see, the results are remarkably similar. (Image credit: drawings – L. da Vinci, others – M. Raissi et al.; research credit: M. Raissi et al.; submitted by Stuart H.)
Boiling Water Using Ice Water
Steve Mould demonstrates a neat thermodynamic trick in this video by using ice water to boil hot water. The key to understanding this is recognizing that the boiling point of water depends both on its temperature and its pressure.
Here’s the set-up (which, to be clear, neither he nor I recommend you try yourself): microwave some water in an open bottle until the water is hot enough to boil. Remove the bottle from the microwave and screw on the lid. At this point, you’ve confined any water vapor coming off the hot water, thereby raising the pressure inside the bottle. Even though it’s still quite hot, the water will stop visibly boiling.
Now pour ice water over the top of the bottle. Because water vapor has a lower heat capacity than liquid water, this will preferentially cool the vapor. As its temperature drops, its pressure will also drop. Liquid water boils at lower temperatures when the pressure is lower. (This is part of why cooking and baking instructions are quite different in Denver than they are in Miami.) When the internal pressure in the bottle drops, the remaining hot water will start to visibly boil. (Image and video credit: S. Mould)
Gliding Birds Get Extra Lift From Their Tails
Gorgeous new research highlights some of the differences between fixed-wing flight and birds. Researchers trained a barn owl, tawny owl, and goshawk to glide through a cloud of helium-filled bubbles illuminated by a light sheet. By tracking bubbles’ movement after the birds’ passage, researchers could reconstruct the wake of these flyers.
As you can see in the animations above and the video below, the birds shed distinctive wingtip vortices similar to those seen behind aircraft. But if you look closely, you’ll see a second set of vortices, shed from the birds’ tails. This is decidedly different from aircraft, which actually generate negative lift with their tails in order to stabilize themselves.
Instead, gliding birds generate extra lift with their maneuverable tails, using them more like a pilot uses wing flaps during approach and landing. Unlike airplanes, though, birds rely on this mechanism for more than avoiding stall. It seems their tails actually help reduce their overall drag! (Image and research credit: J. Usherwood et al.; video credit: Nature News; submitted by Jorn C. and Kam-Yung Soh)
Collapsing Inside a Soap Film
There’s a common demonstration of surface tension where a loop of string is placed in a soap film and then the film inside the loop is popped, making it suddenly form a perfect circle when the outer soap film’s surface tension pulls the string equally from every direction. In this video, researchers study a similar situation but with a few wrinkles.
Here the loop of string is replaced with an elastic ring, which has more internal stiffness and starts out entirely round within the soap film. Then the researchers pop the outer film. That burst instantly creates a stronger surface tension inside the ring, which causes it collapse inward. As the researchers note, this is the equivalent situation to applying an external pressure on the outside of the ring. The form of the buckling ring and film depends on just how large this “pressurization” is.
Thickening the elastic from a ring to a band alters the collapse, too. The thicker the elastic band, the harder it is to buckle in the plane of the soap film. So instead it wrinkles as the film collapses, which creates wrinkles in the soap film, too! (Image, video, and research credit: F. Box et al.; see also F. Box et al. on arXiv)
