fuckyeahfluiddynamics: On extremely hot surfac…


On extremely hot surfaces, droplets will skitter on a layer of their own vapor, thanks to the Leidenfrost effect. This keeps the liquid insulated from contact with the hot surface. But what if the surface isn’t solid?

That situation is what we see above. Instead of soaking into a granular material like a room temperature droplet (left), a drop falling onto a very hot bed of grains digs a hole! As with a typical drop on a super hot surface, the heat vaporizes part of the droplet. As the vapor escapes, it carries sand with it, allowing the boiling drop to burrow its way into the material. As the temperature difference between the sand and droplet changes, the digging slows. Eventually, the drop comes to a rest and boils away. (Video and image credit: J. Zou et al.)



Rhodamine (red) and fluorescein (green) dyes highlight the complex flows around a delta wing. To visualize the flow, researchers painted the apex of the delta wing with rhodamine, which gets drawn into the core of the wing’s leading edge vortex. The green fluorescein dye was added to the wing’s trailing edge, where it gets pulled into the secondary structure of the vortices. A laser illuminates the flow, making even the most delicate wisps of dye shine. As the wake behind the wing develops, the dyes reveal growing instabilities along the vortices. Given time and space, these instabilities will grow large enough to destroy any order in the wake, leaving behind turbulence. (Image and research credit: S. Morris and C. Williamson; see also poster)

fuckyeahfluiddynamics: Pour wine or liquor int…


Pour wine or liquor into a glass, give it swirl, and you can watch as droplets form and dance on the walls. This well-known phenomena, often called “tears” or “legs” in wine, results from an interplay of surface tension and evaporation. Despite its common occurrence, researchers are still discovering interesting subtleties in the physics, as seen in new research on the subject.  

Dianna walks you through the phenomenon step-by-step in this video. The key piece of physics is the Marangoni effect, the tendency of regions with high surface tension to pull flow from areas with lower surface tension. In the wine glass, evaporation creates this surface tension gradient by removing alcohol more quickly from the meniscus than the bulk. That sets up the gradient that lets the wine climb the glass. By preventing or delaying that evaporation, we can see other neat effects, too, like shock fronts that travel through the film. (Video credit: Physics Girl; research credit: Y. Dukler et al.)

fuckyeahfluiddynamics: Over the past couple de…


Over the past couple decades, microfluidic devices have become a staple of medical and biological diagnostics and analysis. Tests that once required large and specialized equipment can now be completed closer to a patient, using only a few drops of sample fluid. Running multiple tests on a single chip can become difficult, though, since flow through the device tends to dissolve and mix the dried reagents used for tests. But a new method cleverly uses fluidic forces to keep reagents separated without the need for complicated microfluidic structures.

The basic concept is outlined in the illustration above. You’re looking down on a microfluidic channel that’s long and very thin. A shallow groove down the middle serves as a barrier by pinning the contact line of the incoming fluid. So when the sample fluid flows in through the inlet on the left, it will only fill the top half of the cell. When it reaches the far right side, it turns the corner and flows to the left, encountering the first of the dried reagents it must dissolve for the device’s tests. The fluid will fill the lower channel quickly and then come to rest while the reagents dissolve. 

With both sides of the channel full of liquid, the shallow barrier can no longer hold, and the fluid will take up the full width of the channel, with two well-dispersed – but separated – regions of reagents. Once that’s happened, a valve – represented by the pale blue line near the right side of the illustration – releases the fluid into the next section of the chip, allowing the analysis to proceed. (Image credit: Nature; research credit: O. Gökçe et al.; submitted by Kam-Yung Soh)

fuckyeahfluiddynamics: Moisture is cotton cand…


Moisture is cotton candy’s natural enemy. The spun sugar dissolves incredibly quickly under the influence of even a couple drops of water. Why that’s so is clearer when looking at a single fiber. Inside the droplet there’s a gradient in the sugar concentration. The more sugary water sinks, and the sugar fiber dissolves more quickly in the upper part of the droplet, where the less sugary water can more easily take up new sugar. 

Once the fiber breaks, capillary forces draw the droplet upward, giving it a fresh section of fiber to dissolve. In a web of fibers, this process can pull droplets apart and together as they quickly eat through the spun sugar. (Image and video credit: S. Dorbolo et al.; submitted by Alexis D.)

The lifespan of an evaporating liquid drop

The lifespan of an evaporating liquid drop

The lifespan of a liquid droplet which is transforming into vapour can now be predicted thanks to a theory developed at the University of Warwick. The new understanding can now be exploited in a myriad of natural and industrial settings where the lifetime of liquid drops governs a process’ behaviour and efficiency.

Water evaporating into vapour forms part of our daily existence, creating plumes emanating from a boiling kettle and bulging clouds as part of the earth’s water cycle. Evaporating liquid drops are also commonly observed, e.g. as the morning dew disappears off a spider’s web, and are critical for technologies such as fuel-injection combustion engines and cutting-edge evaporative cooling devices for next generation electronics.

Researchers from the Mathematics Institute and School of Engineering at the University of Warwick have had the paper “Lifetime of a Nanodroplet: Kinetic Effects & Regime Transitions,” published in the journal Physical Review Letters, in which they explore the lifespan of a liquid droplet.

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fuckyeahfluiddynamics: Reader Matt G asks: [W…


Reader Matt G asks:

[What’s] going on here?

Why’s the pattern square? Just a special case of waves traveling in different directions, and this photo happened to catch some at right angles to one another?

You’re not far off, Matt! This is an example of cross sea, where wave trains moving in different directions meet. Like most ocean waves, these waves originated from wind moving over the water. As the wind blows, it transfers energy to the water, disturbing what would otherwise be a smooth surface and setting up a series of waves. Oftentimes, these waves can outlast the wind that generates them and travel over long distances of open water as a swell.

Cross seas occur when two of these wave systems collide at oblique angles. They’re most obvious in shallow waters like those seen here, where the depth makes their criss-cross pattern clearer. Another name for them is square waves, and although the pattern isn’t a perfect square, it’s usually fairly close. If the waves aren’t separated by a large angle, they’re more likely to merge than to create this sort of pattern.

Neat as cross seas look, they’re quite dangerous, both to ships and swimmers. Ships are built to tackle waves head-on and don’t fare well when they’re forced to take waves from the side. For swimmers, the danger is a little different. Cross seas create intense vorticity under the surface and can generate stronger than usual riptides that sweep the unwary out to sea. (Image credit: M. Griffon)

What is Cavitation? (with AvE) The basics o…

What is Cavitation? (with AvE)

The basics of fluid cavitation, including demonstration from AvE. 

If you subject a fluid to a sudden change in pressure, some interesting things can happen. You can cause tremendous damage to moving parts, or you can harness this destructive power in many beneficial ways. Thanks to AvE for supplying the demonstration for this video. If you like seeing the insides of tools and industrial machinery check out his channel: https://www.youtube.com/channel/UChWv6Pn_zP0rI6lgGt3MyfA

fuckyeahfluiddynamics: Carnivorous pitcher pla…


Carnivorous pitcher plants supplement their nutrient-poor environments by capturing and consuming insects. The viscoelastic fluid inside them helps trap prey, but fluid dynamics plays a role elsewhere on the plant as well. The inner and outer surfaces of the pitcher are covered in macroscopic and microscopic grooves, seen above, oriented toward the interior of the plant. 

Researchers found that these grooves trap droplets on the slippery plant through capillary action. Once adhered, the droplet cannot easily move across the grooves, but it can slip along them, carrying the droplet and any insect stuck to it, into the plant. By replicating pitcher-plant-inspired grooves on manmade surfaces, researchers found they were able to better control droplet motion on slippery, lubricant-infused surfaces than in previous work. (Image and research credit: F. Box et al.; via Royal Society; submitted by Kam-Yung Soh)

Dip-coating: How to predict the thickness of…

Dip-coating: How to predict the thickness of thin films?

The immersion method of applying a coating to an object presents a central challenge: predicting the thickness of the layer that will cover the object. This question raises important financial issues for manufacturers. A team from the CBI laboratory at ESPCI Paris and from the College de France has developed an experimentally verified theoretical framework to predict the coating thickness in the case of yield-stress fluids. The researchers just reported their findings in the journal Physical Review Letters.

To cover an object with a liquid coating, the object is often immersed into a fluid reservoir and then pulled out. This technique, known as dip-coating, has been previously studied both experimentally and numerically when the fluid has a yield stress (such as paints, gels, glues, foams or creams), but a complete description of the behavior has been lacking. The researchers addressed this problem by studying a model system in which axial symmetry plays an important role. It involves plunging a cylindrical rod into a cylindrical bath. They showed that the thickness of the deposited coating results from the fluid flow induced by the movement of the rod. However, this flow depends mainly on the geometry and dimensions of the reservoir. For yield-stress fluids, it is therefore the geometry of the bath that determines the thickness of the deposited film (the dimensions of the withdrawn object also being an important factor).

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