What happens when lava meets ice or water? Artists and geologists are working together to explore these interactions by melting crushed basalt and pouring it onto different substrates. Ice is their classic example; instead of melting instantly through the ice, the lava is so hot that it creates a layer of steam between it and the ice. This steam helps the lava flow due to lower friction while also insulating the ice from the lava. It’s an example of the Leidenfrost effect. The end result is a very bubbly lava flow thanks to the steam trying to escape through the viscous lava. (Video credit: Science Channel; submitted by @jchawner)
Tag: viscous flow
Lava Coiling
It’s tough to get much closer to flowing lava than this video of freshly forming coastline in Hawaii. Lava is complex fluid, with viscous properties that vary significantly with chemical composition, temperature and deformation. Here, despite being very viscous, the lava flows quickly–perhaps even turbulently. Several times it forms a heap and even shows signs of the rope-coiling instability familiar from viscous fluids like honey. All in all, it’s quite mesmerizing. (Video credit: K. Singson; submitted by Stuart B.)
The Kaye Effect
Those who have poured viscous liquids like syrup or honey are familiar with how they stack up in a rope-like coil, as shown in the top row of images above. What is less familiar, thanks to the high speed at which it occurs, is the Kaye effect, which happens in fluids like shampoo when drizzled. Shampoo is a shear-thinning liquid, meaning that it becomes less viscous when deformed. Like a normal Newtonian fluid, shampoo first forms a heap (bottom row, far left). But instead of coiling neatly, the heap ejects a secondary outgoing jet. This occurs when a dimple forms in the heap due to the impact of the inbound jet. The deformation causes the local viscosity to drop at the point of impact and the jet slips off the heap. The formation is unstable, causing the heap and jet to collapse in just a few hundred milliseconds, at which point the process begins again. (Image credit: L. Courbin et al.)
Hydrophobicity and Viscous Flow
Hydrophobic surfaces are great for creating some wild behaviors with water droplets, but they make neat effects with other liquids, too. The viscous honey in the first segment of this Chemical Bouillon video is a great example. Because the honey doesn’t adhere to the hydrophobic surface, the viscoelastic fluid does not maintain the form it had when drizzled on the surface. Instead, the honey contracts, with surface tension driving Plateau-Rayleigh-like instabilities that break the contracting ligaments apart to form nearly spherical droplets of honey on the surface. (Video credit: Chemical Bouillon)
4th Birthday: The Kaye Effect
Today’s post continues my retrospective on mind-boggling fluid dynamics in honor of FYFD’s birthday. This video on the Kaye effect was one of the earliest submissions I ever received–if you’re reading this, thanks, Belisle!–and it completely amazed me. Judging from the frequency with which it appears in my inbox, it’s delighted a lot of you guys as well. The Kaye effect is observed in shear-thinning, non-Newtonian fluids, like shampoo or dish soap, where viscosity decreases as the fluid is deformed. Like many viscous liquids, a falling stream of these fluids creates a heap. But, when a dimple forms on the heap, a drop in the local viscosity can cause the incoming fluid jet to slip off the heap and rebound upward. As demonstrated in the video, it’s even possible to create a stable Kaye effect cascade down an incline. (Video credit: D. Lohse et al.)
Why Ketchup is Hard to Pour
Oobleck gets a lot of attention for its non-intuitive viscous behaviors, but there are actually many non-Newtonian fluids we experience on a daily basis. Ketchup is an excellent example. Unlike oobleck, ketchup is a shear-thinning fluid, meaning that its viscosity decreases once it’s deformed. This is why it pours everywhere when you finally get it moving. Check out this great TED-Ed video for why exactly that’s the case. In the end, like many non-Newtonian fluids, the oddness of ketchup’s behavior comes down to the fact that it is a colloidal fluid, meaning that it consists of microscopic bits of a substance dispersed throughout another substance. This is also how blood, egg whites, and other non-Newtonian fluids get their properties. (Video credit: G. Zaidan/TED-Ed; via io9)
Viscous Fingers
Viscous liquid placed between two plates forms a finger-like instability when the top plate is lifted. The photos above show the evolution of the instability for four initial cases (top row, each column) in which the initial gap between the plates differs. Each row shows a subsequent time during the lifting process. As the plate is pulled up, the viscous liquid adheres to it and air from the surroundings is entrained inward to replace the fluid. This forms patterns similar to the classic Saffman-Taylor instability caused when less viscous fluid is injected into a more viscous one. (Photo credit: J. Nase et al.)
When Jets Collide
When two jets of a viscous liquid collide, they can form a chain-like stream or even a fishbone pattern, depending on the flow rate. This video demonstrates the menagerie of shapes that form not only with changing flow rates but by changing how the jets collide – from a glancing impingement to direct collision. When just touching, the viscous jets generate long threads of fluid that tear off and form tiny satellite droplets. At low flow rates, continuing to bring the jets closer causes them to twist around one another, releasing a series of pinched-off droplets. At higher flow rates, bringing the jets closer to each other creates a thin webbing of fluid between the jets that ultimately becomes a full fishbone pattern when the jets fully collide. The surface-tension-driven Plateau-Rayleigh instability helps drive the pinch-off and break-up into droplets. (Video credit: B. Keshavarz and G. McKinley)
Bubbles Through Constrictions
Surface tension usually constrains bubbles to the smallest area for a given volume – a sphere – but sometimes other forces generate more complicated geometries. The images above show bubbles flowing through microfluidic channels filled with a highly viscous carrier fluid. The bubble size and packing affects the shapes they assume, but so does the geometry of the channel. The narrow constrictions accelerate the flow, elongating the bubbles, whereas the wider channel regions slow the carrier fluid and squish the bubbles together. (Image credit: M. Sauzade and T. Cubaud (Stony Brook University))
The Reynolds Number Illustrated
The dimensionless Reynolds number is a key concept in fluid dynamics, allowing scientists to distinguish regimes of flow between differing geometries and even different fluids. This video gives a great primer on the subject by examining the physics of swimming for a sperm versus a sperm whale. The Reynolds number is essentially a ratio between inertial forces (driven by velocity and size) and viscous forces, and its value can indicate how important different effects are. Sperm and other microbes live at very small Reynolds numbers, meaning that viscosity dominates as the force they must overcome to move. For more on the low Reynolds number world, check out how brine shrimp swim and what happens if a microbe tries to flap its tail. (Hint: it goes nowhere, and this is why.) (Video credit: A. Bhatia/TED Ed; via Jennifer Ouellette)
