A thin spout of water is drawn up through a layer of oil in the photo on the right. This simple version of the selective withdrawal experiment is illustrated in Figure A, in which a layer of viscous oil floats above a layer of water. A tube introduced in the oil sucks fluid upward. At low flow rates, only the oil will be drawn into the tube, but as the flow rate increases (or the tube’s height above the water decreases), a tiny thread of water will be pulled upward as well. The viscous outer fluid helps suppress instabilities that might break up the inner fluid, and their relative viscosities determine the thickness of the initial spout. In this example, the oil is 195 times more viscous than the water. (Photo credit: I. Cohen et al.)
Tag: viscosity
Bubbles With Tails
In water and other Newtonian fluids, a rising bubble is typically spherical, but for non-Newtonian fluids things are a different story. In non-Newtonian fluids the viscosity–the fluid’s resistance to deformation–is dependent on the shear rate and history–how and how much deformation is being applied. For rising bubbles, this can mean a teardrop shape or even a long tail that breaks up into fishbone-like ligaments. The patterns shown here vary with the bubble’s volume, which affects the velocity at which it rises (due to buoyancy) and thus the shear force the bubble and surrounding non-Newtonian fluid experience. (Video credit: E. Soto, R. Zenit, and O. Manero)
Dublin’s Pitch-Drop Experiment
Readers may recall the University of Queensland’s pitch-drop experiment, recognized as the longest continuously running experiment in the world. Back in 1927, a professor started the experiment with the goal of measuring the extremely high viscosity of pitch. Since then, only eight drops have fallen. Queensland’s is not the only version of this experiment, though; Trinity College Dublin has a similar set-up and have just caught a falling pitch drop on camera for the first time ever. Take a look in the video above. Queensland is expecting a drop to fall sometime this year as well. (Video credit: Trinity College Dublin Physics; via SciAm)
Stopping Jet Break-Up
When a stream of liquid falls, a surface tension effect called the Plateau-Rayleigh instability causes small variations in the jet’s radius to grow until the liquid breaks into droplets. For a kitchen faucet, this instability acts quickly, breaking the stream into drops within a few centimeters. But for more viscous fluids, like honey, jets can reach as many as ten meters in length before breaking up. New research shows that, while viscosity does not play a role in stretching and shaping the jet as it falls–that’s primarily gravity’s doing–it plays a key role in the way perturbations to the jet grow. Viscosity can delay or inhibit those small variations in the jet’s diameter, preventing their growth due to the Plateau-Rayleigh instability. In this respect, viscosity is a stabilizing influence on the flow. (Photo credit: Harsha K R; via Flow Visualization)
Spin-Up
With the Oscars just over, it seems like a good time for some movie-trailer-style fluid dynamics. This video shows a rotating water tank from the perspective of a camera rotating with the tank at 10 rpm. Initially, the tank and its contents are at rest. When the tank begins spinning, the fluid inside responds. Pink potassium permanganate crystals at the bottom of the tank show fluid motion as they dissolve, and food coloring is spread on the water’s surface to show motion there. Fluid near the edge of the tank reaches the tank’s rotational velocity fastest, due to friction with the wall, while fluid near the center of the tank takes longer to spin up to speed. This creates the spiral-galaxy-like shape in the dye. Eventually viscosity will transmit the effects of the wall’s motion even into the center of the tank. (Video credit: UCLA Spinlab)
The 9th Pitch Drop is Coming
Remember that 83-year-old pitch drop experiment designed to measure the viscosity of pitch? Well, rumor has it that the ninth drop is due to fall at any time. Will you catch it on the webcam?
The Kaye Effect
The Kaye effect is an instability particular to a falling stream of non-Newtonian fluids with shear-thinning properties. When these fluids are deformed, their viscosity decreases; this, for example, is why ketchup flows out of a bottle more easily once it’s moving. Like most fluids, the falling shampoo creates a heap on the surface. The Kaye effect is kicked off when the incoming jet creates enough shear on part of the heap that the local viscosity decreases, causing the streamer–or outgoing jet–to slip off the side of the heap. As the incoming jet continues, a dimple forms in the heap where the streamer originates. As the dimple deepens, the streamer will rise until it strikes the incoming jet. This perturbation to the system collapses the streamer and ends the Kaye effect. This video also has a good explanation of the physics, along with demonstrations of a stable form of the Kaye effect in which the streamer cascades down an incline. (Video credit: Minute Laboratory; inspired by infplusplus)
INK World v01
In this video, mixtures of inks (likely printer toners) and fluids move and swirl. Magnetic fields contort the ferrofluidic ink and make it dance, while less viscous fluids spread into their surroundings via finger-like protuberances. (Video credit and submission: Antoine Delach)
Reader Question: Snow from Boiling Water?
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Reader kylewpppd asks:
Have you seen the post of a man in Siberia throwing boiling water off of his balcony? Can you provide a better explanation of what’s going on?
As you can see in the video (and in many similar examples on YouTube), tossing near boiling water into extremely cold air results in an instant snowstorm. Several effects are going on here. The first thing to understand is how heat is transferred between objects or fluids of differing temperatures. The rate at which heat is transferred depends on the temperature difference between the air and the water; the larger that temperature difference is the faster heat is transferred. However, as that temperature difference decreases, so does the rate of heat transfer. So even though hot water will initially lose heat very quickly to its surroundings, water that is initially cold will still reach equilibrium with the cold air faster. Therefore, all things being equal, hot water does not freeze faster than cold water, as one might suspect from the video.
The key to the hot water’s fast-freeze here is not just the large temperature difference, though. It’s the fact that the water is being tossed. When the water leaves the pot, it tends to break up into droplets, which quickly increases the surface area exposed to the cold air, and the rate of heat transfer depends on surface area as well! A smaller droplet will also freeze much more quickly than a larger droplet.
What would happen if room temperature water were used instead of boiling water? In all likelihood, a big cold bunch of water would hit the ground. Why? It turns out that both the viscosity and the surface tension of water decrease with increasing temperature. This means that a pot of hot water will tend to break into smaller droplets when tossed than the cold water would. Smaller droplets means less mass to freeze per droplet and a larger surface area (adding up all the surface area of all the droplets) exposed. Hence, faster freezing!
Viscous Fingers
High viscosity silicon oil is sandwiched between two circular plates. As the upper plate is lifted at a constant speed, air flows in from the sides. The initially circular interface develops finger-like instabilities, due to the Saffman-Taylor mechanism, as the air penetrates. Eventually the fluid will completely detach from one plate. (Photo credit: D. Derks, M. Shelley, A. Lindner)
