When a burst of air gets expelled into still surroundings — like when a person coughs — it forms a turbulent puff like the one seen here. Puffs can be surprisingly long-lasting, though these miniature clouds slow down and expand over time. How they behave is critical to understanding the spread of pollution as well as how respiratory illnesses like COVID-19 travel. In this study, researchers found that buoyancy is also a critical factor. When the puff is warmer than its surroundings, it rises higher, lasts longer, and travels further. That might help explain why respiratory illnesses like the flu spread more readily in the winter than in warmer months. (Image and research credit: A. Mazzino and M. Rosti; via Physics World; submitted by Kam-Yung Soh)
Tag: buoyancy
Fun From the Beach
Here’s a neat bit of fluid dynamics derived from a day at the beach! Our experiment begins with well-mixed (and likely compacted) sand grains and sea water in a bottle. When flipped, the sand layer sits at the top of the bottle with the water layer beneath.
Very quickly new layers establish themselves in the bottle. The lower half of the bottle turns into a turbulent churn of water and sand, topped by a thin air bubble, then the thick sand layer, and finally, a layer of filtered water. That air bubble beneath the sand means that the sand layer is compacted enough that surface tension keeps the air from being able to squeeze through the grains. On the other hand, water is able to filter through, eventually making it into that upper region. The compact layer of sand is supported in the bottle by force chains running through the largest grains, which is why only fine sediment settles down through the turbulent layer at this point.
Eventually, the top sand layer erodes enough that it can no longer support its weight, and the sand collapses. As the grains settle out, we end up with fine sediment on the bottom (as previously discussed), followed by a layer of coarse sand from the erosion and collapse of the sand layer, topped with a layer of very fine grains that — due to their light weight — are the very last to settle out of the water. I love that such a simple seaside experiment contains such scientific depth! (Video and submission credit: M. Schich; special thanks to Nathalie V. for helpful input)
Bubbles Rising
Here we see high-speed video of air bubbles rising through sesame oil. The flow rate of air is just right for one bubble to catch up to and merge with the previous bubble. As it the trailing bubble pinches off from the valve, it shoots a small jet through itself and into the prior bubble. For information on how to recreate this and related experiments, check out this article. (Image credit: C. Kalelkar and S. Paul, source; see also C. Kalelkar)
Inside the Blockage of the Suez Canal
In March 2021, the world watched as the Ever Given container ship got stuck in the Suez Canal, disrupting global shipping for more than a week. In this Practical Engineering video, Grady delves into some of the phenomena that may have played a role in the incident of the ship that launched a thousand memes.
Heavy container ships displace a lot of water, and in a narrow, shallow canal, there isn’t much space left for that water to go. To squeeze by, the water must speed up, which (per Bernoulli’s law) creates a pressure drop and suction force on the ship. For a ship too close to a canal bank, that suction will pull the ship further to the side, increasing its chances of lodging in the bank. (Video and image credit: Practical Engineering)
Oil-Coated Bubbles
Bubbles in industrial applications are often more complicated than a simple pocket of air surrounded by water. Here researchers investigate the formation of an air bubble coated in oil before it rises through water. The photo above shows a series of snapshots as the bubble forms. Initially, a droplet of oil sits pinned on the surface. As air gets injected, the oil stretches around the growing bubble. Eventually, buoyancy pulls the bubble off the injector, creating a rising air bubble coated in oil. The team found that oil-coated bubbles could grow much larger than those in water alone. (Image and research credit: B. Ji et al.)
Hedgehogs Atop Waves
Since Michael Faraday, scientists have watched the curious patterns that form in a vibrating liquid. By adding floating particles to such a system, researchers have discovered spiky, hedgehog-like shapes that form near the surface. At low amplitudes, the surface patterns resemble the typical smooth rounded lobes one would expect, but as the wave amplitude increases, spikes form in the tracers, driven by the motion of the waves. (Image and research credit: H. Alarcón et al.; via APS Physics)
Oil in Water
In the decade since the Deepwater Horizons oil spill, scientists have been working hard to understand the intricacies of how liquid and gaseous hydrocarbons behave underwater. The high pressures, low temperatures, and varying density of the surrounding ocean water all complicate the situation.
Released hydrocarbons form a plume made up of oil drops and gas bubbles of many sizes. Large drops and bubbles rise relatively quickly due to their buoyancy, so they remain confined to a relatively small area around the leak. Smaller drops are slower to rise and can instead get picked up by ocean currents, allowing them to spread. The smallest micro-droplets of oil hardly rise at all; instead they remained trapped in the water column, where currents can move them tens to hundreds of kilometers from their point of release. (Image and research credit: M. Boufadel et al.; via AGU Eos; submitted by Kam-Yung Soh)
Floating in Levitating Liquids
When it comes to stability, nature can be amazingly counter-intuitive, as in this case of flotation on the underside of a levitating liquid. First things first: how is this liquid layer levitating? To answer that, consider a simpler system: a pendulum. There are two equilibrium positions for a pendulum: hanging straight down or pointing straight up. We don’t typically observe the latter position because it’s unstable; the slightest disturbance from that perfectly vertical situation will make it fall. But it’s possible to stabilize an inverted pendulum simply by shaking it up and down. The vibration creates a dynamic stability.
The same physics, it turns out, holds for a layer of viscous fluid. With the right vibration, the denser fluid can levitate stably over a layer of air. Inside this vibrating layer, the rules of buoyancy are a little different because the vibration modifies the effects of gravity. As a result, bubbles deep in the liquid layer sink (Image 1). The researchers used this behavior to create their levitating layer (Image 2). The shaking also serves to stabilize objects floating on the underside of the liquid layer, allowing the boat in Image 3 to float upside down! (Image and research credit: B. Apffel et al.; via NYTimes; submitted by multiple sources)
The Magic* Cork
*Spoiler alert: it’s not magic. It’s science!
Just what makes this dropped cork float beneath the surface? Just like a normal cork, it’s buoyancy! But this seemingly straightforward video is hiding a few key elements. Firstly, the cork has been modified; it has a metal sphere inside it so that its effective density is higher than that of water.
Secondly, that liquid is not pure water; notice the hazy swirls near the bottom of the flask when the cork drops in? This is tap water that’s had a layer of salt dissolving in the bottom of it for the last day. That creates a density gradient with denser, salty water at the bottom and lighter, fresh water at the top. In fluid dynamics, we’d say the fluid is stably stratified; “stratified” meaning that there are distinct layers (strata) of different density and “stably” because the heavier ones are at the bottom.
When the cork is dropped in, it settles at the fluid layer that matches its density. Because the surrounding fluid is stably stratified, poking the cork makes it bounce slightly but return to its initial height. Our atmosphere behaves just like this when it’s stably stratified. If you displace a parcel of air, it will oscillate up and down before settling back to equilibrium. In fact, the cork and the air even bounce at the same frequency! (Video and submission credit: F. Croccolo)
Hot Ice, Buoyancy Tricks, and More DIY Fun
Here’s a smorgasbord of DIY experiments from Dianna at Physics Girl. Some are fluidsy, some aren’t, but all of them give you a chance to stretch your science muscles at home. Personally, I think she saved the best for last with her laser-acoustics demo! (Video credit: Physics Girl)
