Seawater froths and foams in ways that freshwater rarely does. A new study pinpoints the ocean’s electrolytes as the reason bubbles resist merging there. By studying the final moments before bubbles coalesce in both pure and salt water, researchers found that dissolved salts slow down the drainage of the thin film of liquid between two bubbles. Once the film reaches a 30-50 nanometer thickness, its electrolyte concentration causes a difference in surface tension that slows the outward flow of liquid in the film. That keeps the film in place longer and makes bubbles form foams instead of merging or popping. (Image credit: P. Kuzovkova; research credit: B. Liu et al.; via APS Physics)
Tag: foam
Stabilizing Foams
Bubbles in a pure liquid don’t last long, but with added surfactants or multiple miscible liquids, bubbles can form long-lasting foams. In soapy foams, surfactants provide the surface tension gradients necessary to keep the thin liquid layers between bubbles from popping. But what stabilizes a surfactant-free foam?
New work finds that foams in mixtures of two miscible fluids only form when the surface tension depends nonlinearly on the concentration of the component liquids. When this is true, thinning the wall between bubbles creates changes in surface tension that stabilize the barrier and keep it from popping.
In mixtures without this nonlinearity, foams just won’t form. The new results are valuable for manufacturing, where companies can avoid unintentional foams simply by careful selection of their fluids. (Image credit: G. Trovato; research credit: H. Tran et al.; via APS Physics; see also Ars Technica, submitted by Kam-Yung Soh)
Simulating Better Breaking Waves
In the ocean, breaking waves trap air into bubbles that then cluster into foam, but conventional simulations don’t capture this foaminess. For bubbles to cluster into foam, there has to be a force preventing — or at least delaying — their coalescence. Typically, this is caused by impurities in the water that help lower the surface tension and thereby lengthen the bubbles’ lifespans. When these features get added to simulation models, bubbles begin to cluster and breaking waves become foamy. (Image and video credit: P. Karnakov et al.)
Sliding Foams
What happens when a foam interacts with a sliding surface? That’s the question at the heart of this study, which finds three major regimes of foam-surface interaction. On smooth surfaces (Image 1), foams will simply slide against the wall without sticking or deforming. When surface roughness is about as large as the foam’s wall thickness (Image 2), the foam will stick to individual asperities, then slip to the next rough spot as the wall moves. But when the surface roughness is large compared to the foam wall (Image 3), the foam will remain anchored to the surface and all the shear from the wall’s movement goes into deforming the bulk of the foam.
Researchers thus found they could change foam’s behavior by changing the surface roughness. They also looked at the reverse situation: a surface with fixed roughness — like, say, a human tongue — and how tuning the size of foam bubbles might alter perception and ease of swallowing. That’s what we’re looking at in the last image, where a spoon slides a foam along a surface with roughness similar to the human tongue. (Image and research credit: M. Marchand et al.)
Tapping a Can Won’t Save Your Beer
It happens to the best of us: sometimes our beer gets shaken up during transit. One common reaction to this is to tap the side of the can repeatedly before opening, but a new scientific study shows that tapping doesn’t affect the volume of beer lost. Danish scientists tested over 1,000 cans of beer in randomized combinations of shaken, unshaken, tapped, and untapped, and observed no difference between tapped and untapped cans.
The foam-up upon opening takes place in shaken beer because carbon dioxide bubbles form in the pressurized beer, especially along defects in the wall where bubbles can nucleate. When the pressure is released, the carbon dioxide becomes supersaturated and comes out of solution, especially into the pre-formed bubbles, which rapidly grow and overflow. In theory, tapping could disturb those bubbles before opening, but in practice, it makes no difference. Your best bet? Give the beer time to settle before you open it. (Image credit: Q. Dombrowski; research credit: E. Sopina et al.; via Ars Technica)
Active Foam
Geometrically, biological tissues and two-dimensional layers of foam share a lot of similarities. To try and understand how active changes in one cell affect neighbors, researchers are studying how foams shift when air is injected (below) at one or more sites. When a foam cell expands, it forces topological changes in neighboring cells, which researchers built an algorithm to track in real-time.
With some processing, they can actually visualize the radially-expanding waves of strain that pass through the foam (bottom image). This allows them to visualize the effects and interaction of multiple injection sites at once, hopefully helping unlock the mechanics behind both the foam’s shifts and those that occur in tissues. (Image and video credit: L. Kroo and M. Prakash)
Foam Collapse
Introduce the right additive and the bubble arrays in foam will collapse catastrophically. What you see above is high-speed video of a quasi-two-dimensional soap bubble foam collapsing. There are two main mechanisms in the collapse. The first is a propagating mode. When one section of the film breaks, a stream of liquid from the broken film can impact an adjacent section, causing it to break as well. This accounts for much of the breakage you see above.
The second mode is through penetration by droplets. Watch carefully, and you’ll see that some of the breaking films generate tiny droplets which can fly through the wall of the next cell and impact against the far side. With the right conditions, that impact can trigger a new break along a non-adjacent film. Together, these two mechanisms can destroy foam in the blink of an eye. (Image and research credit: N. Yanagisawa and R. Kurita)
Finding New Shapes in Foam
In the summer of 2018, a group of researchers announced they’d discovered a new geometrical shape, the scutoid. They found the scutoid, a sort of twisted prism, in the shape of epithelial cells packed between curved surfaces. Having heard of this new geometry, a different group of physicists wondered if they could find scutoids elsewhere, specifically, in the cells of a foam. As shown in the picture above, they did.
To visualize a scutoid, first image a prism. Take two polygons with an equal number of sides and connect them. But if you imagine packing such prisms between two curved surfaces, you’ll quickly see that it won’t work. They just don’t fit together. Instead, one face may adopt, say, six sides, while the other takes on five. To join those two end faces, one of the sides will have to have a Y-shaped junction and a triangular face. This is a scutoid.
You can see two such shapes in the image above. In the left bubble, the far side forms a pentagon, while the near face is a hexagon. On the right, the bubble has six faces in the background and eight in the foreground. And between them, you can just see the triangular face that connects the two scutoids.
It’s not only exciting to find scutoids in a new, non-biological medium; it suggests a physical mechanism behind their formation. Foams are a well-known example of energy minimization. The fact that scutoids are found in a curved foam suggests that the shape itself is connected to energy minimization, something that could help us understand how biological scutoids grow and form. (Image and research credit: A. Mughal et al.; via Physics World; submitted by Kam-Yung Soh)
Foam and Flow
Fluid dynamics often play out on a scale that’s difficult to appreciate from our earthbound perspective, but fortunately, we have tools to aid us. This natural-color satellite image shows Rupert Bay in Quebec, where fresh water stained with sediments and organic matter (right) flows into the saltier water of James Bay (left). White filaments at the edges of these mixing regions are likely foam floating atop the water. The turbulence caused at the intersection of the two bodies of water whips up organic films to form bubbles. The white on the far left of the image is ice chunks still floating in James Bay when the image was taken in early June. Click through to admire the high-resolution version. (Image credit: U.S. Geological Survey; via NASA Earth Observatory)
What Keeps a Foam Intact
Beer, soda, soap, meringue – foams are everywhere in our lives. But have you ever wondered why some foams disappear so quickly while whipped egg whites stick around? That’s the subject of this Gastrofisica video, which is in Spanish but has English captions.
Foams form when air gets introduced into a liquid, but for those bubbles to stick around, they need a certain special something. With soapy water, that ingredient is surfactants, molecules with both hydrophobic (water-fearing) and hydrophilic (water-loving) ends, which line up at the interface of the foam and help hold it together. But surfactants are relatively weak, especially compared to to the albumin proteins in an egg white. By whipping egg whites, you’re effectively untangling those proteins, and, like surfactants, they line up at the interface of the foam so that their hydrophobic and hydrophilic parts can hang out in their preferred mediums. With so many similar molecules crowded together, the proteins coagulate, adding extra strength and stiffness to your whipped egg whites. (Video and image credit: Tippe Top Physics; h/t to MinutePhysics)
