Adding just a few polymers to a liquid can substantially change its behavior. The presence of polymers turns otherwise Newtonian fluids like water into viscoelastic fluids. When deformed, viscoelastic fluids have a response that is part viscous–like other fluids–and part elastic–like a rubber band that regains its initial shape. The collage above shows what happens to a thinning column of a viscoelastic fluid. Instead of breaking into a stream of droplets, the liquid forms drop connected with a thin filament, like beads on a string. In a Newtonian fluid, surface tension would tend to break off the drops at their narrowest point, but stretching the polymers in the viscoelastic fluid provides just enough normal stress to keep the filament intact. If the effect looks familiar, it may be because you’ve seen it in the mirror. Human saliva is a viscoelastic liquid! (Image credit: A. Wagner et al.)
Search results for: “surface tension”
Soap Film Physics
Soap films consist predominantly of water, yet their thin, virtually two-dimensional nature is impossible for water alone to achieve. The small amount of added soap acts as a surfactant, lowering the surface tension of the fluid and preventing it from bursting into droplets. When forming a film, the soap molecules align themselves along the outer surfaces of the film, with their hydrophilic heads among the water molecules and their hydrophobic tails oriented outward. For the most part, the water molecules stay sandwiched between the surfactant layers, forming a film only about as thick as the wavelength of visible light. In fact, the psychedelic colors of a soap film are directly related to the film’s thickness with the black regions being the thinnest. The video above shows a horizontal soap film at the microscopic scale and some of the dynamics exist therein. (Video credit: J. Hart)
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)
Fluid Fingers
Differences in viscosity or surface tension between two fluids can lead to finger-like instabilities. Here food dye placed on corn syrup forms narrow tendrils driven by the differing surface tensions of the two liquids. Similar dendritic shapes can be generated by injecting a low viscosity fluid into a high viscosity one (Saffmann-Taylor instability) or by pulling apart glass plates sandwiched around a high viscosity fluid. (Photo credit: T. Gaskill et al.)
What’s in a Splash?
A droplet falling onto a solid, dry surface seems like a simple situation, one that would be easy to understand. But splashes can be unpredictable. Velocity, viscosity, and surface tension all play clear roles, but the surrounding air also has an impact – drop the air pressure low enough and a droplet won’t splash. A new paper has tackled the problem, producing a mathematical model in agreement with experimental results. So why do some drops splash and others don’t? When a drop falls, its momentum flattens it into a pancake shape while surface tension struggles to hold it together. The spreading edge, called the lamella, can pull away from the surface. When it does, a pocket of high pressure forms beneath it due to lubrication effects, and the faster airflow over the top of the lamella creates a suction effect. This is analogous to a wing producing lift. Like the momentum that spread the droplet, the lift force pulls the lamella and ejecta sheet further up and outward, overcoming the restoring force of surface tension and tearing the droplet apart. For more on the effect, check out the research paper or this Inside Science article. (Video credit: G. Riboux and J. Gordillo; via Inside Science)
Healing Bubbles
Soap bubbles are ephemeral creations. The slightest prick will send them tearing apart in the blink of an eye. It may come as a surprise, therefore, that dropping a water droplet through a bubble will not break it. Instead, the bubble will heal itself using the Marangoni effect. In a soap bubble, the soap molecules act as a surfactant, lowering the surface tension of the water and allowing the fragile structure to hold together. When the water drop impacts the bubble, the local surface tension increases because of the relative lack of soap molecules. This increase in surface tension pulls at the rest of the bubble, drawing more soap molecules toward the point of contact. The effect evens out surface tension across the surface and stabilizes the bubble. You can test the effect at home, too. If you wet your finger, you can poke a soap bubble without popping it. (Video credit: G. Mitchell; via io9)
Paint on Speakers
Paint seems to dance and leap when vibrated on a speaker. Propelled upward, the liquid stretches into thin sheets and thicker ligaments until surface tension can no longer hold the the fluid together and droplets erupt from the fountain. Often paints are shear-thinning, non-Newtonian fluids, meaning that their ability to resist deformation decreases as they are deformed. This behavior allows them to flow freely off a brush but then remain without running after application. In the context of vibration, though, shear-thinning properties cause the paint to jump and leap more readily. For more images, see photographer Linden Gledhill’s website. (Photo credit: L. Gledhill; submitted by pinfire)
The Real Shape of Raindrops
We often think of raindrops as spherical or tear-shaped, but, in reality, a falling droplet’s shape can be much more complicated. Large drops are likely to break up into smaller droplets before reaching the ground. This process is shown in the collage above. The initially spherical drops on the left are exposed to a continuous horizontal jet of air, similar to the situation they would experience if falling at terminal velocity. The drops first flatten into a pancake, then billow into a shape called a bag. The bags consists of a thin liquid sheet with a thicker rim of fluid around the edge. Like a soap bubble, a bag’s surface sheet ruptures quickly, producing a spray of fine droplets as surface tension pulls the damaged sheet apart. The thicker rim survives slightly longer until the Plateau-Rayleigh instability breaks it into droplets as well. (Image credit: V. Kulkarni and P. Sojka)
“The Flow II”
“The Flow II” film by Bose Collins and colleagues features a ferrofluid, a magnetically-sensitive liquid made up of a carrier fluid like oil and many tiny, ferrous nanoparticles. Although ferrofluids are known for many strange behaviors, their most distinctive one is the spiky appearance they take on when exposed to a constant magnetic field. This peak-and-valley structure is known as the normal-field instability. It’s the result of the fluid attempting to follow the magnetic field lines upward. Gravity and surface tension oppose this magnetic force, allowing the fluid to be drawn upward only so far until all three forces balance. (Video credit: B. Collins et al.)
Giant Bubbles
In their latest video, Gavin and Dan of The Slow Mo Guys demonstrate what giant bubbles look like in high-speed video from birth to death. Surface tension, which arises from the imbalance of intermolecular forces across the soapy-water/air interface, is the driving force for bubbles. As they move the wand, cylindrical sheets of bubble film form. These bubble tubes undulate in part because of the motion of air around them. In a cylindrical form, surface tension cannot really counteract these undulations. Instead it drives the film toward break-up into multiple spherical bubbles. You can see examples of that early in the video. The second half of the video shows the deaths of these large bubble tubes when they don’t manage to pinch off into bubbles. The soap film tears away from the wand and the destructive front propagates down the tube, tearing the film into fluid ligaments and tiny droplets (most of which are not visible in the video). Instead it looks almost as if a giant eraser is removing the outer bubble tube, which is a pretty awesome effect. (Video credit: The Slow Mo Guys)
