Search results for: “surface tension”

To Beat Surface Tension, Tadpoles Make Bubbles
For tiny creatures, surface tension is a formidable barrier. Newborn tadpoles are much too small and weak to breach the air-water surface in order to breathe. Researchers found that, instead, the 3 millimeter creatures place their mouths against the surface, expand their mouth to generate suction, and swallow a bubble consisting largely of fresh air.
When they’re especially small, some of these species are essentially transparent (Image 1), allowing researchers to see the bubble directly. But even as the tadpoles aged (Images 2 and 3) and grew strong enough to breach the surface, they observed many instances in which the tadpoles continued this bubble-sucking method to breathe. (Image and research credit: K. Schwenk and J. Phillips; via Cosmos; submitted by Kam-Yung Soh)
February 27, 2020
Surface Tension’s Pop
Surface tension in a liquid arises from molecular forces. Within a liquid like water, a molecule inside the fluid experiences equal tugs from similar molecules in every direction. A molecule at the surface, on the other hand, experiences the pull of similar molecules only on some sides. The net effect of this imbalance is a tensile force along the liquid surface that acts kind of like a sheet of elastic rubber – this is the effect we call surface tension. If you break the surface tension in a soap film like the one shown above, any tear will expand rapidly as the intact surface tension at the edges pulls the interior fluid away from the tear. (Image credit: C. Kalelkar and A. Sahni, source)
March 16, 2017
Reader Question: What is Surface Tension?
Last week reader thesnazz asked:
Is there a difference between surface tension and viscosity, or are they two manifestations of the same process and/or principles? If you know a given fluid’s surface tension, can you predict its viscosity, and vice versa?
I’m tackling this one in parts, and you can click here to read about viscosity.
Surface tension’s intermolecular origins are a bit clearer than those of viscosity. Essentially, within the interior of a water drop, you can imagine water molecules all hanging out with other water molecules. They tug on one another, but because they are surrounded on all sides by other water molecules, the net force of all these interactions on any molecule is zero. Not so at the surface of the drop. The surface is also called an interface; it’s a place where the fluid ends and something else–another fluid or perhaps a solid–begins. For a water molecule at that interface, the forces exerted by neighboring molecules are not balanced to zero. Instead, the imbalance causes the water molecules to be tugged inward. We call this effect surface tension.
Because surface tension is an interfacial effect, it is not completely dependent on the fluid alone. For example, a drop of water sitting on a solid surface can take a variety of shapes depending on the properties of the solid (see also hydrophobicity) and the surrounding air as well as those of the water. This is only one of many manifestations of surface tension. Wikipedia has a pretty good overview of some others, if you’d like to learn more. Like viscosity, surface tension is usually measured rather than calculated from first principles.
In the end, both surface tension and viscosity have molecular origins, but they are two very different and independent properties. Viscosity is inherent to a fluid, whereas surface tension depends on the fluid and its neighboring substance. Both quantities are more easily measured than calculated. Thanks again to thesnazz for a great question! As always, you can ask questions or submit post ideas here on Tumblr or via Twitter or email. (Image credit: Wikimedia)
December 16, 2013
Surface Tension in Action
Surface tension creates a glassy, smooth layer of water over U.S. swimmer Tyler Clary the instant before he surfaces as he competes in the backstroke. Surface tension arises from intermolecular forces between water molecules. In the bulk of the liquid, any given water molecule is being pulled on in every direction by the surrounding molecules, which results in zero net force. At the surface, however, molecules only experience forces from those to the side and below them. As a result, these molecules are pulled inwards, forcing the liquid to take on a form with minimal area. (Photo credit: Getty Images; submitted by drhawkins)
February 7, 2013
“Surface Tension”
From a series called “Surface Tension,” these ink and water drawings by Marguerite French explore the effects of diffusion, surface tension, and evaporation. The forms left by the thin layer of liquids suggest other natural processes like erosion, weathering, and the rings inside trees. (Photo credits: Marguerite French)
October 19, 2012
Surface Tension Floats Coins
Surface tension arises from intermolecular forces along the interface of a fluid, but despite its molecular origins, it can have some substantial macroscopic effects. Here researchers demonstrate how surface tension can hold up metal coins that would otherwise sink. Moreover, when multiple coins are set on the surface of the water, surface tension draws them together into a closely packed array because it reduces the surface energy by creating a single large well instead of many small ones. This is the same reason that your Cheerios tend to clump together on the surface of your milk when you’re eating breakfast! (Video credit: Lawrence Berkeley National Lab)
May 4, 2012
Surface Tension Instability
Droplets of oleic acid spread across a thin film of glycerol on a silicon wafer. The shapes here are driven by hydrodynamic instabilities, particularly Marangoni effects due to the differences in surface tension between the two fluids. (Photo credit: A. Darhuber, B. Fischer and S. Troian)
December 16, 2011
Surface Tension Demo
This simple demonstration shows the power of surface tension, especially at small lengthscales. Another way to break the surface tension holding the water in the sieve would be to spray the top of the jar with soapy water. The soap acts as surfactant, decreasing the surface tension such that the water is unable to counteract the force of gravity.
September 26, 2011
Reader Question: Surface Tension vs. Viscosity
lazenby asks:
How can superfluid liquid Helium have zero viscosity while retaining surface tension? (assuming something like surface tension is required for a liquid to form drops)
The short answer is that surface tension and viscosity are two totally separate properties for a fluid. To illustrate how one can exist without the other in a superfluid, we’ll imagine two different scenarios. For the first, imagine that you have a narrow vertical pipe. Any fluid you put in the pipe will flow downward due to the force of gravity. If you put water through the pipe, you’ll get some rate of outflow. Now imagine putting something like molasses through the pipe. Even with the same external forces on it, the molasses will never move through the pipe as quickly as the water does. This is because the molasses has higher viscosity and resists flowing. In a force balance, viscosity would act like friction, opposing the downward motion of the fluid.
Surface tension arises from a different balance of forces. Now imagine that you have a stationary droplet of one fluid (A) floating in a different fluid (B). Deep inside the droplet, each molecule of Fluid A is being pulled on all sides by other identical molecules of Fluid A. A molecule at the surface of the droplet, though, doesn’t experience that neighborly pull on all sides; it experiences different intermolecular forces from Fluid B. Our imaginary droplet is stationary, though, so all the forces on it and all the forces on its individual molecules have to balance, otherwise there’d be acceleration. Surface tension acts along the interface by pulling molecules of Fluid A in toward one another–much like the elastic of a balloon–thereby balancing the forces in the droplet and equalizing the force across the interface between Fluid A and Fluid B. (Illustration credit: Wikipedia)
In the superfluid, this balance of forces across the interface between air and helium-3 must still exist, despite the superfluid’s lack of viscosity.
January 20, 2011
Surface-Tension Supported Walkers
Nature’s smallest water-walkers use surface tension to keep themselves afloat. This includes hundreds of species of invertebrates like insects and spiders as well as the occasional extremely tiny vertebrate, like the 2-4 cm long pygmy gecko shown above. These animals typically have very thin parts of themselves touching the water – like the spindly legs of the water strider. These skinny appendages curve the air-water interface and that curvature, along with the water’s surface tension, generates the force supporting the animal.
Staying afloat on surface tension does little good if a raindrop or passing splash submerges these tiny water-walkers. To avoid that fate, these animals are also hydrophobic or water repellent. This adaptation keeps them from drowning and helps them enhance the curvature where their feet meet the water.
Those tiny indentations can also be important for the animal’s propulsion. Water striders, for example, use their long middle legs like oars to propel themselves. Any rower will tell you that sticks make poor paddles – they’re just not good at transferring momentum to the water. But curving the surface and then pushing off that curvature works remarkably well. It’s how the water strider creates the vortices in its wake in the image above.
For more on water strider propulsion, I recommend this Science Friday video. If you’d like to see the gecko in action, check out BBC Life’s “Reptiles and Amphibians” episode, which is available on Netflix in the U.S. (Image credits: pygmy gecko, BBC; water strider, J. Bush et al.)
This week FYFD is exploring the physics of walking on water, all leading up to a special webcast on March 5th with guests from The Splash Lab. You don’t want to miss it!
March 1, 2016

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