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)
It is common in many industries to use oil as a defoamer to break up existing foams or prevent foams from forming. But with the right surfactants—additives that change the foam’s surface tension—it’s possible to make aqueous foams that are actually stabilized by the presence of oil. This video explores some of the ways that oil can interact with these kinds of foam, beginning with capillary action, which draws the oil up into the junctions between foam films. For more, see Piroird and Lorenceau. (Video credit and submission: K. Piroird)
Have you ever wondered what happens inside a jet of fluid as it breaks into droplets? Such events are not commonly or readily measured. This video uses a double emulsion—in which immiscible fluids are encapsulated into a multi-layer droplet—to demonstrate interior fluid flow during the Plateau-Rayleigh instability. The innermost drops and the fluid encapsulating them have a low surface tension between them, thanks to the addition of a surfactant to the inner drops. As a result, the inner drops are easily deformed by motion in the fluid surrounding them. Flow on the left side of the jet is clearly parabolic, similar to pipe flow. Closer to the pinch-off, the inner droplets shift to vertical lines, indicating that the interior flow’s velocity is constant across the jet. After pinch-off, the inner droplets return to a spherical shape because they are no longer being deformed by fluid movement around them. The coiling of the inner drops inside the bigger one is due to the electrical charges in the surfactant used. (Video credit: L. L. A. Adams and D. A. Weitz)
The time-lapse video above shows the growth of icicles of various compositions under laboratory conditions. Many icicles in nature exhibit a rippling effect in their shape, which some theories attribute to an effect of lower surface tension in some liquids. Here researchers show the icicle growth of three liquids: pure distilled water, and water with two concentrations of dissolved salt. They found that lowering the surface tension of the freezing liquid with non-ionic surfactants (i.e. not salt) did not produce ripples, but that dissolved ionic impurities like salt strongly affected the growth of ripples. They posit that this may be due to constitutional supercooling, in which growth of the solid-liquid interface is destabilized by the preferential concentration of impurities near the interface. (Video credit: A. S. Chen and S. Morris)
When a drop of water touches a very hot pan, it will skitter across the surface on a thin layer of water vapor due to the Leidenfrost effect. But what happens when another chemical is added to the droplet? Researchers find that adding a surfactant to the water droplets creates some spectacular results. As the water evaporates, the concentration of the surfactant in the droplet increases causing the surfactant to form a shell around the droplet. The pressure inside the droplet increases until the shell breaks in a miniature explosion much like the popping of popcorn. (Video credit: F. Moreau et al.)
Researchers create liquid pearls—a liquid droplet surrounded by a gel-like exterior—by dropping the fluid through a special bath. The initial droplet contains a mixture of the liquid core and an alginate solution. When the drop falls through a bath containing calcium ions, the alginate turns into a hydrogel shell around the liquid core. In order to prevent mixing during the droplet impact, researchers use a surfactant that helps the thin alginate layer persist while gelling takes place. The resulting liquid pearl is permeable to chemicals; researchers hope this may allow them to be used to contain microorganisms or cells in a three-dimensional environment during testing. (Video credit: New Scientist, N. Bremond et al.; see also Gallery of Fluid Motion)
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.