A liquid drop can fold itself up in a thin sheet. The animation above shows a drop of water with an ultra-thin (79nm) circular sheet of polystyrene atop it. As a needle removes water from the underside of the droplet, the shrinking droplet causes wrinkles and folds to form in the sheet. What’s going on here is a competition between the energy required to change the droplet’s shape and the energy needed to bend the sheet. Eventually, the droplet’s volume is small enough that the bending of the sheet overrules surface tension in dictating the droplet’s shape. The result is a tiny empanada-shaped droplet completely encapsulated by the sheet. (Image credit: J. Paulsen et al., source; research paper)
Tag: surface tension
Wrapping Up
It’s often at the intersection of topics that we can learn something new and fascinating. The latest video from The Lutetium Project shows examples of this at the intersection of solid mechanics and fluid dynamics with a look at elastocapillarity. Breaking that word down, that’s where elasticity – that stretchy quality associated with solids – meets capillarity – the surface-tension-dominated behavior of a fluid. In particular, they explore some of the mind-boggling and surprising interactions that happen between drops, bubbles, and thin flexible fibers smaller than the width of a human hair. Check out the full video below. (Images credit: K. Dalnoki-Veress et al.; video credit: The Lutetium Project)
Soap Bubbles Up Close
Watching soap bubbles up close is endlessly fascinating. The iridescent colors reflect the soap film’s thickness, or, in the case of black spots, its lack thereof. The dancing of the colors shows the soap film’s flow and the ever-shifting balance of surface tension necessary to keep the film intact. Even the junctures of the bubbles–so precise and mathematically perfect in structure–reflect the molecular interactions that govern them. (Video credit: Stereokroma; via R. Weston)
Dissolving
It looks like the fiery edge of a star’s corona, but this photo actually shows a dissolving droplet. The droplet, shown as the lower dark region in this shadowgraph image, is a mixture of pentanol and decanol sitting in a bath of water. Pentanol is a type of alcohol that is fully miscible with decanol and is water soluble, so that it will dissolve into the surrounding water over time. Decanol, on the other hand, is immiscible with water, so that part of the droplet won’t mix with the surrounding water.
The bright swirls along the droplet’s edge show areas with more pentanol. As the alcohol dissolves into the water, it forms a buoyant plume at the top of the droplet that rises due to pentanol’s lower density. That rising plume draws fresh water in from the sides, shown by the upper white arrows. Inside the droplet, flow moves in the opposite direction, from the top toward the outer edges. This is a result of uneven surface tension within the droplet. Scientists are interested in understanding the dynamics of these multiple component drops for applications like printing, where it’s desirable for pigments in an ink drop to be distributed evenly as the drop dries. (Image credit: E. Dietrich et al.)
A Water Balloon on a Bed of Nails
If you dropped a water balloon on a bed of nails, you’d expect it to burst spectacularly. And you’d be right – some of the time. Under the right conditions, though, you’d see what a high-speed camera caught in the animation above: a pancake-shaped bounce with nary a leak. Physically, this is a scaled-up version of what happens to a water droplet when it hits a superhydrophobic surface.
Water repellent superhydrophobic surfaces are covered in microscale roughness, much like a bed of tiny nails. When the balloon (or droplet) hits, it deforms into the gaps between posts. In the case of the water balloon, its rubbery exterior pulls back against that deformation. (For the droplet, the same effect is provided by surface tension.) That tension pulls the deformed parts of the balloon back up, causing the whole balloon to rebound off the nails in a pancake-like shape. For more, check out this video on the student balloon project or the original water droplet research. (Image credits: T. Hecksher et al., Y. Liu et al.; via The New York Times; submitted by Justin B.)
Shot Through a Drop
Shoot a sphere through a drop with sufficient speed, and you’ll see something like the composite photo above. Going from right to left, the projectile is initially coated in liquid and stretches the fluid behind it as it continues flying. This forms a thin sheet of fluid called a lamella with a thicker, uneven rim at its far end. The lamella continues stretching until the projectile breaks through and detaches. Now the lamella starts rebounding back on itself as surface tension struggles to keep the fluid together. A new rim forms on the front, and both the front and back rims thicken as the lamella collapses. Along the rims thicker portions start forming droplets – like spikes on a crown – as the surface-tension-driven Plateau-Rayleigh instability starts breaking the structure down. The untenable sheet of fluid will break up into a cloud of smaller, satellite droplets when it can hold together no longer. (Image credit: V. Sechenyh et al., video)
Freezing Drops
A water droplet deposited on a cold surface freezes from the bottom up. As anyone who has made ice cubes knows, water expands when it freezes. But watch the outline of the drop carefully. The drop isn’t expanding radially outward while it freezes. Instead the remaining liquid part of the drop forms what’s known as a spherical cap, a shape like the sliced-off top of a sphere. Surface tension creates that spherical shape, but the water still has to expand when it freezes. The result? The last bit of the drop freezes into a point! This means that surface tension maintains the drop’s spherical shape, for the most part, and all the expansion the water does takes place vertically. (Video credit: D. Lohse et al.)
Ink Drops Spreading
Ink drops atop a layer of glycerol spread in a beautiful fan of blue and white. The ink’s motion is the result of two processes: molecular diffusion and the Marangoni effect. Molecular diffusion is the mixing that occurs due to the random background motion of molecules. Since glycerol is a very viscous liquid, the ink is quite slow to spread in this manner.
The second factor, the Marangoni effect, is driven by differences in surface tension. The ink and glycerol have different surface tensions, and the exact values depend on concentration. Notice how the ink drops spread fastest from areas where the ink is densely concentrated. This tells us that the ink’s surface tension is lower than the glycerol’s. As a result, the glycerol’s higher surface tension tends to pull ink toward it. As the ink spreads and its concentration decreases relative to the glycerol, the ink-glycerol mixture’s surface tension increases. Since the difference between the surface tension of the mixture and the pure glycerol is not as large, the Marangoni force is reduced and the spreading slows. (Image credit: C. Kalelkar, source)
“Chemical Poetry”
In “Chemical Poetry” artists Roman Hill and Paul Mignot use fluid dynamics to create incredible and engaging visuals. With a stunningly close eye to fluids mixing and chemicals reacting, their imagery feels like gazing on primordial acts of creation or destruction. There’s even a sequence that feels like you’re watching an explosion in slow-motion, but there’s no CGI in any of it. This is just the beauty of physics laid bare, revealing the dances driven by surface tension, the undulations of a fluid’s surface, and the dendritic spread of one fluid into another – all cleverly lit and filmed for maximum effect. It is well worth taking the time to watch the whole video and check out more of their work. (Image/video credit and submission: NANO; GIFs via freshphotons)
Coarsening in a Soap Film
Flow in a soap film is driven by gravity’s efforts to thin the film and surface tension’s attempts to stabilize variations in thickness. Because evaporation guarantees that the soap film will eventually dry out, gravity typically wins the battle and causes a soap film to rupture. This video takes a close look at what happens in the film just before it ruptures. Black dots form in the thinnest region of the flow. These areas are not holes, but they appear black because they are thinner than any wavelength of visible light. Before rupture, the black dots begin coalescing with one another, first due to diffusion and later more rapidly due to convection in the soap film. Ultimately, the black dots are the harbingers of doom for the fragile bubble. (Video credit: L. Shen et al.)
