A droplet falling at high speed can pass through a soap film without breaking it. On impact, the drop stretches the soap film and ultimately only passes through by getting coated with a thin shell of soap film fluid. That liquid shell is separated from the original droplet by an extremely thin air layer. This air layer isn’t typically visible, but we know that it’s there from what happens when that soap-film-shelled droplet later impacts a liquid pool. As seen above, the droplet sits on the surface until the soap film shell coalesces with the pool. This sucks the drop under, but the drop itself does not coalesce. Instead it becomes an antibubble – a submerged liquid drop surrounded by a shell of air. (Image credit: J. Zou et al., source)
Tag: soap film
Schooling in Soap Films
In sports, flocks of birds, and schools of fish, we’re accustomed to thinking that the followers get an aerodynamic or hydrodynamic advantage over the leaders, but this may not always be the case. Here are two flags placed one after another in a soap film flowing from top to bottom. The flags are passive, meaning that their motion is entirely dependent on the flow around them; they cannot exert any resistive force of their own. In this case, scientists observe an effect known as inverted drafting. The lead flag actually experiences less drag – by as much as 50% – than the following flag. This seems to be a result of flow around the second flag having an upstream influence on the motion of the first. (Image and research credit: L. Ristroph and J. Zhang, pdf)
Stabilizing Films
Liquids don’t typically survive very long as thin films. If you try to make one from water, gravity drains it away immediately. (Not so in space.) To make a liquid film stick around, we add surfactants like soap. These extra molecules congregate at the surface of the film and provide a stabilizing force to oppose gravity’s drainage. Exactly what that stabilizing force is depends on the surfactant.
Surfactants that are insoluble are often quite viscous. These molecules distribute themselves across the interface and then they stay. They resist both gravity or even just moving thanks to their high viscosity. That produces a soap film pattern like the one on the right – symmetric and slow to change.
Other surfactants may be soluble in the film and have no appreciable viscosity themselves. These surfactants constantly move and shift on the interface as surface tension variations occur. When weak spots form, the surfactant molecules shift, via the Marangoni effect, to stabilize the film. This creates a film pattern like the familiar one on the right, with an ever-shifting palette of colors. (Image and research credit: S. Bhamla et al., source; submission by S. Bhamla)
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)
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)
“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.)
Soap Film Turbulence
The brilliant colors of a soap film reveal the fluid’s thickness, thanks to a process known as thin film interference. The twisting flow of the film depends on many influences: gravity pulls down on the liquid and tends to make it drain away; evaporation steals fluid from the film; local air currents can push or pull the film; and the variation in the concentration of molecules – specifically the surfactants that stabilize the film – will change the local surface tension, causing flow via the Marangoni effect. Together these and other effects create the dancing turbulence captured above. (Video credit: A. Filipowicz)
Bubbles and Films Merging
As we’ve seen before, a water droplet can merge gradually with a pool through a coalescence cascade. It turns out that the coalescence of a soap bubble with a soap film can follow a similar process! Initially, the bubble and film are separated by a thin layer of air. Once that air drains away and the bubble contacts the fluid, it starts to coalesce. But the bubble pinches off before its entire volume merges, leaving behind a daughter bubble with about half the radius of the previous bubble. This process repeats until the bubble is small enough that it merges completely. To see more great high-speed footage of this bubble merger, check out the full video below. (Image/video credit: D. Harris et al.)
Inside a Popping Bubble
Popping a soap bubble is more complicated than what the eye can see. In high-speed video, we find that the action is very directional, with the soap bubble film pulling away from the point of rupture. As it does so, waves, like those in a flapping flag, appear along the surface and strings of fluid form along the edge of the film before breaking into droplets. This video takes matters a step further, looking at what happens to air inside a bubble when it pops. Those subtle waves and strings of fluid we see in the high-speed rupture have a distinctive effect on air inside the bubble. As the film pulls away, it leaves behind a rippled, wavy surface rather than a smooth sphere of foggy air. (Video credit: Z. Pan et al.)
