Water droplet art celebrates the infinite forms created from the impact of drops with a pool and rebounding jets. It’s a still life captured from split second interactions between inertia, momentum, and surface tension. These examples from photographer Markus Reugels are among some of the most complex shapes I’ve seen captured. Be sure to check out his website for more beautiful examples of liquids frozen in time. (Photo credits: Markus Reugels; via Photigy)
Any phenomenon in fluid dynamics typically involves the interaction and competition of many different forces. Sometimes these forces are of very different magnitudes, and it can be difficult to determine their effects. This video focuses on capillary force, which is responsible for a liquid’s ability to climb up the walls of its container, creating a meniscus and allowing plants and trees to passively draw water up from their roots. Being intermolecular in nature, capillary forces can be quite slight in comparison to gravitational forces, and thus it’s beneficial to study them in the absence of gravity.
In the 1950s, drop tower experiments simulating microgravity studied the capillary-driven motion of fluids up a glass tube that was partially submerged in a pool of fluid. Without gravity acting against it, capillary action would draw the fluid up to the top of the glass tube, but no droplets would be ejected. In the current research, a nozzle has been added to the tubes, which accelerates the capillary flow. In this case, both in terrestrial labs and aboard the International Space Station, the momentum of the flow is sufficient to invert the meniscus from concave to convex, allowing a jet of fluid out of the tube. At this point, surface tension instabilities take over, breaking the fluid into droplets. (Video credit: A. Wollman et al.)
Here fluid is ejected as two flat plates collide, creating a sheet of silicone oil. The initially smooth sheet forms a thicker ligament about the edge. Gravity causes the sheet to bend downward like a curtain, and growing instabilities along the ligament cause the development of a wavy edge. The points of the waves develop droplets that eject outward. Not long after this photograph, the entire liquid sheet will collapse into ligaments and flying droplets. (Photo credit: B. Chang, B. Slama, and S. Jung)
In the collage above, successive frames showing the bouncing and break-up of liquid droplets impacting a solid inclined surface coated with a thin layer of high-viscosity fluid have been superposed. This allows one to see the trajectory and deformation of the original droplet as well as its daughter droplets. The impacts vary by Weber number, a dimensionless parameter used to compare the effects of a droplet’s inertia to its surface tension. A larger Weber number indicates inertial dominance, and the Weber number increases from 1.7 in (a) to 15.3 in (d). In the case of (a), the impact of the droplet is such that the droplet does not merge with the layer of fluid on the surface, so the complete droplet rebounds. In cases (b)-(d), there is partial merger between the initial droplet and the fluid layer. The impact flattens the original droplet into a pancake-like layer, which rebounds in a Worthington jet before ejecting several smaller droplets. For more, see Gilet and Bush 2012. (Photo credit: T. Gilet and J. W. M. Bush)
Droplet impacts on granular surfaces and water interactions with superhydrophobic surfaces are not unfamiliar topics for FYFD. But this behavior of water droplets falling on a superhydrophobic powder is unusual, to say the least. When the droplets impact in powder, they rebound with a partial coating of powder. In the case of the superhydrophobic powder, the shape of the droplet is “frozen” by the powder. A satellite droplet is ejected from the region not coated in powder and the resultant main drop falls back to the surface and comes to rest with little to no deformation. The researchers report a critical velocity at which the behavior is observed. (Video credit: J. Marston et al.; via Physics Buzz)
The Leidenfrost effect occurs when a liquid encounters a solid object much hotter than the liquid’s boiling point, like when water skitters on a hot griddle or someone plunges a hand in liquid nitrogen. A thin layer of vapor forms between the liquid and the solid, thereby (briefly) insulating the remaining liquid. The Leidenfrost effect can be static—like a droplet sitting on a pan—or dynamic, like the video above in which a droplet impacts the hot object. The video shows both a top and a side view of a droplet striking a plate that is over five times hotter than the liquid’s boiling point. On impact, the droplet spreads and flattens, and a spray of even tinier droplets is ejected before rebound. (Video credit: T. Tran and D. Lohse, from a review by D. Quere)
In the photo sequence above, a bubble is created at the interface between two immiscible liquids—water on top and denser hydrofluroether (HFE) below. Initially, the bubble expands explosively due to the vaporization of water generated by a short laser pulse. As the bubble collapses, a jet forms and accelerates into the HFE. After collapse, the bubble remnants injected in the HFE cause the formation of a jet that shoots back into the water above. Surface instabilities make the jet assume a mushroom or crown-like structure that detaches from the jet. Eventually gravity will return the system to its initial undisturbed fluid-fluid interface. (Photo credit: S. Avila et al. 1,2)
Microgravity continues to be a fascinating playground for observing surface tension effects on the macroscale without pesky gravity getting in the way. Here astronaut Don Pettit has created a sphere of water, which he then strikes with a jet of air from a syringe. Initially, the momentum from the jet of air creates a sharp cavity in the water, which rebounds into a jet of water that ejects one or more satellite drops. Surface waves and inertial waves (inside the water sphere) reflect back and forth until the fluid comes to rest as a sphere once more. Note how similar the behavior is to the pinch-off of a water column. Both effects are dominated by surface tension, but on Earth we can only see this behavior with extremely small droplets and high-speed cameras! (Video credit: Don Pettit, Science Off the Sphere)
To the human eye, the burst of a soap bubble appears complete and instantaneous, but high-speed video reveals the directionality of the process. Surface tension is responsible for the spherical shape of the bubble, and, when the bubble is pierced, surface tension is broken, causing the soap film that was the bubble to contract like elastic that’s been stretched and released. Droplets of liquid fly out from the edges of the sheet until it atomizes completely.