Tag: droplets

When Seeing a Flow Changes It
Adding dye to a flow is a common technique for visualization. After all, many flows in fluids like air and water are invisible to our bare eyes. But for some classes of flows — especially those driven by variations in surface tension — adding dye can have unforeseen effects. A recent study shows how true this is for bursting Marangoni droplets, where evaporation and alcohol concentration can pull a water-alcohol droplet apart.
As more dye is added to the experiment, the daughter droplets grow larger and more ligaments form. In the first three images, a dashed black line has been added to show the location of the droplet rim.
Without dye, it’s nearly impossible to see the phenomenon since the refractive indices of the two component liquids are so close. But the researchers found that, as they added more methyl blue dye, it did more than increase the contrast in the flow. It changed the flow, making the droplets larger and creating ligaments between them. They believe that the dye’s own surface tension creates local gradients that alter the flow. It’s a reminder that experimentalists have to be careful to consider how our efforts to measure and observe a flow can change it. (Image credit: top – The Lutetium Project, bottom – C. Seyfert and A. Marin with modification; research credit: C. Seyfert and A. Marin)
June 21, 2022
Merging Along Wires
As oil slides down two slowly converging wires, the droplets will merge into a sheet that stretches between both wires. When this happens can vary somewhat but occurs somewhere around the liquid’s capillary length.
In the poster above, the leftmost image (not the illustration) shows three possible merger points. To the right of the image, is a teal curve; this is a probability density function. Essentially, this curve shows where the merger is most likely to occur. The peak of the curve corresponds to the most probable point of merger.
The following two composite images show the same system — same oil flow rate, same wire spacing — with gas blowing upward along the wires. As the gas’s flow rate increase, the oil drops get larger, making the oil films thinner. The result? The wires have to get closer to one another before the oil merges. That’s reflected in the yellow and orange probability density functions, which have peaks further along the wires than the no-gas-flow case. (Image credit: C. Wagstaff et al.)
June 6, 2022
Self-Stopping Leaks
A leak can actually stop itself, as shown in this video. To demonstrate, the team used a tube pierced with a small hole. When filled, water initially shoots out the hole in a jet. The pressure driving the jet comes from the weight of the fluid sitting above the hole. As the water level drops, the pressure drops, causing the jet to sag and eventually form a rivulet that wets the side of the tube. As the water level and driving pressure continue to fall, the rivulet breaks up into discrete droplets, whose exact behavior depends on how hydrophobic the tube is. Eventually, a final droplet forms a cap over the hole and the leak stops. At this point, the flow’s driving pressure is smaller than the pressure formed by the curvature of the capping droplet. (Image and video credit: C. Tally et al.)
May 30, 2022
Dripping Impact
How does water drip, drip, dripping onto stones erode a crater? Water is so much more deformable that it seems impossible for it to wear harder materials away, even over thousands of impacts. To investigate this, a team of researchers developed a new measurement technique: high-speed stress microscopy. In the process, they found that water owes its incredible erosive power to three factors: 1) The drop’s impact creates surface shock waves along the material, which helps increase erosive power; 2) After the shock wave passes, a decompression wave in the material helps loosen surface matter; and 3) The spreading drop sends a non-uniform wave of stress across the material that simultaneously presses and scrubs at the surface. Together, these factors enable simple, repetitive droplet impacts to wear away at hard surfaces. (Image credit: cottonbro; research credit: T. Sun et al.; via Cosmos; submitted by Kam-Yung Soh)
May 26, 2022
Coalescence Symmetry
When droplets coalesce, they perform a wiggly dance, gyrating as the capillary waves on their surface interfere. When the droplets have matching surface tensions, like the two water droplets in the animation on the lower left, the coalescence dance is symmetric. But for differing droplets, like the water and ethanol droplets merging on the lower right, coalescence is decidedly asymmetric.
Two water droplets merge symmetrically.
A water droplet and an ethanol droplet merge asymmetrically.
The asymmetry arises from the droplets’ different surface tensions. The size and speed of the capillary waves that form on a droplet depend on surface tension, so droplets of different liquids have inherently different capillary waves. During merger, the interference of these capillary waves causes the asymmetry we see. (Image credit: top – enfantnocta, coalescence – M. Hack et al.; research credit: M. Hack et al.)
May 24, 2022
Raindrops on the Windshield
When I was a child, I was fascinated by the raindrops that shimmied along the windshield of our car. Some would slide up the glass. Some would run down. And some just seemed to wiggle in place, until the car’s speed changed. As common as this sight is, the physics of these droplets is quite complicated and not completely understood.
Each droplet has a host of forces on it: gravity flattening it or pulling it down an incline; a drag force from the wind flowing over it; and friction between the drop and the surface trying to pin it in place. Recently, scientists have developed a new mathematical model that captures some of the behaviors behind these drops. The work describes the wind speed necessary to move a drop of a given size sitting on a flat surface. The authors also explored how that critical wind speed changes when a drop sits on a tilted surface aligned or against the wind. (Image credit: P. Gupta; research credit: A. Hooshanginejad and S. Lee; via Science News; submitted by Kam-Yung Soh)
May 17, 2022
“ColorLover”
“ColorLover,” a short film by artist Rus Khasanov, is a delightful liquid rainbow. The video’s ingredients seem to be ink, paint, oil, and a bit of superhydrophobic coating primed to reveal a heart. I love that latter touch; it’s a cool way to use regular materials in a way that some might assume involved digital effects! (Video credit: R. Khasanov)
April 29, 2022
Within the Bubble’s Pop
To our eyes, a soap bubble appears to pop instantly, but when observed in high-speed video, the process is far more complex. In this video, the Slow Mo Guys pop human-sized bubbles, giving us an opportunity to appreciate the rupture process at speeds up to 50,000 frames per second.
Once the rupture starts, the hole spreads very symmetrically. But as the hole grows, the remaining soap film starts distorting. As Gav and Dan observe, the far side of the bubble actually wrinkles up before the rupture front arrives and tears the remaining fluid into droplets! (Image and video credit: The Slow Mo Guys)
April 19, 2022
Splashing on Spheres
The splash of a droplet is a surprisingly complex phenomenon, depending not only on the droplet’s characteristics but also the surrounding air pressure, the roughness and temperature of the impact surface, and the surface’s curvature. In this study, researchers investigated the effects of surface curvature on splashing, finding that it’s harder for a drop to splash on spheres of smaller radius than ones with a larger radius of curvature.
In Image 1, the falling droplet coats the 2-mm sphere with no sign of splashing. But as the radius gets larger (Images 2 and 3), splashing becomes more and more pronounced. They found that the splash suppression is due to a modification of the lift force on the leading edge of the lamella, the thin liquid layer created as the drop impacts and spread. (Image, research, and submission credit: T. Sykes et al.; also available here)
March 9, 2022
Stopping The Drop
When a droplet falls on a mesh surface, some of the liquid can burst through the holes (top row). But subsequent drops have a harder time penetrating the prewetted mesh. After a few drops have impacted (rows 2-3), the wetted mesh can completely suppress penetration (rows 4-5). The authors found that the taller drops sitting atop the mesh were better at stopping penetration from an incoming drop. (Image and research credit: L. Xu et al.)
February 16, 2022