Falling raindrops get distorted by the air rushing past them, ultimately breaking large droplets into many smaller ones. This research poster shows how variable this process is by showing two different raindrops, both of the same 8-mm initial diameter. On the left, the drop is prolate — longer than it is wide — and on the right, the drop is oblate — wider than it is long. Moving from bottom to top, we see a series of snapshots of each drop’s shape as it deforms and, eventually, breaks into smaller drops. The overall process is similar for each: the drop flattens, dimples, and then inflates like a sail, with part of the drop thinning into a sheet and ultimately breaking into smaller droplets. Yet, each drop’s specific details are entirely different. (Image credit: S. Dighe et al.)
Tag: droplet breakup
Hitting Molten Steel
Watching droplets burst is often fascinating, but it’s rare that we get to watch droplets of molten metal. In this Slow Mo Guys video, though, they’re shattering globs of molten steel and filming the results in slow motion. It’s the kind of starburst that breaks compression algorithms but remains beautiful regardless. (Video and image credit: The Slow Mo Guys)
Falling Pancake Drops
Despite their round appearance, the droplets you see here are actually shaped like little pancakes. They’re sandwiched inside a Hele-Shaw cell, essentially two plates with a viscous fluid between them. As these droplets fall through the cell, some remain steady and rounded (Image 1), while others experience instabilities (Images 2 and 3). By varying the ratio of the ambient fluid’s viscosity relative to the drop, the authors found two different kinds of breakup. In the first type (Image 2), droplet breakup occurred due to perturbations inside the drop itself. In the second type (Image 3), the viscosity of the ambient fluid is closer to that of the drop and intrusions of the ambient fluid into the drop break it apart. (Image and research credit: C. Toupoint et al.)
Paintball Collisions
In their latest video, the Slow Mo Guys collide paintballs in mid-air, creating some pretty great paint splashes. The high-speed video does a nice job of revealing some of the typical stages a splash goes through. Initially, the paint spreads in a liquid sheet. As it expands and (necessarily) thins, holes form and grow, driving the paint into string-like ligaments. These ligaments are also stretching and eventually break up into an spray of droplets, much like the jet dripping from your faucet does. If you’d like to see some more awesome high-speed liquid collisions, check out what happens when a solid projectile hits a falling drop and then look at when a laser pulse hits a droplet. (Image and video credit: The Slow Mo Guys; submitted by Omar M.)
Liquid Fragmentation
From spilling coffee to driving through puddles, our daily lives are full of examples of liquids fragmenting into drops. A recently published study describes how this break-up occurs and predicts what the distribution of droplet sizes will be for a given fluid. Viscoelasticity is the property that governs this droplet size distribution. Viscoelasticity describes two aspects of a fluid–its viscosity, which acts like internal friction, resisting motion–and its elasticity, the fluid’s ability to return to its original shape after stretching. Most fluids have a little bit of each of these properties, which makes them somewhat sticky, both in the sense of not-flowing-easily and in the sense of sticking-to-itself. These same properties cause viscoelastic fluids to wind up with a broader droplet size distribution, ultimately creating both more small droplets and more large droplets than a Newtonian liquid like water. (Video credit: MIT News; research credit: B. Keshavarz et al.; submitted by mrvmt)
A Particle-Filled Splash
A drop of water that impacts a flat post will form a liquid sheet that eventually breaks apart into droplets when surface tension can no longer hold the water together against the power of momentum flinging the water outward. But what happens if that initial drop of water is filled with particles? Initially, the particle-laden drop’s impact is similar to the water’s – it strikes the post and expands radially in a sheet that is uniformly filled with particles. But then the particles begin to cluster due to capillary attraction, which causes particles at a fluid interface to clump up. You’ve seen the same effect in a bowl of Cheerios, when the floating O’s start to group up in little rafts. The clumping creates holes in the sheet which rapidly expand until the liquid breaks apart into many particle-filled droplets. To see more great high-speed footage and comparisons, check out the full video. (Image credit and submission: A. Sauret et al., source)
Shooting Droplets with Lasers
Last week we saw what happens when a solid projectile hits a water droplet; today’s video shows the impact of a laser pulse on a droplet. Several things happen here, but at very different speeds. When the laser impacts, it vaporizes part of the droplet within nanoseconds. A shock wave spreads from the point of impact and a cloud of mist sprays out. This also generates pressure on the impact face of the droplet, but it takes milliseconds–millions of nanoseconds–for the droplet to start moving and deforming. The subsequent explosion of the drop depends both on the laser energy and focus, which determine the size of the impulse imparted to the droplet. The motivation for the work is extreme ultraviolet lithography–a technique used for manufacturing next-generation semiconductor integrated circuits–which uses lasers to vaporize microscopic droplets during the manufacturing process. (Video credit: A. Klein et al.)
Shooting Droplets
This animation shows high-speed video of a polystyrene particle striking a falling water droplet. Under the right conditions, the particle rips through the droplet, stretching the water into a bell-shaped lamella extending from a thicker rim. When the particle detaches, surface tension rapidly collapses the lamella into a ring which destabilizes. Thin ligaments and droplets fly off the crown-like ring as momentum overcomes surface tension’s ability to hold the droplet together. Be sure to check out the full video on YouTube or later next month at the APS Division of Fluid Dynamics meeting. (Yes, I will be there!) (Image credit: V. Sechenyh et al., source video)
The Real Shape of Raindrops
We often think of raindrops as spherical or tear-shaped, but, in reality, a falling droplet’s shape can be much more complicated. Large drops are likely to break up into smaller droplets before reaching the ground. This process is shown in the collage above. The initially spherical drops on the left are exposed to a continuous horizontal jet of air, similar to the situation they would experience if falling at terminal velocity. The drops first flatten into a pancake, then billow into a shape called a bag. The bags consists of a thin liquid sheet with a thicker rim of fluid around the edge. Like a soap bubble, a bag’s surface sheet ruptures quickly, producing a spray of fine droplets as surface tension pulls the damaged sheet apart. The thicker rim survives slightly longer until the Plateau-Rayleigh instability breaks it into droplets as well. (Image credit: V. Kulkarni and P. Sojka)
The March of Drops
I love science with a sense of humor. This video features a series of clips showing the behavior of droplets on what appears to be a superhydrophobic surface. In particular, there are some excellent examples of drops bouncing on an incline and droplets rebounding after impact. For droplets with enough momentum, impact flattens them like a pancake, with the rim sometimes forming a halo of droplets. If the momentum is high enough, these droplets can escape as satellite drops, but other times the rebound of the drop off the superhydrophobic surface is forceful enough to overcome the instability and draw the entire drop back off the surface. (Video credit: C. Antonini et al.)
