Inside a cavity with a square cross-section, bubbles form an array. The shapes of their edges are determined by surface tension and capillarity (lower half of center image). Adding an elastic ribbon into the bubbles (upper half of center image) means that the bubbles’ shapes are determined by a competition between the elasticity of the ribbon and the capillarity of the fluid. Researchers found that they could tune the rigidity of the ribbon to dictate the shape of the bubble array, or, conversely, they could use the bubbles to set the shape of a UV-curable ribbon. (Image and research credit: M. Jouanlanne et al., see also)
Tag: 2022gosmp
“Keeping Our Sheet Together”
When two liquid jets collide, they form a falling liquid sheet. Here researchers explore how that sheet breaks up when the liquids involved contain polymers. The intact areas of the sheet show as dark red or almost black. The edges of the sheet appear in brighter red and yellow, outlining the holes that form and grow during breakup. The type of breakup observed depends on the concentration of polymer in the liquid. (Image credit: C. Galvin et al.)
A Levitated Boil
When acoustically levitated, objects tend to clump together and move like a single, large solid. But researchers found more fluid-like states for their levitated particles when the particles were smaller. At low acoustic power, the particles behave like a liquid and shift primarily within a plane. But as the acoustic power increases, the granular liquid begins to “boil” and transition into a gaseous state, with particles moving in all directions. It’s amazing how often these metaphors (e.g., treating a group of particles as a “liquid”) hold true when observing different physical systems! (Image and video credit: B. Wu et al.)
Escaping the Flood
Fire ants clump together into giant rafts to stay alive during floods. But these rafts won’t form with just any number of ants. Researchers found that individual ants will actually kick one another away. It’s not until there are about ten ants that the raft formation becomes stable. In this video, the team lays out their experiments and models for fire ant rafting, showing that capillary action helps draw the raft together and individual ants’ activity can destabilize rafts if they’re too small. (Image and video credit: H. Ko and D. Hu)
Rifts in Rafts
A raft of particles floating on water has some natural cohesion from particle attraction and capillary action. But when the raft is pulled apart, what happens? Does it break cleanly in one spot? Does it stretch and deform? That’s what this video explores. It turns out that the speed you pull the raft at determines how it holds together. Every particle cluster has a preferred relaxation rate, and by choosing the pulling speed, you determine which relaxation rate — and therefore cluster size — can survive most effectively. (Image and video credit: K. Tô and S. Nagel)
Stably Jammed
Granular materials like sand, gravel, and medications can become a rigid mass when squeezed or sheared. Even with a relatively loose packing, these materials can jam together to act like a solid if the contacts between grains no longer allow particles to shift or rotate. In this video, researchers explore how stable these jammed states are by repeatedly shearing the mixture and observing how it changes.
Most of the videos are set up as a triptych, where all three panels show the same material. On the left, you see a simple view showing the position of each particle. In the middle, the disks are viewed through polarized filters, so that the material looks brightest where it is stressed. This view lets us see the force chains that run through the material. On the right, UV-sensitive ink on each marker glows to show any rotation particles experience.
In the first sample, repeated shearing slowly unjams the mixture and allows it to shift and flow once more. We see this from the decreasing brightness in the middle panel. The slow fade to black means that the force chain network has disappeared entirely. In contrast, the second sample ultimately reaches an “ultra-stable” jammed state, in which further shear cycles cause no change to the network. Once again, this is easiest to observe in the middle image, where the bright force network stops changing after 2,000 cycles or so. (Image and video credit: Y. Zhao et al., research pre-print)
Recreating Flocks
Birds, fish, and other creatures form amazing, undulating swarms of individuals. How these collectives comes together and move continues to fascinate scientists. Here, researchers look at simple particles with two “instructions,” if you will. One causes the particle to self-navigate toward a target; the other causes short-range repulsion if the particle gets too close to another one. With only these two simple guidelines, a flock of these particles forms complex, ever-changing flows! (Image and video credit: M. Casiulis and D. Levine)
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.)
Inside Viscous Fingers
Sandwich a viscous fluid between two transparent plates and then inject a second, less viscous fluid. This is the classic set-up for the Saffman-Taylor instability, a well-studied flow in which the interface between the two fluids forms a wavy edge that develops into fingers. Despite its long history, though, there is still more to learn, as shown in this video. Here, researchers alternately injected a dyed and undyed version of the less viscous fluid. The result (Image 3) is a set of concentric dye rings that show how the fluid moves far from the fingers along the edge. Notice that the waviness of the fingers appears in the flowing fluid well before it approaches the interface. (Image and video credit: S. Gowan et al.)
Squishy Actuators
Hard materials don’t always work well in robotics. Here, researchers build soft actuators that can bend, curl, and tighten in order to manipulate objects. They begin by injecting liquid elastomer into a tube (Image 1), followed by a bubble of air. Buoyancy makes the air bubble rise within the tube, creating an asymmetric cross-section where the solidified elastomer has a thin shell along one side and a thicker wall along the other (Image 2). When high-pressure air is pumped into the soft tubes, their asymmetric cross-section makes them bend and twist (Image 3). The team found that they can tune the elastomer tubes to form complex shapes good for gripping and flexing — perfect for a soft robot! (Video and image credit: T. Jones et al.; research credit: T. Jones et al.)
