Check out the winners of this year’s Gallery of Soft Matter Physics

Scientific research often produces striking visuals, and this year's winners of the Gallery of Soft Matter Physics are no exception. Selected during the American Physical Society March Meeting last week in Las Vegas, Nevada, the winning video entries featured the Cheerios effect, the physics of clogs, and exploiting the physics behind wine tears to make bubbles last longer. Submissions were judged on the basis of both striking visual qualities and scientific interest. The gallery contest was first established last year, inspired in part by the society's hugely successful annual Gallery of Fluid Motion. All five of this year's winners will have the chance to present their work at next year's March meeting in Minneapolis, Minnesota.

Mermaid Cereal

As we've previously reported, the "Cheerios effect" describes the physics behind why those last few tasty little "O"s of cereal tend to clump together in the bowl: either drifting to the center or to the outer edge. The effect can also be found in grains of pollen (or mosquito eggs) floating on top of a pond or small coins floating in a bowl of water. The culprit is a combination of buoyancy, surface tension, and the so-called "meniscus effect." It all adds up to a type of capillary action. Basically, the mass of the Cheerios is insufficient to break the milk's surface tension. But it's enough to put a tiny dent in the surface of the milk in the bowl, such that if two Cheerios are sufficiently close, they will naturally drift toward each other. The "dents" merge and the "O"s clump together. Add another Cheerio into the mix, and it, too, will follow the curvature in the milk to drift toward its fellow "O"s. Measuring the actual forces at play on such a small scale is daunting, since they're on about the same scale as the weight of a mosquito. Typically, this is done by placing sensors on objects and setting them afloat in a container, using the sensors to deflect the natural motion. But Cheerios are small enough that this was not a feasible approach. Building on prior work by former undergraduate Ian Ho (now a graduate student at Stanford), Brown University postdoc Alireza Hooshanginejad and cohorts used two 3D-printed plastic disks, roughly the size of a Cheerio, and placed a small magnet in one of them. Then they set the disks afloat in a small tub of water, surrounded by electric coils, and let them drift together (attraction). The coils in turn produced magnetic fields, pulling the magnetized disk away from its non-magnetized partner (repulsion). Hooshanginejad et al. were able to derive a scaling law from their experiments relating the strength of the capillary action in the Cheerios effect to the mass, diameter, and spacing of the disks. For instance, they found that at a certain spacing between the disks, the two opposing forces balance, so the disks settle into a standoff. They also noted that certain patterns formed under different conditions. For instance, repulsion is the dominant force when the density of particles is low, so the particles form a crystal lattice. Increase the density, and the attractive force gains sway because the particles are closer together. That's when the particles form clusters. Increase the attractive force even more, and the particles will form stripes.

To Clog or Not To Clog?

Clogs are the bane of many different sectors, from inkjet printer nozzles, sinks, and toilets, to blood clots, sewers, and the flow of grains draining through a silo, as well as traffic flow and crowd control. So naturally they are of great interest to researchers. There are three basic mechanisms behind clogging. Sieving occurs when particles are too big to pass through a constriction; bridging is when particles get jammed at the constriction and form a stable arch; and aggregation occurs when small cohesive particles build up at a constriction. The dynamics in all three scenarios are influenced by the shape and size of the particles, as well as how much they deform. Ben McMillan and colleagues at the University of Cambridge focused on the "bridging" scenario: the way plastic (polyurethane) disks jam together while passing through a small hole. It's similar to the physics of a keystone arch in architecture: The pressure from the weight above presses particles below more firmly together. For their experiments, McMillan et al. used a vertical hopper with a funnel-shaped opening at the bottom and monitored how the disks occasionally jammed together to form a clog as they slid down the funnel. To get over the challenge of analyzing opaque granular materials, McMillan et al. exploited the fact that their polyurethane disks revealed the patterns of light within when viewed between opposite circular polarizers (photoelasticity)—the result of changes in the refractive index. That pattern depends on the strength and direction of each force acting on a given disk, so they were able to quantify the force between each particle. The team let the disks (or particles) flow until an arched clog formed. They observed both stable and metastable arch formations, in which the clog eventually collapses spontaneously. Some metastable clogs persisted longer than others. That photoelasticity enabled them to see how the various forces evolved over time in each arch. They concluded it is the fluctuations in force strength that determine whether an arch will be stable, enabling them to predict when one will occur.

The Life of a Thermal Marangoni Bubble

Bubbles are inherently ephemeral. Most burst within minutes in a standard atmosphere. Over time, the pull of gravity gradually drains the liquid downward, and at the same time, the liquid component slowly evaporates. As the amount of liquid decreases, the "walls" of the bubbles become very thin. The combination of these two effects is called "coarsening." Adding some kind of surfactant keeps surface tension from collapsing bubbles by strengthening the thin liquid film walls that separate them. And last year, French physicists succeeded in creating "everlasting bubbles" out of plastic particles, glycerol, and water, one of which survived for a record-breaking 465 days. Saurabh Nath and other MIT colleagues figured out a new method for extending the lifetime of bubbles: exploiting the so-called Marangoni effect, in which a liquid flows from a low surface tension area to a higher surface tension area. It's the phenomenon behind "wine tears" (aka wine legs or "fingers") and the coffee ring effect. Spread a thin film of water on your kitchen counter and place a single drop of alcohol in the center, and you'll see the water flow outward, away from the alcohol. The difference in their alcohol concentrations creates a surface tension gradient, driving the flow. For their experiments, Nath et al. produced bubbles out of silicone oil injected with air and used an infrared camera to monitor how they formed and burst. The temperature of the oil bath proved crucial. If the temperature was lower (27° Celsius), the bubbles burst almost immediately. At higher temperatures (about 68° Celsius), they lasted longer. The warmer oil produced a temperature gradient, similar to the surface tension gradient behind wine tears, between the top and bottom of the bubble. That resulted in an upward Marangoni flow to counter the gravity-induced coarsening. Nath et al. followed up by having the bubbles adhere to a metal wire hung just over the surface of the oil. They found that the oil flowing upward formed a liquid meniscus around the wire that eventually became unstable—at which point a "tear drop" of oil formed and dropped back down into the bath. The researchers were able to determine the volume of the Marangoni flow by measuring the size and frequency of those tear drops.

Winning posters

There were also two posters honored in this year's Gallery of Soft Matter Physics. The first ("Dry Hard: Controlling Cracks in Drying Suspension Drops") was submitted by Mario Ibrahim and colleagues in MIT's Fluid Lab. The poster featured their exploration of crack patterns in drying droplets, similar to how layers of mud and paint often crack and dry, or the coffee ring effect. The droplets are colloidal suspensions of silica nanoparticles in water. The droplets are placed on a glass substrate to dry, and as they evaporate, the resulting flow generates a strong negative pressure up to 100 times Earth's atmosphere. This in turn produces cracks that spread via avalanche dynamics. The deposits form different crack patterns depending on whether the initial droplet had a large or a small contact angle with the substrate, forming, for instance, a pattern that resembles a blooming flower or delicate circular deposits (pictured, above right) that resemble the wings of a dragonfly. That sensitivity makes drying cracks difficult to control. The second poster ("Colloidal Bananas Get to Form Colloidal Vortices") was submitted by Carla Fernández-Rico and Roel Dullens from the University of Oxford and shows the results of their study of the self-organization of particles into crescent-shaped liquid crystal patterns known as "colloidal bananas." First discovered about 20 years ago, there are more than 50 "banana phases" catalogued so far, determined by the degree of molecular curvature and crystal sizes. It's challenging to directly observe how the banana particles self-assemble. So Fernández-Rico and Dullens developed an optical microscopy system to determine the positions and orientations of banana-shaped particles with different curvatures. Specifically, they found that by mixing high curvature with low curvature "bananas," the particles self-organize into colloidal vortices (three configurations are pictured, above left) that bear a striking resemblance to the brush strokes in Vincent van Gogh’s The Starry Night.