Scientific research often produces stunning images, and this year’s Gallery of Soft Matter Physics winners are no exception. Selected during the American Physical Society’s March meeting last week in Las Vegas, Nevada, the winning video entries showcased the Cheerios Effect, the physics of clogs, and using 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 competition was first launched 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 opportunity to present their work at next year’s March meeting in Minneapolis, Minnesota.
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 toward the center or the outer rim. The effect can also be found in pollen grains (or mosquito eggs) floating on a pond, or small coins floating in a bowl of water. This is due to a combination of buoyancy, surface tension and the so-called “meniscus effect”. It all adds up to a kind of capillary action. Basically, the mass of the Cheerios is not enough to break the surface tension of the milk. But making a small dent in the surface of the milk in the bowl is enough so that when two Cheerios get close enough, they naturally drift towards each other. The “dents” merge and the “O”s clump together. Add another Cheerio to the mix and he too will follow the curve in the milk to drift towards his other “O”s.
Measuring the actual forces at work on such a small scale is daunting, as they are roughly equivalent to the weight of a mosquito. Typically this is done by attaching sensors to objects and floating them in a container, using the sensors to deflect natural movement. But Cheerios are small enough that this wasn’t a viable approach. So Brown University postdoc Alireza Hooshanginejad and his colleagues used two 3D-printed plastic discs about the size of a Cheerio and placed a small magnet in one of them. Then they let the discs float in a small water tub surrounded by electric coils and float together (attraction). The coils, in turn, create magnetic fields that pull the magnetized disk away from its non-magnetized partner (repulsion).
Hooshanginejad et al. were able to derive a scaling law from their experiments that relates the strength of the capillary action in the Cheerios effect to the mass, diameter and spacing of the disks. For example, they found that at a certain distance between the discs, the two opposing forces balance out, causing the discs to move apart. They also found that certain patterns formed under different conditions. For example, repulsion is the dominant force when the density of the particles is low such that the particles form a crystal lattice. Increase the density and the attraction gains weight because the particles are closer together. Then the particles form clusters. Increase the gravity even more and the particles will form streaks.
To clog or not to clog?
Blockages are the bane of many different sectors, from inkjet printer nozzles, sinks and toilets to blood clots, sewers and the flow of grain draining through a silo, to traffic flow and crowd control. Therefore, they are of course of great interest for research. There are three basic mechanisms behind constipation. Sieving occurs when particles are too large to pass through a restriction; Bridging is when particles are pinched at the constriction and form a stable arc; and aggregation occurs when small cohesive particles build up at a neck. The dynamics in all three scenarios are affected by the shape and size of the particles, as well as how much they deform.
Ben McMillan and colleagues from the University of Cambridge focused on the ‘bridging’ scenario: the way plastic (polyurethane) discs clamp together as they pass through a small hole. It’s similar to the physics of a keystone arch in architecture: the pressure of the upper weight squeezes the underlying particles tighter.
For your experiments, McMillan et al. used a vertical hopper with a funnel-shaped opening at the bottom and observed the discs occasionally jamming into a clog as they slid down the hopper. To meet the challenge of analyzing opaque granular materials, McMillan et al. exploited the fact that their polyurethane discs showed the light patterns inside when viewed between opposing circular polarizers (photoelasticity) – the result of changes in the refractive index. This pattern depends on the magnitude and direction of each force acting on a given disk, allowing them to quantify the force between each particle.
The team allowed the disks (or particles) to flow until an arc-shaped clog formed. They observed both stable and metastable arc formations, where the plug eventually collapses spontaneously. Some metastable clogs lasted longer than others. This photoelasticity allowed them to see how the different forces evolved over time in each arc. They concluded that it is the fluctuations in force magnitude that determine whether an arc is stable, allowing them to predict when one will occur.
The Life of a Marangoni Thermal Bubble
Bubbles are inherently ephemeral. Most burst within minutes in a standard atmosphere. Over time, gravity gradually drains the liquid downwards, 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 type of surfactant keeps surface tension from collapsing the bubbles by strengthening the thin liquid film walls that separate them. And last year, French physicists managed to create “eternal bubbles” out of plastic particles, glycerin and water, one of which survived for a record-breaking 465 days.
Saurabh Nath and other MIT colleagues have developed a new way to extend bubble life: exploiting what’s known as the Marangoni effect, in which a liquid flows from an area of low surface tension to an area of higher surface tension. It’s the phenomenon behind “wine tears” (aka wine shanks 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 will see the water flow outwards, away from the alcohol. The difference in their alcohol concentrations creates a surface tension gradient that drives flow.
For her experiments, Nath et al. made bubbles from air-injected silicone oil and watched them form and burst with an infrared camera. The temperature of the oil bath proved to be crucial. When the temperature was lower (27° Celsius), the blisters burst almost immediately. At higher temperatures (approx. 68° Celsius) they lasted longer. The warmer oil created a temperature gradient between the top and bottom of the bubble, similar to the surface tension gradient behind wine tears. This resulted in an upward Marangoni flow to counteract the coarsening caused by gravity.
Nath et al. The bubbles are then attached to a metal wire that hangs just above the surface of the oil. They found that the upward flowing oil formed a liquid meniscus around the wire that eventually became unstable – at which point a “teardrop” of oil formed and dripped back into the bath. The researchers were able to determine the volume of the Marangoni River by measuring the size and frequency of these teardrops.
Two posters were also awarded 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 at MIT’s Fluid Lab. The poster featured their study of cracking 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 creates a severe negative pressure, up to 100 times that of Earth’s atmosphere. This in turn creates cracks that propagate through avalanche dynamics. Depending on whether the initial droplet had a large or small contact angle with the substrate, the deposits form different crack patterns, forming, for example, a pattern resembling a blooming flower, or delicate circular deposits (top right image) resembling the wings of a Dragon-fly. This sensitivity makes it difficult to control drying cracks.
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 on the self-assembly of particles into crescent-shaped liquid crystal patterns known as “colloidal bananas”. First discovered about 20 years ago, more than 50 “banana phases” have been cataloged so far, which are determined by the degree of molecular curvature and crystal size.
It is difficult to directly observe how the banana particles themselves assemble. So Fernández-Rico and Dullens developed an optical microscopy system to determine the positions and orientations of banana-shaped particles with different curvatures. In particular, they found that by mixing high-curvature and low-curvature “bananas”, the particles self-organize into colloidal vortices (three configurations are shown above left) that bear a striking resemblance to the brushstrokes in Vincent van Gogh’s The starry night.