FYFD

Category: Research

  • Storing Memory in Bubbles

    Storing Memory in Bubbles

    Soft systems like this bubble raft can retain memory of how they reached their current configuration. Because the bubbles are different sizes, they cannot pack into a crystalline structure, and because they’re too close together to move easily, they cannot reconfigure into their most efficient packing. This leaves the system out of equilibrium, which is key to its memory. 

    By shearing the bubbles between a spinning inner ring (left in image) and a stationary outer one (not shown) several times, researchers found they they could coax the bubbles into a configuration that was unresponsive to further shearing at that amplitude. 

    Once the bubbles were configured, the scientists could sweep through many shear amplitudes and look for the one with the smallest response. This was always the “remembered” shear amplitude. Effectively, the system can record and read out values similar to the way a computer bit does. Bubbles are no replacement for silicon, though. In this case, scientists are more interested in what memory in these systems can teach us about other, similar mechanical systems and how they respond to forces. (Image and research credit: S. Mukherji et al.; via Physics Today; submitted by Kam-Yung Soh)

  • Condensing Halos

    Condensing Halos

    Drops that impact a very hot surface will surf on their own vapor, and ones that hit a very cold surface will freeze almost immediately. But what happens when the temperature differences aren’t so extreme? Scientists explored this (above) by dropping room-temperature water droplets onto a cool surface – one warmer than the freezing point but cooler than the dew point at which water condenses. 

    They found that impacting drops formed a triple halo of condensate (right).  The first and brightest ring forms at the radius of the drop’s maximum extent during impact. The second band forms from water vapor that leaves the droplet at impact. As that vapor cools, it condenses into a second band. The final, dimmest band forms as the droplet stabilizes and cools. It’s the result of water vapor near the droplet continuing to cool and condense. (Image and research credit: Y. Zhao et al.; via Nature News; submitted by Kam-Yung Soh)

  • Dandelion Flight, Continued

    Dandelion Flight, Continued

    Not long ago, we learned for the first time that dandelion seeds fly thanks to a stable separated vortex ring that sits behind their bristly pappus. Building on that work, researchers have now published a mathematical analysis of flow around a simplified dandelion pappus. Despite their simplifications, the model captures the flow observed in the previous experiments (bottom image: experiments on left; model on right). 

    The model also allowed researchers to test various features – like the number of filaments in the pappus – and see how they affected the flow. Interestingly, they found that dandelion flight was most stable with about 100 filaments, which is right around the number of a typical pappus! (Image credits: dandelion – Pixabay, figure – P. Ledda et al.; research credit: P. Ledda et al.; via APS Physics; submitted by Kam-Yung Soh and Marc A.)

  • Prehistoric Filter Feeders

    Prehistoric Filter Feeders

    Earth’s earlier ages are filled with enduring mysteries about the plants and creatures that lived and died long before humanity. Many of these organisms, like the aquatic Ernietta shown above, are known only from scattered fossil remains. Yet fluid dynamics is helping us understand how Ernietta lived and fed some 545 million years ago.

    Ernietta were sack-like organisms consisting of stitched-together tubular elements. They had no way to move around and no obvious method for transporting nutrients into their bodies. Scientists hypothesized that they likely used one of two feeding methods: either Ernietta relied on its surface area to extract nutrients directly from the water or its shape enabled it to trap larger particles to feed on from the flow. To decide between these modes, scientists turned to computational fluid dynamics.

    By modelling both single Ernietta and small groups, they found that the shape of the organism generates a rotating current inside the bag that pulls flow down along one side and back up the other. Moreover, being near one another enhanced this effect, helping downstream Ernietta catch more particles than they otherwise would. All in all, the results suggest not only Ernietta’s likely feeding method but also that they lived in colonies and practiced one of the earliest known examples of communal feeding! (Image credit: D. Mazierski, source; research credit: B. Gibson et al.; via ArsTechnica; submitted by Kam-Yung Soh)

  • Urban Centers During Hurricanes

    Urban Centers During Hurricanes

    As the climate warms, many urban centers are facing stronger and more frequent storms. Some, like New York City, are using numerical simulations to better understand the interactions of their complicated urban geometries with hurricane force winds. 

    Above you see a simulation showing predicted wind speeds in a Lower Eastside neighborhood. The incoming wind speed (from the left) is roughly 60 m/s (~134 mph), but the speeds around and between buildings are as much as 2 times higher than that. That means that, even if a storm is Category 3 or 4, there will be areas of a neighborhood that receive sustained winds well beyond the range of a Category 5 hurricane. Urban planners need this sort of data both for devising building requirements and for understanding what storm conditions warrant mandatory evacuations for residents. (Video and image credit: X. Jiang et al.)

  • Transporting Droplets

    Transporting Droplets

    Transporting droplets easily and reliably is important in many microfluidic applications. While this can be done using electric fields, those fields can impact biological characteristics researchers are trying to measure. As an alternative, a group of researchers have developed the concept of “mechanowetting,” a technique that uses surface tension forces to hold droplets on a traveling wave.

    Now visually, it’s a bit tough to see what’s going on here. In the animations, it looks like the droplets are just sticking to a moving surface, but that’s an illusion. The surface the droplet is sitting on is fixed and unmoving. It’s a thin silicone film that covers a ridged conveyor belt. The belt underneath can (and does) move. This creates a traveling wave. Instead of that wave simply passing beneath the droplet, it triggers an internal flow and restoring force that helps the drop follow the wave. The effect is strong enough that small droplets are even able to climb up vertical walls or stick upside-down. (Image, research, and submission credit: E. de Jong et al.)

  • Oil-on-Water Impact

    Oil-on-Water Impact

    Although many people have studied droplet impacts over the years, there’s been remarkably little work done with oil-on-water impacts. One of the things that makes this situation different is that the oil and water are completely immiscible, which means we can see aspects of the impact process that are invisible with, say, water-on-water impacts.

    The animation above shows an underwater view of the oil droplet’s impact. The energy of the initial impact creates an expanding crater and an unstable crown splash. That crown splash contains both water and oil. After it ejects some droplets, the rim stabilizes, but we can still see small perturbations along its edge as it starts to retract. In the water, high surface tension damps out these perturbations. Not so for the oil! As the crater retracts, the small disturbances along the rim get stretched into mushroom-shaped fingers that point inward toward the impact site. Because the index of refraction is different between oil and water, we can see the fingers clearly near the end of the animation. (Image and research credit: U. Jain et al.; submitted by Utkarsh J.)

  • Artificial Microswimmers

    Artificial Microswimmers

    In a 1959 lecture entitled “There’s Plenty of Room at the Bottom”, Richard Feynman challenged scientists to create a tiny motor capable of propelling itself. Although artificial microswimmers took several more decades to develop, there are now a dozen or so successful designs being researched. The one shown above swims with no moving parts at all.

    These microswimmers are simple cylindrical rods, only a few microns long, made of platinum (Pt) on one side and gold (Au) on the other. They swim in a solution of hydrogen peroxide, which reacts with the two metals to generate a positively-charged liquid at the platinum end and a negatively-charged one at the gold end. This electric field, combined with the overall negative charge of the rod, causes the microswimmer to move in the direction of its platinum end. 

    Depending on the hydrogen peroxide concentration, the microswimmers can move as quickly as 100 body lengths per second, and they’re capable of hauling cargo particles with them. One planned application for artificial microswimmers is drug delivery, though this particular variety is not well-suited to that since the salty environment of a human body disrupts the mechanism behind its motion. (Image credits: swimmers – M. Ward, source; diagram – J. Moran and J. Posner; see also Physics Today)

  • Floccing Particles

    Floccing Particles

    Adding particles to a viscous fluid can create unexpected complications, thanks to the interplay of fluid and solid interactions. Here we see a dilute mixture of dark spherical particles suspended in a layer of fluid cushioned between the walls of an inner and outer cylinder. Initially, the particles are evenly distributed, but when the inner cylinder begins to rotate, it shears the fluid layer. Hydrodynamic forces assemble the particles together into loose conglomerates known as flocs. Once the particles form these log-like shapes, they remain stable thanks to the balance between viscous drag on particles and the attractive forces that pull particles toward one another. (Image and research credit: Z. Varga et al.; submitted by Thibaut D.)

  • Crepe-Making Physics

    Crepe-Making Physics

    If you buy a crêpe from a vendor, chances are that they’ll use a blade like the one above to spread the batter evenly across an immobile griddle. But for those of us making our own crêpes at home, this method is impractical. (After all, who wants to purchase a special griddle and utensil just for making one meal?) Instead most of us make our crêpes or pancakes in a standard pan and we use gravity to help us spread the batter.

    Now researchers have described this crêpe-making process mathematically and calculated the optimal method for getting a perfect, uniformly-thin crêpe. Their model even accounts for the fact that the viscosity of the batter changes as the crêpe cooks.

    For optimal crêpe-making, add the batter to the center of the pan. Then immediately tilt the pan to one side to spread the batter all the way to the edge. Keeping the pan inclined, rotate once to fill in the full circumference. Then continue the rotation at a slighter incline to fill in any holes until the pan is horizontal and the crêpe is cooked through. This is what’s shown in the lower animation, where the colormap indicates the crêpe thickness and the arrows show the effective direction of gravity. (Image credit: crêpe-making – taleitan, simulated crêpe – E. Boujo and M. Sellier; research credit: E. Boujo and M. Sellier; via APS Physics; submitted by Kam-Yung Soh)