FYFD

Tag: science

  • 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.)

  • Featured Video Play Icon

    Ink Explosion

    Sometimes beautiful flows come from simple combinations. Here the artists of Chemical Bouillon combine ink and hydrocarbons to create lovely explosions of color. Eschewing quick cuts between views, they allow us to linger and explore the flow ourselves as it changes. Differences in surface tension drive streaming flows along the surface, but there seem to be some chemical reactions contributing as well. Watch along the edges and you may even see convection pulling ink down and back. The whole video is only 2 minutes long and worth a full watch. (Image and video credit: Chemical Bouillon)

  • 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)

  • Featured Video Play Icon

    Experimenting with Speakers

    In her ongoing quest to explore natural resonance, Dianna has enlisted some very nice, very expensive speakers to find out just what happens when the bass drops. If you ever wondered what the natural frequency of your eyeballs is, then this one’s for you.

    If you’re more intrigued by the idea of putting out fires with sound (and/or explosions), I’ve got some posts on that including a sound-based fire extinguisher and a supersonic cannon capable of blowing out fires. (Video credit: Physics Girl)

  • 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)

  • Using Bubbles to Keep Clean

    Using Bubbles to Keep Clean

    Keeping produce clean of foodborne pathogens is a serious issue, and delicate fruits and vegetables like tomatoes cannot withstand intense procedures like cavitation-based cleaning. But a new study suggests that simple air bubbles may have the power to keep our produce free of germs.

    In particular, researchers studied air bubbles injected into water as they bounced and slid along an inclined solid surface. They found that as a bubble approaches a tilted surface, it squeezes a thin film of liquid between itself and the surface. That flow creates a shear stress that pushes contaminants like E. coli away from the point of impact. When the bubble bounces away, fluid gets sucked back into the void left behind, creating more shear stress. In their experiments and simulations, the team measured shear stresses greater than 300 Pa, more than double what’s needed to remove foodborne bacteria like Listeria. (Image credit: Pixabay; research credit: E. Esmaili et al.)

  • Featured Video Play Icon

    How Spillways Work

    Human infrastructure like dams have the challenge of standing up to whatever nature can throw at them. It’s expensive, if not outright impossible, to build to every single contingency, so engineers have developed methods of dealing with problems like excess flow caused by a storm. For dams, one of the ways of dealing with this are spillways, which allow a method of controlled release from a reservoir. 

    Spillways come in many shapes and sizes, as seen in the video, but there are two general types: those that are actively managed and those that are automatic. An automatic spillway is like the “morning glory” type seen in the middle animation. There’s no on or off for a spillway like this. Instead, once the water level is high enough, water naturally flows out. In that sense, it’s like the overflow holes found in many bathroom sinks.

    Controlled spillways are usually managed with gates that can be opened or closed as operators need them. This technique gives more granular control and can even end up being cheaper in some situations because it requires less space to implement. (Video and image credit: Practical Engineering)

  • Salty Comets

    Salty Comets

    Many of the products we use every day in our homes behave like solids until the right force is applied. These yield-stress fluids are like hand sanitizer – strong enough to suspend millimeter-sized particles when still but capable of flowing easily when pumped. In hand sanitizer, this is because the fluid is made up of swollen microgel particles that are jammed together. To rearrange, they need a certain amount of force applied. The weight of the sugars, capsules, and particulates added to the product aren’t heavy enough to move the jammed microgels, so they stay suspended.

    But researchers found that if they add a salt crystal of the same size and weight (bottom image), it sinks steadily through the gel. The salt’s velocity is constant; it doesn’t change with size as we might expect. That’s because it’s not falling by forcing the microgel particles to move. Instead, its salinity forces the microgel to release its absorbed liquid; basically, it’s collapsing the jammed particles. It falls steadily because it takes a given amount of time to collapse each gel particle.  (Image credits: microgel – N. Sharp; salt comet – A. Nowbahar et al.; research credit: A. Nowbahar et al.)

  • Polygonal Droplets

    Polygonal Droplets

    Spheres are a special shape; they provide the smallest possible surface area necessary to contain a given volume. And since surface tension tries to minimize surface energy by reducing the surface area, drops and soap bubbles are, generally, spherical. There’s subtlety here, though: namely, what if reducing the surface area doesn’t minimize the surface energy?

    That’s the issue at the heart of this study. It looks at microscale oil droplets, like the ones above, that are floating in water and stabilized by surfactants. We’d expect droplets like these to be round, and above a critical temperature, they are. But as the temperature drops, the surfactant molecules along the droplet’s interface crystallize. The drop itself is still liquid, but interface is not.

    This changes the rules of the game. There’s no way for the surfactant molecules to form a sphere when solidified; they simply can’t fit together that way. So instead defects form along the interface and the drop becomes faceted. As the temperature drops further, the energy relationship between the water, oil, and surfactants continues shifting, causing the droplet to change shape – even to increase its surface area – all to minimize the overall energy. The effect is reversible, too. Raise the temperature back up above the critical point, and the interface “thaws” so that the drop becomes round again. (Image and research credit: S. Guttman et al.; via Forbes; submitted by Kam-Yung Soh)