FYFD

Category: Research

  • 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.)

  • 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)

  • Order in Chaos

    Order in Chaos

    Although turbulent flow is chaotic, it’s not completely disordered. In fact, order can emerge from turbulence, though exactly how this happens has been a long-enduring mystery. Take the animations above. They show the flow that develops between two plates moving in opposite direction that are separated by a small gap. (The formal name for this is planar Couette flow.) The visualization is taken in a plane at a fixed height between the plates.

    Initially (top), the flow shows narrow bands of turbulence, shown in green, separated by calmer, laminar zones in black. As time passes, these areas of laminar and turbulent flow self-organize, eventually forming diagonal stripes that are much longer than the gap between plates (bottom), the natural length-scale we would expect to see in the flow. Researchers have wondered for years why these distinctive stripes form. What sets their spacing, and why are they along diagonals?

    To answer those questions, researchers explored the full Navier-Stokes equations, searching for equilibrium solutions that resemble the striped patterns seen in experiments and simulation. And for the first time, they’ve found a mathematical solution that matches. What the work shows is that the pattern emerges naturally from the equations; in fact, given the characteristics of the solution, the researchers found that many disturbances should lead to this result, which explains why the pattern appears so frequently. (Image and research credit: F. Reetz et al., source; via phys.org; submitted by Kam-Yung Soh)

  • The Shape of Splashes

    The Shape of Splashes

    When a wedge falls into a pool, it creates a distinctive, doubly-curved splash. Here’s how it works. When the front of the wedge first enters the water, it creates a thin sheet of fluid that gets ejected diagonally upward. As the wedge sinks further, the sheet thickens and ejects at a more vertical angle. That creates a low pressure zone in the air beside the splash, which causes outside air to flow inward, generating a sort of Venturi effect under the splash. Because the outer part of the splash sheet is thinner, it’s more strongly affected by the air flow beneath it, and it gets pulled downward, enhancing the splash’s curvature.

    This doubly-curved splash is particular to wedges of the right angle. To see what kind of splashes other shapes make, check out the video below. (Image and video credit: Z. Sakr et al.; for more, see L. Vincent et al.)

  • Featured Video Play Icon

    Active Foam

    Geometrically, biological tissues and two-dimensional layers of foam share a lot of similarities. To try and understand how active changes in one cell affect neighbors, researchers are studying how foams shift when air is injected (below) at one or more sites. When a foam cell expands, it forces topological changes in neighboring cells, which researchers built an algorithm to track in real-time. 

    With some processing, they can actually visualize the radially-expanding waves of strain that pass through the foam (bottom image). This allows them to visualize the effects and interaction of multiple injection sites at once, hopefully helping unlock the mechanics behind both the foam’s shifts and those that occur in tissues. (Image and video credit: L. Kroo and M. Prakash)

  • Bubble Break-Up

    Bubble Break-Up

    When bubbles burst, they spray a myriad of tiny droplets into the air. In general, the older a bubble gets, the thinner it is, thanks to gravity draining its liquid away. When older bubbles burst, they create tinier and more numerous droplets (upper right) compared to a younger bubble (upper left). But there are more forces than just gravity at play.

    Bubbles also undergo evaporation – most effectively at the apex. Evaporation cools the cap of the bubble, increasing its surface tension and triggering a Marangoni flow that helps restore fluid to the bubble film. This stabilizes an aging bubble. 

    Contamination plays a role as well. The bright spots in the bottom image reveal bacteria in the bubble’s cap. Compared to a clean bubble, these contaminated ones can survive far longer and, when burst, produce 10 times as many droplets as a clean bubble of the same age. That has major implications for disease transmission, especially for bacteria that spend a significant portion of their life cycle in liquids. (Image and research credit: S. Poulain and L. Bourouiba; see also Physics Today)

  • Pluto’s Subsurface Ocean

    Pluto’s Subsurface Ocean

    Since the New Horizons probe visited Pluto in 2015, scientists have suspected that Sputnik Planitia (a.k.a. Pluto’s Heart), shown above, may hide a subsurface ocean. But it’s tough to explain how that ocean could stay warm enough to be liquid while the surface ice remains cold and viscous enough to support the variations in thickness we see. One theory cites the possibility of ammonia in the ocean, essentially serving as anti-freeze, but that would require much higher concentrations of ammonia than have been observed in comets – which, like Pluto, spend most of their time in the icy, frigid regions of the Kuiper Belt.

    A new study suggests another theory: a layer of gas-trapping hydrates between the liquid ocean and its icy cap. A thin layer of clathrate hydrates, as proposed by the authors, would trap gases like methane and create a thermally-insulating layer between a warm ocean and much colder ice cap. Because heat would struggle to cross the insulation layer, the water beneath would stay above the freezing point without the cold ice above leeching all of its warmth away.

    It would likely require future missions to Pluto or other potential ocean worlds to confirm the presence of such a hydrate layer, but, for now, the theory provides a possible new explanation for how icy objects like Pluto maintain liquids. (Image credit: NASA/JHU Applied Physics Laboratory/SwRI; research credit: S. Kamata et al.; via Gizmodo)

  • Guiding Particles with Chladni Patterns

    Guiding Particles with Chladni Patterns

    During the 19th century, Ernst Chladni and Michael Faraday independently explored the patterns formed by particles of different sizes placed on a vibrating plate. Faraday found that large particles accumulated at nodes of the plate, where there was no vertical vibration, whereas smaller particles moved toward anti-nodes, where air currents caused by the large vibration amplitude lifted them up.

    The situation becomes a little different if you submerge the vibrating plate in water. Then large, heavy particles gather at the anti-nodes. Drag keeps the particles on the plate, while acoustic forces and gravity conspire to move the particles horizontally toward the anti-nodes (top). Because anti-node patterns change with frequency, this actually provides a way to manipulate particle’s trajectories. The researchers demonstrated this by steering a particle through a maze (bottom) as well as by manipulating an entire swarm of beads. (Image and research credit: K. Latifi et al.; via Physics World; submitted by Kam-Yung Soh)

  • Giving Chocolate that Smooth Finish

    Giving Chocolate that Smooth Finish

    Anyone who’s tried to make chocolate confections at home can tell you that achieving that perfect smooth consistency isn’t easy. It was only after Rodolphe Lindt invented the process of conching in 1879 that anyone enjoyed smooth chocolate. Conching is what allows granular solids like sugar, milk and cocoa powders to mix with liquid cocoa butter into a smooth, homogeneous liquid. Although the process has been known for more than a century, it’s only recently that researchers have unraveled the underlying physics that enables it.

    One of the key parameters to conching is the a mixture’s jamming volume fraction; in other words, the point where the fraction of solid particles in the mixture is too high for it to flow freely. In the first stage of conching, the solid particulates and a small amount of liquid are stirred and slowly heated. The mechanical action of stirring breaks up aggregates and raises the jamming volume fraction. By the end of the dry conche, the mixture could flow, in theory, except that it fractures at a lower stress than what’s necessary to flow.

    At this point, chocolatiers add the remainder of the liquid ingredients. That infusion of moisture decreases the friction between solid particles and further raises the jamming volume fraction. With the system now far below that jamming point, the mixture transforms into a freely-flowing, smooth fluid. By understanding the intricacies of the process, scientists hope to reduce the energy necessary in chocolate production and similar industrial processes.  (Image credit: A. Stein; research credit: E. Blanco et al.; via Physics World; submitted by Kam-Yung Soh)