FYFD

Tag: polymer effects

  • Studying Active Polymers Using Worms

    Studying Active Polymers Using Worms

    I’ve covered some odd studies in my time, but this might be the strangest: to understand how active polymers affect viscosity, researchers loaded drunk worms into a rheometer. Active polymers are long-chain molecules that, like worms, can move on their own using stored energy or by extracting energy from their surroundings. Their dynamics are tough to study, though, because individual polymers are almost impossible to observe while a suspension of them is being deformed.

    Enter the humble sludge worm. Often sold as fish food, these worms — like the polymers they’re meant to imitate — are individually quite wiggly but, given their size, are far easier to observe. Researchers placed them in a custom rheometer in a solution of water and observed how the worm mass responded when sheared by a spinning top plate (Image 3). Like active polymers, the worms exhibited shear-thinning; the faster the plate spun, the lower the worms’ viscosity, likely because the additional force helps align the worms.

    But how do active worms compare with passive ones? The obvious solution would be to repeat their tests with dead worms, but the researchers found a more humane method: by adding some alcohol to the water, they temporarily reduced the worms’ activity, allowing them to compare active and passive worms (Image 2). Once rinsed in water, the worms sobered up and returned to their normal activity levels.

    The researchers found that both the active and passive worms exhibited shear-thinning as the force on them increased, but the shear-thinning in the active worms was not as pronounced, presumably because the movements of individual worms prevented them from aligning smoothly. (Image and research credit: A. Deblais et al.; via Gizmodo and APS Physics)

  • Perfecting Giant Bubbles

    Perfecting Giant Bubbles

    Whether young or old, everyone enjoys blowing soap bubbles, and the bigger the bubble, the more impressive it is. Researchers have been on a quest to discover how bubbles can survive with volumes measured in the tens of meters and thicknesses of mere microns.

    The key to these behemoth bubbles are the polymer chains inside them. The long molecules of polymers get entangled with one another and resist further stretching, which strengthens the soap film. The researchers found that a mixture of polymer lengths are even better for long-lasting bubbles because they entangle more fully than polymers that are all the same size.

    But if what you really want are practical results, I have good news for you: the researchers have released their recommended recipe for making the best giant soap bubbles. It’s included in the video below, but I’ve also reproduced it in text for easier recreation (with thanks to Ars Technica):

    Giant Soap Bubble Solution
    From the Burton Lab, via Ars Technica

    Ingredients
    1 liter of water (about 2 pints)
    50 milliliters of Dawn Professional Detergent (a little over 3 TBSP)
    2-3 grams of guar powder, a food thickener (about 1/2 heaping TSP)
    50 milliliters of rubbing alcohol (a little more than 3 TBSP)
    2 grams of baking powder (about 1/2 TSP)

    Directions
    Mix the guar powder with the alcohol and stir until there are no clumps.

    Combine the alcohol/guar slurry with the water and mix gently for 10 minutes. Let it sit for a bit so the guar hydrates. Then mix again. The water should thicken slightly, like thin soup or unset gelatin.

    Add the baking powder and stir.

    Add the Dawn Professional Detergent and stir gently to avoid causing the mixture to foam.

    Dip a giant bubble wand with a fibrous string into the mixture until it isf fully immersed and slowly pull the string out. Wave the wand slowly or blow on it to create giant soap bubbles.

    Happy bubble making! (Image credit: Burton Lab; video credit: Emory University; research credit: S. Frazier et al.; via Ars Technica; submitted by Kam-Yung Soh)

  • Making Giant Soap Bubbles

    Making Giant Soap Bubbles

    Making soap bubbles is fun, but there’s something about gigantic soap bubbles that brings out the child in everyone. The world’s largest freestanding soap bubble had more than 100 square meters of surface area, which begs an important question: how can such a thin film stay stable at that size?

    The solutions used for giant bubbles have a few main ingredients: water, naturally; detergent, used for its surfactants; and polymers like polyethylene glycol that help stabilize the soap film. Exactly why polymers helped was a bit of mystery, but a new pre-print study aims to answer that.

    Researchers studied how polymer concentrations affected 1) how much solution could be drawn in as bubbles formed, and 2) how long a film of solution lasted before gravity and evaporation thinned it to breaking. They found that intermediate polymer concentrations actually worked best. This gave the solution the viscoelasticity needed to draw in more solution as bubbles grew without having so much polymer that it negatively affected film lifetime. (Image credit: Pixabay; research credit: S. Frazier et al.; via MIT Tech Review; submitted by Kam-Yung Soh)

  • Viscoelasticity and Liquid Armor

    Viscoelasticity and Liquid Armor

    One proposed method for improving bulletproof armor is adding a layer of non-Newtonian fluid that can help absorb and dissipate the kinetic energy of impact. Thus far researchers have focused on shear-thickening fluids – like cornstarch-based oobleck – filled with particles that jam together if anything tries to deform them quickly. But is it really the shear-thickening properties that matter for high-speed impacts?

    To test this, researchers studied projectile impact on three fluids: water (left), a cornstarch mixture (not shown), and a shear-thinning polymer mixture (right). Water is Newtonian, and it slows down the projectile but doesn’t stop it. Both the shear-thickening cornstarch and the shear-thinning polymer mixture do stop the projectile. And by modeling the impacts, researchers concluded that the key to that energy dissipation isn’t their shear-related behaviors: it’s the fact that both fluids are viscoelastic.

    That means that these fluids show both viscous (fluid-like) and elastic (solid-like) responses depending on the timescale of an impact. The high speed of the impact triggered a strong viscous response in both fluids, bringing the projectile to a halt. And if, as the researchers suggest, it’s a fluid’s viscoelasticity that matters most, that widens the field of candidates when it comes to developing a fluid-based armor. (Image and research credit: T. de Goede et al.)

  • Using Instabilities for Manufacturing

    Using Instabilities for Manufacturing

    Manufacturing textured, flexible surfaces can be difficult, but researchers are exploring ways to use fluid dynamical instabilities to make the process easier. They begin with a pourable polymer mixture that cures and solidifies over time. By putting the mixture on a cylinder and rotating it, engineers trigger the Rayleigh-Taylor instability – the same instability that makes dense fluids sink into lighter ones. Here, the instability is driven not only by gravity but by the added acceleration caused by centrifugal force. It causes the fluid film to drain and form arrays of droplets, which then cure into dimples. The researchers can control the size, shape, and spacing of the droplets by changing parameters like the spin rate. And by repeating the process multiple times on the same piece, they can build up spikier shapes, like the ones shown on the poster below. (Image and research credit: J. Marthelot et al., poster)

    image

    Reminder for those at the APS DFD meeting! My talk is tonight at 5:10PM in Room B206. You’ll probably want to come early if you want a seat!

  • Pressing Non-Newtonian Fluids

    Pressing Non-Newtonian Fluids

    For many fluids, the relationship between force and deformation is not simple. The catch-all name for these materials is non-Newtonian fluids. In a recent episode, the Hydraulic Press Channel did some experiments extruding a couple non-Newtonian fluids: oobleck and a temperature-sensitive putty. What they demonstrated is that a fluid’s response to the forces it experiences can change depending on the rate at which force is applied.

    Take their putty example from the latter half of the video. When the hydraulic press pushes the putty slowly, it extrudes in a smooth, semi-solid string. When they increase the pressure driving the hydraulic press, it pushes the putty more quickly, causing it to spray out of the die in a shredded mess. What they actually did here is surpass a threshold for what’s known in manufacturing as the sharkskin instability. This behavior occurs due to long-chain polymer molecules in the fluid. Inside the die, flow near the walls is slowed down by friction but moves freely in the middle of the pipe. When the walls are suddenly gone, flow at the outside accelerates to match the inside of the stream, which stretches the polymers until they can snap free of the die. The result is the rough, saw-tooth-like pattern seen here. (Video and image credit: Hydraulic Press Channel, source)

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    Non-Newtonian Splashes

    What happens when a stream of liquid falls through a screen? As the above video shows, water creates a beautiful flower-like burst of fluid when it hits a screen. Adding a little polymer to the water makes it non-Newtonian and more viscous. When hitting the screen, this slows it down but doesn’t prevent the fluid from flowing.

    Add enough polymer, though, and the fluid becomes what’s known as a yield-stress fluid. These fluids behave much like a solid–they don’t flow–until you apply a certain amount of stress. Then they’ll flow. If you’ve ever tried to get ketchup out of a glass bottle, then you’re familiar with how these yield-stress fluids act. When dropped onto a screen, the yield-stress fluid just forms a pile–unless the impact speed is high enough to create the necessary force to get the fluid to flow! (Video credit: B. Blackwell et al.)

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    Making Droplets Stick

    Lots of plants have evolved leaves that are superhydrophobic – that is, water repellent. For a plant, this makes a lot of sense. A superhydrophobic leaf will make water bounce and run off, draining down to where the plants roots can drink it up. But this same feature can be a frustration to farmers who spread pesticides by spraying plants. They need the pesticide to stick to the leaves if it’s to deter insects, and the superhydrophobicity of the leaves forces them to spray more pesticides in the hopes of getting some to stick. Researchers at MIT are looking to change this status quo with a few biodegradable polymer additives that can counter the leaves’ superhydrophobic tendencies and help droplets stick to the surface. This could reduce the amount of pesticides needed to protect crops. (Video credit: MIT)

  • Climbing Up the Walls

    Climbing Up the Walls

    You may have noticed when baking that fluids don’t always behave as expected when you agitate them. If you put a spinning rod into a fluid, we’d expect the rod to fling fluid away, creating a little vortex that stirs everything around. And for a typical (Newtonian) fluid, this is what we see. The fluid’s viscosity tries to resist deforming the fluid, but the momentum imparted by the rod wins out. With a viscoelastic fluid, on the other hand, the story is much different. As before, the spinning of the rod deforms the fluid. But the viscoelastic fluid contains long chains of polymers. As those polymers get stretched by the deformation, they generate their own forces, including forces parallel to the rod. Instead of being flung outward, the viscoelastic fluid starts climbing up the rod, with the stretchy elasticity of the polymers helping pull more fluid up and up.  (Image credit: Ewoldt Research Group, source)

  • Whiskey Stains

    Whiskey Stains

    Photographer Ernie Button discovered that whiskey left behind intriguing patterns after it evaporated. Unlike coffee rings, the whiskey leaves behind a more uniform residue. Curious, he contacted researchers at Princeton, who were eventually able to explain why whiskey and coffee dry so differently. They observed three major effects in drying whiskey mixtures. Firstly, the alcohol in whiskey evaporates faster than other components, creating differences in concentration and, therefore, surface tension along the droplet. These variations in surface tension create Marangoni flow, which tends to mix the droplet. Coffee, being non-alcoholic, does not do this.

    Whiskey also contains surfactants, low surface tension chemicals, which help pull particulates away from the edge of the droplet so they aren’t trapped there like in coffee. And finally, they found that the polymers in whiskey helped glue particles to the glass so that they were less likely to be carried by the flow. Taken together, these three ingredients – alcohol, surfactants, and polymers – all help make the whiskey stain more uniform. For more, watch the video below, see Button’s website, or check out the research paper. (Image credit: E. Button; research credit: H. Kim et al.; video credit: C&EN; submitted by @tommyjwilson)