FYFD

Tag: lubrication

  • Surface Fat Gives Chocolate’s Mouthfeel

    Surface Fat Gives Chocolate’s Mouthfeel

    Understanding the interactions of food and our mouths is incredibly difficult. There are lots of changes going on: shape changes from chewing, viscosity changes as saliva lubricates the food, and, sometimes, phase changes from the heat of our bodies. Add to that the sensitivity of our papillae-covered tongues, and it’s a lot to manage all at once. Recently, researchers have turned to 3D-printing to create a more realistic lab version of our mouths.

    The team 3D-printed a papillae pattern matching the size and distribution of an actual human tongue, then molded that pattern onto a silicone elastomer. The result? A replica tongue that matches a human one in terms of softness, wettability, and surface roughness. They then attached their tongue to a rheometer to measure the friction between the tongue and dark chocolate.

    Their experiments simulated licking, eating, and swallowing the confection. During licking and eating, they found that the chocolate was lubricated by a layer of fat directly between the tongue and the food. Their results suggest that one way to make healthier chocolate options is to concentrate fat into the surface layer of the chocolate while lowering the fat content inside the bar. (Image credit: D. Ramoskaite; research credit: S. Soltanahmadi et al.; via APS Physics)

  • Box Closing Physics

    Box Closing Physics

    My fellow board game aficionados (and anyone else who regularly opens and closes lidded boxes) have probably noticed the way a lid drops slowly onto its box once aligned. The weight of the lid pressurizes air inside the box, driving a flow through the narrow gap between the walls of the box and the lid. Researchers found that the time it takes for a box to slide closed is closely related to the size and shape of the gap between the walls. Despite gaps of less than 1 millimeter, air moving out of the box typically flows at about 1 meter per second!

    With their mathematical model of the flow from a closing box, the group was also able to determine the optimal shape for a fast-closing box, something that may be of interest to manufacturers as well as fans of board games. (Image credit: N. Sharp; research credit: J. de Ruiter et al.; via APS Physics)

  • Beijing 2022: Sliding on Snow

    Beijing 2022: Sliding on Snow

    Skiing and snowboarding events rely on the peculiar physics of sliding on snow. According to classical lubrication theory, that sliding shouldn’t be nearly as low in friction as what we observe. The key here is that snow is soft and porous; it’s compressible, but it can also trap air (or water) in the pores between flakes. Because the passage of a skier or snowboarder is so fast, the air doesn’t have the time to slip out of the pores. Instead, it gets pressurized, providing lift that keeps the slider’s friction low. In the end, it isn’t the snow holding the slider up, it’s the air trapped in the snow beneath them! (Image credit: skier in powder – J. Andersson, snowboarder – Visit Almaty, halfpipe – P. T’Kindt; research credit: Z. Zhu et al.)

  • Levitating Cylinders by Lubrication

    Levitating Cylinders by Lubrication

    Here’s a surprising example of defying gravity: if you coat a vertical treadmill in oil, a cylinder held next to it will levitate! A new paper delves into the mathematics behind this surprising situation, showing that the key to keeping the cylinder aloft is the pressure that forms where the oil layer splits around the disk. For a given cylinder size and mass, there’s a unique treadmill speed that will levitate it. By experimentally testing a range of cylinder sizes and masses, the authors validated their model and showed a simply scaling argument for predicting the belt speed needed for levitation. (Image and research credit: M. Dalwadi et al.; via Nature; submitted by Kam-Yung Soh)

  • Kugel Fountains

    Kugel Fountains

    At science museums and tourist attractions around the world, visitors can spin the multi-tonne spheres of kugel fountains with the brush of their hand. The secret of the sphere’s mobility is aquaplaning – the same phenomenon that can cause cars to lose traction in wet conditions. In these fountains, the massive sphere sits in a precisely-shaped cup, with their surfaces separated by a thin layer of water. The entire system acts like a hydrostatic bearing, which allows the sphere to move freely. But even a relatively small disruption can destroy the effect, as happened to the Science Museum of Virginia’s original Grand Kugel after it cracked. (Image credit: E. Roberts; via Atlas Obscura; submitted by Kam-Yung Soh)

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    Hydrodynamic Bearings

    If you twirl a glass syringe, it spins quite nicely, lubricated on a micron-thin layer of air. This is an example of a hydrodynamic bearing, a device where the viscosity of a fluid and relative motion of two closely-spaced surfaces provides the cushion necessary to keep the surfaces separate. In this video, Steve Mould explains the phenomenon in more detail and shares some awesome examples of this hydrodynamic levitation in action. (Image and video credit: S. Mould; submitted by clogwog)

  • 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)

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    Recreating Pyroclastic Flow

    One of the deadliest features of some volcanic eruptions is the pyroclastic flow, a current of hot gas and volcanic ash capable of moving hundreds of kilometers an hour and covering tens of kilometers. Since volcanic particles have a high static friction, it’s been something of a mystery how the flows can move so quickly. Using large-scale experiments (top), researchers are now digging into the details of these fast-moving flows.

    What they found is that the two-phase flow results in a pressure gradient that tends to force gases downward. This creates a gas layer with very little friction near the bottom of the pyroclastic flow (bottom), essentially lubricating the entire flow with air. This helps explain why pyroclastic flows are so fast and long-lived despite their inherent friction and the roughness of the terrain over which they flow. (Image and research credit: G. Lube et al.; video credit: Nature; submitted by Kam-Yung Soh)

  • Landslide Lubrication

    Landslide Lubrication

    In 2008, an 8.2 magnitude earthquake in China caused the enormous Daguangbao landslide, which loosed over one cubic kilometer of rocks and debris. That material rushed down the mountainside, running more than 4 kilometers before coming to a stop. A new study uses field measurements and laboratory experiments to explain how the landslide could run so far from its source.

    The researchers found that friction between the sliding material and the stable rock heated that layer to over 850 degrees Celsius, hot enough to start decomposing the dolomite in the fall. That vaporized carbon dioxide out of the rock, which helped lower the friction. Simultaneously, the high temperatures and high pressures within in the landslide caused recrystallization in the falling rocks; this created a viscous layer that helped lubricate the slide. The team estimated that the two mechanisms working in tandem enabled the landslide to reach an estimated 60 m/s. (Image and research credit: W. Hu et al.; via Nature; submitted by Kam-Yung Soh)

  • What Makes Joints Pop?

    What Makes Joints Pop?

    Cracking one’s knuckles produces an unmistakable popping noise that satisfies some and disconcerts others. The question of what exactly causes the popping noise has persisted for more than fifty years. It’s generally agreed that separating the two sides of a joint causes low enough pressures to form a cavitation bubble in the sinovial fluid of the joint. But researchers have been divided on whether it’s the formation or the collapse of this bubble that’s responsible for the sound. Studying the phenomenon firsthand is difficult with today’s imaging technologies – none of them are fast enough to capture a behavior that takes only 300 milliseconds. As a result, scientists are turning to mathematical modeling and numerical simulation.

    A recent study tackled the problem by modeling a joint that already contains a bubble and examining the bubble’s response to changes in pressure inside the joint. The pressure changes alter the bubble’s size and cause it to generate sound. When compared to experiments of people cracking their knuckles, the simulated sounds are remarkably similar in both amplitude and frequency. It’s not even necessary for the bubble to collapse completely to make the noise. Just a partial collapse is enough to sound just like that old, familiar pop. (Image credit: G. Kawchuk et al.; research credit: V. Chandran Suja and A. Barakat; via Gizmodo)