We’ve all experienced the frustration of ketchup refusing to leave the bottle or toothpaste that shoots out suddenly. These materials are yield stress fluids, which transition from solid-like behavior to liquid flow once the right amount of force is applied. A new study suggests that — despite their wide range of characteristics — these fluids share a universal relation: their yield transition (when they start to flow) depends on their characteristics when at rest. Interestingly, this relationship seems to hold not only for polymeric fluids like the one in the study but also nonpolymeric ones. (Image credit: haideyy; research credit: D. Keane et al.; via APS Physics)
Tag: rheology
Measuring Mucus by Dragging Dead Fish
A fish‘s mucus layer is critical; it protects from pathogens, reduces drag in the water, and, in some cases, protects against predators. But little is known about how mucus could affect terrestrial locomotion in species like the northern snakehead, which can breathe out of the water and move across land. So researchers explored the snakehead’s mucus layer by measuring the force required to drag them (and two other non-terrestrial species) across different surfaces.
The team tested the same, freshly euthanized fish twice: once with its mucus layer intact and again once the mucus was washed off. Unsurprisingly, the fish’s friction was much lower with its mucus. But they also found that the snakehead was slipperier than either the scaled carp or the scale-free catfish. The biologists suggest that the snakehead could have evolved a slipperier mucus to help it move more easily on land, thereby extending the distance it can cover.
As a fluid dynamicist, I think fish mucus sounds like a great new playground for the rheologists among us. (Image and research credit: F. Lopez-Chilel and N. Bressman; via PopSci)
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Baseball’s Mysterious Rubbing Mud
Since 1938, every ball in Major League Baseball has been covered in a special “rubbing mud” harvested from a secret location in New Jersey. Although the league has tried in the past to replace the mud with an alternative, it’s never stuck. Researchers wondered just what makes this mud so special, so naturally, they brought some to the lab to study its composition and rheology.
The mud consists of clay, silt, and sand with a smattering of organic particles. The make-up was pretty typical of river mud in the region, although researchers noted a drop-off in large particle sizes that probably indicates some sieving. In terms of rheology, the mud is shear-thinning, meaning it behaves a bit like lotion. It sits solidly in the hand until it’s deformed, at which point it smoothly coats the surface as a liquid would.
So how does the mud change the baseballs? The researchers found three effects. First, the mud’s shear-thinning allowed it to fill in any pores or imperfections in the ball’s surface, creating a more uniform surface. Second, the dried mud’s residue doubled the ball’s contact adhesion. And, finally, the occasional large sand particles glued to the ball by the dried mud added friction. As the researchers put it, the rubbing mud “spreads like skin cream and grips like sandpaper.” (Image credit: L. Juarez; research credit: S. Pradeep et al.; via EOS)
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Cooking Perfect Cacio e Pepe
In cooking, sometimes the simplest recipes are the toughest to master. Cacio e pepe — a classic three-ingredient Italian pasta — is an excellent example. Made properly, the sauce of cheese and black pepper combines with starchy water to coat the pasta in a uniform, cheesy sauce. Or, if you’re me, you wind up with a pasta sauce flecked with stringy clumps of melted cheese. Fortunately for those of us who have yet to master this one, a new research paper has us covered with tips to make the perfect cacio e pepe.
The key to that elusive silky sauce, they found, is the starch – water – cheese combination. Your water needs just the right amount of starch — they found that between 1 – 4% starch by (cheese) mass worked. If the starch concentration is too low (which can easily happen in pasta water), you’ll get the clumpy cheese mess that so frequently happens in my kitchen. Temperature is also critical; if the water is too hot when it’s added, then it can destabilize the sauce. Check out the pre-print’s Section V for the scientific, supposedly foolproof, recipe. I know I’ll be trying it! (Image credit: O. Kadaksoo; research credit: G. Bartolucci et al. pre-print; via APS News)
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Jamming Soft Grains
Hard granular materials — sand, gravel, glass beads, and so on — can flow, but, in narrow regions or under large forces, they can also jam up, essentially turning into a solid. Soft particles can also flow and jam, but do so under different conditions than hard particles. One group of researchers used a custom-built rheometer to measure jamming in soft particles like the hydrogel beads pictured here. They found that they could extend existing models for jamming in hard particles, but they had to rescale the mathematics to account for the way soft particles change their shape under pressure. (Image credit: Girl with red hat; research credit: F. Tapia et al.; via APS Physics)
Dendritic Painting Physics
In the art of Akiko Nakayama, colors branch and split in a tree-like pattern. In studying the process, researchers found the physics intersected art, soft matter mechanics, and statistical physics. In dendritic painting, the process starts with an underlying layer of acrylic paint, diluted with water. Atop this wet layer, you place a drop of acrylic ink mixed with isopropyl alcohol.
The combination of both layers is key. The alcohol-acrylic drop on a Newtonian substrate will show spreading, driven by Marangoni forces, but no branching. It’s the slightly shear-thinning nature of the diluted acrylic paint substrate that allows dendrites to form. As the overlying drop expands, it shears the underlayer, changing its viscosity and allowing the branches to form. You can see video of the process here. (Image credit: A. Nakayama; research credit: S. Chan and E. Fried; via Physics World)
How Hagfish Slime Clogs
When attacked, the eel-like hagfish slimes its predator, clogging the fish’s gills so that it can escape. A recent study looks at just what makes the slime so effective. There are two main (non-seawater) components to hagfish slime: mucus and threads. The team’s experiments showed that the slime’s clogging is due almost entirely to the mucus; the clogging power of full slime and mucus-only slime is almost identical.
So what are the threads for? They make it harder for the mucus to get washed away. Mucus alone isn’t able to clog as effectively after a single rinse, but, with the threads included, the slime hardly budges. That staying power makes it all the harder for a predator to clear its gills once slimed. In fact, it’s still unclear to scientists whether a slimed fish can free itself from the clogging. After all, the attacker can’t use the hagfish’s trick to free itself from slime. (Image credit: dirtsailor2003/Flickr; research credit: L. Taylor et al.)
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)
Mixing the Perfect Batter
In baking, there’s a point when wet and dry ingredients get combined to form the batter (or dough) that eventually becomes a tasty treat. Experienced bakers know that the ratio of wet-to-dry must be just right for the final product. Too dry and the mixture won’t come together; too wet and the final product is a soggy mess.
Mixing liquids and powders is ubiquitous outside the kitchen, too. Ceramics, concrete, laundry detergent, chocolate — all involve this critical step. To understand how these mixtures transition from fluid to clustered granules to granulations (think wet sand), researchers carefully studied a mixture of glass spheres and glycerol. When there were relatively few particles in the mixture (in technical terms, a smaller “particle volume fraction”), the mixture was fully fluid (top image, orange background). When the ratio of particles-to-liquid was high, the mixture was granular (blue background). And in-between these ratios, whether the mixture formed clumps, or granules, depended on how it was mixed (green background). Vigorous mixing (top row) formed large granules, which consisted of a wet, jammed interior and an outer layer of dry particles (lower image).
Their observations allowed the researchers to predict what ratio of liquid and powder is needed, and how much mixing is necessary, to create a desired outcome. (Image and research credit: D. Hodgson et al.; via Physics Today)
A cross-section of a granule, showing the wet, jammed interior (left) surrounded by a region of dry particles (center, enclosed between red dashes). 
Making Yeast-Free Pizza
Yeast is a key ingredient in many pizza doughs; as the yeast ferment sugars in the dough, they produce carbon dioxide which bubbles into the dough, creating the light and airy texture necessary for a good crust. It’s a slow process, though, often requiring several hours for the dough to rise. Recently, researchers studied an alternative pizza-making method that generates bubbles in the dough via pressurization — with no yeast required.
The new technique is similar to the process used to carbonate sodas. The team mixed flour, water, and salt and placed the dough in an autoclave, which allowed them to control both temperature and pressure during baking. They dissolved gas into the dough at high pressure and then carefully released the pressure during baking, allowing the bubbles to grow. They used rheological measurements to compare the characteristics of yeasted and yeast-free doughs at various stages in the leavening and baking processes.
Now that they have the methodology down, they’ve purchased a food-grade autoclave and are looking forward to taste testing their yeast-free creations — none more so than their team member who has a yeast allergy! Since the pressures required for their method are quite mild, they hope it’s a technique that restaurants will take on. (Image credit: B. Huff; research credit: P. Avallone et al.)
