The thick and thin of viscosity

Viscosity is a liquid’s property of resisting deformation, such as flow. At home, you probably know this property as the ‘thickness’ of a fluid. The word comes from the Latin word “viscum”, meaning ‘mistletoe’, the berries of which were used in ancient times to create glue.

Viscosity can be a weird concept to wrap your head around, because it makes fluids act not like fluids. Such a large part of our intuitive understanding of fluids relies on them behaving a certain way. Water, the quintessential liquid, flows immediately when poured; it takes the shape of its container, and if there’s no container, it spills everywhere. It gets absorbed by sponges or soil; it bubbles with gusto when boiling.

On the other hand, pitch can easily be confused for a solid. Because it’s so highly viscous, it barely flows, has quite an easy time maintaining its rough shape, and barely percolates through either soil or sponges.

So let’s see what this property which makes fluids un-fluid-like is.

The source of viscosity

In strictly physical terms, viscosity is a fluid’s shear stress over its shear rate, and is expressed in poise (P), a measure of pressure per second. It shows how much force per unit of area you need to apply to make a fluid move (shear stress) and how the internal layers of that fluid will move in respect to one another (shear rate).

In short, it shows how much energy you need to apply to make a fluid flow. The field of science that studies the flow patterns of matter is called rheology from the Greek term “rheo”, meaning ‘flow’.

Viscosity is the product of internal friction between the fluid’s molecules. Both gases and liquids have viscosity, but the molecules in liquids are packed in more tightly — making them interact more and, thus, have higher viscosity than gases.

Shear rate is important in this equation because it describes how successive ‘layers’ of molecules in fluid interact with one another. A great way to see it in action is to try pouring honey out of a jar. It flows much more slowly than water because it is more viscous than it. But you’ll also notice that the honey in the center will flow out before the honey on the sides of the jar does as well.

This is due to the friction between the layers we mentioned earlier. Honey molecules that are right next to the glass will be attracted to the molecules in glass strongly enough that they’re basically stationary (solids are dense bunches of immobile molecules). They will generate friction with other honey molecules, in turn.

Friction always opposes the direction of motion so, in effect, this stops the fluid from sliding off the glass. Further layers of honey also experience friction, but against layers of fluid that are more mobile than the glass — so there’s less friction. Keep the model going and honey right in the middle of the jar will be most flowy because it’s experiencing little friction (against other relatively mobile molecules of honey).

It doesn’t only slow down pouring fluid — viscosity also prevents objects from passing through. A spoon will go through water with almost no effort, but not so through honey.

A good way to think about viscosity is that all the molecules in a fluid have hands, and they’re using them to hold onto their neighbors. The better these molecules can latch on, the more viscous the substance will be. In physical terms, these hands make up the cohesive force, being generated by chemical and physical interactions between molecules.

Types of viscosity

True solids cannot be vicious. In solids, molecules cannot really move in relation to one another. If one moves, they all move, or break apart. Since viscosity is defined in relation to flow (where particles move independently from one another), it can’t by definition be applied to a true solid. It would be like trying to define what light tastes like. That being said, not all fluids are born equal in regard to viscosity.

The most common class of viscous material you’ll encounter are Newtonian fluids. These always behave like fluids should, having a relatively constant viscosity. Although their resistance to flow does increase as more force is applied, this is proportional to the force being applied. Newtonian fluids, for all intents and purposes, keep acting like fluids no matter how much force you exert on them.

Air is a Newtonian fluid, although viscosity has no bearing on the speed of sound through air.

On the other end of the spectrum, we have non-Newtonian fluids. With them, viscosity varies very strongly with the force being applied, but not necessarily in the same way between different materials. Silly putty is a good example; left to itself, it’s somewhat flabby, but when you apply force against the putty, it hardens up. There are three large categories of non-Newtonian fluids: dilatant (apparent viscosity increases with exerted force), pseudoplastic (apparent viscosity decreases with exerted force), and generalized Newtonian fluids (which, ironically enough, are non-Newtonian). Ketchup is also non-Newtonian, but it becomes less viscous when force is applied.

Electrorheological and magnetorheological fluids increase their viscosity in the presence of electrical and magnetic fields respectively and can become near-solid under the right conditions, and quickly reverting back to a liquid if needed.

What has viscosity ever done for you?

Perhaps one of the most important impacts viscosity has on our lives is in our biology. The size, shape, and structure of our hearts, circulatory system and virtually every other tissue, as well as the structures and functionality of our cells, are all shaped to take advantage of or work around viscosity. Our immune system relies in no small part on our lymph channels. Lacking the propelling power of our heart, lymph vessels have to be designed around the viscosity of lymph to keep everything in working order.

Each morning, when you go brush your teeth, it’s viscosity preventing the paste in the tube from splattering everywhere. Without viscosity, your ketchup would drain out of the bottle almost instantly, and the lubricants in your car wouldn’t be able to coat (and thus, lubricate) anything.

Viscosity is what makes liquids want to pool together, and it was a key player in applications such as pill manufacturing. Industries involved in the production of food, chemicals, adhesives, biofuels, paints, medicine, and petroleum processing all keep a close eye on the viscosity of their products.

Finally, viscosity is always affecting us, although we’ve evolved to not feel it. But whenever you make even the slightest of motions, the air’s viscosity works against it — it produces ‘air drag’. Swimming, too, lets you experience a somewhat more concentrated flavor of viscosity.

Personally, I find that processes happening mostly in the background of our awareness, the unsung underdogs of physics (such as viscosity) are the most fascinating ones to study. We may take them for granted, we may not even be aware they exist, but they silently play a part in everything we do. Even a ‘humbler’ one, like viscosity, has directly shaped life on Earth into what it is today.

Meet your new organ: the interstitium

Doctors have identified a previously unknown feature of human anatomy with many implications for the functions of most organs and tissues, and for the mechanisms of most major diseases.

Structural evaluation of the interstitial space. (A) Transmission electron microscopy shows collagen bundles (asterisks) that are composed of well-organized collagen fibrils. Some collagen bundles have a single flat cell along one side (arrowheads). Scale bar, 1 μm. (B) Higher magnification shows that cells (arrowhead) lack features of endothelium or other types of cells and have no basement membrane. Scale bar, 1 μm. (C) Second harmonics generation imaging shows that the bundles are fibrillar collagen (dark blue). Cyan-colored fibers are from autofluorescence and are likely elastin, as shown by similar autofluorescence in the elastic lamina of a nearby artery (inset) (40×). (D) Elastic van Gieson stain shows elastin fibers (black) running along collagen bundles (pink) (40×).

A new paper published on March 27th in Scientific Reports, shows that layers of the body long thought to be dense, connective tissues — below the skin’s surface, lining the digestive tract, lungs, and urinary systems, and surrounding arteries, veins, and the fascia between muscles — are instead interconnected, fluid-filled spaces.

Scientists named this layer the interstitium — a network of strong (collagen) and flexible (elastin) connective tissue fibers filled with fluids, that acts like a shock absorber to keep tissues from rupturing while organs, muscles, and vessels constantly pump and squeeze throughout the day.

This fluid layer that surrounds most organs may explain why cancer spreads so easily. Scientists think this fluid is the source of lymph, the highway of the immune system.

In addition, cells that reside in the interstitium and collagen bundles they line, change with age and may contribute to the wrinkling of skin, the stiffening of limbs, and the progression of fibrotic, sclerotic and inflammatory diseases.

Scientists have long known that more than half the fluid in the body resides within cells, and about a seventh inside the heart, blood vessels, lymph nodes, and lymph vessels. The remaining fluid is “interstitial,” and the current paper is the first to define the interstitium as an organ in its own right and, the authors write, one of the largest of the body, the authors write.

A team of pathologists from NYU School of Medicine thinks that no one saw these spaces before because of the medical field’s dependence on the examination of fixed tissue on microscope slides. Doctors examine the tissue after treating it with chemicals, slicing it thinly, and dyeing it in various colorations. The “fixing” process allows doctors to observe vivid details of cells and structures but drains away all fluid. The team found that the removal of fluid as slides are made makes the connective protein meshwork surrounding once fluid-filled compartments to collapse and appear denser.

“This fixation artifact of collapse has made a fluid-filled tissue type throughout the body appear solid in biopsy slides for decades, and our results correct for this to expand the anatomy of most tissues,” says co-senior author Neil Theise, MD, professor in the Department of Pathology at NYU Langone Health. “This finding has potential to drive dramatic advances in medicine, including the possibility that the direct sampling of interstitial fluid may become a powerful diagnostic tool.”

Researchers discovered the interstitium by using a novel medical technology — Probe-based confocal laser endomicroscopy. This new technology combines the benefits of endoscopy with the ones of lasers. The laser lights up the tissues, sensors analyze the reflected fluorescent patterns, offering a microscopic real-time view of the living tissues.

When probing a patient’s bile duct for cancer spread, endoscopists and study co-authors Dr. David Carr-Locke and Dr. Petros Benias observed something peculiar — a series of interconnected spaces in the submucosa level that was never described in the medical literature.

Baffled by their findings, they asked Dr. Neil Theise, professor in the Department of Pathology at NYU Langone Health and co-author of the paper for help in resolving the mystery. When Theise made biopsy slides out of the same tissue, the reticular pattern found by endomicroscopy vanished. The pathology team would later discover that the spaces seen in biopsy slides, traditionally dismissed as tears in the tissue, were instead the remnants of collapsed, previously fluid-filled, compartments.

Researchers collected tissues samples of bile ducts from 12 cancer patients during surgery. Before the pancreas and the bile duct were removed, patients underwent confocal microscopy for live tissue imaging. After recognizing this new space in images of bile ducts, the team was able to quickly spot it throughout the body.

Theise believes that the protein bundles seen in the space are likely to generate electrical current as they bend with the movements of organs and muscles, and may play a role in techniques like acupuncture.

Another scientist involved in the study was first author Rebecca Wells of the Perelman School of Medicine at the University of Pennsylvania, who determined that the skeleton in the newfound structure was comprised of collagen and elastin bundles.

NASA plans to use repellent coatings to coax liquids into flowing even when there is no “down”

Scientists are tackling an unusual problem: how to make liquids flow out of their tanks. The catch is that they’re trying to do this in space, where the absence of gravity means there’s nothing draining the liquids which just stick to the walls of tanks.

Astronaut Clayton Anderson facing a water bubble in 0 G aboard Discovery.
Image credits NASA.

Emptying a container full of liquid is pretty easy right? Just point the hole down and it will flow out. Easy, right?

Well, yes, on Earth. But NASA found that it’s incredibly difficult to drain containers in space because liquids just stick to the walls of tanks like some weird Jell-O in the absence of gravity. Spacecraft today have to employ an orchestra of vanes, sponges, channels, screens, and so on, just to get fuel into the engine for example. This process requires a lot of mechanical equipment that’s heavy, bulky, and can break down — and you don’t want that to happen on the way to Mars.

“The sponges, vanes, baffles and other structures placed inside fuel tanks to move liquid where it is needed are all susceptible to breakage,” said Brandon Marsell, NASA’s Launch Services Program chief investigator.

“If we can replace these complicated metallic mechanisms with a coating, it will reduce the potential for things to break, as well as save weight and money.”

To address that problem, NASA has started the Slosh Coating investigation. Basically, they’ll be testing to see if coating the inside of containers with a liquid-repellant substance will allow them to better move liquids in microgravity. Researchers will compare how liquid behaves in two tanks, one with the coating and one without, on board the ISS. They’ll be using small, transparent tanks, filled with colored water. A series of high-definition cameras will record the liquids’ motion as the containers are put through various motions.

Space-flow

In low-gravity environments, liquid propellant spreads out into a coat against the walls of storage tanks. This creates two problems, Marsell says. First off, heat on the outside of the tanks can set the liquid to boil — wasting resources such as water or fuel — and the liquids may not flow out to where they’re needed.

“We thought if we painted liquid-repelling material on the walls of the tanks, theoretically, instead of sticking to the wall, fluid will stick to the sump at the bottom of the tank, where we want it,” Marsell said.

Right now, NASA knows that the coating will successfully repel liquids, but they have no idea how they will behave beyond this point. It’s possible that the fluids will bounce off the wall but stick to the sump — the tank’s bottom — where there is no coating. One thing the agency plans on finding out with this investigation is how “well it sticks, how easy or difficult it is to dislodge the liquid from the sump when it sloshes around,” said co-investigator Jacob Roth, who is also with the LSP.

If all goes according to plan, the coatings could be used to build more efficient storage tanks for fuel or other fluids during long-range space flights. Keeping cryogenic propellants off the walls would also prevent them from boiling — which leads to fuel losses.

The end result would be much more efficient crafts, so we could travel more on the same fuel.

The Marangoni Effect – an affair with surface tension

The Marangoni Effect says that fluid will want to flow from areas of lower surface tension to areas of higher surface tension.

Soap has a lower surface tension than Water/ Milk. And as a result, when soap is placed on the surface of a fluid (as it is, in these animations), it wants to flow away to areas of higher surface tension. A more in-depth explanation of the Marangoni effect and surface tension in general can be found a previous ZME Science post.

And this propels the small boat, causes the pepper flakes to spread away, makes the string to expand, and the dye to fan out. It is also responsible for the Tears of Wine phenomenon that you might have already witnessed. : )

PC: Flow Visualization at UC Boulder, source video, MIT, Dan Quinn