Slip layer dynamics reveal why some fluids flow faster than expected

New microscopy technique provides unprecedented insight into nanoscopic slip layers formed in flowing complex liquids

Whether it is oil gushing through pipelines or blood circulating through arteries, how liquids flow through tubes is perhaps the most fundamental problem in hydrodynamics. The challenge is to maximize transport efficiency by minimizing the loss of energy to friction between the moving liquid and the stationary tube surfaces. Counterintuitively, adding a small amount of large, slow moving polymers to the liquid, thus forming a ‘complex liquid’, leads to faster, more efficient transport. This phenomenon was speculated to arise from the formation of thin layer around the internal wall of the tube, known as depletion layer or split layer, in which the polymer concentration was significantly lower than in the bulk solution. However, given the inherently thinness of this layer, which is only a few nanometers thick, on the order of the polymer size, direct experimental observation was difficult, and so progress in the field relied heavily on bulk measurements and computer simulations.

Researchers at the Center for Soft and Living Matter, within the Institute for Basic Science (IBS, South Korea), made a significant advance in the field by successfully imaging the depletion layer in polymer solutions flowing through microchannels. Their study, published in the Proceedings of the National Academy of Sciences, relied on the development of a novel super-resolution microscopy technique that allowed the researchers to see this layer with unprecedented spatial resolution.

The first observation of this phenomenon was made nearly a century ago. Experimental studies on high molecular weight polymer solutions revealed a puzzling observation: there was an apparent discrepancy between the measured viscosity of the polymer solution and the rate at which it flowed through a narrow tube. The polymer solution would always flow faster than expected. Furthermore, the narrower the tube, the larger this discrepancy. This sparked an interest which persists to this day.

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When droplets walk across a liquid surface

When droplets walk across a liquid surface

When a container of silicone oil or other similar liquid is vertically shaken at a regular frequency, 1-millimeter-sized droplets of the same liquid placed on the liquid’s surface appear to “walk” across the surface at speeds of about 1 cm/second, propelled by their own waves. In a new study, physicists have found that these walking droplets can be much larger (up to 2.8 mm in diameter) and faster (5 cm/second) than previously observed. These “superwalkers” exhibit a wide range of never-before-seen behaviors, including novel synchronized movements.

One of the interesting features of walkers is that, while their movements can be completely explained by classical mechanics, some of their behaviors mimic certain quantum phenomena that typically exist only at the atomic scale. Examples of such features include tunneling, quantized orbits, and correlations among multiple droplets. In a sense, droplet mechanics could be viewed as a combination of classical and quantum mechanics, where both particle and wave behaviors coexist.

Researchers Rahil Valani and Anja Slim at Monash University, and Tapio Simula at Swinburne University of Technology, have published a paper on the superwalkers in a recent issue of Physical Review Letters.

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fuckyeahfluiddynamics: Laser-induced forward t…


Laser-induced forward transfer (LIFT) is an industrial printing technique where a laser pulse aimed at a thin layer of ink creates a tiny jet that deposits the ink on a surface. In practice, the technique is plagued with


issues, in part because it’s difficult to produce only a single cavitation bubble when aiming a laser at the liquid layer. This is what we see above. 

The laser pulse creates its initial bubble just above the middle of the liquid layer. Shock waves expand from that first bubble and quickly reflect off the liquid surface (top) and wall (bottom). When reflected, the shock waves become rarefaction waves, which reduce the pressure rather than increasing it. This helps trigger the clouds of tiny bubbles we see above and below the main bubble. 

The effect is worst along the path of the laser pulse because that part of the liquid has been weakened by pre-heating, but impurities and dissolved gases in the liquid layer are also prone to bubble formation, as seen far from the bubble. The trouble with all these unintended bubbles is that they can easily rise to the surface, burst, and cause additional jets of ink that splatter where users don’t intend. (Image and research credit: M. Jalaal et al.; submitted by Maziyar J.)

fuckyeahfluiddynamics: Soft systems like this …


Soft systems like this bubble raft can retain memory of how they reached their current configuration. Because the bubbles are different sizes, they cannot pack into a crystalline structure, and because they’re too close together to move easily, they cannot reconfigure into their most efficient packing. This leaves the system out of equilibrium, which is key to its memory. 

By shearing the bubbles between a spinning inner ring (left in image) and a stationary outer one (not shown) several times, researchers found they they could coax the bubbles into a configuration that was unresponsive to further shearing at that amplitude. 

Once the bubbles were configured, the scientists could sweep through many shear amplitudes and look for the one with the smallest response. This was always the “remembered” shear amplitude. Effectively, the system can record and read out values similar to the way a computer bit does. Bubbles are no replacement for silicon, though. In this case, scientists are more interested in what memory in these systems can teach us about other, similar mechanical systems and how they respond to forces. (Image and research credit: S. Mukherji et al.; via Physics Today; submitted by Kam-Yung Soh)

Solving a condensation mystery

Solving a condensation mystery

Condensation might ruin a wood coffee table or fog up glasses when entering a warm building on a winter day, but it’s not all inconveniences; the condensation and evaporation cycle has important applications.

Water can be harvested from “thin air,” or separated from salt in desalination plants by way of condensation. Due to the fact condensing droplets take heat with them when they evaporate, it’s also part of the cooling process in the industrial and high-powered computing arenas. Yet when researchers took a look at the newest method of condensation, they saw something strange: When a special type of surface is covered in a thin layer of oil, condensed water droplets seemed to be randomly flying across the surface at high velocities, merging with larger droplets, in patterns not caused by gravity.

“They’re so far apart, in terms of their own, relative dimensions"—the droplets have a diameter smaller than 50 micrometers—"and yet they’re getting pulled, and moving at really high velocities,” said Patricia Weisensee, assistant professor of mechanical engineering & materials science in the McKelvey School of Engineering at Washington University in St. Louis.

“They’re all moving toward the bigger droplets at speeds of up to 1 mm per second.”

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Researchers discover traditional fluid flow …

Researchers discover traditional fluid flow observations may miss the big picture

Before and after comparisons don’t tell the full story of chemical reactions in flowing fluids, such as those in a chemical reactor, according to a new study from a collaboration based in Japan.

The researchers published their paper on May 6 in the Journal of Physical Chemistry B, a journal of the American Chemical Society. The results were featured on the journal’s cover.

The team examined how a solution of dissolved polymers changed after the addition of Fe3+ solution. These types of solutions are used to better control variables in several fields, including manufacturing. In automobile manufacturing, for instance, the solutions help achieve a thorough evenness of paint coverage and control over how much a material expands or contracts under various temperatures.

Traditionally, researchers examine a solution before a reactant, such as Fe3+solution, is added, and again after the reaction takes place.

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fuckyeahfluiddynamics: Transporting droplets e…


Transporting droplets easily and reliably is important in many microfluidic applications. While this can be done using electric fields, those fields can impact biological characteristics researchers are trying to measure. As an alternative, a group of researchers have developed the concept of “mechanowetting,” a technique that uses surface tension forces to hold droplets on a traveling wave.

Now visually, it’s a bit tough to see what’s going on here. In the animations, it looks like the droplets are just sticking to a moving surface, but that’s an illusion. The surface the droplet is sitting on is fixed and unmoving. It’s a thin silicone film that covers a ridged conveyor belt. The belt underneath can (and does) move. This creates a traveling wave. Instead of that wave simply passing beneath the droplet, it triggers an internal flow and restoring force that helps the drop follow the wave. The effect is strong enough that small droplets are even able to climb up vertical walls or stick upside-down. (Image, research, and submission credit: E. de Jong et al.)

fuckyeahfluiddynamics: In a 1959 lecture entit…


In a 1959 lecture entitled “There’s Plenty of Room at the Bottom”, Richard Feynman challenged scientists to create a tiny motor capable of propelling itself. Although artificial microswimmers took several more decades to develop, there are now a dozen or so successful designs being researched. The one shown above swims with no moving parts at all.

These microswimmers are simple cylindrical rods, only a few microns long, made of platinum (Pt) on one side and gold (Au) on the other. They swim in a solution of hydrogen peroxide, which reacts with the two metals to generate a positively-charged liquid at the platinum end and a negatively-charged one at the gold end. This electric field, combined with the overall negative charge of the rod, causes the microswimmer to move in the direction of its platinum end. 

Depending on the hydrogen peroxide concentration, the microswimmers can move as quickly as 100 body lengths per second, and they’re capable of hauling cargo particles with them. One planned application for artificial microswimmers is drug delivery, though this particular variety is not well-suited to that since the salty environment of a human body disrupts the mechanism behind its motion. (Image credits: swimmers – M. Ward, source; diagram – J. Moran and J. Posner; see also Physics Today)



Adding particles to a viscous fluid can create unexpected complications, thanks to the interplay of fluid and solid interactions. Here we see a dilute mixture of dark spherical particles suspended in a layer of fluid cushioned between the walls of an inner and outer cylinder. Initially, the particles are evenly distributed, but when the inner cylinder begins to rotate, it shears the fluid layer. Hydrodynamic forces assemble the particles together into loose conglomerates known as flocs. Once the particles form these log-like shapes, they remain stable thanks to the balance between viscous drag on particles and the attractive forces that pull particles toward one another. (Image and research credit: Z. Varga et al.; submitted by Thibaut D.)

fuckyeahfluiddynamics: A jet of falling liquid…


A jet of falling liquid doesn’t remain a uniform cylinder; instead, it breaks into droplets. In this video, Bill Hammack explores why this is and what engineers have learned to do to control the size of the droplets formed. 

The technical name for this phenomenon is the Plateau-Rayleigh instability. It begins (like many instabilities) with a tiny perturbation, a wobble in the falling jet. This begins a game of tug of war. One of the competitors, surface tension, is trying to minimize the surface area of the liquid, which means breaking it into spherical droplets. But doing so requires forcing some of the the liquid to flow upward, against both gravity and the liquid’s inertia. The battle takes some time, but eventually surface tension wins and the jet breaks up.

That’s not necessary a bad thing. It’s actually key to many engineering processes, like ink-jet printing and rocket combustion, as Bill explains in the full video. (Video and image credit: B. Hammack; submitted by @eclecticca)

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