A buoyant plume of smoke rises from a stick of incense. At first the plume is smooth and laminar, but even in quiescent air, tiny perturbations can sneak into the flow, causing the periodic vortical whorls seen near the top of the photo. Were the frame even taller, we would see this transitional flow become completely chaotic and turbulent. Despite having known the governing equations for such flow for over 150 years, it remains almost impossible to predict the point where flow will transition for any practical problem, largely because the equations are so sensitive to initial conditions. In fact, some of the fundamental mathematical properties of those equations remain unproven. (Photo credit: M. Rosic)
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Transition to Turbulence
Smoke introduced into the boundary layer of a cone rotating in a stream highlights the transition from laminar to turbulent flow. On the left side of the picture, the boundary layer is uniform and steady, i.e. laminar, until environmental disturbances cause the formation of spiral vortices. These vortices remain stable until further growing disturbances cause them to develop a lacy structure, which soon breaks down into fully turbulent flow. Understanding the underlying physics of these disturbances and their growth is part of the field of stability and transition in fluid mechanics. (Photo credit: R. Kobayashi, Y. Kohama, and M. Kurosawa; taken from Van Dykeโs An Album of Fluid Motion)
Smoke Transition
Smoke issuing from a round jet undergoes transition from laminar to turbulent flow. As the smoke moves past the unmoving ambient air, the friction between these two layers creates shear and triggers a Kelvin-Helmholtz instability, recognizable by the formation and roll up of vortices along the edges of the jet. Those vortices then roll together in pairs, detach, and devolve into a generally turbulent flow. Because turbulence is far more efficient at mixing than a laminar flow is, the smoke seems to disappear.
Osborne Reynolds and Transition
How and when flow through a pipe becomes turbulent has been a conundrum for fluid mechanicians since the days of Osbourne Reynolds (~1870s):
Typically, the laminar-to-turbulence transition is studied mathematically by linearizing the Navier-Stokes equations, the governing equations of fluid dynamics, then perturbing the system. These perturbations will gradually disappear in laminar flow, but if the flow is turbulent, theyโll grow and produce chaotic motion. The transition, then, is the critical point between these two.
However, for pipe flows, this linearized approach shows that the perturbations decay for all Reynolds numbers, even though this doesnโt happen in actual experiments. In the real world, as the Reynolds number increases, small, turbulent puffs begin to split and interact, and their lifetimes increase. Eventually, these puffs carry enough turbulence to transition the flow entirely. # (submitted by David T)
Reynolds on Transition
For although only the disciplined motion is recognized in military tactics, troops have another manner of motion when anything disturbs their order. And this is precisely how it is with water: it will move in a perfectly direct disciplined manner under some circumstances, while under others it becomes a mass of eddies and cross streams, which may be well likened to the motion of a whirling, struggling mob where each individual particle is obstructing the others. The larger the army, and the more rapid the evolutions, the greater the chance of disorder; so with ๏ฌuid, the larger the channel, and the greater the velocity, the more chance of eddies.
Urinal Dynamics Win Ig Nobel Prize
Thirteen years ago, I made a prediction that work on how to avoid urinal splashback would win an Ig Nobel Prize. Today, I am, at last, vindicated. Randy Hurd, Zhao Pan, Tadd Truscott, and Kaveeshan Thurairajah shared the 2026 Ig Nobel Prize in Physics for their work designing a splash-free urinal.
The culmination of this decade-plus of research are two urinal designs, the Cornucopia (“Cornucopeea”) and the Nautilus (“Nauti-Loo”). Both designs minimize splash, in part, through their geometry. As you may have noticed when rinsing dishes, having a stream of droplets hit a surface at a high impact angle creates lots of splash. But at a low impact angle, very little splash occurs. The team took this observation and created designs that minimized impact angle no matter where a user aimed.
How splashback varies with impact angle. High and medium impact angles (left and middle, respectively) generate a lot of splashing from a stream of impacting droplets. In contrast, below a critical impact angle, the splashing is negligible (right). Naturally, they tested the two new designs, alongside two existing urinal designs, finding that the new urinals reduced splashing by as much as 95% across a range of flow rates and user heights. Although the Cornucopia was the least splashy urinal, the team gave the Nautilus an overall edge because its design is easier to clean and works for children, adults, and wheelchair users.
Considering the estimated 1 million liters of urine contemporary urinals splash across U.S. restrooms daily, the Nautilus could save significant labor and cleaning costs, if implemented. (Image credits: urinals and experiment – K. Thurairajah et al., poster – R. Hurd et al.; research credit: K. Thurairajah et al. and R. Hurd et al.)
P.S. – As indicated, I’ve followed this work for a long time. In addition to this post, we did a webcast (10 years ago, yikes!) that touched on the topic. But my most in-depth coverage of the story is still to appear in print; you’ll get to enjoy the whole tale–stretching all the way back to 2012–in a chapter of my forthcoming book. More on that soon! In the meantime, please enjoy this gem of a scientific poster from the project’s early days in 2013:
One of the best research posters of all time, designed to look like it’s been written on a tiled bathroom wall. The text reads, “Confessions of a Sitzpinkler. Though Sitzpinklers, men who sit to urinate, are held in low-esteem within the male community, they have reasonable scientific justification for their actions. Due to the Plateau-Rayleigh instability, a simulated average male urine stream breaks into droplets approximately 15-20 cm after emerging from the urethra. For a typical male and toilet, the opening of the urethra is 13 cm or less above the surface of the water when sitting. The urine stream does not fully transition into droplets before it enters the water as shown in the image on the left. This stream-surface interaction causes bubble entrainment and limited splashing. The resulting satellite droplets lack the necessary momentum to rise above the rim of a typical toilet, not to mention that they toilet bowl is covered when sitting.
In contrast, the average male urinates from a standing height of 64 cm above the water surface with the urine stream breaking into droplets 44-49 cm above the water surface. In the image on the right, the rapid procession of droplets impacts the surface violently, creating splash curtains, deep cavities and jets. These dynamic events collectively contribute to the emission of relatively high-momentum satellite droplets, capable of traversing beyond the rim of a typical toilet bowl.
Sitzpinklers around the world should rest easy knowing that the hygienic benefits of sitting during urination far outweigh the negative social implications.”Fediverse Reactions
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Dropping Oobleck
Oobleck is a peculiar substance. Formed from a suspension of cornstarch particles in water, it can flow like a liquid at low shear rates or jam into a solid under impact. Here, researchers explore what happens to a droplet of oobleck impacting a surface. As they expected, the team found that dilute drops could spread like a normal liquid during impact (top), and denser suspensions could impact like a solid would (below). But at the right conditions, they found that cornstarch-rich droplets could show liquid-like behavior at high shear rates and transition to solid-like behavior once the shear rate slowed down. (Image and research credit: A. Mobaseri et al.; via APS)
Making a Star-Shaped Droplet
We usually think of surface tension turning droplets into spheres in order to minimize their area. But spheres aren’t the only shape surface tension can enforce. Here, researchers suspend tiny droplets of oil in a soapy fluid. At the right temperature, these droplets form a crystalline surface while the fluid within remains liquid. As in the fully liquid droplet, surface tension tries to minimize the shell’s surface energy, enabling it to take on many different shapes.
The droplet’s transition from hexagon to star and back. The shape changes occur as the liquid’s temperature changes, thereby affecting its surface tension. In this study, researchers demonstrate that the shell-enclosed droplets can even change, reversibly, from a hexagon to a six-pointed star and back. The transformation is shown above, in an experiment that gradually changes the droplet’s temperature–and, thus, its surface tension.
Although shape changes similar to these have been described before, this experiment was the first where the shell’s defects–the vertices of the hexagon–don’t shift during the transformation. (Video, image, and research credit: C. Quilliet et al.; via APS)
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Milano Cortina 2026: How Ski Skins Work
The 2026 Olympics include the debut of ski mountaineering (a.k.a. skimo), a sprint race heading both up and down the mountain on skis. During the uphill segment of the race, competitors use skins on their skis to help them climb; these skins then get ripped off (see below) before skiing back down.
As their name suggests, the first climbing skins used on skis were made from seal skin. By angling the seal fur, skiers could glide in the forward direction and resist sliding backwards. Modern skins may have animal or synthetic fibers, but they use the same physical mechanism. The angled hairs let skis slide forward easily, then grip and resist sliding backward. (Image credits: touring – H. Morkel, skins – Josefka, video – NBC Bay Area)
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Inside Solidification
As children, we’re taught that there are three distinct phases of matter–solid, liquid, and gas–but the reality is somewhat more complicated. In the right–often exotic–conditions, there are far more phases matter takes on. In a recent study, researchers described a metal that sits somewhere between a liquid and a solid.
In a liquid, atoms are free to move. During solidification, atoms lose this freedom, and their frozen positions relative to one another determine the solid’s properties. Atoms frozen into orderly patterns form crystals, whereas those frozen haphazardly become amorphous solids. In their experiment, researchers instead observed atoms in liquid metal nanoparticles that remained stationary throughout the transition from liquid to solid. The number and position of stationary atoms affected whether the final solid crystallized or not.
By tracking these stationary atoms and their influence, the team hopes to better control the material properties of the final solidified metal. (Image credit: U. of Nottingham; research credit: C. Leist et al.; via Gizmodo)
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