The volcanoes of the South Sandwich Islands, located in the South Atlantic, have a notable effect on cloud formation in this satellite photo. Visokoi Island, on the right, sheds a wake of large vortices that distort the cloud layer above it. ย On the left, Zavodovski Islandโs volcano does the same, with the added effect of low-level volcanic emissions, which include aerosols. These tiny particles provide a nucleus around which water droplets form, causing an marked increase in cloud formation visible in the bright tail streaming off the island. (Photo credit: NASA, via Earth Observatory)
Search results for: “water droplet”
Icing on Airplane Wings
Icing on airplane wings remains little understood and a major hazard. These photos show examples of ice formation along the leading edge of a swept wing. If an aircraft flies through a cloud of supercooled water droplets, the droplets will freeze shortly after impact with the aircraftโs wings. As ice continues to build up in strange shapes, the aerodynamic profile of the wing changes, which can lead to disastrous effects as the stall and control characteristics of the wing shift.ย (Photo credit: NASA Glenn Research Center)
Reader Question: Rocket Propulsion
staunchreality-deactivated20120ย asks:
Hey there – Love the blog. Most interesting science blog I follow ๐ This may be a silly question – is propulsion through space purely a function of exit velocity and catching gravity slingshots around planets, or is there enough of anything to push against for rocket propulsion?
Thanks! Glad you enjoy the blog. And your question is not silly at all.
Whether in the atmosphere or not, rocket engines always operate on the same principle: Newtonโs 3rd law. ย For every force exerted, there is an equal and opposite reaction force. ย For a rocket, this means that the momentum of the rocket exhaust provides forward momentumโthrustโfor the rocket. ย When acting in an atmosphere, the exhaust doesnโt push against the atmosphere in order to move the rocketโin fact, rockets have to overcome aerodynamic drag when in the atmosphere, which opposes their thrust.
While the operating principle of a rocket remains the same regardless of its surrounding, the ambient pressure (essentially zero in space and non-zero in an atmosphere) does affect the efficiencyย of the rocketโs nozzle, which can affect the exit velocity of the exhaust, and, thus, the efficiency of the rocket. Under ideal conditions, the exhaust should exit the nozzle at the same pressure as the ambient conditionsโwhatever they are. If the exhaust pressure is lower than the ambient, the exhaust can separate from the nozzle, causing instabilities in the flow and potentially damaging the nozzle. On the other hand, if the exhaust pressure is too high, then there is exhaust that could be turned into thrust that is going to waste. Unfortunately, matching the exhaust pressure to the ambient pressure is a function of the geometry of the nozzle, which is usually fixed. Engineers of rockets intended to fly from within the atmosphere to space usually have to pick a particular altitude to design around and deal with the inefficiencies while the rocket flies at other ambient conditions.
Outside of the physical mechanics of how thrust is produced, propulsion in space is dominated by the influence of orbital mechanics. Once in an orbit, a spacecraft will stay on that orbital path without expending any thrust. ย To change between orbits, it is necessary for the spacecraftโrocket or otherwiseโto change its velocityโtypically referred to as delta-vโby firing an engine or thruster. Itโs also possible to change orbits using the gravity of other celestial bodies (Jupiter is a popular one) to change a spacecraftโs delta-v without expending propellant. However, fluid dynamics donโt play a big role in the process aside from the problems of fuel sloshing aboard the spacecraft and the actual mechanism by which thrust is produced.
That said, if anyone is interested in getting a better feel for how orbit mechanics work, I have two recommendations. ย The first is to watch this videoย of water droplets โorbitingโ a charged knitting needle aboard the ISS. And the second is to play the game Osmos. It is like rocket propulsion and orbit mechanics in action!
(Photo credits: NASA, The Aerospace Corporation, Hemisphere Games)
Freezing in a Microchannel
Fluid mechanics at the microscale can behave quite differently than in our everyday experience. Microfluidic devicesโsometimes known as labs on a chipโare becoming increasingly important in research and daily life. For example, the test strips used by diabetics to check their blood sugar levels are microfluidic devices. ย In this video, researchers use a microfluidic channel to observe the freezing of supercooled water droplets. As the droplet first passes into the cold zone of the channel, it flash freezes, filling from the inside out with ice crystals. As it continues through the cold zone, the drop freezes fully, beginning at the outside surface and working inward. As it does so, the ice droplet fractures due to stresses. (Video credit: Stan et al)
Hot Spheres Sink Faster
New research shows that the Leidenfrost effectโwhich causes water droplets to skitter across a hot panโcan drastically reduce the drag on objects moving through a liquid. When raised to a high enough temperature, a sphere falling water will be coated in a protective layer of vapor (see video above) that acts like a lubricant as the sphere moves through the water. If the temperature of the object drops too low, the vapor layer will dissolve into a mess of bubbles (~35 secs into video). One way that this mechanism reduces drag is by keeping flow attached to the sphere for longer as shown inย this video. Preventing this flow separation increases the pressure recovered after the point of lowest pressure (the shoulders of the sphere), which reduces overall drag.
See also:
- PRL Article and Supplemental Materials
- Wired article
- The Photonist
Happy Anniversary
ESA astronaut Pedro Duque shown refracted through a water droplet in microgravity. Today marks the 50th anniversary of human space flight. #
Frosting on Superhydrophobic Surfaces
Icing on airplane wings can be disastrous for lift and control, and thus how ice initially forms on a wing is an active area of research. New work shows that superhydrophobic (water-fearing) surfaces may actually promote ice buildup. Superhydrophobic surfaces are prone to frostingโcollecting ice that forms directly from a vaporous stateโand that fine layer of frost is conducive to further ice buildup from a liquid state. The photo above shows a water droplet striking a dry superhydrophobic surface (top) and a frosted superhydrophobic surface (bottom). (via Gizmodo) #
Airplanes Creating Snow
Scientists now think that that airplanes may be responsible for increasing local snowfall by flash-freezing supercooled water vapor in clouds. Water droplets can persist in the atmosphere to temperatures of -42 degrees Celsius. But when an airplaneโs wing passes through moist air, the acceleration of the air passing over the wing causes a pressure decrease that can drop the temperature by as much as 19 C, causing the water droplets to form ice crystals immediately. (The particulate matter in the aircraft exhaust probably also aids this process.) The same behavior can also create holes in clouds and cause ice to form on the wings. #ย (Related behavior: vapor cones)
Photo credit: lhoon
Superhydrophobic Carbon Nanotubes
Carbon nanotubes form a superhydrophobic (super water repellent) surface that interacts with water droplets in interesting ways. The droplet is unable to wet the surface and thus the bounces along. When the impact velocities are too great for surface tension to hold the decelerating mass together, it breaks into many, smaller droplets that also bounce along the surface. # (via @JetForMe and @Vinnchan)
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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