In this video, the HouseholdHacker heads to the kitchen and uses milk, food coloring, and dish soap to create some solutal convection much like this one with cream and liqueur. The food coloring serves as a tracer for the fluid motion; it’s really the interaction of the milk and the dish soap that drives the motion. The dish soap lowers the surface tension of the milk, causing motion via the Marangoni effect. That motion redistributes some of the dish soap as well, causing further motion in the form of a surface tension instability. As with the cream and liqueur experiment, the fat in the milk is important; you won’t see much (if any) effect with fat free milk because its surface tension properties aren’t as dissimilar to dish soap. (via misterhonk.de)
Category: Phenomena
Marangoni Convection in Space
In this Saturday Morning Science video, astronaut Don Pettit demonstrates Marangoni convection in microgravity using a water film with tracer particles, a soldering iron, and a flashlight. This same effect occurs on earth but is masked behind the much stronger effect of buoyant convection.
Vortex Street
A flow visualization behind a cylinder shows the formation of a von Karman vortex street. The frequency of vortex shedding in the wake is directly related to the speed of the airflow–the higher the velocity, the faster vortices will shed from the cylinder. This relationship is expressed in the Strouhal number, which remains constant for any cylinder. (via freshphotons)
Waves on Cornstarch
A thin layer of the non-Newtonian fluid oobleck on a vibrating surface (in this case, a speaker) is a great way to show off nonlinear standing waves known as Faraday waves. The waves form because, under these circumstances, the flat surface of the air/oobleck interface has actually become unstable.
Thermal Convection
This video turbulent convection in a vertical channel. Buoyancy and the density variations caused by small differences in temperature are what drive the behavior.
The Roaming Rocks of Death Valley
The dry lake beds of Death Valley National Park in California are home to a perplexing phenomenon: roaming rocks. These rocks, some of which weigh hundreds of pounds, leave long furrows in the dirt but have no obvious means of propulsion. One theory posits that the rocks glide on collars of ice around their base. The ice acts like a flotation device when rain wets the valley and then the rocks slide so easily that high winds can move them across the surface. #
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 airplane’s 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. # (Related behavior: vapor cones)
Photo credit: lhoon
Water Drops at 10,000 FPS
We’ve seen water droplets join a larger pool at 2,000 frames per second, but what about 10,000 frames per second? (via Gizmodo)
Mach Diamonds
Joe asks:
Why does this rocket have that repeating pattern in its exhaust? I’m amazed that it’s so stable for so far as distance from the nozzle.
Excellent question! The diamond-shaped pattern seen in the rocket’s exhaust is actually a series of reflected shock waves and expansion fans. The rocket’s nozzle is designed to be efficient at high altitudes, which means that, at its nominal design altitude, the shape of the nozzle is such that the exhaust gases will be expanded to the same pressure as the ambient atmosphere. At sea level, the nozzle is overexpanded, meaning that the exhaust gases have been expanded to a lower pressure than the ambient. The supersonic exhaust has to reach ambient pressure, and it does so through an oblique shock right at the exit of the nozzle. However, the oblique shock, in addition to raising the pressure, turns the gases toward the exhaust centerline. To ensure flow symmetry, two additional oblique shocks form. But then the exhaust is at a higher pressure than ambient. Expansion fans form to reduce the pressure, but those, too, affect the direction the exhaust gases flow. The pattern, then, is a series of progressively weaker oblique shocks and expansion fans that raise the exhaust gas pressure to that of the ambient atmosphere.
Crown Breakup
When a droplet falls into a pool of similar fluid, one often observes a crown-like impact effect. This student video shows high-speed footage of different fluids crowning and explores the effects of surface tension on crown breakup.
