This timelapse video shows a particulate suspension as it dries and the pattern formation that results. The mixture of silicon dioxide particles and water is spread over a glass slide. As the water evaporates, capillary action generates a flow toward the edges, but the fluid meniscus pins larger particles to the glass, trapping them. As more and more water evaporates, smaller particles are trapped, causing the formation of uneven stripes in the particulate deposits. You’ve probably seen these patterns before on the side of a muddy car after a rainy day! (See also: how coffee rings form; Video credit: Bjornar Sandnes)
Tag: science
Worthington Jet
A drop of sugar syrup falls into a pool of methylated spirits, producing a Worthington jet and several ejected droplets. Although surface tension holds the jet in a smooth shape, the refractive index of the spirits reveals the turbulent mixing within the jet. (Photo credit: Rebecca Ing)
Fragmenting Raindrops
This numerical simulation demonstrates the fragmentation of droplets of water falling through a quiescent medium–essentially how a raindrop behaves. As the initial droplet falls, drag forces deform the droplet, contorting it until surface tension causes it to break into smaller droplets, which can themselves be broken up by the same mechanisms.
Star-Shaped Nozzles
Efficient mixing of fluids is vital for many applications, including fuel injection for all types of combustion and masking the exhaust of stealth fighters. Star-shaped lobed nozzles can produce jets that mix more effectively than conventional jets. This photo shows cross-sections of the jet at several downstream distances from the nozzle exit. (Photo credit: H. Hu et al)
Breakup of an Annular Sheet
A thin annular sheet of water is sandwiched between two concentric air streams. This airflow on either side of the water causes shearing and Kelvin-Helmholtz-type instabilities develop, causing the sinuous waves along the water surface. Periodic behavior of the sort observed here is frequently observed in fluid mechanical instabilities. #
Viscous Fluid Falling on a Moving Belt
In this video a very viscous (but still Newtonian) fluid is falling in a stream onto a moving belt. Initially, the belt is moving quickly enough that the viscous stream creates a straight thread. As the belt is slowed, the stream begins to meander sinusoidally and ultimately begins to coil. Aside from some transient behavior when the speed of the belt is changed very quickly, the behavior of the thread is very consistent within a particular speed regime. This is indicative of a nonlinear dynamical system; each shift in behavior due to the changing speed of the belt is called a bifurcation and can be identified mathematically from the governing equation(s) of the system. (Video credit: S. Morris et al)
Separation and Stall
This flow visualization of a pitching wind turbine blade demonstrates why lift and drag can change so drastically with angle of attack. When the angle the blade makes with the freestream is small, flow stays attached around the top and bottom surfaces of the blade. At large (positive or negative) angles of attack, the flow separates from the turbine blade, beginning at the trailing edge and moving forward as the angle of attack increases. The separated flow appears as a region of recirculation and turbulence. This is the same mechanism responsible for stall in aircraft. (Submitted by Bobby E)
Splash Sheets
When a falling liquid jet hits a horizontal impacter, it is deflected into a sheet. The shape of the sheet is dependent upon the velocity of the jet and the viscosity of the fluid. At sufficiently high speeds the sheet will be circular; at lower speeds it may sag into a bell-shape. The circular sheets can also develop an instability that causes them to become polygonal, as shown in the photos above. The fluid then flows out along the sheet, into and along the rim, and then spouts outward in jets at the polygon’s corners. For some conditions, the jets at the corners even form a sort of fluid chain (top photo). (Photo credit: R. Buckingham and J. W. M. Bush; via 14-billion-years-later)
Viscoelastic Fluids in Space
In honor of astronaut Don Pettit’s launch to the International Space Station (and in the hope that he’ll do more neat microgravity fluids demonstrations while in space!), here’s a look a the behavior of viscoelastic fluids in microgravity. The elasticity of these fluids means that, when strained, the fluid deforms instantaneously and then returns to its initial shape when the strain is removed. Pettit demonstrates both Plateau-Rayleigh instability behavior, where a column of fluid breaks apart due to surface tension variations, and die swell, where a fluid jet expands beyond the diameter of nozzle from which it was extruded. Such swelling is commonly caused by the stretching and relaxation of polymers in the fluid as they react to forces caused by the nozzle opening.
Wave Clouds Over Alabama
Last week, Birmingham, Alabama got treated to a special cloudy day, thanks to some Kelvin-Helmholtz waves, shown above. When a layer of faster moving fluid shears a slower moving fluid, this instability can form and cause some spectacular mixing. In this case, the lower, slower fluid was cool and moist enough to contain clouds, enabling us to see the effect with the naked eye. The same mechanism is responsible for the shape of breaking ocean waves and can even be seen in the atmospheres of gas giants like Saturn and Jupiter. (submitted by David B)
