Destin from Smarter Every Day has a great new video exploring acoustic levitation. With carefully placed speakers, you can create a standing wave with sound that’s capable of levitating lightweight objects against the force of gravity. Around 4:00, Destin demonstrates this with colored water droplets, which is where the real fireworks start. As he turns up the volume on the speakers, the big droplets explode. This happens when surface tension can no longer hold the drop together. But the high-speed footage offers other clues about what’s going on. Notice how the drops flatten out as the sound volume increases. If you look back to the standing wave animation at 1:33, you’ll notice that just to either side of the nodes (the spots that don’t move), the wave is still oscillating back and forth a little bit. As you increase the sound volume, that standing wave gets stretched to a larger amplitude, which means that those little oscillations just to either side of the node get stronger (and steeper), too. This change in acoustic pressure squishes the drops into pancakes as the fluid tries to stay right at the node. Eventually the droplet is just too flattened for surface tension to keep it together and it bursts into smaller droplets. (Video credit: Smarter Every Day; submitted by Matthew P.)
Month: May 2015
The Free Surface of a Typhoon
Gazing across the top of of Typhoon Maysak highlights the three-dimensionality of the storm. Like a swirling vortex seen in a bathtub, hurricanes are a kind of free surface vortex with a surface indentation near their eye. To understand this shape, imagine spinning a container of water on a rotating plate. Like the vortex, the water’s surface would take on a parabolic shape. The two forces acting on the rotating water are gravity in the downward direction and centrifugal force in the radial direction. By taking on a parabolic shape, the fluid remains perpendicular to the combination of these two forces at every point along the surface, thereby ensuring that pressure is a constant across the free surface of the fluid. (Image credits: S. Cristoferreti/ESA/NASA; T. Virts/NASA)
Drops on a Porous Surface
The splashing of a drop upon impact is a remarkably complicated phenomenon. Perhaps surprisingly, the air around the impacting drop plays a major role in determining which drops splash and which don’t. Lowering the air pressure, for example, stops a drop from splashing. The layer of air that gets trapped beneath the spreading edge of a drop during impact seems to be responsible for splashing. As seen in the video above, drops that impact on a leaky surface, where air can escape, do not splash. By varying where leakage is possible on the surface, the researchers can localize where trapping the air matters most. There’s a critical radius during the drop’s spread where, without leakage, air will be trapped and cause the drop to splash. (Video credit: Y. Liu et al.)
