Geological evidence shows that millions of years ago, the Mediterranean Sea nearly dried out. In fluid mechanics, we’d describe this problem using one of our fundamental equations: conservation of mass, also known as continuity.
Imagine a volume containing the entire Mediterranean. To describe the amount of sea water in that volume, you need to keep track of two major quantities: how much water is flowing into the volume and how much is leaving it. For the prehistoric (as well as today’s) Mediterranean, the sources feeding the sea are 1) an inflow from the Atlantic through the Strait of Gibraltar; 2) inflows from rivers; and 3) rainfall. Water is lost primarily to evaporation.
As explained in the video, the Mediterranean’s dry spell was heralded by tectonic changes that sealed the Strait of Gibraltar, depriving it of its largest source of inflow. At the same time, warmer temperatures and less rainfall reduced influx from rivers and the atmosphere while increasing evaporation rates. The result? Water levels in the Mediterranean dropped by hundreds of meters, creating massive salt deposits, wiping out native marine life, and allowing mass migration by land-dwelling animals. Eventually, though, the Strait re-opened, creating what might have been a massive flood. (Video and image credits: PBS Eons)
Acoustic levitation and optical tweezers both use waves — of sound and light, respectively — to trap and control particles. Water waves also have the power to move and capture objects, as shown in this award-winning poster from the 2019 Gallery of Fluid Motion. The central image shows a submerged disk, its position controlled by the arc-shaped wavemaker at work on the water’s surface. The complicated pattern of reflection and refraction of the waves we see on the surface draws the disk to a focal point and holds it there.
On the bottom right, a composite image shows the same effect in action on a submerged triangular disk driven by a straight wavemaker. As the waves pass over the object, they’re refracted, and that change in wave motion creates a flow that pulls the object along until it settles at the wave’s focus. (Image and research credit: A. Sherif and L. Ristroph)
Getting enough water in arid climates can be tough, but Western diamondback rattlesnakes have a secret weapon: their scales. During rain, sleet, and even snow, these rattlesnakes venture out of their dens to catch precipitation on their flattened backs, which they then sip off their scales.
Researchers found that impacting water droplets tend to bead up on rattlesnake scales, forming spherical drops that the snake can then drink. Compared to other desert-dwelling snakes, Western diamondbacks have a far more complicated microstructure to their scales, with labyrinthine microchannels that provide a sticky, hydrophobic surface for impacting drops. (Video and image credit: ACS; research credit: A. Phadnis et al.; via The Kid Should See This)
The splash of a drop impacting a surface depends on many factors — among them droplet speed and size, air pressure, and surface characteristics. In this award-winning video from the 2019 Gallery of Fluid Motion, we see how the geometry of a superhydrophobic surface can alter a splash.
When a drop falls on a protruding superhydrophobic surface, like the apex of a cone, it can be pierced from the inside, completely changing how the droplet rebounds and breaks up. The variations the video walks us through are all relatively simple, but resulting splashes may surprise you nevertheless. (Image and video credit: The Lutetium Project)