FYFD

Bubbles in industrial applications are often more complicated than a simple pocket of air surrounded by water. Here researchers investigate the formation of an air bubble coated in oil before it rises through water. (Image and research credit: B. Ji et al.)

When a tire spins over a wet roadway, pressure at the front of the tire generates a lifting force; if that lift exceeds the weight of the car, it will start hydroplaning. To prevent this, the grooves of a tire's tread are designed to redirect the water. (Image credit: tires - S. Warid, others - D. Cabut et al.; research credit: D. Cabut et al.; via Physics World; submitted by Kam-Yung Soh)

Today, let's take a look at the fluid dynamics of disease transmission, an important aspect of the current coronavirus pandemic. (Video credit: TEDMED; image credit: Bourouiba Research Group, source; research credit: L. Bourouiba et al., see also S. Poulain and L. Bourouiba, pdf)

Sneezing can be a major factor in the spread of some illnesses. Not only does sneezing spew out a cloud of tiny pathogen-bearing droplets, but it also releases a warm, moist jet of air. Flows like this that combine both liquid and gas phases are called multiphase flows, and they can be a challenge to study because of the interactions between the phases. For example, the buoyancy of the air jet helps keep smaller droplets aloft, allowing them to travel further or even get picked up and spread by environmental systems. Researchers hope that studying the fluid dynamics and mathematics of these turbulent multiphase clouds will help predict and control the spread of pathogens. Check out the Bourouiba research group for more. (Video credit: Science Friday)

Sneezing and coughing are major contributors to the spread of many pathogens. Both are multiphase flows, consisting of both liquid droplets and gaseous vapors that interact. The image on the left shows a sneeze cloud as a turbulent plume. The kink in the cloud shows that plume is buoyant, which helps it remain aloft. The right image shows trajectories for some of the larger droplets ejected in a sneeze. Like the sneeze cloud, these droplets persist for significant distances. The buoyancy of the cloud also helps keep aloft some of the smaller pathogen-bearing droplets. Researchers are building models for these multiphase flows and their interactions to better predict and counter the spread of such airborne pathogens. For similar examples of fluid dynamics in public health, see what coughing looks like, how hospital toilets may spread pathogens, and how adjusting viscoelastic properties may counter these effects. For more about this work, see the Bourouiba research group's website. (Image credit: L. Bourouiba et al.)

Three basic components are necessary for a geyser: water, an intense geothermal heat source, and an appropriate plumbing system. In order to achieve an explosive eruption, the plumbing of a geyser includes both a reservoir in which water can gather as well as some constrictions that encourage the build-up of pressure. A cycle begins with geothermally heated water and groundwater filling the reservoir. As the water level increases, the pressure at the bottom of the reservoir increases. This allows the water to become superheated--hotter than its boiling point at standard pressure. Eventually, the water will boil even at high pressure. When this happens, steam bubbles rise to the surface and burst through the vent, spilling some of the water and thereby reducing the pressure on the water underneath. With the sudden drop in pressure, the superheated water will flash into steam, erupting into a violent boil and ejecting a huge jet of steam and water. For more on the process, check out this animation by Brian Davis, or to see what a geyser looks like on the inside, check out Eric King's video. (Video credit: Valmurec; idea via Eric K.)

A labyrinthine pattern forms in this timelapse video of a multiphase flow in a Hele-Shaw cell. Initially glass beads are suspended in a glycerol-water solution between parallel glass plates with a central hole. Then the fluid is slowly drained over the course of 3 days at a rate so slow that viscous forces in the fluid are negligible. As the fluid drains, fingers of air invade the disk, pushing the beads together. The system is governed by competition between two main forces: surface tension and friction. Narrow fingers gather fewer grains and therefore encounter less friction, but the higher curvature at their tips produces larger capillary forces. The opposite is true of broader fingers. Also interesting to note is the similarity of the final pattern to those seen in confined ferrofluids.  (Video credit and submission: B. Sandnes et al. For more, see B. Sandes et al.)

Varying the rate of injection of air into a wet granular mixture contained in a Hele Shaw cell results in very different flow patterns. At low injection rates, stick-slip bubbles form. As the injection rate increases, patterns are affected by "temporal intermittency" where continuous motion is occasionally interrupted by jamming. Increasing the injection rate still further results in Saffman-Taylor-like fingering. #