Video Prizes for Odor Plumes and Quantum Imposters

Dilip Maity recalls being “rained on” when childhood friends would shake a wet tree branch above his head as a prank. “I always wondered how such a tiny shake or gust of wind could suddenly [release] a droplet sitting so peacefully on a leaf,” he says. Now a postdoc at King Abdullah University of Science and Technology in Saudi Arabia, he and his colleagues have studied the process by which a drop is liberated from a wire when the wire is moved rapidly upward, mimicking what occurs when the wind shakes a tree or a spider web after a rainstorm. There is little information on the process, since most previous work has examined droplets sliding slowly on fibers.

The team discovered several new behaviors: Rather than elongating into a simple tube of liquid, the drop spreads into a thin sheet of fluid stretched between two fatter “lips” at the two edges of the sheet. As the drop and wire separate, these so-called rims are pulled together, generating a long, thin jet above the drop. The researchers determined the effects of viscosity, surface tension, and wire motion on the separation process. They hope that an improved understanding of drop separation will be useful for studies of the spread of pathogens across agricultural fields and of fog harvesting nets, which collect water from condensation in areas where water is scarce.

Many insects use scents to find food and mates, and researchers want to learn exactly how they navigate through complex odor plumes. As part of that research, PhD student Elle Stark of the University of Colorado, Boulder, and her colleagues are studying the interactions between a chaotic breeze and an odor plume, such as that emanating from a flower as it lures in bees or other pollinators. The team has produced the first data set that includes both the detailed, wispy tracks of a plume and the simultaneous velocity field of the surrounding air under natural conditions. The high-resolution data required two different types of imaging—one measuring the concentration of an acetone vapor plume and the other measuring the velocity fluctuations of the surrounding mildly turbulent air.

The combination allowed the team to directly observe the way in which patterns in the airflow drive the swirls and folds of odor filaments. Stark and her colleagues expect their datasets to provide important inputs for studies of insect behavior in response to such odor plumes. They say that the work could also benefit applications such as explosive detection and gas-leak-source identification.

Quantum particles can interact like waves, but sometimes classical particles can too. Millimeter-scale fluid droplets bouncing on a vibrating fluid generate surface waves that allow them to interact with other droplets at a distance. These interacting droplets can mimic certain quantum behaviors. But they have a habit of coalescing when they touch, which precludes quantum analogue experiments studying particles interacting at close range. Now undergraduate student Joseph Clampett of the University of North Carolina (UNC) at Chapel Hill and his colleagues have shown that running the experiments under high air pressure prevents coalescence and allows observations of new phenomena.

The team tracked the motions of many droplets placed on a vibrating fluid surface inside a circular corral. The pressure was raised to 10 atmospheres, allowing the droplets to continuously scatter and interact with one another through their wave fields for several hours. The average density (the spatial probability distribution) over the duration of the experiment exhibited two concentric rings as maxima, as would be expected for a quantum system, such as a Bose-Einstein condensate. Team leader Pedro Sáenz of UNC-Chapel Hill says that working with these droplets under pressure can help researchers better understand the crossover between classical and quantum regimes.