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Remote control of heat nanosources motion and thermal-induced fluid flows by using light forces

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Remote control of heat nanosources motion and thermal-induced fluid flows by using light forces
a, Multiple gold NPs (spheres of 200 nm radius) are confined by a ring-shaped laser trap (wavelength of 532 nm) and optically transported around it. These NPs rapidly assemble into a stable group of hot particles creating a confined heat source (G-NP) of temperature ~500 K. Free (not trapped) gold NPs acting as tracer particles are dragged toward the G-NP by the action of the thermal-induced water flow created around it (see Video S5 of the paper). The speed of the G-NP is controlled by the optical propulsion force which is proportional to the phase gradient strength tailored along the laser trap as displayed in b, corresponding to the transport state 1. This non-uniform propulsion force drives the G-NP reaching a maximum speed of 42 μm/s. b, Sketch of the switching of the phase gradient configuration (state 1 and 2) enabling a more sophisticated manipulation of the heat source: split and merge of the G-NP. (c), The opposite averaged propulsion forces in the split region (see state 3 at ~0 deg, shown in b) separate the NPs belonging to the original G-NP thus creating G-NP1 and G-NP2, as observed in the displayed sequence (see Video S6 of the paper). These two new heat sources are propelled by the time averaged propulsion force corresponding to state 3 in opposite directions toward the region where they finally merge into a joint G-NP again. Complex transport trajectories for G-NP delivery, for example in form of knot circuit (see Video S7 of the paper), can be created enabling spatial distribution of moving heat sources across a target network Credit: José A. Rodrigo, Mercedes Angulo and Tatiana Alieva

Today, optofluidics is one of the most representative applications of photonics for biological/chemical analysis. The ability of plasmonic structures (e.g., colloidal gold and silver nanoparticles, NPs) under illumination to release heat and induce fluid convection at the micro-scale has attracted much interest over the past two decades. Their size- and shape-dependent as well as wavelength-tunable optical and thermal properties have paved the way for relevant applications such as photothermal therapy/imaging, material processing, biosensing and thermal optofluidics to name a few. In-situ formation and motion control of plasmon-enhanced heat sources could pave the way for further harnessing of their functionalities, especially in optofluidics. However, this is a challenging multidisciplinary problem combining optics, thermodynamics and hydrodynamics.


In a recent paper published in Light Science & Applications, Professor Jose A. Rodrigo and co-workers from Complutense University of Madrid, Faculty of Physics, Department of Optics, Spain, have developed a technique for jointly controlling the formation and motion of heat sources (group of gold NPs) as well as of the associated thermal-induced fluid flows created around them. The scientists summarize the operational principle of their technique, “The technique applies a structured laser-beam trap to exert an optical propulsion force over the plasmonic NPs for their motion control, while the same laser simultaneously heats up them. Since both the shape of the laser trap and the optical propulsion forces are easily and

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science 

Wildfires can cause dangerous debris flows

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Wildfires can cause dangerous debris flows
Time lapse images of a 2019 debris flow in the burn scar of the Holy Fire near Lake Elsinore. Credit: James Guilinger/UCR

Wildfires don’t stop being dangerous after the flames go out. Even one modest rainfall after a fire can cause a deadly landslide, according to new UC Riverside research.


“When fire moves through a watershed, it creates waxy seals that don’t allow water to penetrate the soil anymore,” explained environmental science doctoral student and study author James Guilinger.

Instead, the rainwater runs off the soil surface causing debris flows, which are fast-moving landslides that usually start on steep hills and accelerate as they move.

“The water doesn’t behave like water anymore, it’s more like wet cement,” Guilinger said. “It can pick up objects as big as boulders that can destroy infrastructure and hurt or even kill people, which is what happened after the 2018 Thomas fire in Montecito.”

Guilinger and his team of mentors and collaborators wanted to understand in detail how multiple storm cycles affect an area that’s been burned by wildfire, since Southern California tends to have much of its rain in the same season.

The team headed to the burn scar caused by the 23,000-acre Holy Fire near Lake Elsinore to observe this phenomenon, and their results have recently been published in the Journal of Geophysical Research: Earth Surface.

“It’s only recently that technology has advanced to the point that we can directly monitor soil erosion at extremely small scales,” said Andrew Gray, assistant professor of watershed hydrology and Guilinger’s advisor. Gray’s laboratory works to understand how wildfire impacts the movement of water and sediment through landscapes after wildfire.

Wildfires can cause dangerous debris flows
House damaged by debris flows generated in Los Angeles County’s Mullally Canyon in response to a rainstorm on February 6, 2010. Credit: Susan Cannon/USGS

Even with the latest technology, the data was not easy to obtain. To deploy their ground-based laser scanner, which uses visible and infrared waves to reconstruct surfaces down to millimeter accuracy, the scientists had to climb steep hill slopes. They also deployed drones in collaboration with Nicolas Barth, assistant professor of geomorphology, in order to zoom out and see up to 10 hectares of land after the storms.

What they found is that most of the soil in channels at the bottom of valleys between hill slopes eroded during the first few rains, even though the rains were relatively modest. The channels fill with material during the years between fires as well as in response to fire, with rain then causing rapid erosion resulting in the debris flows.

“This proves the first storm events that strike an area are the most critical,” Guilinger said. “You can’t really mitigate them at the source. Instead, people downstream need to be aware of the dangers, and land managers need hazard modeling tools to help them respond effectively and create a plan to catch the sediment as it flows.”

U.S. Geological Survey models incorporate widely available 10-meter data for watershed slopes and information about burn severity

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