Simulations show two droplets hitting a micro‑structured hydrophobic surface bounce differently depending on spacing and angle. This dynamic is useful for surface engineering.

🔗 https://pubs.aip.org/aip/pof/article-abstract/38/2/027133/3380441/Dynamics-of-double-droplet-centered-impacting-a

#dropletimpact #hydrophobic #leidenfrosteffect #fluidmechanics #microstructure

Day 1 recap at #LeidenForce Winter School:

🔹David Quéré (ESPCI): overview of Leidenfrost regimes and phenomena.

🔹Detlef Lohse @utwente : Leidenfrost impacts, jet formation, impact phases & key parameters.

#Leidenfrost #WinterSchool #FluidDynamics #DropletImpact #Physics

Charged Drops Don’t Splash

When a droplet falls on a surface, it spreads itself horizontally into a thin lamella. Sometimes — depending on factors like viscosity, impact speed, and air pressure — that drop splashes, breaking up along its edge into myriad smaller droplets. But a new study finds that a small electrical charge is enough to suppress a drop’s splash, as seen below.

The drop’s electrical charge builds up along the drop’s surface, providing an attraction that acts somewhat like surface tension. As a result, charged drops don’t lift off the surface as much and they spread less overall; both factors inhibit splashing.* The effect could increase our control of droplets in ink jet printing, allowing for higher resolution printing. (Image and research credit: F. Yu et al.; via APS News)

*Note that this only works for non-conductive surfaces. If the surface is electrically conductive, the charge simply dissipates, allowing the splash to occur as normal.

#dropletImpact #droplets #electricalField #electrohydrodynamics #fluidDynamics #lamella #physics #science #splashes #splashing

Hot Droplets Bounce

In the Leidenfrost effect, room-temperature droplets bounce and skitter off a surface much hotter than the drop’s boiling point. With those droplets, a layer of vapor cushions them and insulates them from the hot surface. In today’s study, researchers instead used hot or burning drops (above) and observed how they impact a room-temperature surface. While room-temperature droplets hit and stuck (below), hot and burning droplets bounced (above).

In this case, the cushioning air layer doesn’t come from vaporization. Instead, the bottom of the falling drop cools faster than the rest of it, increasing the local surface tension. That increase in surface tension creates a Marangoni flow that pulls fluid down along the edges of the drop. That flow drags nearby air with it, creating the cushioning layer that lets the drop bounce. In this case, the authors called the phenomenon “self-lubricating bouncing.” (Image and research credit: Y. Liu et al.; via Ars Technica)

#bouncingDroplets #dropletImpact #entrainment #fluidDynamics #marangoniEffect #physics #science

Drops on the Edge

Drops impacting a dry hydrophilic surface flatten into a film. Drops that impact a wet film throw up a crown-shaped splash. But what happens when a drop hits the edge of a wet surface? That’s the situation explored in this video, where blue-dyed drops interact with a red-dyed film. From every angle, the impact is complex — sending up partial crown splashes, generating capillary waves that shift the contact line, and chaotically mixing the drop and film’s liquids. (Video and image credit: A. Sauret et al.)

#2024gofm #crownSplash #dropletImpact #droplets #flowVisualization #fluidDynamics #fluidsAsArt #physics #science #wetting

Non-Newtonian Raindrops

Fluids like air and water are called Newtonian because their viscosity does not vary with the force that’s applied to them. But many common fluids — almost everything in your fridge or bathroom drawer, for example — are non-Newtonian, meaning that their viscosity changes depending on how they’re deformed.

Non-Newtonian droplets can behave very differently than Newtonian ones, as this video demonstrates. Here, their fluid of choice is water with varying amounts of silica particles added. Depending on how many silica particles are in the water, the behavior of an impacting drop varies from liquid-like to completely solid and everything in between. Why such a great variation? It all has to do with how quickly the droplet tries to deform and whether the particles within it can move in that amount of time. Whenever they can’t, they jam together and behave like a solid. (Image, video, and research credit: S. Arora and M. Driscoll)

#2019GOFM #deformation #dropletImpact #fluidDynamics #jamming #nonNewtonianFluids #physics #science

When a raindrop hits a leaf, it spreads out into a rimmed sheet that breaks up into droplets. These tiny drops can carry dust, spores, and even pathogens as they fly off. But many leaves aren’t smooth-edged; instead they have serrations or teeth. How does that affect a splash? That’s the question at the heart of today’s study.

To simplify from a leaf’s shape, the team studied water dropping onto star-shaped pillars. As seen above and below, the pillar’s edge shaped the splash sheet, with the sheet extending further in the edge’s troughs. This asymmetry extends into the rim also, concentrating the liquid — and the subsequent spray of droplets — along lines that extend from the edge’s troughs and peaks.

The team found that, in addition to sending drops along a preferred direction, the shaped edge made the droplets larger and faster than a smooth edge did. (Image and research credit: T. Bauer and T. Gilet)

https://fyfluiddynamics.com/2024/09/shaped-splashes/

#dropletEjection #dropletImpact #droplets #flowVisualization #fluidDynamics #physics #science #splashes