Making a Star-Shaped Droplet

We usually think of surface tension turning droplets into spheres in order to minimize their area. But spheres aren’t the only shape surface tension can enforce. Here, researchers suspend tiny droplets of oil in a soapy fluid. At the right temperature, these droplets form a crystalline surface while the fluid within remains liquid. As in the fully liquid droplet, surface tension tries to minimize the shell’s surface energy, enabling it to take on many different shapes.

In this study, researchers demonstrate that the shell-enclosed droplets can even change, reversibly, from a hexagon to a six-pointed star and back. The transformation is shown above, in an experiment that gradually changes the droplet’s temperature–and, thus, its surface tension.

Although shape changes similar to these have been described before, this experiment was the first where the shell’s defects–the vertices of the hexagon–don’t shift during the transformation. (Video, image, and research credit: C. Quilliet et al.; via APS)

A Bubbly Heart

Next time you fill your water bottle, watch closely and see if you can spot a bubble heart like these. When a jet falls into a pool, it pulls air in with it. The low pressure of the jet pulls bubbles inward, even as shear pulls the bubbles downward with the sinking liquid. If the bubbles are large and there’s enough momentum in the jet, the lower portion of the bubble will get pulled into a conical shape, while the upper portion remains a hemisphere. That forms one lobe of the heart. The other half requires a second bubble. But with a little patience and luck, you can form a complete heart. Happy Valentine’s Day! (Image credit: S. Tuley et al.)

- Capillary number: balancing viscosity and surface tension -

The capillary number quantifies how viscosity and surface tension interact in droplets. Explore this key concept in the ESPCI Paris - PSL MOOC video.

🎥 https://www.youtube.com/watch?v=bbEOcd977ec&list=PLcbz7zf4dTyk9BqlBPLpgI48i9TiorpEi&index=12

#CapillaryNumber #Viscosity #SurfaceTension #FluidPhysics #LeidenfrostEffect

Caught in a Spider’s Web

Grains of pollen are caught amid droplets on a spider’s web in this award-winning image by John-Oliver Dum. How droplets behave on fibers has been a popular topic in recent years with research on how droplets nestle into corners, how they slide on straight or twisted wires, the patterns formed by streams of falling drops, and what happens to a droplet on a plucked string. (Image credit: J. Dum; via Ars Technica)

Superwalking Droplets

When placed on a vibrating oil bath, droplets have many wild behaviors, some of which mirror quantum mechanics. Even big droplets — bigger than 2 millimeters in diameter — can get in on the fun. This video shows several of these “jumbo superwalkers” in action, both singly and in groups. (Video and image credit: Y. Li and R. Valani; via GFM)

Marangoni Effect in Biology

For decades, biologists have focused on genetics as the key determiner for biological processes, but genetic signals alone do not explain every process. Instead, researchers are beginning to see an interplay between genetics and mechanics as key to what goes on in living bodies.

For example, scientists have long tried to unravel how an undifferentiated blob of cells develops a clear head-to-tail axis that then defines the growing organism. Researchers have found that, rather than being guided purely by genetic signals, this stage relies on mechanical forces–specifically, the Marangoni effect.

The image above shows a mouse gastruloid, a bundle of stem cells that mimic embryo growth. As they develop, cells flow up the sides of the gastruloid, with a returning downward flow down the center. This is the same flow that happens in a droplet with higher surface tension in one region; the Marangoni effect pulls fluid from the lower surface tension region to the higher one, with a returning flow that completes the recirculation circuit.

The same thing, it turns out, happens in the gastruloid. Genes in the cells trigger a higher concentration of proteins in one region of the bundle, creating a lower surface tension that causes tissue to flow away, helping define the head-to-tail axis. (Image credit: S. Tlili/CNRS; research credit: S. Gsell et al.; via Wired)

A Soft Cell in Microgravity

There are many shapes that can be tiled to fill space, but nearly all of them have sharp corners. Last year, mathematicians identified a new class of shapes, known as “soft cells,” that feature curved edges and faces but very few sharp corners. Like traditional polyhedrals, soft cells can tile to fill a space completely without overlapping or gapping.

Now the researchers, with some help from astronauts aboard the ISS, have brought one of their soft cells to life. Using an edge skeleton to guide the shape, astronaut Tibor Kapu filled the skeleton with water, which, in microgravity, formed a perfect soft cell, complete with faces curved by surface tension to their minimal area. See it in action below. (Image and video credit: HUNOR/NASA; research credit: G. Domokos et al.; via Oxford Mathematics)

https://www.youtube.com/shorts/EyMbqPUKl80

Marangoni Bursting With Surfactants

A few years ago, researchers described how an alcohol-water droplet atop an oil bath could pull itself apart through surface tension forces. Dubbed Marangoni bursting, this phenomena has shown up several times since. Here, researchers explore a twist on the behavior by adding surfactants to see how they affect the bursting phenomenon. (Video and image credit: K. Wu and H. Stone; via GFM)

#flowVisualization #fluidDynamics #instability #MarangoniBursting #physics #science #surfaceTension #surfactant

- Superhydrophobicity: when droplets slide -

Superhydrophobic surfaces repel water and promote drop sliding – key to understanding the Leidenfrost effect. To see in this video from the "Dynamics of fluid interfaces" MOOC by ESPCI Paris - PSL.

🎥 https://www.youtube.com/watch?v=T8AFO2iXsmE&list=PLcbz7zf4dTyk9BqlBPLpgI48i9TiorpEi&index=10

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#Superhydrophobic #SurfaceTension #FluidPhysics #HydrohphobicTextures #LeidenfrostEffect

The Balvenie

Photographer Ernie Button explores the stains left behind when various liquors evaporate. This one comes from a single malt scotch whisky by The Balvenie. The stain itself is made up of particles left behind when the alcohol and water in the whisky evaporate. The pattern itself depends on a careful interplay between surface tension, evaporation, pinning forces, and internal convection as the whisky puddle dries out. (Image credit: E. Button/CUPOTY; via Colossal)

#alcohol #deposition #evaporation #fluidDynamics #fluidsAsArt #physics #science #surfaceTension