Search Results for “droplet” – Page 36

Blooming Deposits

As the drop evaporates, particle deposits build up and form a bent, flower-like solid

As the drop dries out, cracks form in the solid

Evaporate a droplet full of silica nanoparticles, and you’ll get beautiful, flower-like films. As the water evaporates, dry nanoparticles build up in a solid deposit. The evaporation creates a pressure gradient that pulls toward the center of the drop, forcing the deposit to bend. As stress builds in the deposit, cracks form petal-like segments. The number of cracks is indicative of how much of the drop was solid material; the higher the volume fraction of particles is, the fewer cracks form and the less the deposit bends. (Image, video, and research credit: P. Lilin et al.)

December 23, 2019

Falling Drops and Forming Stalagmites

A droplet of water falls from a cave's stalactite

A drop of water falls onto the wide top of a stalagmite and creates a crown-like splash

The vast stalactites and stalagmites found in caves take millennia to form. Mineral-rich water seeps down the icicle-like stalactites and then drips onto stalagmites below, each drop depositing a little more calcite onto the growing rock. By observing this dripping action first-hand, researchers found that most falling drops create a splash that’s much smaller than the width of the stalagmite they fall onto. So how do stalagmites end up so wide?

It turns out that there’s a large variance in where drops hit the stalagmite. There’s no wind in these caves to push the droplets, so researchers concluded the drop’s trajectory depends on the vortices it sheds as it falls. A drop that falls from a short height will have a vertical trajectory. But once the drop is falling tens of meters, it can end up as many as several centimeters to the side of where it would fall in a vacuum. This scatter-shot variation in drop impacts is what enables stalagmites to grow so wide. (Image and research credit: J. Parmentier et al., source; via NYTimes; submitted by Kam-Yung Soh)

December 12, 2019

CU Flow Vis 2019

I love when science and art come together, which is why I’ve long been a fan of the Flow Vis course at CU Boulder. Some of my earliest posts on FYFD date from previous editions of the course. Here are a few of my favorite images from the Fall 2019 class, from the top:

Ferrofluid and India ink merge in this colorful photo. A magnet underneath the mixture on the left side causes the dark spikes of ferrofluid, but without magnetic influence, the ink and ferrofluid form cell-like droplets.
Although it looks like a shower head, this is actually fluorescent oobleck dripping through a strainer. A relatively long exposure time means that it’s impossible to tell whether the oobleck is falling in a fluid stream or broken-up chunks.
These colorful water droplets are sitting on a hydrophobic surface, hence their extremely rounded edges. I particularly like how this makes each one like a little lens for the light shining through them and into their shadows.
A thin layer of ferrofluid reacts to the magnet beneath. Gotta love those little streaks left behind the flow.

For those in the Front Range area, the Flow Vis class will be showcasing their work on Saturday, December 14th at the Fiske Planetarium. Snacks are at 4:30 pm and the show starts at 5 pm. For those not nearby, you can peruse the art from this semester and previous ones at your leisure online. (Image credits: colorful ferrofluid – R. Drevno; falling oobleck – A. Kumar; droplets – A. Barron; macro ferrofluid – A. Zetley)

December 9, 2019

Making Drops Stick

As you may have noticed when washing vegetables, many plants have superhydrophobic leaves. Water just beads up on their surface and slides right off. This is a useful feature for plants that want to direct that water toward their roots, but it’s a frustration in agriculture, where that superhydrophobicity means extra spraying of pesticides in order to get enough to stick to the plant.

But that may not be the case for much longer. Researchers have found that adding a little polymer to water droplets (right) can suppress their ability to rebound (left) from superhydrophobic surfaces. Above a critical concentration, the high shear rate of the droplet as it tries to detach activates the viscoelastic properties of the polymer. That viscoelasticity suppresses the rebound, keeping the droplet attached. That’s good news for everyone, since it means less spraying is needed to protect crops. (Image and research credit: P. Dhar et al.)

November 18, 2019

Whiskey Stains

Complex fluids leave behind fascinating stains after they evaporate. We’ve seen previously how coffee forms rings and whisky forms more complicated stains as surface tension changes during evaporation drive particles throughout the droplet. Now researchers are considering the differences between traditional Scottish whisky, which ages in re-used, uncharred barrels, and American whiskeys like bourbon, which are required to age in new, charred white oak barrels.

When diluted, the American whiskeys form web-like patterns – seen above – that vary from brand to brand, like a fingerprint. The charring of the barrels allows American whiskeys to pick up more water-insoluble molecules compared to whisky aged in uncharred barrels. Since the webbed patterns form in American whiskey but not Scotch whisky, it’s likely those molecules play an important role in the evaporation dynamics and subsequent staining. (Image credit: S. Williams et al.; research credit: S. Williams et al.; via APS Physics; submitted by Kam-Yung Soh)

November 15, 2019

“Mocean”

Ocean waves are endlessly fascinating to watch. In “Mocean,” cinematographer Chris Bryan captures them in ways few ever see, thanks to his high-speed camera. Honestly, this film is so gorgeous that I don’t want to distract you with the science, so just go watch!

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All done? Pretty wonderful, right? There’s nothing quite like seeing those holes break and expand through sheets of water, tearing what looked solid into a spray of droplets that bleed salt into the atmosphere. Or how about those rib vortices underneath the waves? Or the cloud-like turbulence of the waves breaking overhead? How fortunate we are to see and capture and share such beauty! (Video and image credit: C. Bryan; via RedShark; submitted by Michael F.)

November 14, 2019

Leidenfrost Stars

Atop a very hot surface, liquids can instantly vaporize, leaving a drop levitating on a layer of its own vapor. These Leidenfrost droplets demonstrate all kinds of interesting behaviors, including self-propulsion, explosion, and star-shaped oscillations, like those above. The oscillation is driven by feedback between the drop and its vapor layer.

Interestingly, the drops are capable of sustaining more than one mode of oscillation at once, as seen above. The obvious mode (m=5) corresponds to the 5 star-like points pushing out on the drop. But notice that the drop is also stretching into an oval shape that moves up and down, back and forth. This is the second mode (m=2) present. It moves slower than the m=5 mode, completing a cycle only once for every four cycles the other has. (Image and research credit: J. Bergen et al.)

November 13, 2019

Drops That Dig

On extremely hot surfaces, droplets will skitter on a layer of their own vapor, thanks to the Leidenfrost effect. This keeps the liquid insulated from contact with the hot surface. But what if the surface isn’t solid?

That situation is what we see above. Instead of soaking into a granular material like a room temperature droplet (left), a drop falling onto a very hot bed of grains digs a hole! As with a typical drop on a super hot surface, the heat vaporizes part of the droplet. As the vapor escapes, it carries sand with it, allowing the boiling drop to burrow its way into the material. As the temperature difference between the sand and droplet changes, the digging slows. Eventually, the drop comes to a rest and boils away. (Video and image credit: J. Zou et al.)

November 8, 2019

Pollock Avoided Coiling

Streaks of black and gray in the Jackson Pollack painting the researchers studied.

Artists are often empirical masters of fluid dynamics, as they must be to achieve the effects they want. Jackson Pollock was particularly known for his so-called dripping technique, in which he dropped filaments of paint from brushes, cans, and even syringes as he moved around a horizontal canvas. (Scientifically speaking, this wasn’t really dripping since the paint wasn’t breaking up into droplets for the most part, but that’s another story.)

What Pollock was doing, fluid dynamically speaking, is the subject of a new study. Researchers analyzed historical footage of Pollock painting to measure the typical heights from which he dropped paint and the speed at which he moved. Then they built their own apparatus to mimic the painting style with modern paints and study the flow regime Pollock’s technique falls into.

Since much of the paint falls in a steady stream, like syrup falling onto pancakes, the researchers wondered whether the paint was likely to coil the way other viscous fluids do. What they found, however, is that Pollock’s choice of height and speed when applying paint seems deliberately designed to avoid the coiling instability. That fact suggests that art historians might identify forged paintings in part from the presence of too much coiling among the paint filaments. (Image credits: photo – M. Holmes/LIFE, painting – J. Pollock; research credit: B. Palacios et al; via Ars Technica; submitted by Kam-Yung Soh)

November 7, 2019

Themed Series

Occasionally, FYFD will feature a series of posts on a special theme. This page serves as an archive of these themed series. Got an idea for theme? You can always suggest one via Tumblr, Twitter, or email.