Search results for: “water droplet”

How Water Droplets Charge Up
Rubbing a balloon on your hair can build a significant electrical charge. Water droplets have the same issue when they slide across a hydrophobic, electrically-insulated surface. A new study models why these charges build up and tests the model both experimentally and through simulation. They focused their theory on three effects that determine how much charge builds up. The first is a two-way chemical reaction that continuously creates charge at the interface, with positive charge building in the drop. Secondly, the drop’s contact angle with the surface sets how many protons can build up at the contact line, thereby affecting the electrical field they generate. And, finally, fluid motion at the rear of the drop deflects protons upward, shifting the electrical field. In particular, their model predicts that the higher contact angles of hydrophobic surfaces should increase charge build-up and faster sliding velocities should slow charge build-up, both of which agree with experiments.
The model should help researchers understand various charging scenarios, like those found on self-cleaning surfaces, in inkjet printing, and in semiconductor manufacturing. In the last scenario, rinsing semiconductor wafers in ultrapure water can build up charges in the kilovolt range, which is enough to damage the product. (Image credit: D. Carlson; research credit: A. Ratschow et al.; via APS Physics)
July 11, 2024
Jumping Water Droplets
Superhydrophobic surfaces resist wetting from water, but it turns out they can also trigger interesting behaviors in the tiny droplets condensing on the surface. High-speed video reveals that when two condensate droplets coalesce, the energy released by surface tension causes the new droplet to jump off the surface. The phenomenon is the same as one observed in some types of mushroom–when a condensate droplet touches a wetted spore, the spore is ejected from the mushroom. (Video credit: J. Boreyko)
April 12, 2012
Playing Pac-Man with Water Droplets
The vibrations of a plate in the horizontal and vertical directions can be used to control the motion of a drop placed on the surface. Here a droplet of water on a superhydrophobic surface is controlled by joystick a la Pacman. For more, see papers here and here.
November 10, 2011
Steering Water Droplets
At the microscale, fluid behavior can be quite different than what we witness in everyday life. Mechanisms that have little effect on the macroscale suddenly become extremely important in a channel only a few hundred microns wide. Here, water droplets in oil are steered and controlled using lasers.
December 21, 2010
Water Walking, Exploding Droplets, and Colliding Vortices
Every year I look forward to the APS DFD conference in November. It brings thousands of researchers together to share the latest in fluid dynamics. So much goes on in those three days that it’s impossible to capture, but last year I teamed up with Tom Crawford and the Journal of Fluid Mechanics to attempt just that. We interviewed 50 researchers on their projects, and we’ll be bringing you their work, in their words, each month leading up to the 2018 APS DFD meeting.
This first video focuses on some of the awesome entries to the 2017 Gallery of Fluid Motion. Watch to learn about oil droplets that go flying everywhere when you’re cooking, balls that walk on water, the water music of Vanuatu and more! To see the videos we discuss and all the other entries, go to gfm.aps.org. (Video credit: N. Sharp and T. Crawford)
January 24, 2018
“Droplet on a Plucked Wire”
What happens to a droplet hanging on a wire when the wire gets plucked? That’s the fundamental question behind this video, which shows the effects of wire speed, viscosity, and viscoelasticity on a drop’s detachment. With lovely high-speed video and close-up views, you get to appreciate even subtle differences between each drop. Capillary waves, viscoelastic waves, and Plateau-Rayleigh instabilities abound! (Video and image credit: D. Maity et al.)
Fediverse Reactions
June 23, 2025
The Mystery of the Binary Droplet
What goes on inside an evaporating droplet made up of more than one fluid? This is a perennially fascinating question with lots of permutations. In this one, researchers observed water-poor spots forming around the edges of an evaporating drop, almost as if the two chemicals within the drop are physically separating from one another (scientifically speaking, “undergoing phase separation“). To find out if this was really the case, they put particles into the drop and observed their behavior as the drop evaporated. What they found is that this is a flow behavior, not a phase one. The high concentration of hexanediol near the edge of the drop changes the value of surface tension between the center and edge of the drop. And that change is non-monotonic, meaning that there’s a minimum in the surface tension partway along the drop’s radius. That surface tension minimum is what creates the separated regions of flow. (Video and image credit: P. Dekker et al.; research pre-print: C. Diddens et al.)
January 20, 2025
Predicting Droplet Sizes
Squeeze a bottle of cleaning spray, and the nozzle transforms a liquid jet into a spray of droplets. These droplets come in many sizes, and predicting them is difficult because the droplets’ size distribution depends on the details of how their parent liquid broke up. Shown above is a simplified experimental version of this, beginning with a jet of air striking a spherical water droplet on the far left. In less than 3 milliseconds, the droplet has flattened into a pancake shape. In another 4 milliseconds, the pancake has ballooned into a shape called a bag, made up of a thin, curved water sheet surrounded by a thicker rim. A mere 10 milliseconds after the jet and drop first meet, the liquid is now a spray of smaller droplets.
Researchers have found that the sizes of these final droplets depend on the balance between the airflow and the drop’s surface tension; these two factors determine how the drop breaks up, whether that’s rim first, bag first, or due to a collision between the bag and rim. (Image credit: I. Jackiw et al.; via APS Physics)
December 4, 2024
“Dew Point” Deposits Droplets
Artist Lily Clark loves to work in water. One of her recent sculptures, “Dew Point,” uses superhydrophobic ceramic to grow and manipulate water droplets over and over and over. Droplets coalesce in four corners until they grow large enough for gravity to pull them into a circular depression. Given their limited contact with the ceramic, the falling water droplets zip and slide on their way to a return slit in the center of the piece. You can see more of the action in the video below. Personally, I’m reminded of coins falling into a collection box! (Video credit: L. Turczan; artwork by: L. Clark; via Colossal)

May 24, 2024
Gravity Changes Droplet Shapes
With small droplets, gravity usually has little effect compared to surface tension. An evaporating water droplet holds its spherical shape as it evaporates. But the story is different when you add proteins to the droplet, as seen in this recent study.
The protein-filled sessile drop starts out largely spherical, but as the drop evaporates, the concentration of proteins reaches a critical point and an elastic skin forms over the drop. From this point onward, the drop flattens.
As a protein-doped droplet sitting on a surface evaporates, it starts out spherical, like its protein-free cousin. But, as the water evaporates, it leaves proteins behind, gradually increasing their concentration. Eventually, they form an elastic skin covering the drop. As water continues to evaporate, the droplet flattens.
For a hanging droplet, the shape again starts out spherical. But as the drop’s water evaporates and the proteins concentrate, it also forms an elastic skin. As the drop evaporates further, the skin wrinkles.
In contrast, a hanging droplet with proteins takes on a wrinkled appearance once its elastic skin forms. The key difference, according to the model constructed by the authors, is the direction that gravity points. Despite these droplets’ small size, gravity makes a difference! (Image, video, and research credit: D. Riccobelli et al.; via APS Physics)
August 22, 2023

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