Researchers from Northwestern University and the Massachusetts Institute of Technology (MIT) have studied individual water droplets and discovered a miniature version of the "water hammer," an effect that produces the familiar radiator pipe clanging in older buildings.
In piping systems, the water hammer occurs when fluid is forced to stop abruptly, causing huge pressure spikes that can rupture pipe walls. Now, for the first time, the researchers have observed this force on the scale of microns: such pressure spikes can move through a water droplet, causing it to be impaled on textured superhydrophobic surfaces, even when deposited gently.
This insight of how droplets get stuck on surfaces could lead to the design of more effective superhydrophobic, or highly water-repellant, surfaces for condensers in desalination and steam power plants, de-icing for aircraft engines and wind turbine blades, low-drag surfaces in pipes and even raincoats. In certain cases, improved surfaces could improve energy efficiency on many orders of magnitude. (About half of all electricity generated in the world comes from steam turbines.)
The research is published by the journal Physical Review Letters.
"We want to design surface textures that will cause the water to really hate those surfaces," said Neelesh A. Patankar, associate professor of mechanical engineering at Northwestern's McCormick School of Engineering and Applied Science. "Improving current hydrophobic materials could result in a 60 percent drag reduction in some applications, for example."
Patankar collaborated with Kripa K. Varanasi, the d'Arbeloff Assistant Professor of Mechanical Engineering at MIT. The two are co-corresponding authors of the paper. Patankar initiated this study in which he and Varanasi led the analytical work, and the experiments were conducted at MIT in Varanasi's lab. Other co-authors are MIT mechanical engineering graduate students Hyuk-Min Kwon and Adam Paxson.
In designing superhydrophobic surfaces, one goal is to produce surfaces much like the natural lotus leaf. Water droplets on these leaves bead up and roll off easily, taking any dirt with them. Contrary to what one might think, the surface of the leaves is rough, not smooth. The droplets sit on microscopic bumps, as if resting on a bed of nails.
"If a water droplet impales the grooves of this bumpy texture, it becomes stuck instead of rolling off," Patankar said. "Such transitions are well known for small static droplets. Our study shows that the impalement of water is very sensitive to the dynamic 'water hammer' effect, which was not expected."
To show this, the researchers imaged millimeter-scale water droplets gently deposited onto rough superhydrophobic surfaces. (The surfaces were made of silicon posts, with spacing from post edge to post edge ranging from 40 to 100 microns, depending on the experiment.) Since these drops were on the millimeter scale and being deposited gently, prior understanding was to assume that gravitational force is not strong enough to push the water into the roughness grooves. The Northwestern and MIT researchers are the first to show this is not true.
They observed that as a droplet settles down on the surface (due to the drop's own weight) there is a rapid deceleration in the drop that produces a brief burst of high pressure, sending a wave through the droplet. The droplet is consequently pinned on the rough surface. That's the powerful mini water hammer effect at work.
By understanding the underlying physics of this transition, the study reveals that there is actually a window of droplet sizes that avoid impalement. Although focused on drop deposition, this idea is quite general and applies to any scenario where the water velocity is changing on a short (less than a millisecond) time scale. This insight can lead to the design of more robust superhydrophobic surfaces that can resist water impalement even under the dynamic conditions typical in industrial setups.
"One way to reduce impalement is to design a surface texture that results in a surface that sustains extremely high pressures," Patankar said. "It is the length scale of the roughness that is important." To resist impalement, the height of a bump and the distance between bumps need to be just right. Hundreds of nanometer scale roughness can lead to robust surfaces.
"Our ultimate goal," he added, "is the invention of textured surfaces such that a liquid in contact with it will, at least partially, vaporize next to the surface -- or sustain air pockets -- and self-lubricate. This is similar to how an ice skater glides on ice due to a cushion of thin lubricating liquid film between the skates and the ice. A critical step is to learn how to resist impalement of water on the roughness. Our work on water hammer-induced impalement is a crucial advance toward that goal of ultra-slippery vapor stabilizing surfaces."
The paper is titled "Rapid Deceleration-driven Wetting Transition During Pendant Drop Deposition on Superhydrophobic Surfaces."
Megan Fellman | EurekAlert!
Electron tomography technique leads to 3-D reconstructions at the nanoscale
24.05.2018 | The Optical Society
These could revolutionize the world
24.05.2018 | Vanderbilt University
The more electronics steer, accelerate and brake cars, the more important it is to protect them against cyber-attacks. That is why 15 partners from industry and academia will work together over the next three years on new approaches to IT security in self-driving cars. The joint project goes by the name Security For Connected, Autonomous Cars (SecForCARs) and has funding of €7.2 million from the German Federal Ministry of Education and Research. Infineon is leading the project.
Vehicles already offer diverse communication interfaces and more and more automated functions, such as distance and lane-keeping assist systems. At the same...
A research team led by physicists at the Technical University of Munich (TUM) has developed molecular nanoswitches that can be toggled between two structurally different states using an applied voltage. They can serve as the basis for a pioneering class of devices that could replace silicon-based components with organic molecules.
The development of new electronic technologies drives the incessant reduction of functional component sizes. In the context of an international collaborative...
At the LASYS 2018, from June 5th to 7th, the Laser Zentrum Hannover e.V. (LZH) will be showcasing processes for the laser material processing of tomorrow in hall 4 at stand 4E75. With blown bomb shells the LZH will present first results of a research project on civil security.
At this year's LASYS, the LZH will exhibit light-based processes such as cutting, welding, ablation and structuring as well as additive manufacturing for...
There are videos on the internet that can make one marvel at technology. For example, a smartphone is casually bent around the arm or a thin-film display is rolled in all directions and with almost every diameter. From the user's point of view, this looks fantastic. From a professional point of view, however, the question arises: Is that already possible?
At Display Week 2018, scientists from the Fraunhofer Institute for Applied Polymer Research IAP will be demonstrating today’s technological possibilities and...
So-called quantum many-body scars allow quantum systems to stay out of equilibrium much longer, explaining experiment | Study published in Nature Physics
Recently, researchers from Harvard and MIT succeeded in trapping a record 53 atoms and individually controlling their quantum state, realizing what is called a...
25.05.2018 | Event News
02.05.2018 | Event News
13.04.2018 | Event News
25.05.2018 | Event News
25.05.2018 | Machine Engineering
25.05.2018 | Life Sciences