Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Not just for raincoats

09.02.2011
Discovery of mini 'water hammer' effect could lead to materials that water really hates

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!
Further information:
http://www.northwestern.edu

More articles from Materials Sciences:

nachricht Hidden talents: Converting heat into electricity with pencil and paper
20.02.2018 | Helmholtz-Zentrum Berlin für Materialien und Energie

nachricht Contacting the molecular world through graphene nanoribbons
19.02.2018 | Elhuyar Fundazioa

All articles from Materials Sciences >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: In best circles: First integrated circuit from self-assembled polymer

For the first time, a team of researchers at the Max-Planck Institute (MPI) for Polymer Research in Mainz, Germany, has succeeded in making an integrated circuit (IC) from just a monolayer of a semiconducting polymer via a bottom-up, self-assembly approach.

In the self-assembly process, the semiconducting polymer arranges itself into an ordered monolayer in a transistor. The transistors are binary switches used...

Im Focus: Demonstration of a single molecule piezoelectric effect

Breakthrough provides a new concept of the design of molecular motors, sensors and electricity generators at nanoscale

Researchers from the Institute of Organic Chemistry and Biochemistry of the CAS (IOCB Prague), Institute of Physics of the CAS (IP CAS) and Palacký University...

Im Focus: Hybrid optics bring color imaging using ultrathin metalenses into focus

For photographers and scientists, lenses are lifesavers. They reflect and refract light, making possible the imaging systems that drive discovery through the microscope and preserve history through cameras.

But today's glass-based lenses are bulky and resist miniaturization. Next-generation technologies, such as ultrathin cameras or tiny microscopes, require...

Im Focus: Stem cell divisions in the adult brain seen for the first time

Scientists from the University of Zurich have succeeded for the first time in tracking individual stem cells and their neuronal progeny over months within the intact adult brain. This study sheds light on how new neurons are produced throughout life.

The generation of new nerve cells was once thought to taper off at the end of embryonic development. However, recent research has shown that the adult brain...

Im Focus: Interference as a new method for cooling quantum devices

Theoretical physicists propose to use negative interference to control heat flow in quantum devices. Study published in Physical Review Letters

Quantum computer parts are sensitive and need to be cooled to very low temperatures. Their tiny size makes them particularly susceptible to a temperature...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

VideoLinks
Industry & Economy
Event News

2nd International Conference on High Temperature Shape Memory Alloys (HTSMAs)

15.02.2018 | Event News

Aachen DC Grid Summit 2018

13.02.2018 | Event News

How Global Climate Policy Can Learn from the Energy Transition

12.02.2018 | Event News

 
Latest News

New tech for commercial Lithium-ion batteries finds they can be charged 5 times fast

20.02.2018 | Power and Electrical Engineering

Hidden talents: Converting heat into electricity with pencil and paper

20.02.2018 | Materials Sciences

Rare find from the deep sea

20.02.2018 | Life Sciences

VideoLinks
Science & Research
Overview of more VideoLinks >>>