Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

That's the way the droplets adhere

20.02.2013
Understanding exactly how droplets and bubbles stick to surfaces — everything from dew on blades of grass to the water droplets that form on condensing coils after steam drives a turbine in a power plant — is a "100-year-old problem" that has eluded experimental answers, says MIT's Kripa Varanasi. Furthermore, it's a question with implications for everything from how to improve power-plant efficiency to how to reduce fogging on windshields.

Now this longstanding problem has finally been licked, Varanasi says, in research he conducted with graduate student Adam Paxson that is described this week in the journal Nature Communications. They achieved the feat using a modified version of a scanning electron microscope in which the dynamic behavior of droplets on surfaces at any angle could be observed in action at high resolution.

Previous attempts to study droplet adhesion have been static — using drops of a polymer that are allowed to harden and then sliced in cross-section — or have been done only at very low resolution. The ability to observe the process in close-up detail and in full motion is an unprecedented feat, says Varanasi, the Doherty Associate Professor of Ocean Utilization.

Normally, scanning electron microscopes observe materials on a fixed horizontal stage and under a strong vacuum, which causes water to evaporate instantly. The MIT team was able to adapt the equipment to operate with a weaker vacuum, and with the ability to change the surface angle and to push and pull droplets across the surface with a tiny wire.

Paxson and Varanasi found that a key factor in determining whether a droplet sticks to the surface is the angle of the droplet's leading and trailing edges relative to the surface. Nobody had been able to observe these angles dynamically at microscale before, while theorists had not predicted their importance.

The MIT researchers also found that on rough surfaces, surface texture is crucial to adhesion. Surprisingly, they found that too much roughness can make droplets stick more — contrary to the widely held belief that greater roughness always improves a surface's ability to shed water. It all depends on the details of the texture, they found.

For many applications, it's important that droplets fall away from a condensing surface as quickly as possible; for others, it's best to "pin" them in place as long as possible so they can grow and spread. The new analysis, which led to a mathematical system for precisely predicting droplet behavior, can be used to optimize a surface in either way. (Bubbles, such as those on the bottom of a pan of boiling water, behave in essentially the same way).

"People have only been able to make sketches" of how droplet adhesion works, Paxson says. With the new high-resolution imagery, it is now clear that as a droplet peels away from a rough surface, the round droplet forms a series of tiny "necks" adhering to each of the high points on the surface; these necks (which the researchers call "capillary bridges") then gradually stretch, thin and break. The more high spots on the surface, the more of these tiny necks form. "That's where all the adhesion occurs," Paxson says.

The MIT authors say the phenomenon is "self-similar," like fractal structure: Each neck or capillary bridge can consist of several capillary bridges at finer length scales; it is the cumulative effect that dictates the overall adhesion. This self-similarity is exploited by some biological structures for lowering adhesion.

There had been two leading theories on how to calculate the adhesion of droplets: One held that the areas of contact and energy levels of the molecules were key; the other, that the length of the edge of a drop on a surface was critical. The evidence produced by this research strongly supports the second theory. "I think we have now closed a decades-old debate on this one," Varanasi says.

In general, Paxson says, "complicated shapes tend to be more sticky," because of their greater edge-length.

Droplets and bubbles are ubiquitous in many engineering applications. This work could be applied to engineering industrial surfaces with controlled adhesion in applications ranging from large desalination and power plants to consumer products such as fabrics, packaging and medical devices. While some applications, such as condensers, strive to shed droplets quickly from a surface, others — such as ink droplets sprayed onto paper in an inkjet printer — require the reverse. The new methodology might help in improving both functions, the researchers say.

Paxson and Varanasi's formulas can also explain variability among natural textured surfaces — such as lotus leaves, which shed water efficiently, and rose petals, which do not. Finally, the new research could advance our understanding of certain biological processes — such as how water spiders, which make an air bubble to house themselves under the surface of a body of water, control the surface tension to penetrate the bubble.

The work was supported by the National Science Foundation and the DuPont-MIT Alliance.

Written by David Chandler, MIT News Office

Sarah McDonnell | EurekAlert!
Further information:
http://www.mit.edu

More articles from Materials Sciences:

nachricht New pop-up strategy inspired by cuts, not folds
27.02.2017 | Harvard John A. Paulson School of Engineering and Applied Sciences

nachricht Let it glow
27.02.2017 | Okinawa Institute of Science and Technology (OIST) Graduate University

All articles from Materials Sciences >>>

The most recent press releases about innovation >>>

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

Im Focus: Safe glide at total engine failure with ELA-inside

On January 15, 2009, Chesley B. Sullenberger was celebrated world-wide: after the two engines had failed due to bird strike, he and his flight crew succeeded after a glide flight with an Airbus A320 in ditching on the Hudson River. All 155 people on board were saved.

On January 15, 2009, Chesley B. Sullenberger was celebrated world-wide: after the two engines had failed due to bird strike, he and his flight crew succeeded...

Im Focus: Breakthrough with a chain of gold atoms

In the field of nanoscience, an international team of physicists with participants from Konstanz has achieved a breakthrough in understanding heat transport

In the field of nanoscience, an international team of physicists with participants from Konstanz has achieved a breakthrough in understanding heat transport

Im Focus: DNA repair: a new letter in the cell alphabet

Results reveal how discoveries may be hidden in scientific “blind spots”

Cells need to repair damaged DNA in our genes to prevent the development of cancer and other diseases. Our cells therefore activate and send “repair-proteins”...

Im Focus: Dresdner scientists print tomorrow’s world

The Fraunhofer IWS Dresden and Technische Universität Dresden inaugurated their jointly operated Center for Additive Manufacturing Dresden (AMCD) with a festive ceremony on February 7, 2017. Scientists from various disciplines perform research on materials, additive manufacturing processes and innovative technologies, which build up components in a layer by layer process. This technology opens up new horizons for component design and combinations of functions. For example during fabrication, electrical conductors and sensors are already able to be additively manufactured into components. They provide information about stress conditions of a product during operation.

The 3D-printing technology, or additive manufacturing as it is often called, has long made the step out of scientific research laboratories into industrial...

Im Focus: Mimicking nature's cellular architectures via 3-D printing

Research offers new level of control over the structure of 3-D printed materials

Nature does amazing things with limited design materials. Grass, for example, can support its own weight, resist strong wind loads, and recover after being...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

Booth and panel discussion – The Lindau Nobel Laureate Meetings at the AAAS 2017 Annual Meeting

13.02.2017 | Event News

Complex Loading versus Hidden Reserves

10.02.2017 | Event News

International Conference on Crystal Growth in Freiburg

09.02.2017 | Event News

 
Latest News

New pop-up strategy inspired by cuts, not folds

27.02.2017 | Materials Sciences

Sandia uses confined nanoparticles to improve hydrogen storage materials performance

27.02.2017 | Interdisciplinary Research

Decoding the genome's cryptic language

27.02.2017 | Life Sciences

VideoLinks
B2B-VideoLinks
More VideoLinks >>>