Tag Archives: David Chandler

When wrinkles are good for us

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I like the video animation that the scientists at the Massachusetts Institute of Technology (MIT) have provided so much (particularly the raisins), I’m going to start with it,

The August 1, 2012 MIT news release on EurekAlert provides some additional detail,

This basic method, they say, could be harnessed for a wide variety of useful structures: microfluidic systems for biological research, sensing and diagnostics; new photonic devices that can control light waves; controllable adhesive surfaces; antireflective coatings; and antifouling surfaces that prevent microbial buildup.

A paper describing this new process, co-authored by MIT postdocs Jie Yin and Jose Luis Yagüe, former student Damien Eggenspieler SM ’10, and professors Mary Boyce and Karen Gleason, is being published in the journal Advanced Materials.

The process uses two layers of material. The bottom layer, or substrate, is a silicon-based polymer that can be stretched, like canvas mounted on a stretcher frame. Then, a second layer of polymeric material is deposited through an initiated chemical vapor deposition (iCVD) process in which the material is heated in a vacuum so that it vaporizes, and then lands on the stretched surface and bonds tightly to it. Then — and this is the key to the new process — the stretching is released first in one direction, and then in the other, rather than all at once.

When the tension is released all at once, the result is a jumbled, chaotic pattern of wrinkles, like the surface of a raisin. But the controlled, stepwise release system developed by the MIT team creates a perfectly orderly herringbone pattern.

The David Chandler Aug.  1, 2012 article (written for MIT) which originated the news release notes,

Many techniques have been used to create surfaces with such tiny patterns, whose dimensions can range from nanometers (billionths of a meter) to tens of micrometers (millionths of a meter). But most such methods require complex fabrication processes, or can only be used for very tiny areas.

The new method is both very simple (consisting of just two or three steps) and can be used to make patterned surfaces of larger sizes, the team says. “You don’t need an external template” to create the pattern, says Yin, the paper’s lead author.

…
John Hutchinson, a professor of engineering and of applied mechanics at Harvard University who was not involved in this research, says, “Wrinkling phenomena are highly nonlinear and answers to questions concerning pattern formation have been slow to emerge.” He says the MIT team’s work “is an important step forward in this active area of research that bridges the chemical and mechanical engineering communities. The advance rests on theoretical insights combined with experimental demonstration and numerical simulation — it covers all the bases.”

The work was funded by the King Fahd University of Petroleum and Minerals in Saudi Arabia.

It’s nice to see wrinkles being appreciated.

Anti-fogging, self-cleaning, and glare-free: glass

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They raise my hopes then dash them to the ground; still, this is very exciting news for anyone wanting self-cleaning windows. The April 26, 2012 news item on Nanowerk features some of the latest work from MIT (Massachusetts Institute of Technology) on nanotextures and ‘multifunctional’ glass,

One of the most instantly recognizable features of glass is the way it reflects light. But a new way of creating surface textures on glass, developed by researchers at MIT, virtually eliminates reflections, producing glass that is almost unrecognizable because of its absence of glare — and whose surface causes water droplets to bounce right off, like tiny rubber balls.

The new “multifunctional” glass, based on surface nanotextures that produce an array of conical features, is self-cleaning and resists fogging and glare, the researchers say. Ultimately, they hope it can be made using an inexpensive manufacturing process that could be applied to optical devices, the screens of smartphones and televisions, solar panels, car windshields and even windows in buildings.

Here’s what they mean by ‘conical features’,

Through a process involving thin layers of material deposited on a surface and then selectively etched away, the MIT team produced a surface covered with tiny cones, each five times taller than their width. This pattern prevents reflections, while at the same time repelling water from the surface. Image: Hyungryul Choi and Kyoo-Chul Park

David Chandler in his April 26, 2012 news release for MIT explains,

The surface pattern — consisting of an array of nanoscale cones that are five times as tall as their base width of 200 nanometers — is based on a new fabrication approach the MIT team developed using coating and etching techniques adapted from the semiconductor industry. Fabrication begins by coating a glass surface with several thin layers, including a photoresist layer, which is then illuminated with a grid pattern and etched away; successive etchings produce the conical shapes. The team has already applied for a patent on the process.

Since it is the shape of the nanotextured surface — rather than any particular method of achieving that shape — that provides the unique characteristics, Park and Choi [MIT mechanical engineering graduate students Kyoo-Chul Park and Hyungryul Choi] say that in the future glass or transparent polymer films might be manufactured with such surface features simply by passing them through a pair of textured rollers while still partially molten; such a process would add minimally to the cost of manufacture.

If you’re guessing that nature inspired some of this, read on (from Chandler’s MIT news release),

The researchers say they drew their inspiration from nature, where textured surfaces ranging from lotus leaves to desert-beetle carapaces and moth eyes have developed in ways that often fulfill multiple purposes at once. Although the arrays of pointed nanocones on the surface appear fragile when viewed microscopically, the researchers say their calculations show they should be resistant to a wide range of forces, ranging from impact by raindrops in a strong downpour or wind-driven pollen and grit to direct poking with a finger. Further testing will be needed to demonstrate how well the nanotextured surfaces hold up over time in practical applications.

The chief excitement seems to centre around applications with solar panels (from Chandler’s MIT news release),

Photovoltaic panels, Park explains, can lose as much as 40 percent of their efficiency within six months as dust and dirt accumulate on their surfaces. But a solar panel protected by the new self-cleaning glass, he says, would have much less of a problem. In addition, the panel would be more efficient because more light would be transmitted through its surface, instead of being reflected away — especially when the sun’s rays are inclined at a sharp angle to the panel. At such times, such as early mornings and late afternoons, conventional glass might reflect away more than 50 percent of the light, whereas an anti-reflection surface would reduce the reflection to a negligible level.

While some earlier work has treated solar panels with hydrophobic coatings, the new multifunctional surfaces created by the MIT team are even more effective at repelling water, keeping the panels clean longer, the researchers say. In addition, existing hydrophobic coatings do not prevent reflective losses, giving the new system yet another advantage.

More testing is needed and while they do fantasize about wider applications (car windows, microscopes, cameras, smartphones, building windows, etc. mentioned earlier in this posting)  for this technology there are no immediate plans to fulfill my dream of self-cleaning apartment windows and mirrors.

Two-dimensional Dirac cones, bismuth-antimony films, and graphene

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Two researchers at MIT (Massachusetts Institute of Technology) have developed a new material, a bismuth-antimony film. From the April 24, 2012 news item by David Chandler for MIT News,

Now, researchers at MIT have found another compound that shares many of graphene’s unusual characteristics — and in some cases has interesting complementary properties to this much-heralded material.

The material, a thin film of bismuth-antimony, can have a variety of different controllable characteristics, the researchers found, depending on the ambient temperature and pressure, the material’s thickness and the orientation of its growth. The research, carried out by materials science and engineering PhD candidate Shuang Tang and Institute Professor Mildred Dresselhaus …

Like graphene, the new material has electronic properties that are known as two-dimensional Dirac cones, a term that refers to the cone-shaped graph plotting energy versus momentum for electrons moving through the material. These unusual properties — which allow electrons to move in a different way than is possible in most materials — may give the bismuth-antimony films properties that are highly desirable for applications in manufacturing next-generation electronic chips or thermoelectric generators and coolers.

In such materials, Tang says, electrons “can travel like a beam of light,” potentially making possible new chips with much faster computational abilities. The electron flow might in some cases be hundreds of times faster than in conventional silicon chips, he says.

Similarly, in a thermoelectric application — where a temperature difference between two sides of a device creates a flow of electrical current — the much faster movement of electrons, coupled with strong thermal insulating properties, could enable much more efficient power production. This might prove useful in powering satellites by exploiting the temperature difference between their sunlit and shady sides, Tang says.

Such applications remain speculative at this point, Dresselhaus says, because further research is needed to analyze additional properties and eventually to test samples of the material. This initial analysis was based mostly on theoretical modeling of the bismuth-antimony film’s properties.

PhD candidate Shuang Tang, left, and Institute Professor Mildred Dresselhaus Photo: Dominick Reuter

While possible applications are purely speculation, the new material appears to have some interesting properties, from the April 24, 2012 news item on Nanowerk,

While it turns out that the thin films of bismuth-antimony can have some properties similar to those of graphene, changing the conditions also allows a variety of other properties to be realized. That opens up the possibility of designing electronic devices made of the same material with varying properties, deposited one layer atop another, rather than layers of different materials.

The material’s unusual properties can vary from one direction to another: Electrons moving in one direction might follow the laws of classical mechanics, for example, while those moving in a perpendicular direction obey relativistic physics. This could enable devices to test relativistic physics in a cheaper and simpler way than existing systems, Tang says, although this remains to be shown through experiments.

“Nobody’s made any devices yet” from the new material, Dresselhaus cautions, but adds that the principles are clear and the necessary analysis should take less than a year to carry out.

Here’s what another researcher (not affiliated with this work) had to say about the new material, from the April 24, 2012 MIT article,

Joseph Heremans, a professor of physics at Ohio State University who was not involved in this research, says that while some unusual properties of bismuth have been known for a long time, “what is surprising is the richness of the system calculated by Tang and Dresselhaus. The beauty of this prediction is further enhanced by the fact that system is experimentally quite accessible.”

Heremans adds that in further research on the properties of the bismuth-antimony material, “there will be difficulties, and a few are already known,” but he says the properties are sufficiently interesting and promising that “this paper should stimulate a large experimental effort.”

So, in about one year, we should know more.