What you have just observed is an example of the Leidenfrost effect, named for Johann Gottlob Leidenfrost, an 18th-century German physician and scientist. The phenomenon occurs when a liquid, upon approaching an object that is much hotter than the liquid's boiling point, produces a vapor which insulates the liquid from the surface of the object. This repulsive force, say scientists, has two consequences. It prevents droplets of the liquid from making physical contact with the surface, causing them instead to hover over the surface. And it causes the droplets to boil off more slowly than they would on a surface with a lower temperature that is still above the liquid's boiling point. Researchers in Hong Kong and at Lehigh University recently demonstrated that it is possible to exploit the Leidenfrost effect to control the direction and destination of liquid droplets on a surface and thus to cool it more efficiently. They achieved this by lithographically patterning a surface with microscale features that convert excess surface tension into a kinetic energy that propels droplets to "hot spots" on the surface. The discovery, say Zuankai Wang of the City University of Hong Kong and Manoj Chaudhury of Lehigh, has the potential to improve technologies that involve microfluidics, heat transfer, heat exchange, micro-heat exchange, water management and thermal management. "Many applications, such as power plant reactors, require the management and control of the movement of water droplets at very high temperatures," says Wang, an associate professor of mechanical and biomedical engineering at City University. "Typically, the cooling of extremely hot surfaces has been accomplished with spray cooling. You spray a lot of water droplets onto a surface and as they boil, they take away the heat. "At a high temperature, however, this doesn't work because the Leidenfrost effect prevents the droplets from making sufficient contact with the surface to cool it. Thus it takes too long to cool a surface by boiling off water." Wang, Chaudhury and their colleagues reported their results today (Feb. 1) in Nature Physics, a journal of Nature magazine, in an article titled "Directional transport of high-temperature Janus droplets mediated by structural topography." The article's lead author is Jing Li, a Ph.D. candidate in the department of mechanical and biomedical engineering at City University. Scientists in the last 20 years have learned to control the movement of liquid droplets on a solid surface by breaking the wetting symmetry that results from the impact of a droplet on a surface. They have accomplished this by harnessing gradients of surface energy and by utilizing light, temperature, electric force and mechanical vibration. Chaudhury, the Franklin J. Howes Jr. Distinguished Professor of Chemical and Biomolecular Engineering at Lehigh, for example, has published articles with his students in Science and Langmuir describing their successful efforts to direct the movement of water droplets on surfaces. But scientists have not yet achieved this control on surfaces heated to Leidenfrost temperatures and above, or on surfaces with extremely hot local spots. Two years ago, Wang came up with the idea of creating topographical contrasts on a silicon wafer by etching the wafer surface with micropillars and arranging the pillars in zones that vary according to the density of the pillars and the contact angle of the pillars with the surface. "The Leidenfrost Effect has been extensively studied for drag reduction, while the presence of the undesired vapor layer also prevents efficient heat transfer," says Wang. "Thus, we came up with the idea of creating an asymmetric surface to control droplet motion at high temperatures." In their Nature Physics article, the researchers reported that their experiments, which were conducted in Hong Kong, showed that "judicious control of the structural topography and operating temperature range of the solid substrate" served to break the wetting symmetry of droplets. The group also reported a "new physical phenomenon in which two concurrent wetting states—Leidenfrost and contact-boiling—can be designed in a single droplet [heated] above its boiling point." The droplet, the researchers wrote in Nature Physics, "exhibits a contrasting (or Janus) thermal state with a lower contact angle in the boiling region, but a higher angle in the Leidenfrost region." This contrast generates "a gradient of curvature, and thus a gradient of Laplace pressure." "As the [droplet's] viscous dissipation is minimal," the researchers wrote, "the resulting excess surface energy of the droplet is converted to kinetic energy, naturally causing it to dislodge from the surface and take flight into the air. The droplet eventually gets deposited in the contact-boiling region." The researchers liken this phenomenon to the action of a slingshot and note that something similar occurs with a filamentous mushroom called Basidiomycota. Each spore of the fungus is part hydrophobic, with a shape like a thin film, and part hydrophilic, with a shape like a sphere. When the two regions make contact, they coalesce, and the tension between hydrophobic and hydrophilic regions creates a force that carries the entire spore into the air. "This is a chemical effect that nature produces," says Shuhuai Yao, an associate professor of mechanical and aerospace engineering at the Hong Kong University of Science and Technology. "At the point of coalescence, there is an asymmetry but the desire for symmetry causes a transitory state and generates a force that propels the spore." The contrasting topographies of the micropillars on Wang's silicon wafer create a similar phenomenon. At a high temperature, as one part of the droplet is boiling and one is non-boiling, an asymmetry is created. But as with the Basidiomycota, the natural tendency toward symmetry creates the slingshot effect that propels the droplet. There is, however, a critical difference between the two phenomena, the researchers say. While nature makes no effort to guide a spore but instead merely to release it, a droplet on a bio-inspired surface can be targeted to a specific place and made to land on a hot spot and boil off. More information: Jing Li et al. Directional transport of high-temperature Janus droplets mediated by structural topography, Nature Physics (2016). DOI: 10.1038/NPHYS3643
Liquid metal batteries, invented by MIT professor Donald Sadoway and his students a decade ago, are a promising candidate for making renewable energy more practical. The batteries, which can store large amounts of energy and thus even out the ups and downs of power production and power use, are in the process of being commercialized by a Cambridge-based startup company, Ambri. Now, Sadoway and his team have found yet another set of chemical constituents that could make the technology even more practical and affordable, and open up a whole family of potential variations that could make use of local resources. The latest findings are reported in the journal Nature Communications, in a paper by Sadoway, who is the John F. Elliott Professor of Materials Chemistry, and postdoc Takanari Ouchi, along with Hojong Kim (now a professor at Penn State University) and PhD student Brian Spatocco at MIT. They show that calcium, an abundant and inexpensive element, can form the basis for both the negative electrode layer and the molten salt that forms the middle layer of the three-layer battery. That was a highly unexpected finding, Sadoway says. Calcium has some properties that made it seem like an especially unlikely candidate to work in this kind of battery. For one thing, calcium easily dissolves in salt, and yet a crucial feature of the liquid battery is that each of its three constituents forms a separate layer, based on the materials’ different densities, much as different liqueurs separate in some novelty cocktails. It’s essential that these layers not mix at their boundaries and maintain their distinct identities. It was the seeming impossibility of making calcium work in a liquid battery that attracted Ouchi to the problem, he says. “It was the most difficult chemistry” to make work but had potential benefits due to calcium’s low cost as well as its inherent high voltage as a negative electrode. “For me, I’m happiest with whatever is most difficult,” he says — which, Sadoway points out, is a very typical attitude at MIT. Another problem with calcium is its high melting point, which would have forced the liquid battery to operate at almost 900 degrees Celsius, “which is ridiculous,” Sadoway says. But both of these problems were solvable. First, the researchers tackled the temperature problem by alloying the calcium with another inexpensive metal, magnesium, which has a much lower melting point. The resulting mix provides a lower operating temperature — about 300 degrees less than that of pure calcium — while still keeping the high-voltage advantage of the calcium. The other key innovation was in the formulation of the salt used in the battery’s middle layer, called the electrolyte, that charge carriers, or ions, must cross as the battery is used. The migration of those ions is accompanied by an electric current flowing through wires that are connected to the upper and lower molten metal layers, the battery’s electrodes. The new salt formulation consists of a mix of lithium chloride and calcium chloride, and it turns out that the calcium-magnesium alloy does not dissolve well in this kind of salt, solving the other challenge to the use of calcium. But solving that problem also led to a big surprise: Normally there is a single “itinerant ion” that passes through the electrolyte in a rechargeable battery, for example, lithium in lithium-ion batteries or sodium in sodium-sulfur. But in this case, the researchers found that multiple ions in the molten-salt electrolyte contribute to the flow, boosting the battery’s overall energy output. That was a totally serendipitous finding that could open up new avenues in battery design, Sadoway says. And there’s another potential big bonus in this new battery chemistry, Sadoway says. “There’s an irony here. If you’re trying to find high-purity ore bodies, magnesium and calcium are often found together,” he says. It takes great effort and energy to purify one or the other, removing the calcium “contaminant” from the magnesium or vice versa. But since the material that will be needed for the electrode in these batteries is a mixture of the two, it may be possible to save on the initial materials costs by using “lower” grades of the two metals that already contain some of the other. “There’s a whole level of supply-chain optimization that people haven’t thought about,” he says. Sadoway and Ouchi stress that these particular chemical combinations are just the tip of the iceberg, which could represent a starting point for new approaches to devising battery formulations. And since all these liquid batteries, including the original liquid battery materials from his lab and those under development at Ambri, would use similar containers, insulating systems, and electronic control systems, the actual internal chemistry of the batteries could continue to evolve over time. They could also adapt to fit local conditions and materials availability while still using mostly the same components. “The lesson here is to explore different chemistries and be ready for changing market conditions,” Sadoway says. What they have developed “is not a battery; it’s a whole battery field. As time passes, people can explore more parts of the periodic table” to find ever-better formulations, he says. “This paper brings together innovative engineering advances in cell design and component materials within a strategic framework of ‘cost-based discovery’ that is amenable to the massive scale-up required of grid-scale applications,” says Richard Alkire, a professor of Chemical and Biomolecular Engineering at the University of Illinois, who was not involved in this research. Because this work builds on a base of well-developed electrochemical systems used for aluminum production, Alkire says, “the path forward to grid-scale applications can therefore take advantage of a large body of existing engineering experience in areas of sustainability, environmental, life cycle, materials, manufacturing cost, and scale-up.” The research was supported by the U.S. Department of Energy’s Advanced Research Projects Energy (ARPA-E) and by the French energy company Total S.A.
Just as the single-crystal silicon wafer forever changed the nature of communication 60 years ago, a group of researchers from Cornell University is hoping its work with the nanocrystals known as quantum dots can help usher in a new era in electronics. The team, led by Tobias Hanrath, associate professor in Cornell’s School of Chemical and Biomolecular Engineering, and graduate student Kevin Whitham, has fashioned two-dimensional superstructures out of these single-crystal building blocks. Using a pair of chemical processes, the lead-selenium nanocrystals can be synthesized into larger crystals, which are then fused together to form atomically-coherent square super-lattices. The difference between these and previous crystalline structures is the atomic coherence of each crystal, which is just 5nm in size. They're not connected by a substance between each crystal – they're connected to each other. This means the electrical properties of these superstructures are potentially superior to existing semiconductor nanocrystals, with anticipated applications in energy absorption and light emission. "As far as level of perfection, in terms of making the building blocks and connecting them into these superstructures, that is probably as far as you can push it," Hanrath said, referring to the atomic-scale precision of the process. A paper on this research is published in Nature Materials. This latest work grew out of previously published research by the Hanrath group, including a 2013 paper in Nano Letters that reported a new approach to connecting quantum dots through controlled displacement of a connector molecule, called a ligand. That paper described ‘connecting the dots’ – i.e. electronically coupling each quantum dot – as one of the most persistent hurdles still to be overcome. That barrier now seems to have been cleared with this new research. The strong coupling of the nanocrystals leads to the formation of energy bands that can be manipulated via the crystals' makeup, and could be the first step towards discovering and developing other artificial materials with controllable electronic structures. Still, Whitham said, more work needs to be done to bring the group's work out of the laboratory and into the real world. The structure of the Hanrath group's super-lattice, while superior to ligand-connected nanocrystal solids, still has multiple sources of disorder due to the fact the nanocrystals are not all identical. This creates defects, which limit electron wave function. "I see this paper as sort of a challenge for other researchers to take this to another level," Whitham said. "This is as far as we know how to push it now, but if someone were to come up with some technology, some chemistry, to provide another leap forward, this is sort of challenging other people to say, 'How can we do this better?'" Hanrath said the discovery can be viewed in one of two ways, depending on whether you see the glass as half empty or half full. "It's the equivalent of saying, 'Now we've made a really large single-crystal wafer of silicon, and you can do good things with it,'" he said, referencing the game-changing communications discovery of the 1950s. "That's the good part, but the potentially bad part of it is we now have a better understanding that if you wanted to improve on our results, those challenges are going to be really, really difficult." This story is adapted from material from Cornell University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.
Young J.D.,Chemical and Biomolecular Engineering
Metabolic Engineering X | Year: 2014
Novel software and experimental protocols have been developed to enable 13C MFA in cyanobacteria and plants Requires isotopically nonstationary MFA Does not require pool size measurements • 13C INST-MFA was extended to a terrestrial plant system for the first time. PR flux increased from 17% to 28% of net C02 assimilation with HL acclimation Calvin cycle flux was inversely correlated with intermediate pool sizes. Source
Li X.,Chemical and Biomolecular Engineering |
Lim K.H.,Chemical and Biomolecular Engineering
ChemCatChem | Year: 2012
Density functional theory has been used in this study to investigate the steam reforming of formaldehyde and its reaction intermediates on Cu(221), PdZn(100), and Ir(100) surfaces. The adsorption complexes (CH 2O, H 2COOH, H 2COO, HCOO, COOH, etc.), binding energies, and reaction energies involved in the steam reforming and dehydrogenation of formaldehyde have been systematically characterized. Our results showed that CH 2O dehydrogenation is the dominant pathway for Ir(100). In contrast, desorption of CH 2O is more favorable than dehydrogenation on regular Cu and PdZn(100) surfaces and the steam reforming reaction is likely to occur at defect sites such as the Cu(221) surface. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Source