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Posted: Sep 02, 2016
Chemical engineer receives NSF grant to study self-assembly of large-scale particles
(Nanowerk News) The National Science Foundation (NSF) has awarded Joseph McCarthy, William Kepler Whiteford Professor and Vice Chair for Education in the Department of Chemical and Petroleum Engineering at the University of Pittsburgh, a $404,187 grant for research into the self-assembly of materials into complex structures at sizes much larger than the nanoscale.
The self-assembly of materials is a phenomenon in which component parts of a system spontaneously organize themselves into a uniform and desired structure. This process is similar to how a number of coin-shaped magnets might assemble themselves into a cylinder if they are jostled. At the nanoscale, particles arrange themselves into organized and stable structures whereby the "jostling" is accomplished simply through natural thermal (Brownian) motion. Because nanoparticles exhibit this behavior on their own, they can easily be used to build biological and chemical sensors, computer chips with more computing power and a variety of photonic devices.
The researchers created an ordered array of both 100 and 10 micro-meter particles. After chemically etching away the 100 micro-meter particles, what remains is a highly ordered porous material with both large and small pores whose sizes are completely tunable (via choice of initial particles). From left to right the panels show progressively wider frames. (Image: Joseph McCarthy) (click on image to enlarge)
Larger particles are more difficult for scientists and engineers to manipulate, and they have not yet shown the potential for the same range of applications that has caused the explosion of nanotechnology in recent years. However, the results of McCarthy's research have already suggested the self-assembly of larger particles is possible.
"Fabricating the self-assembly of larger particles had been done a handful of times before we started trying it, but we've pushed the possibilities a lot further," said McCarthy. "Other researchers noticed the phenomenon occurring empirically, but we are trying to formalize it. We are working with particles that are at least 100 times bigger than anything that has been done before."
An array of problems has prevented researchers from exploring the possibilities of engineering larger structures. While nanoparticles respond dramatically to Brownian motion, larger particles often have too much mass to self-assemble in a useful way. McCarthy and his team artificially thermalize the larger particles to allow them to arrange themselves into different sizes and shapes. The results could open up new engineering possibilities across multiple fields.
"Cells are typically about 10 microns. If we took a traditional approach to forming tissue engineering scaffolding via self-assembly, the pores between the components would be much too small for the cells to infiltrate. The methods we will be experimenting with and modeling would allow us to create scaffolding with pore sizes similar to those of cells and which also helps keeps the cells alive by promoting good nutrient flow due to the regularity of the pore structure," said McCarthy.
Another field that might benefit greatly from large-scale self-assembly is microelectronics. Next generation batteries with higher charge capacities suffer from phase changes, meaning the cycles of charging and discharging cause changes to the battery's internal structure. These variations hinder the battery's performance and eventually prevent it from holding a charge at all. Chemical engineers would be able to apply large-scale self-assembly to create batteries in which ions were able to be transferred more precisely, potentially resulting in a longer life spans.
Funding from NSF began on July 15th, 2016. The study, "Realizing Hierarchically Ordered Porous Function Materials from the Crystallization of Both Large-scale and Colloidal Particles," will attempt to both advance the fundamental understanding of large-scale self-assembly and test applications of some of the materials already engineered by McCarthy and his team.