Nanochannels have important applications in biomedicine, sensitivity and many other fields. Although engineers working in the field of nanotechnology have been making these tiny tube-like structures for years, much remains unknown about their properties and behavior.
Now, Maryland University associate professor of mechanical engineering Siddhartha Das and a group of his PhD students have published surprising new findings in the journal ACS Nano. Using simulations at the atomic level, Das and his team were able to show that charge properties as well as charge-induced fluid flow within a functionalized nanochannel do not always behave as expected.
“We discovered a new context for nanochannels that are functionalized by grafting their inner walls with charged polymer molecules (also known as polyelectrolytes or PE),” Das said, referring to the process of grafting polymers or other substances onto a nanochannel to make it work. “The functionalization of nanochannels is not new. But we have come to a paradigm shift in terms of understanding the behavior and properties of such systems in the context of their charge properties and their ability to regulate fluid flow.
“For example,” Das said, “we discovered a new kind of flow behavior in such functionalized nanochannels; by increasing the magnitude of the electric field applied to the nanochannel, the direction of that electric field-guided flow (often known as electroosmotic current) can be reversed.”
The work of Dasa and his students describes three specific discoveries in detail. First, they showed that when polyelectrolytes (PE) are cleaved in the form of a layer on the inner wall of the nanochannel, this PE layer will, under certain conditions, suffer a surprising reversal of the electric charge. Typically, if PE negative molecules are attached to a nanochannel, the nearby PE layer should have a net negative charge. Das and his students, however, identified situations in which the charge becomes reversed and the net charge within the layer positive due to attracting more positive ions (than is necessary for the PE layer charge screen) within the layer — this phenomenon is known as “excessive screen “.
The team then investigated how this overflow affects the external flow of the electric field (known as the electroosmotic or EOS flow) within the nanochannel. Surprisingly, they found that in such situations, the flow is driven by ions that have the same charge as the Dog grafted onto the canal walls; therefore, the negatively charged polymer creates a net positive field in its vicinity, but the flow is driven by negative ions.
“We call this‘ koion-guided electroosmosis, ’and our work marks the first time this phenomenon has been identified,” Das said.
Finally, the team showed unexpected results of increasing the magnitude of the electric field: the PE molecules attached to the nanochannel become deformed, and the ions that caused the case of the excessive screen begin to emerge from the PE layer. This causes the screen to stop, and also reverses the flow direction in the channel: if, for example, it was moving left to right, it switches right-left. “No one predicted this,” Das said.
The findings are significant, Das said, because much of the interest in nanochannels relates to their ability to transport molecules. “Because flow is so important, a new discovery in this area allows us to build on our understanding of how nanochannels work and what we can do with them,” Das said. “There are other methods of reversing the flow, but so far it was not known that we could achieve this by increasing the strength of the field.”
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Turash Haque Pial et al., Screen, Ion-dominated electroosmosis and redirection of electric field mediated in nanochannels functionalized with a polyelectrolyte brush, ACS Nano (2021). DOI: 10.1021 / acsnano.0c09248
Provided by the University of Maryland
Citation: Surprising ionic and flow behaviors with functionalized nanochannels (2021, April 20) retrieved April 20, 2021 from https://phys.org/news/2021-04-ionic-behaviors-functionalized-nanochannels.html
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