Assemblies of nanoparticles at the interface of two different liquids have potential for the creation of so-called structured liquids, which could be three-dimensionally printed and used to build responsive coatings, sensors, drug-delivery systems, and liquid batteries. To design such creations, engineers need to understand the dynamics of how the nanoparticles assemble at the liquid interfaces so they can know how to manipulate them into the arrangement they want. One group of researchers, using the Advanced Photon Source at Argonne National Laboratory, has studied how the behavior of such nanoparticles evolves as the assembly takes place. Their results were published in the journal ACS Nano.
The researchers used gold nanoparticles with an average diameter of 14.3 ± 1.7 nm, which they functionalized by adding ligands of heptaethylene glycol with carboxylic acid on the end, making it easier to hold them at the interface between water and oil. When the researchers changed the shape of the liquid, the interface between the water and oil decreased, and the nanoparticles were jammed together, making them immobile and turning them essentially into a dense, disordered film. The jammed nanoparticles then provided structural support for the liquid to hold its shape. Altering the chemistry of one or both liquids gives the scientists control over its behavior and function, and the nanoparticles themselves can have optical, electrical, or magnetic properties that researchers can take advantage of when designing a device.
To view the dynamics of the nanoparticles’ movement and assembly, the researchers used grazing incidence small-angle x-ray photon correlation spectroscopy, which they performed at the APS X-ray Science Division Dynamics & Structure Group’s x-ray beamline 8-ID-I at the APS, a Department of Energy Office of Science user facility at Argonne National Laboratory. Because the x-ray beam from that beamline is coherent, the x-rays striking the sample at a small angle show a frequency shift that can be used to measure the motion of the particles. They chose gold nanoparticles because gold provides a high x-ray contrast that allowed them to perform the experiment with great accuracy. They could measure motions over distances from 10 nm to nearly a micrometer, with each measurement lasting tens of seconds (Fig. 1).
They found that different collections of nanoparticles move at different speeds, a phenomenon that is common in soft, glass-forming materials. They saw that the speed at which the nanoparticles move changes as they congregate at an interface. As the particles cluster at the surface of the liquids, they slow down, and the speed at which they relax to an equilibrium state is five orders of magnitude slower than for free particles. When the interface is almost completely full of particles, jamming takes over, and the time for relaxation increases suddenly. After that, the dynamics slow exponentially, until the particles gradually rearrange themselves so that the whole system is collectively jammed.
The heterogeneity of the assembly, with some groups of nanoparticles moving slower and others more quickly, was a surprise to researchers, who expected the behavior to be more uniform. Researchers were also surprised by the slowness of the motion. Beyond their use in building structured liquids, the nanoparticle assemblies can help physicists examine questions about the behavior of glass materials.
For future work, these scientists would like to run similar trials on particles that are in some way different from each other. For instance, they might use nanoparticles of distinctly different sizes. Or they might mix particles that are chemically different. They could, for example, combine gold nanoparticles, which are conductive, with silica nanoparticles, which are insulating, allowing them to create a patterned surface. That would allow them to build structured droplets that they could alter chemically and treat the droplets as larger particles that could also interact. ― Neil Savage
See: Paul Y. Kim1, Zachary Fink2, Qingteng Zhang3, Eric M. Dufresne3, Suresh Narayanan3, and Thomas P. Russell1*, “Relaxation and Aging of Nanosphere Assemblies at a Water−Oil Interface,” ACS Nano 16, 896 (2022). DOI: 10.1021/acsnano.2c00020
Author affiliations: 1Lawrence Berkeley National Laboratory, 2University of Massachusetts Amherst, 3Argonne National Laboratory
Correspondence: * [email protected]
This work was supported by the U.S. Department of Energy (DOE) Office of Science-Basic Energy Sciences, Materials Sciences and Engineering Division under Contract No. DE-AC02-05- CH11231 within the Adaptive Interfacial Assemblies Towards Structuring Liquids program (KCTR16). This work was done in collaboration with Lawrence Berkeley National Laboratory and Argonne National Laboratory and supported by the National Science Foundation and the DOE Science Graduate Student Research Program. This research used resources of the Advanced Photon Source, a U.S. DOE Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.
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