A study sheds new light on the assembly of materials in confinement

Cornell researchers used computer simulations to show how the assembly of truncated-vertex tetrahedra is affected when confined inside a spherical container. The results offer materials scientists a new method to control the assembly structure and characteristics of the resulting material. Credit: Rachael Skye

Hacking multiple pairs of shoes into a vacation suitcase, twisting and flipping them in different arrangements to fit each pair needed, is a familiar optimization problem faced by time-pressed travelers. This same problem is well known to engineers: when given a number of objects with a particular shape, how can they be packed into a container? And what pattern will this packaging form?

Unlike the contents of a suitcase, the way the microscopic particles are put together can be used to design the characteristics of the materials they form; for example, how light or electricity travels. Materials scientists have long studied how particle assembly in confined space can be used as a tool to give materials new capabilities, but how uniquely shaped particles interact with a barrier remains poorly understood.

A new study by researchers from Cornell University’s Department of Materials Science and Engineering has used computer simulations to show how assembling truncated-vertex tetrahedra – a particle shape that has four hexagonal faces and four triangular faces – is affected when confined inside a spherical container. The results, published in the journal soft materialoffer materials scientists a new method to control the assembly structure and characteristics of the resulting material.

A study sheds new light on the assembly of materials in confinement

Simulations of 10,000 particles in spherical containers, (a–c) seen from the outside and (d–f) in cross sections. Three different shapes are highlighted: Platonic tetrahedra (a and d), space-filling truncated tetrahedra (b and e) and Archimedean truncated tetrahedra (c and f). The coloring corresponds to local particle environments: blues represent particles that are mostly vertex-to-vertex, and oranges correspond to the majority vertex-to-edge. White particles are not classified. Simulations show that a wall can alter the behavior of nearby particles, allowing researchers to selectively assemble different structures. Credit: Rachael Skye

“Previously, theorists mostly did simulations with spheres because most particles are roughly spherical, and it was computationally simpler,” said Rachael Skye, PhD student and first author of the study, “but experimenters keep coming up with exciting ways to control shape and now they can create colloidal particles like tetrahedrons, octahedrons or cubes. With advanced computing power, we can simulate these shapes, but also go further and predict what new particles not yet synthesized might do.

To help fill the knowledge gap about how these particle shapes come together in confinement, Skye and the study’s lead author, Julia Dshemuchadse, assistant professor of materials science and engineering, simulated assemblies of tetrahedral particles in spherical containers. Each contained as few as four particles and up to 10,000. In each simulation, the container would shrink as much as possible with the programmed number of particles inside.

“This simulation mimics the way some colloidal materials are produced, with particles placed inside a liquid droplet that contracts as it evaporates,” Dshemuchadse said.

These particles can fit together in many ways, but there are two distinct patterns: aligned, with hexagonal faces adjacent, or anti-aligned, with a hexagonal face adjacent to a triangular face. Each pattern results in an overall structure that conforms differently to container borders.

Une étude apporte un éclairage nouveau sur l'assemblage des matériaux en confinementNat Common“/>

An example of a self-assembling colloidal cluster confined in a water-in-oil emulsion droplet, a project led by the Friedrich-Alexander-University of Erlangen-Nürnberg. Cornell’s simulations could help control the assembly of future colloidal materials. Credit: Wang, J., Mbah, CF, Przybilla, T. et al. Colloidal clusters of magic numbers as minimal free energy structures. Nat Common

“If you have these anti-aligned particles, you can very well form flat layers and stack them infinitely, which makes it a very good crystal,” said Dshemuchadse, who added that this pattern is favored when simulating a large number of particles because the larger container size has a smaller curvature,” but if you have the particles aligned, the structure can form a curved pattern that fits better into a spherical shell. At a small number of particles, the aligned pattern is favored because the smallest containers have large curvatures.”

The results provide materials scientists with a method to grow large crystals in particle systems that do not typically assemble into ordered structures. Other methods of obtaining a well-ordered crystal involve techniques such as “seeding” the material with particles strained in specialized orientations that result in the corresponding structure, but these methods require the fabrication of new types of particles, which would be less simple in an experimental realization of these systems. In contrast, forming crystals on a flat substrate is often the norm, and this study shows how this technique can benefit the resulting structure.

“Colloidal crystals tend to be small and full of flaws, but for them to be useful in most applications they need to be fairly large and flawless,” Skye said. “The idea is that by choosing your container or wall correctly, you can create a much larger and higher quality crystal than you otherwise could.”

Skye added that in fields such as plasmonics and photonics, this assembly technique can be used to orient the same particle in two different ways, allowing engineers to create devices that have different responses depending on formation. chosen assembly.

Bottom-up construction with a 2D twist could produce new materials

More information:
Rachael S. Skye et al, Tuning of hard-form assembly structures in confinement via interface curvature, soft material (2022). DOI: 10.1039/D2SM00545J

Provided by Cornell University

Quote: Study sheds new light on materials assembly in containment (August 19, 2022) Retrieved August 19, 2022 from https://phys.org/news/2022-08-materials-confinement.html

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