Control of colloidal shapes may lead to hierarchical materials design

LAURA LEAY: Welcome to MRS Bulletin’s Materials News Podcast, providing breakthrough news & interviews with researchers on the hot topics in materials research. My name is Laura Leay.  So many people have missed out on face-to-face conferences over the last few years. Newly published research from a long running collaboration shows how important they are to making new discoveries. Laura Rossi works on novel techniques to create complex colloidal particles but it was only after seeing a talk from Erin Teich that the collaboration was formed. Erin is part of Greg van Anders’ research group and her talk was about simulating the packing of polyhedra inside a sphere. It gave rise to a ground-breaking new research direction for Laura.

LAURA ROSSI: Experimentally we had these superballs that can interpolate between a sphere and a perfect cube, right.

LAURA LEAY: The superballs are about a micron wide and Laura’s group had originally been working to understand how they packed in two dimensions and in the bulk. The talk by Erin led the two research groups to look at what happens when the superballs are confined.

LAURA ROSSI: These are dispersed in water and then we can simply emulsify the water that contains our particles and then we evaporate the water selectively so that we compress the particles together and then we obtain a variety of clusters formed by a different number of particles. Once the particles are touching and all of the water is evaporated then van der Waals forces keep the clusters in shape. 

LAURA LEAY: The clusters formed by the superball particles have been observed under vacuum in a scanning electron microscope so it’s likely that the van der Waals forces are sufficient to hold the clusters together, preserving their defined structures. The structures that were seen experimentally by Laura’s group were confirmed by simulations from Greg’s group.

GREG VAN ANDERS: In simulation we try to model these evaporating emulsion droplets. We model them as rigid spheres and then the particles inside as rigid particles. So then our simulation protocol was to translate and rotate the particles inside the hard sphere as we were basically reducing the size of the sphere slowly. We were only looking at the shape of the particles and the arrangement of the particles within the droplet with no interactions other than the – basically - hard, excluded volume between the particles. 

LAURA LEAY: Since simulations are able to look only at the effects of shape, they show that particle shape rather than interatomic forces is the governing factor in determining the structure of these clusters. Experimental work by Laura with magnetic superballs, made by depositing silica on hematite cubes, further proved that shape is important to the formation of these clusters. The simulations also looked at what would happen if you allow these clusters to come together and form larger structures.

GREG VAN ANDERS: We found that we got bulk results that were quite different than what you would get if you just looked at the individual particles. We saw on large length scales, structures that had body-centered cubic and hexagonally close packed arrangements which are not the structures that Laura had seen in any of her experiments before where they looked at individual particles.

LAURA LEAY: These insights could lead to hierarchical materials with different symmetries at different length scales that can be created in a controlled manner and, as Laura explains, this can be done without controlling the chemistry.

LAURA ROSSI: One reason why that is very powerful is also because there is absolutely no surface functionalization of the particles, right. So what we obtain is simply by packing the particles into different building blocks which makes things easier if you think of scaling up and maybe applying this method from bulk principles.

LAURA LEAY: The larger structures predicted by simulations are less dense than bulk structures formed directly from the individual particles. These results could lead to the design of lightweight materials and what’s really exciting is that they help shed light on the fundamental principles behind the structure of matter. This work was published in a recent issue of Science Advances. My name is Laura Leay from the Materials Research Society. For more news, log onto the MRS Bulletin website at mrsbulletin.org and follow us on twitter, @MRSBulletin. Don’t miss the next episode of MRS Bulletin Materials News – subscribe now. Thank you for listening.