Atherton Group Residency in Australia, Week 2

As reported in a previous blog post, the Atherton group has been awarded an International Research Grant  to be resident in Patrick Spicer‘s lab at the University of New South Wales, Sydney, Australia. This post is about what we got up to in the second and last week of the residency. 

One thing that the residency has made possible is the sort of informal interactions with Patrick’s group—located on the opposite side of the world!—that we take for granted with our colleagues at Tufts. During one of our discussions, Patrick talked about some of his recent work showing that emulsions with elongated droplets are more effectively deposited on a surface, which is important for consumer products like shampoo but also potentially for drug delivery.

Conveniently, Chris Burke has been working for a while on studying how spherical particles on an ellipsoidal droplet can stabilize its shape, as you can read about on previous blog posts. Normally, an ellipsoidal droplet relaxes into a spherical shape as shown. It does so in order to minimize the surface area and hence the amount of contact between the two fluids. The idea is that if enough particles can be attached to the fluid interface, forming what is called a Pickering Emulsion, the particles become crowded as the surface is reduced and jam, preventing the droplet from relaxing further. The process is actually quite reminiscent of Patrick’s work on arresting the coalescence of two droplets.

One of the most exciting moments of the trip came as Patrick called us into the lab one afternoon: his student Zengyi Wei had actually managed to produce one of the predicted droplets! We were delighted, and our objectives now are to reproduce the experiment and also apply the image analysis program shown in our first post. Once we have more data, we will be able to achieve our goal in this project, which is to classify the detailed microstructure—how the particles are packed—and to correlate this to the stability of the droplets.

So what are we looking for in the packings? In the last blog post, we talked about “scars”, chains of defects that are necessary to accommodate the curvature. Another tool we can bring to look at is what symmetries are present in the packings, which requires us to use the mathematical language of group theory. Chris has already run many thousands of simulations on the Tufts high performance computing cluster, generating possible packings. During the residency, he wrote a program to classify the number of symmetries present in a configuration, that is how many possible rotations, inversions and flips and combinations leave the configuration unchanged.

Below, we show a plot of packings with different particle number and aspect ratio of the ellipse where the colors represent how many symmetries exist (deeper colors mean more symmetries)—blue is for achiral configurations and orange is for chiral structures. You can see a rich variety of highly symmetric structures for particular values of ellipsoid shape and particle number, with two representatives depicted. We’re interested in these really symmetric structures because we expect them to be extremely stable—the symmetric structures of some viruses actually occurs for similar reasons.

 

With two weekends included in the trip, we were also able to explore quite a few cultural aspects of Sydney, thus fulfilling one of the additional objectives of the trip, to facilitate intercultural exchange. We all got to go surfing at Manly, NSW, a beautiful beach accessible by public transit from Sydney. Chris and Kate hadn’t surfed before, so they got to experience the fascinating and highly complex physics of surfing first hand, ably guided by instructors from the Manly surf school. Tim checked out the amazing Art Gallery of New South Wales and was impressed by the richness of Australian art; Kate and Chris went to the zoo and aquarium, being enchanted by Australian fauna.

We were also delighted to contribute—if in a very minor way!—to another worthy science project while we were in Sydney: Awoken by shrieking sounds from our balcony one morning, we were intrigued to meet Cockatoo 013 Travis (pictured) who is being tracked as part of the Wingtags project to monitor their movements, breeding and habitat preferences. We reported the sighting using the cool Wingtags iPhone App.

 

We had an amazingly productive trip to Sydney—the results and programs developed during the residency will fuel several scientific papers that we hope you’ll look out for. We’d like to thank: Patrick, Zengyi and UNSW for hosting us; all the wonderful people we met in Sydney who made us feel so much at home; Tufts and the anonymous donor for the Grant for making this residency possible. Many thanks also to Kate Voorhes for taking some wonderful photos—so multitalented!