Atherton Group Residency in Australia, Week 1

The Soft Matter Theory Group was recently awarded an International Research Grant from Tufts. These prestigious grants, funded by an anonymous donor, are intended to provide sufficient resources to enable research teams in the sciences to carry out research at a laboratory outside the USA. In our case, we’re visiting Patrick Spicer’s group in the school of Chemical Engineering at the University of New South Wales in Sydney, Australia. Spicer is an experimentalist: his research portfolio includes understanding how the microscopic details of emulsions, oil-water mixtures encountered in consumer products like shampoo and ice cream, affects their performance, e.g. how well the shampoo is absorbed onto hair. As theoretical physicists, our task is to develop models of how these properties are determined by the processes at work in the emulsion. By working with an experimentalist like Spicer, we’re able to test our models and also to provide guidance on likely strategies to make better materials.

Our visiting team consists of PI Tim Atherton, graduate student Chris Burke and undergraduate summer scholar Kate Voorhes. Because we’ve been able—thanks to the unique nature of this grant—to bring so many people, we’ve had a very rich interaction with Patrick and his student Zengyi Wei. The task we’ve set ourselves for the trip is to figure out how best to connect theory to experiment, i.e. what can we calculate that can be measured? What subtleties present in the experiments might we need to include in our models? Our time has therefore been spent in discussion, performing experiments together and developing computer codes.

The system that we’re looking at is called a Pickering Emulsion, in which colloidal particles are added to an emulsion. The particles tend to be move to the oil-water interface because in doing so they reduce the total amount of contact area between the two immiscible fluids. Looking at the system under the microscope, as we show below, you can see the oil drop in surrounding water with the surface decorated by particles. Notice the packing of the particles is quite regular—and that’s what we’re interested in. In fact, the first thing we did this week is to write a computer program in Mathematica to analyze such images and try to analyze the regularity of the packing quantitatively.

Sample output is shown in the colored image above on the right—the program has located each particle and colored it by how many neighbors each particle has. Most particles on the drop are red, indicating 6 neighbors, but some are blue or green, representing 5 or 7 neighbors respectively. Looking closely at the red particles, you’ll see that they’re packed in a triangular arrangement. Around the blue or green particles, the packing is less regular and so these are called defects. The defects arise partly because the droplet’s surface is curved. You’ll also notice that the blue and green defects form chains (look at the top of the image) which have been discovered before by the Bowick group and are called scars.

As you can read in other blog posts and on our Youtube channel, my postdoc Badel Mbanga and Chris Burke have developed a code to study possible packing of particles on droplets. Looking at a sample packing from the program (right), you can see similar features to those observed in the experiments, although the simulated packing is rather more ordered as it has fewer defects.

These initial results are very encouraging—they show that we can apply the same quantitative tools to experiments and theory. We’re now ready to examine more interesting problems such as what happens when the emulsion particles combine together or coalesce, as Patrick has already studied experimentally. Since the coalescence process is what ultimately causes the emulsion to destabilize—anyone who’s left ice cream out for too long will know about this!—we have a great toolkit to determine how the nature of the particle packings change during the coalescence, and ultimately to control the stability of the emulsion.

So much for the science: An important additional component to the visit has been for the research group to participate in intercultural exchange, experiencing some of Australia’s rich culture. One way we’ve done this is to visit Sydney’s Museum of Contemporary Art, conveniently open late on thursday evenings. We were greatly impressed with the variety of media and artists represented: One of the many pieces that caught our interest was Ross Manning’s Fixational Eye (Vertical) shown on the right; a dynamic sculptural piece using light and the motion of a wire. It may just because we’re physicists, but we were quite entranced by the incredible wave-like motions made by the piece. You can see for yourselves in this video made by the artist.

We’ve had a greatly productive first week in Sydney—look out for another update next week. Thanks to Kate Voorhes for some of the images; as well as a talented physics major, she’s also a great photographer!