The Physics of Attractive Colloids

The behavior of attractive colloidal particles is the subject of my PhD thesis, advised by Prof. David A. Weitz at Harvard University. Colloids are microscopic objects that are small enough to have their dynamics driven by thermal energy, like atoms. But unlike atoms, colloids are big enough to see with light, and their interactions can be very finely controlled. This makes them an ideal platform for investigating the properties of different kinds of materials, one particle at a time. In the laboratory, these colloidal particles (1 micron spheres) are visible and their positions can be precisely located using a confocal microscope (see this book chapter for an introduction to the technique), enabling precise studies of the dynamics of colloidal structures. We also launch colloidal samples into space, running experiments aboard the International Space Station, to look at long-time behavior in the absence of gravity.

 

Probing Colloidal Phase Transitions, Aggregation and Gelation with Confocal Microscopy at Harvard University

Attractive Colloids on Earth

We use confocal microscopy to determine the three-dimensional positions of thousands of particles as a function of time. We control the interactions between the particles to make them attractive, and can control the range and strength of this attraction potential. We are able to create and observe a number of phases, including equilibrium fluids, a kinetically-arrested gels, and large clusters that persist (see paper in Physical Review Letters, and movies of cluster and gel phases). More recently, we have found that gelation is universally driven by liquid-gas spinodal decomposition — a thermodynamic instability leading to equilibrium liquid-gas phase separation — for short-ranged attractive particles (see paper in Nature, and movies of a fluid, spinodal decomposition and arrested gel). Our ultimate aim is to establish a general framework for understanding the behavior of attractive colloid systems, and which physics drives their formation and properties. One particular limit, equilibrium phase separation near the critical, takes a very long time to observe, and we therefore conduct experiments in space, where we don't have to worry about long-time issues of sedimentation.

 

Exploring Phase Transitions with Astronauts onboard the International Space Station

Attractive Colloids in Space

Under normal conditions on earth, the separation between liquids and gases is rather uninteresting to watch: the level of water in a drinking glass simply falls as the liquid evaporates into a gas, since the liquid is denser. Onboard the International Space Station (ISS), orbiting the earth, gravity is so weak that this density difference does not draw the liquid to the bottom, and instead complex and beautiful patterns form. By looking at these patterns, and how they evolve over time, we investigate the behavior around the critical point, a specific set of conditions (temperature, and pressure), where the properties of liquids and gases become the same. Under these conditions, fluids have interesting properties that have been used for a wider range of applications, from extracting the delicate molecules of new pharmaceuticals from plants, to providing an environmentally-friendly solvent for dry-cleaning, to powering the rocket engines that send payloads throughout our solar system. And the experiment itself is a lot of fun to perform: I even got to a few phone calls from space, from the astronauts aboard the ISS, which you can hear below!

 

Related Links

Original research paper on colloidal gelation in Nature [ pdf ]
Original research paper on colloidal clusters in Physical Review Letters [ pdf ]
Official NASA brochure of the BCAT3 experiment [ pdf ]
Conversations with astronauts in space:
  • ISS Call from Astronaut Mike Foale 1 [ mp3 ]
  • ISS Call from Astronaut Mike Foale 2 [ mp3 ]
  • ISS Call from Astronaut Mike Fincke [ mp3 ]
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