Gelation of Particles with Short-ranged Attraction
Nanoscale or colloidal particles are exceptionally important in many realms of science and technology. They can dramatically change the properties of materials, imparting solid-like behavior to a wide variety of complex fluids, from yoghurt to cast ceramics. This behavior arises when particles aggregate to form mesoscopic clusters and networks. The essential component leading to aggregation is an interparticle attraction, which can be generated by many physical and chemical mechanisms. In the limit of irreversible aggregation, infinitely strong interparticle bonds lead to diffusion-limited cluster aggregation (DLCA), long-understood as a purely kinetic phenomenon, which can form solid-like gels at arbitrarily low particle density. Far more important technologically are systems with weaker attractions, where gel formation requires higher colloid densities. Numerous scenarios for gelation have been proposed, including DLCA, kinetic or dynamic arrest, phase separation, percolation, and jamming. No consensus has emerged, and despite its ubiquity and significance, gelation is far from understood; even the location of the gelation phase boundary is not agreed upon. I will show that gelation of spherical particles with isotropic short-ranged attractions is initiated by spinodal decomposition; this thermodynamic instability triggers the formation of density fluctuations, leading to spanning clusters that dynamically arrest to create a gel. This simple picture of gelation does not depend on microscopic system-specific details, and should thus apply broadly to any short-ranged attractive particle system. Our results suggest that gelation, often previously considered a purely kinetic phenomenon, is in fact a direct consequence of equilibrium liquid-gas phase separation.
Peter J. Lu received his AB summa cum laude in physics (2000) from Princeton University, and AM (2002) and PhD (2008) in physics from Harvard University. He is presently a post-doctoral research fellow in the Department of Physics and in the School of Engineering and Applied Sciences at Harvard University; his main focus is on the physics of attractive colloids and the integration of high-performance imaging and analysis techniques. He has conducted a series of experiments aboard the International Space Station, examining phase separation of colloid mixtures in the absence of gravity. He has also published his discoveries of modern quasicrystal geometry in medieval Islamic architectural tilings; the first precision compound machines, from ancient China; the first use of diamond, in prehistoric China; and the first quasicrystalline mineral found in nature.
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