Compact expansion of a repulsive suspension

Short-range repulsion governs the dynamic behavior of matter across length scales, from atoms to animals. As the density increases, the dynamics transition from nearest-neighbor to many-body interactions, posing a challenge for an analytical description. Here we use theory, simulations, and experiments to show that a suspension of particles with short-range repulsion spreads compactly. Unlike the diffusive boundary of a spreading drop of Brownian particles, a compact expansion is characterized by a density profile that is strictly zero beyond a cutoff distance. Starting from the microscopic interactions, we derive an effective, non-linear diffusion equation and find that the dynamics exhibit two distinct transitions: (1) when very dense, particle-particle interactions extend beyond nearest neighbors, and the ensemble grows in a self-similar fashion as time to the power of 1/4. (2) at lower densities, nearest-neighbor interactions dominate, and the expansion slows to logarithmic growth. We examine the second regime experimentally by monitoring the expansion of a dense suspension of charge-stabilized colloids. Using simulations of thousands of particles, we observe the continuous crossover between the self-similar and the logarithmic dynamics. Our results are general and robust, with practical implications in engineering and pharmaceutical industries, where suspensions must operate at extreme densities

Progress in the Self-Assembly of Arbitrarily Designed Functional Colloidal Structures

Are we “at home in the universe”? Is life the way we know it is nothing but a statistical fluke, or is it part of a deterministic process begging to happen in a diverse molecular soup? Geological evidence of cyanobacteria existence dates back 3.7 billion years ago, suggesting that life did not wait much for luck to emerge on our 4.5 billions years old planet. And perhaps life indeed should be expected as a deterministic consequence of complexity. The seemingly spontaneous, some say divine, formation of complex structures with diverse function out of mud intrigued philosophers and scientists from the beginning of history. This notion of “order for free” and self-organization has recently became appealing for physicist and material scientists dealing with complex matter. The world is complex – not only in the philosophical sense, but also in the very material sense. Most materials we encounter daily are non-crystalline, dynamic, or gooey — i.e. complex materials; from the ink we use to write a check, to the very skin we live in, from the patterns on a seashell, to the membrane surrounding a living cell. A physicist treating self assembly tries to establish an experimental and a theoretical framework for emergent, complex phenomena built on the foundations of the underlying geometry, energy source, and constituent compounds. Complex, self-organize systems aspire to explore systems where a “plain vanilla” description is not enough. Adopting Feynman’s spirit “What I can not make I do not understand”, we program the self assembly of complex matter. By making, we hope to discover the underlying laws and formulating set of rules and relations that would help biologists and engineers to unravel and to re-invent the fabric of our existence