Angelo Cacciuto, PhD , Columbia University
In the first case, we systematically studied how deviations from the spherical shape of hard particles affect their ability to spontaneously organize into ordered structures when compressed at high densities. In the second case we explored what is the effect of breaking the dogma of mutual non-penetrability between nanoparticles on their phase behavior.
(1) Far from being a purely mathematical problem, understanding how aspherical particles pack together, and specifically establishing geometric features at the particle level that can be used to sort out which shapes are expected to form ordered phases and which ones are expected to form unstructured fluids, is a very important problem. In fact, issues of packing and crystallization find application in a variety of fields including extraction of oil through porous materials, storage of grains, but also memory storage, protein crystallization and production of photonic materials. The research performed in our group on this problem is the first of its kind, and we managed not only to establish clear guidelines for the manufacturing of nanoparticles shape, but also to compute the phase behavior of monodisperse and polydisperse systems of hard aspherical components. We have shown that below a treshold degree of polydispersity the phase diagram has universal features that are independent of the specific particle shape, and we introduced the new concept of shape screening in the context of crystal formation.
(2) Understanding the phase behavior of ultra-soft nanoparticles is also a problem at the cutting-edge of the field of complex fluids. Indeed, the ability of particles to be squeezed over each other at a constant free energy cost is an inherent property of polymer-based nanoparticles such as charged or neutral star polymers, dendrimers or microgels. Surprisingly, the simple relaxation of excluded volume constraints results in an extremely rich and unexpected phase behavior. Our research explored several aspects of these systems. We systematically studied the packing of soft nanoparticles in two dimensions and uncovered a plethora of novel close-packed phases, including snub kogame and other open structure crystals. Our work exposed how phase behavior and structural diversity are affected by the functional form of the potential, and established clear guidelines linking them to each other.
Overall I believe that the PRF grant was greatly beneficial for my career and for that of the graduate student that it supported. Our research produced so far two publication and a third one is about to be submitted. The key point here is that the PRF grant allowed us to perform some highly exploratory research for which no previous work had been previously published. Specifically, I believe that our work on aspherical particles will establish a new direction in field of crystallization and self-assembly of nanoparticles. What we generated were the very first steps, but much more work that we have already planned for next year needs to be done. Our research on ultra-soft nanoparticles is also very relevant as the link between structure and pair interactions is a crucial factor in the phase behavior of these systems. Our plan is to keep on exploring the features of these phase diagrams to better understand the limit of validity of these pair potentials when describing such complex mesoparticles with internal degrees of freedom, and eventually develop systematic coarse-graining strategies to better describe their physical properties.
The graduate student working on these projects has mastered the most advanced techniques in computer simulations for free energy calculations of complex fluids, and has been exposed to some very intriguing scientific puzzles whose realm goes well beyond the standard studies of a physical chemistry graduate program. He also attended an international summer school on related material over the summer.