431398 Anisotropic Effects in Non-Equilibrium Self-Assembly

Wednesday, November 11, 2015: 10:30 AM
Canyon B (Hilton Salt Lake City Center)
Amir Vahid, Chemical and Bimolecular Engineering, Northwestern University, Evanston, IL, Bartosz A. Grzybowski, Chemistry, UNIST, Ulsan, South Korea and Fateme Sadat Emami, Chemical Engineering, Northwestern University, Evanston, IL

In this research, non-equilibrium self-assemblies (NESAs) are studied using the tools of non- equilibrium thermodynamics and molecular simulation techniques. The proposed mesoscopic model captures the relation between microscopic structure and macroscopic behavior. Until now, NESA in artificial systems has been achieved by modifying the interactions between the constituent objects/parts. In the proposed methodology, we looked at the problem from a fundamentally different perspective – namely, we will keep the interactions between the components intact but instead change the agitation mode to achieve different structures. For this purpose, we aim to control the particle self-assembly (SA) in non-equilibrium regime by applying local anisotropic vibrations (for the solvent or particles or both), which effectively translating into anisotropic temperature that can be considered through hydrodynamic effects. The hydrodynamic effects along with dynamic anisotropy bias the particles toward the desired configuration. Several simulations with various settings were performed including change of the long-range interactions between the beads, changing the density of the system, relative diameters. The anisotropy in temperature would act as an external field that can form various patterns of self-assembled structures including hexagonal and rod shaped considering hydrodynamic and friction effects. The anisotropic temperature is imposed through changing the velocity of the particles. It is found that different agitation modes play crucial role in obtaining different SA patterns. According to the equipartition theorem at thermal equilibrium, kinetic energy of the particles is equally divided into all their degrees of freedom translating into isotropic temperature. This theorem does not hold outside of equilibrium and, in principle, one can envision situations in which different degrees of freedom have different “thermal” energies. We have first studied such hypothetical situations by means of modeling. Molecular dynamic validated the hypothesis that the ultimate shape of the assemblies can be controlled by the mode of anisotropic agitation. We implemented multiscale simulations that bridge the gap between the molecular (or nano) scale of the non-equilibrium agitating particles moving anisotropically, and the larger particles that are being agitated. During the simulations, we will perform agglomeration tests by measuring the orientational ordering (to probe and quantify the structure of the growing assemblies). We also calculated energies and entropies at various stages of NESA and compare the results with those of non-equilibrium coarse-grained field theories. The ultimate objective – of this work, is that we will be able to engineer self-assembling structures of arbitrary shapes and properties by designing appropriate agitation “schedules”. This work will create a novel experimental test-bed for studying non-equilibrium self-assembly under well-defined conditions. It can also lead to practical development in designing efficient bottom-up micro- and nanomanufacturing schemes. 

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