377937 Nonequilibrium Self-Assembly and Structures

Sunday, November 16, 2014
Galleria Exhibit Hall (Hilton Atlanta)
Amir Vahid, Non-Equilibrium Energy Research Ctr, Chemical and Biological Engineering Dept, Northwestern University, Evanston, IL

My research focuses on nonequilibrium self-assemblies (NESAs) that require energy supply to maintain their order or, as cells, to “survive”. Nonequilibrium assemblies are energy (external field) driven systems that reside in steady states or long-lived metastable structures, some of which can be highly dissipative. DNA transcription machinery, self-propelling micelles, liquid crystals under external fields, cytoskeletal fibers, and molecular motors are all examples of various aspects NESAs. In my research, these complex assemblies are studied using the tools of nonequilibrium thermodynamics. Until now, NESA in artificial systems has been achieved by modifying the interactions between the constituent objects/parts. In the proposed methodology, I will look at the problem from a fundamentally different perspective – namely, I will keep the interactions between the components intact but instead change the agitation mode to achieve different structures. For this purpose, I aimed to control the particle SA in nonequilibrium regime by applying local anisotropic vibrations (for the solvent or particles or both), which effectively translating into anisotropic temperature. 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.  In my research, I have first studied such hypothetical situations by means of modeling. These experiments validated my hypothesis that the ultimate shape of the assemblies can be controlled by the mode of anisotropic agitation. With this foundation, my current and proposed research will focus both on understanding the fundamental implications of having anisotropic temperature as well as implementing such NESA systems in practice. In terms of theory and modeling, I will implement 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, I will perform agglomeration tests by measuring the orientational ordering (to probe and quantify the structure of the growing assemblies). I will also calculate energies and entropies at various stages of NESA and compare the results with those of nonequilibrium coarse-grained field theories.  Simultaneously, I will implement several system in laboratory practice – these systems will be based on larger assembly components being surrounded by smaller magnetic particles that can be anisotropically “jiggled” by time-varying external fields. These magnetic particles are the non-Brownian particles effectively creating an anisotropic temperature field experienced by the larger particles. The ultimate objective – and, indeed, great hope – of my work, is that I will be able to engineer self-assembling structures of arbitrary shapes and properties by designing appropriate agitation “schedules”. Of course, magnetic agitation is but the first step in these directions, and I also envision other classes of systems based on acoustic or pressure waves, and also on rapidly varying external temperature gradients. 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.

Molecular dynamics simulation has employed to test the above hypothesis. 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 (SA) structures including hexagonal and rod shaped considering hydrodynamic and friction effects. It is found that different agitation modes play crucial form in obtaining different SA patterns. These patterns are found in DNA-coated particles, colloids, polymers, and supercooled fluids.

As a faculty candidate, I would apply my beyond equilibrium knowledge to the SA in magnetic fluids in the presence of a magnetic field as an external force and study different effects such as static and alternative field to develop a complet theoretical approach for dynamic self-assembly far from equilibrium. The results of this study would be a basis for more complex NESA systems such as photoswitchable nanoparticles or to control NESA systems that are coupled with chemical reaction networks.

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