Thermodynamics and Dynamics of Mesophases from Novel Self-Assembling Building Blocks

Cornell University, Ithaca NY

Investigators

Abstract

1033349 Escobedo Intellectual Merit: Motivated by the growing ability to experimentally produce particles of almost any imaginable shape in the nano- to micro-size range, the goal of this proposal is to develop and apply novel molecular simulation methods to study the thermodynamic and dynamic properties of partially ordered phases (mesophases) of systems containing rigid colloidal particles of polyhedral shapes. This goal lies within the scope of nanotechnology that seeks to achieve greater control of orderly assembly of nanoscale objects; specifically, by elucidating how multifaceted building blocks form novel self-assembled structures. In this context, particle shape complementarity plays the role of an ?entropic bonding? that helps orient and position particles in regular patterns (even in the absence of chemical selectivity). The particle shapes to be investigated are convex space filling polyhedrons such as polygonal prisms, truncated octahedron, and rhombic dodecahedron. Selected binary mixtures of these particle types will also be studied, including mixtures of triangular and hexagonal prisms (which may template photonic band-gap materials), and mixtures of octahedra and tetrahedra (which would lead to model nano assembled 3D compounds). The models used are coarse grained representations of colloidal particles whose effective inter-particle interactions can be tuned by surface functionalization or by the composition of the solvent media. It is expected that many of these systems will exhibit a liquid crystalline phase or a plastic solid in between the isotropic phase (at low concentrations) and crystal phase (at high concentrations). To identify such mesophases, advanced Monte Carlo methods and order parameters will be used to outline their phase boundaries and characterize their structure. To elucidate how such mesophases form and melt, the kinetics and mechanism of isotropicmesophase transitions will be investigated via novel path sampling methods. To elucidate how particles and defects move in such phases, molecular dynamic simulations will be performed to track and characterize their motion at equilibrium conditions and under steady shear flow. Some of these mesophases may exhibit unusual shear response like flow directionality and yield stress. The methodological developments to be pursued are: (i) optimization of novel forward flux sampling to study the kinetics of order disorder phase transitions and to identify good orderparameters to characterize mechanism, and (ii) extension of expanded ensemble methods to simulate mesophase transitions in pure and binary systems using suitable order parameters. The proposed research can thus be seen as having a dual scope. The primary goal is to elucidate the thermodynamic and dynamic behavior of model rigid building blocks that have potential uses in the nanotechnology of self assembly. The secondary goal is to formulate novel numerical statistical mechanics techniques that have potentially widespread applications. Broader Impacts: This work is complementary to experimental efforts by collaborators who will try to realize the predicted novel phases and test their mechanical, optical, and rheological properties. In the long term, the results could impact the ceramic, plastics, and semiconductor industries by helping broaden the approaches available to develop strong nano composites with high particle loadings, sieves with regular topology, liquid armors, colloid based mesocrystals for light control in photonic materials, sensors and lubricants sensitive to stress directionality, and nanocrystal arrays for photovoltaics. Advances in simulation methods should also help materials modelers to improve product properties by predicting and exploiting meso-scale, entropy aided self-assembly. The graduate and undergraduate students involved with this project will get ample exposure to the physics and engineering of colloids while acquiring a significant expertise on multiple molecular and mesoscopic modeling techniques. They will also coordinate with the Cornell Center for Material Research (CCMR) to create a teaching module on Nano-Lego engineering: harnessing entropy to create order? for use in local high schools. Our scientific results will be disseminated through professional meetings and an industrial outreach program organized by CCMR. Results of this investigation will be used in at least two courses: a new course on molecular simulations and the advanced Chemical Engineering thermodynamics core course.