Browsing by Author "Zhang, K"

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    ADVANCES IN THE MOLECULAR SIMULATION OF MICROPHASE FORMERS
    (2022-01-01) Charbonneau, P; Zhang, K
    This chapter details the different experimental microphase formers and provides a minimal theoretical framework to present the simulation challenges associated with studying model microphase formers. Block copolymers are by far the most studied microphase formers. The chapter focuses on the phenomenological field theory description of the universality of the microphase formation and of the nature of the order-disorder transition. The chapter describes molecular simulation methods that have been specifically designed to achieve equilibrium in the periodic microphase regime. It details the thermodynamic framework and a free energy integration simulation method, followed by a concrete introduction to the ghost particle/cluster switching method. The chapter discusses several classical Monte Carlo algorithms to enhance the efficiency of simulating disordered microphases. It presents three models for which quantitative results have been obtained: a one-dimensional, a lattice, and an off-lattice microphase former. Fine-tuning colloidal suspensions to allow the formation of periodic microphases thus remains an open experimental problem.
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    Erratum: [N]pT ensemble and finite-size-scaling study of the critical isostructural transition in the generalized exponential model of index 4
    (Physical Review E - Statistical, Nonlinear, and Soft Matter Physics, 2012-12-05) Zhang, K; Charbonneau, P
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    Monte Carlo approach for studying microphases applied to the axial next-nearest-neighbor Ising and the Ising-Coulomb models
    (Physical Review B - Condensed Matter and Materials Physics, 2011-06-09) Zhang, K; Charbonneau, P
    The equilibrium phase behavior of microphase-forming systems is notoriously difficult to obtain because of the extended metastability of their modulated phases. In this paper we present a systematic simulation methodology for studying layered microphases and apply the approach to two prototypical lattice-based systems: the three-dimensional axial next-nearest-neighbor Ising (ANNNI) and Ising-Coulomb (IC) models. The method involves thermodynamically integrating along a reversible path established between a reference system of free spins under an ordering field and the system of interest. The resulting free-energy calculations unambiguously locate the phase boundaries. Simple phases are not found to play a particularly significant role in the devil's flowers and interfacial roughening plays at most a small role in the ANNNI layered regime. With the help of generalized order parameters, the paramagnetic-modulated critical transition of the ANNNI model is also studied. We confirm the XY universality of the paramagnetic-modulated transition and its isotropic nature. © 2011 American Physical Society.
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    [N]pT ensemble and finite-size-scaling study of the critical isostructural transition in the generalized exponential model of index 4.
    (Phys Rev E Stat Nonlin Soft Matter Phys, 2012-10) Zhang, K; Charbonneau, Patrick
    First-order transitions of system where both lattice site occupancy and lattice spacing fluctuate, such as cluster crystals, cannot be efficiently studied by traditional simulation methods, which necessarily fix one of these two degrees of freedom. The difficulty, however, can be surmounted by the generalized [N]pT ensemble [J. Chem. Phys. 136, 214106 (2012)]. Here we show that histogram reweighting and the [N]pT ensemble can be used to study an isostructural transition between cluster crystals of different occupancy in the generalized exponential model of index 4 (GEM-4). Extending this scheme to finite-size scaling studies also allows us to accurately determine the critical point parameters and to verify that it belongs to the Ising universality class.