### Browsing by Author "Heinonen, Vili"

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Item Computational study on nanoscale ice flows(2011) Heinonen, Vili; Hynninen, Teemu; Teknillisen fysiikan laitos; Perustieteiden korkeakoulu; School of Science; Ala-Nissilä, TapioCrystal ice with its multitude of different phases and exotic surface physics is complicated beyond most of the homogeneous materials. The complex nature of ice is rooted in the hydrogen bonding that is keeping the water molecules of ice in an ordered crystal structure. The fact that ice can be melted by increasing pressure gives rise to critical behaviour when ice is pushed around some obstacle as the ice undergoes a phase transition and starts to flow. Atomistic molecular dynamics (MD) simulations are a good tool to study complex systems such as flows of ice. The complex dynamics on the boundary of ice can be reconstructed as the positions and the orientations of all the molecules are known at any time. Recently developed all-atom models for water have increased the efficiency of the computation allowing simulations of large systems. This thesis consists of two parts. In the first part a flow of ice around several different types of spheres is studied using all-atom MD simulations. The focus is in a depinning transition caused by a phase transition in ice when a large enough driving force is applied. The second part proposes a continuum flow model for ice based on the idea of pressure melting of ice. The depinning transition caused by breaking layers of ice was seen to be completely different from recent studies of flows around nanowires. The flows around spheres with same radii with the wires of earlier simulations were dominated by thermal effects as the high curvature of the spheres caused the ice to flow around the spheres with great ease. The dynamics with spheres of larger sizeswere affected by large-scale fractures in the ice and single layers of ice ceased to have any real impact on the dynamics. The continuum flow model was dominated by effects of Newtonian flows and was seen unfit for ice flows.Item Cutting Ice: Nanowire Regelation(American Physical Society (APS), 2010) Hynninen, Teemu; Heinonen, Vili; Dias, Cristiano L.; Karttunen, Mikko; Foster, Adam S.; Ala-Nissilä, Tapio; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of ScienceEven below its normal melting temperature, ice melts when subjected to high pressure and refreezes once the pressure is lifted. A classic demonstration of this regelation phenomenon is the passing of a thin wire through a block of ice when sufficient force is exerted. Here we present a molecular-dynamics study of a nanowire cutting through ice to unravel the molecular level mechanisms responsible for regelation. In particular, we show that the transition from a stationary to a moving wire due to increased driving force changes from symmetric and continuous to asymmetric and discontinuous as a hydrophilic wire is replaced by a hydrophobic one. This is explained at the molecular level in terms of the wetting properties of the wire.Item An energy stable, hexagonal finite difference scheme for the 2D phase field crystal amplitude equations(Academic Press Inc., 2016-09-15) Guan, Zhen; Heinonen, Vili; Lowengrub, John; Wang, Cheng; Wise, Steven M.; Department of Applied Physics; Multiscale Statistical and Quantum Physics; University of California, Santa Barbara; University of Massachusetts Dartmouth; University of Tennessee SystemIn this paper we construct an energy stable finite difference scheme for the amplitude expansion equations for the two-dimensional phase field crystal (PFC) model. The equations are formulated in a periodic hexagonal domain with respect to the reciprocal lattice vectors to achieve a provably unconditionally energy stable and solvable scheme. To our knowledge, this is the first such energy stable scheme for the PFC amplitude equations. The convexity of each part in the amplitude equations is analyzed, in both the semi-discrete and fully-discrete cases. Energy stability is based on a careful convexity analysis for the energy (in both the spatially continuous and discrete cases). As a result, unique solvability and unconditional energy stability are available for the resulting scheme. Moreover, we show that the scheme is point-wise stable for any time and space step sizes. An efficient multigrid solver is devised to solve the scheme, and a few numerical experiments are presented, including grain rotation and shrinkage and grain growth studies, as examples of the strength and robustness of the proposed scheme and solver.Item Phase field crystal models and fast dynamics(Aalto University, 2016) Heinonen, Vili; Ala-Nissilä, Tapio, Prof., Aalto University, Department of Applied Physics, Finland; Teknillisen fysiikan laitos; Department of Applied Physics; Multiscale Statistical Physics group; Perustieteiden korkeakoulu; School of Science; Ala-Nissilä, Tapio, Prof., Aalto University, Department of Applied Physics, FinlandEmergent microscale structural properties of crystalline materials have been puzzling scientist since the dawn of modern solid state physics. The microstructure of crystalline materials is affected by disorder: topological defects such as vacancies and grain boundaries can fundamentally change both the electronic and structural properties of materials. The microstructure is determined by the manufacturing process, the most famous being solidification from melt. Understanding the solidification process can be used to produce materials with specific properties in a controlled way. Classical density functional theory (CDFT) and its computationally more efficient counterpart, the phase field crystal (PFC) model, have emerged as efficient tools for studying microscopic crystalline properties of materials at diffusive time scales, avoiding the unnecessary short time scale of thermal vibrations. In this thesis we further develop the PFC methodology. First, we use the DFT and PFC methods to calculate the free energy of the solid–liquid boundary of the well-known Yukawa system. In the second part – the main body of the work – we develop fast dynamics for the PFC framework. The last part is dedicated to computational work: we propose an efficient numerical scheme for solving the dynamics of the PFC amplitude system. Developing fast dynamics tackles an old problem of diffusive dynamical systems. The presence of collective vibrations of atoms is characteristic of metallic materials. In principle all the dynamical theories that describe elasticity of materials should also include these vibrations. However, diffusive dynamics are unable to describe such vibrations. To this end, we propose two approaches. In the first approach the elastic excitations that create these vibrations are explicitly equilibrated. In the second approach we develop a hydrodynamic theory for the PFC framework based on physical conservation laws. In both cases we demonstrate the viability and the importance of the approach. The latter theory is more general and can be used to describe a larger spectrum of fast phenomena such as advective mass transport. The theories developed in this thesis allow studying previously inaccessible problems such as fast solidification. In addition they should produce better understanding of many problems where fast dynamics are present. These include problems where relaxation of elastic excitations is important, such as polycrystalline coarsening, and problems where mass transport is important e.g. certain type of dendritic solidification. Another advantage of the methods presented here is that the theoretical framework is general and can be extended to different systems.Item Phase-field crystal model for heterostructures(American Physical Society, 2019-10-16) Hirvonen, Petri; Heinonen, Vili; Dong, Haikuan; Fan, Zheyong; Elder, Ken R.; Ala-Nissila, Tapio; Centre of Excellence in Quantum Technology, QTF; Massachusetts Institute of Technology MIT; Bohai University; Oakland University; Department of Applied PhysicsAtomically thin two-dimensional heterostructures are a promising, novel class of materials with ground-breaking properties. The possibility of choosing many constituent components and their proportions allows optimization of these materials to specific requirements. The wide adaptability comes with a cost of large parameter space making it hard to experimentally test all the possibilities. Instead, efficient computational modeling is needed. However, large range of relevant time and length scales related to physics of polycrystalline materials poses a challenge for computational studies. To this end, we present an efficient and flexible phase-field crystal model to describe the atomic configurations of multiple atomic species and phases coexisting in the same physical domain. We extensively benchmark the model for two-dimensional binary systems in terms of their elastic properties and phase boundary configurations and their energetics. As a concrete example, we demonstrate modeling lateral heterostructures ofgraphene and hexagonal boron nitride. We consider both idealized bicrystals and large-scale systems with random phase distributions. We find consistent relative elastic moduli and lattice constants, as well as realistic continuous interfaces and faceted crystal shapes. Zigzag-oriented interfaces are observed to display the lowest formation energy.Item Power Flow Simulations On Integrating Large Scale Wind Power Production Into Finnish Power Grid(2010) Heinonen, Vili; Informaatio- ja luonnontieteiden tiedekunta; Lund, Peter