The final programme is given below,
and it may also be downloaded as a pdf file
.
The list of participants with talk/poster titles and
abstracts is given
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| Jabr Aljedani | The University of Adelaide | Variational Model of a Rippled Graphene Sheet |
| Abstract: The calculus of variations is utilised to study the behaviour of a rippled graphene sheet supported on a metal substrate. We propose a model that is underpinned by two key parameters, the bending rigidity of graphene and the van der Waals interaction strength. Three cases are considered, each of which addresses a specific case of a rippled graphene sheet located on a flat substrate. The transitional case assumes a fixed length for both the graphene sheet and the substrate. The substrate constrained case assumes only the substrate has a fixed length. Finally, the graphene constrained case assumes only the length of the graphene sheet is fixed. Numerical results are presented for each case. The interpretation of all these cases demonstrates a continuous relationship between the total energy and the substrate length, that incorporates all three cases. |
| Sobin Alosious | PhD Scholar, Swinburne University of Technology, Hawthorn | Prediction of Kapitza length at solid-fluid interfaces. |
| Abstract: Understanding the interfacial heat transfer and thermal resistance at an interface between two dissimilar materials is of great importance in the development of nanoscale systems. This paper introduces a new and highly accurate method for calculating interfacial thermal resistance or Kapitza resistance in solid-fluid interfaces with the help of equilibrium molecular dynamics (EMD) simulations. The theoretical predictions are validated against classical molecular dynamics (MD) simulations. MD simulations are carried out in a Lennard-Jones (L-J) system with fluid confined between two solid slabs. Different types of interfaces are tested by varying the solid-fluid interactions (wetting coefficient) at the interface.It is observed that the Kapitza length decreases monotonically with an increasing wetting coefficient as expected. The theory is further validated by simulating under different conditions such as channel width, density, and temperature. Our method allows us to directly
determine the Kapitza length from EMD simulations by considering the temperature fluctuation and heat flux fluctuations at the interface. The predicted Kapitza length shows an excellent agreement with the results obtained from both equilibrium and non-equilibrium molecular dynamics (EMD/NEMD) simulations. |
| Stuart Anderson | University of Adelaide | |
| Hywel Bennett | University of Adelaide | |
| Debra Bernhardt (Searles) | The University of Queensland | Irreversibility for arbitrary protocols - fluctuation theorems as a sufficient but not necessary condition |
| Abstract: In the past it has been shown numerically that in nonequilibrium systems, if a time-asymmetric protocol is used for the application of the driving force or field, the usual primitive fluctuation theorem is not obeyed. From the fluctuation theorem the so-called second law inequality can be derived. We consider if it is possible to derive that inequality without using the fluctuation theorem. This could expand the range of systems that the second law inequality can be applied it and would provide a second law inequality for time-asymmetric protocols. |
| Belinda Boehm | The University of Adelaide | Understanding solution-phase aggregation of organic semiconductors |
| Abstract: The use of organic semiconductors is a promising means by which cheap, flexible, and lightweight electronic devices may be achieved. Although already commonly used in electronic displays, a better understanding of how these molecules pack and interact, and the influence this may have on their electronic properties, would be beneficial for the more rapid development of other interesting technologies. To this end, understanding how these molecules behave in solution and how the choice of processing conditions can affect the solution-phase aggregation properties is important, as many devices are fabricated through solution processing methods. Furthermore, as solution-phase aggregation has been shown to be important for controlling orientation at important interfaces in organic semiconductor based devices which has implications for charge transport and device performance, it is hoped that understanding solution aggregation may lead to a better understanding of aggregation-controlled interfacial alignment. Computer simulations can facilitate this understanding, but conventional all-atom simulations are unfeasible for studying mesoscale assembly processes. We address this issue by using molecular dynamics simulations of coarse-grained models of representative semiconducting polymers to examine how aggregation occurs in solution and how the molecular properties and processing conditions can influence solution-phase behaviour. The importance of molecular anisotropy and whether this plays a role in determining aggregation properties is also considered in order to better understand the principles driving aggregation, and how it may be predicted from molecular structure and properties. |
| Patrick Bowe | University of Adelaide | Modelling Carbon Nanotube Cap Formation via Carbon Vapour Deposition |
| Abstract: One method by which carbon nanotubes can be synthesised is carbon vapour deposition. Since many potential applications of carbon nanotubes require populations with specific properties, the selectivity of this method is important. By developing a model of the formation of carbon caps on metallic catalysts during the carbon vapour deposition process, we can study the selectivity of this process under different conditions. Using this model, we examine the effects of various parameters on the formation of carbon caps, and determine when a cap is likely to lift off from a catalyst to form a carbon nanotube. |
| Chris Bradly | University of Melbourne | Phase boundaries and universality in solvent-dependent polymer adsorption |
| Abstract: A long-standing question about the critical behaviour of the adsorption transition of polymer molecules in dilute solutions concerns whether the crossover exponent has a universal value in all dimensions and how it is affected by solvent quality, modeled by a monomer-monomer bulk interaction.
We consider the phase diagram of self-avoiding walks (SAWs) and similar models on the simple cubic lattice subject to surface and bulk interactions. In this work we simulate SAWs at specific bulk interaction strengths to focus on locating certain transitions and their critical behaviour. Recent results proposed that the large changes to the crossover exponent and the adsorption temperature as the bulk interaction strength is increased is due to multicriticality. We show that this behaviour coincides with the appearance of other phases where bulk collapse is just as important as surface adsorption and the critical temperatures are not the adsorption temperature but are indicative of a transition to these other phases. This implies a simpler picture of the variation of critical exponents due to strength of the bulk interaction and reflects the normal universal behaviour of exponents. |
| Gary Bryant | RMIT University | Understanding dynamics in complex suspensions using Light Scattering and Differential Dynamic Microscopy. |
| Abstract: Dynamic light scattering (DLS) is a well established technique for measuring dynamics in colloidal suspensions, and is routinely employed for particle sizing to complement imaging techniques such as electron microscopy. However, there are a range of challenging systems that are difficult to study with DLS, including: non-spherical particles; concentrated (turbid) suspensions, and active colloids such as bacteria. Differential Dynamic Microscopy (DDM) is a relatively new technique that uses white light microscopy to measure dynamics by Fourier transformation of subtracted frames (see figure 1), and has some advantages over DLS for such systems.
In this talk we will explain the principles behind the techniques, and present novel application of both DLS and DDM to three systems: particle sizing in turbid colloidal suspensions; the measurement of dynamics of nanorods, and the characterization of bacterial motility. We discuss the advantages and disadvantages of each of the techniques.
|
| Junbo Chen | UNSW | |
| Nathan Clisby | Swinburne University of Technology | Universal properties of polymer melts from high resolution Monte Carlo simulations of Hamiltonian paths |
| Abstract: We study a model for dense polymer systems via a novel highly efficient Monte Carlo algorithm for sampling Hamiltonian paths, i.e. single space-filling lattice chains. The underlying lattice is simple cubic, and confined to a three-dimensional cubic box with periodic boundary conditions. We obtain extremely precise numerical results for long chains, and focus on intra-chain correlations. In the long-chain limit, we reproduce random-walk statistics, as expected from Flory screening of excluded-volume interactions in dense systems. The finite-chain corrections, indicative of not-yet perfect screening, behave like a power-law tail. On scales small compared to the box size, we find the same behaviour that has been numerically established and theoretically predicted previously for dense many-chain systems. Conversely, on scales significantly larger than the box size we find a much more rapid decay of correlations, indicative of a dramatic enhancement of Flory screening due to finite box size effects. This is a somewhat surprising result that so far lacks any theoretical explanation.
This is joint work with Burkhard Dünweg of the Max Planck Institute for Polymer Research in Mainz. |
| Barry Cox | University of Adelaide | Graphene wrinkles |
| Abstract: Chemical vapour deposition is a popular technique for producing high-quality graphene sheets on a substrate. However, the cooling process causes the graphene sheet to undergo a strain-induced, out-of-plane buckling resulting in graphene wrinkles. These wrinkles often lead to undesirable effects on the properties of the graphene sheet. In this talk we construct a mathematical model to understand the conformations of these wrinkles. Initially an arch-shaped wrinkle is modelled and this is then generalised to incorporate graphene self-adhesion through van der Waals interactions across the wrinkle sides. Variational techniques are utilised to determine the lowest-energy conformation for both models. We find these models predict lowest-energy structures similar to those seen in experiments. |
| Peter Daivis | RMIT University | Energy flow in thermostatted nonequilibrium molecular dynamics simulations |
| Abstract: Energy transfers between internal kinetic and potential energy reservoirs in a simple liquid are studied by setting the temperature of one energy reservoir to a different value from that of the others and computing the resultant energy flows. In the first set of simulations, the x−directional kinetic temperature was artificially raised above the other five, and in the second, the x−directional configurational temperature was artificially raised above the other five. In both cases, external energy flows balanced, but unexpected energy flows between different directional components of the potential energy were observed. Additional simulations showed that these energy flows occurred regardless of the arrangement of thermostats imposed on the six degrees of freedom and the addition of shear. Heat flow between degrees of freedom that were ostensibly at the same temperature was anomalously observed. It was concluded that a different breakdown of the contributions to the configurational energy that is consistent with the definition of the directional configurational temperatures is required. |
| Len Davidson | DST Group | |
| Nicolas de Souza | Australian Nuclear Science and Technology Organisation | Soft-matter from neutron backscattering spectroscopy at ACNS |
| Abstract: - |
| Leo de Yong | DST Group | |
| Kirill Glavatskiy | The University of Sydney | Interfacially driven transport theory: a way to unify Marangoni and osmotic flows |
| Abstract: We show that the solvent behaviour in both diffusio-osmosis and Marangoni flow can be derived from a simple model of colloid–interface interactions. We demonstrate that the direction of the flow is regulated by a single value of the attractive parameter covering the purely repulsive and attractive–repulsive interaction cases. The proposed universality between diffusio-osmosis and Marangoni flow is extended further to include diffusio-phoresis. In particular, an object immersed to a colloidal solution moves towards the low concentration of the colloidal particles in the case of colloid–interface repulsion and towards the high concentration of the colloidal particles in the case of colloid–interface attraction. The approach combines the methods of fluid dynamics, molecular physics and transport phenomena and provides a tractable explanation of how the colloid–interface interactions affect the momentum balance and the transport phenomena (interfacially driven transport). |
| Michael Gruenwald | University of Utah | Crystallization and spontaneous resolution of chiral molecules |
| Abstract: Predicting the crystallization behavior of solutions of chiral molecules is a major challenge in the chemical sciences. In this talk, I will describe our recent theoretical efforts to understand chiral crystallization with the help of a family of coarse-grained models of chiral molecules with a broad range of molecular shapes and interactions. Our simulations reproduce the experimental crystallization behavior of real chiral molecules, including racemic and enantiopure crystals, as well as amorphous solids. Using efficient algorithms for the packing of shapes, we enumerate millions of low energy crystal structures for each model and analyze the thermodynamic landscape of polymorphs. In agreement with recent conjectures, our analysis shows that the ease of crystallization is largely determined by the number of competing polymorphs with low free energy. We find that this number, and hence crystallization outcomes, depend on molecular interactions in a simple way: Strongly heterogeneous interactions across molecules promote crystallization and favor the spontaneous resolution of racemic mixtures. |
| Tobias Hain | Murdoch University | Thermodynamics of the Quantizer Problem: the Voronoi Liquid |
| Abstract: Optimization problems in spatial cellular media are important to many
applications from biology to physics and material science. Here we focus
on the Quantizer problem, searching for minimal moment of inertia of the
cells in a tessellation of the entire space. Intuitively speaking, it prefers
similarly sized ‘sphere-like’ cells. Recent work found that starting from
a broad range of initial conditions, including arbitrarily large long-range
density fluctuations, a minimization of the Quantizer energy - using a local
algorithm - results in a fully amorphous state with a strong suppression
of large-scale density fluctuations as measured by a range of order-metrics
with (Klatt et al., Nature Communications 2019). Using theoretical models
and molecular dynamics simulations, previous work studied the Quantizer
system at finite temperatures, naming it the Voronoi Liquid, and showed
both similarities and differences to “normal” liquids (Ruscher et al., EPL,
2015). We use Monte Carlo methods to study the Quantizer system at both
finite and zero temperature. In particular, we study the phase behavior of
the system, including a recently found order transition. We thus shed light
on the intriguing energy landscape of the Quantizer problem and the role of
hyperuniformity in this system. |
| Dr Ellie Hajizadeh | Lecturer/University of Melbourne | Multiscale Simulations of polymer-bridged colloidal latex particle suspensions |
| Abstract: We developed a hierarchy of simulation methods to investigate the linear viscoelastic response and phase behaviour of suspensions of colloids bridged reversibly by telechelic polymers such as latex particles bridged by polyethylene oxide urethanes. We used Brownian dynamics simulation with explicit polymer chains and identified at least four relaxation times for the system, next we developed a novel hybrid population balance - Brownian dynamics simulation technique with implicit chain and investigated the microstructure development in the system, followed by an effective potential modelling, which resulted in predicting the phase behaviour of these systems. Modelling results compared against experimental results provided to us by our collaborators at the Dow Chemicals Company, and guided their new laboratory formulations of one of their product portfolios, namely coating fluids. |
| Peter Harrowell | Univeristy of Sydney | How Useful is Structure in Amorphous Materials? |
| Abstract: A set of general strategies for the analysis of structure in amorphous materials and a general approach to assessing the utility of any selected structural description are presented . Two measures of structure are defined, “diversity” and “utility,” and applied to two model glass forming binary atomic alloys, Cu50Zr50 and a Lennard-Jones A80B20 mixture. We show that the change in diversity associated with selecting Voronoi structures with high localization or low energy, while real, is too weak to support claims that specific structures are the prime cause of these local physical properties. |
| Jordan Hill | RMIT | |
| David Huang | The University of Adelaide | |
| Ravi Jagadeeshan | Monash University | Internal friction can be measured with the Jarzynski equality |
| Abstract: Conformational transitions in polymer molecules lead to a dissipation of energy due to frictional resistance from surrounding solvent molecules, and due to the presence of internal friction within molecules. The latter is linked to configurational rearrangements on an underlying energy landscape. In a rheological context, internal friction is conjectured to lead a shear-rate dependent viscosity, a finite limiting value for the infinite frequency limit of the dynamic viscosity, and to instantaneous stress jumps at the inception of steady shear flow. In a biological context, it is believed to be responsible for the slowing down of the process of protein folding, to influence stretching transitions in single biomolecule force spectroscopy, and to affect the dynamics of intermolecular interactions in intrinsically disordered proteins. The experimental quantification of internal friction has remained challenging. A novel methodology is proposed here for the experimental determination of the internal friction coefficient of polymer chains. Essentially, the average work dissipated as the molecule is stretched repeatedly is calculated by applying the Jarzynski equality, and the internal friction coefficient is then estimated in the hypothetical limit of zero solvent viscosity. The validity of the protocol is established through Brownian dynamics simulations of a single-mode spring-dashpot model for a polymer that incorporates both internal friction and solvent-mediated friction. Single-molecule manipulation techniques, such as optical tweezer-based pulling, can be used to implement the suggested protocol experimentally, and it is argued that the proposed methodology provides a means for the first time of directly estimating the magnitude of the internal friction coefficient in polymers. |
| Owen Jepps | Griffith University | |
| Matthew King | Griffith University | Fluctuations in a polygonal channel billiards model |
| Abstract: The Fluctuations Relations (FRs) describes the relative probabilities of positive and negative entropy production over time for systems with reversible dynamics. Currently there are two distinct characterisations of the conditions for FRs to hold: chaos (as characterised by Gallavotti and Cohen) or mixing (as characterised by Evans and Searles). In this talk we present work based on a polygonal billiard channel system which involves a point particle travelling through a system of polygonal sawtooth walls. The system is periodically perturbed in order to stop the formation of periodic orbits in the trajectory of the particle moving through the system and to allow us to study the fluctuations that occur over long time periods. This builds upon previous work using the Ehrenfest Wind-Tree system which involves a point particle moving through a hexatic lattice of polygonal scatterers. Although this channel billiard system and the Wind-Tree system share a similar dynamics, the transport behaviours of the two systems appear to behave substantially differently along with their sensitivity to perturbations. |
| Amith Kunhunni | Swinburne University of Technology | |
| Yawei Liu | Univeristy of Sydney | Dynamic Simulations of Rod-Shaped Colloidal Particles: Phase behaviour, self-assembly, diffusion and electrophoresis |
| Abstract: Here, we present a coarse-grained model for dynamic simulations of rod-shaped colloidal particles. Individual rods are represented by a rigid linear chain consisting of overlapping spheres which interact through a pseudo-hard-core. Non-adsorbed polyremes are modelled as freely inter-penetrable spheres with respect to each other, while there is the pseudo-hard-core repulsion between the polymer and rod spheres. Solvents are considered implicitly or explicitly with a dissipative particle dynamics (DPD) model. The phase behaviour of this model, obtained from continuous compression and expansion simulations, reproduces previous predictions based on theoretical calculations and Monte Carlo simulations. The brute-force simulations and forward-flux-sampling simulations are performed with this model to investigate the nucleation mechanism and to predict the nucleation rate/barrier for the self-assembly process of hard-rod colloidal particles in non-adsorbed polyreme solutions. The diffusion coefficients of a single rod-shaped colloidal particle measured with this model are in good agreement with the predictions based on the continuum theory. We also use this mode to study the motion of a single charged rod-shaped colloidal particle in the electric-filed to reveal the orientation of the particle in electrophoresis. |
| Hartmut Lowen | University of Dusseldorf | Active particles near substrates: from biofilms to colloids in motility patterns |
| Abstract: Self-propelled "active" particles typically aggregate on two-dimensional planar substrates performing quasi two-dimensional motion. Here we discuss recent results for active particles near such substrates. First of all we propose a simple model for biofilms exploring the hydrodynamic flow towards the film as induced by the self-propulsion of the particles in the film [1]. Second we consider active colloids in various motility patterns [2] and discuss a colloidal brazil nut effect [3] as well as motility-induced flashing [4] and trapping.
References
[1] A. J. T. M. Mathijssen, F. Guzman-Lastra, A. Kaiser, H. Löwen, Physical Review Letters 121, 248101 (2018)
[2] C. Lozano, B. ten Hagen, H. Löwen, C. Bechinger, Nature Communications 7, 12828 (2016)
[3] S. Jahanshahi, B. ten Hagen, C. Lozano, C. Bechinger, H. Löwen, J. Chem. Phys. 150, 114902 (2019)
[4] C. Lozano, B. Liebchen, B. ten Hagen, C. Bechinger, H. Löwen, Soft Matter 15, 5185 (2019)
|
| Luca Maffioli | Swinburne University of Technology | Three-body entropy computation for an atomic fluid |
| Abstract: We used equilibrium molecular dynamics to compute the two and three-body contribution to the
excess entropy, expressed via the Green’s expansion, of a system of particles interacting with
Weeks-Chandler-Andersen potential. The three-body correlation function and its contribution to the
entropy are computed with a new method which obviates the cumbersome calculations involved in
the normalization and integration operations. We analyzed the numerical instability emerging in the
calculation of the three-body entropy, and we discuss the best choice for the parameters involved in
the computation (number of samples, spacial definition of the three-body histogram) in order to
remove the divergence or to minimize its magnitude.
We observed that the two and three-body entropies have a generally smaller magnitude than the
ones of a Lennard-Jones system. Close to the freezing point, the two-body contribution alone is
approximately the same as the total excess entropy. |
| Andreas Menzel | Heinrich Heine University Dusseldorf | Statistical characterization of the collective behaviour in active suspensions of self-propelled microswimmers |
| Abstract: Using statistical tools, we investigate the properties of intrinsically nonequilibrium active colloidal suspensions. These suspensions consist of active microswimmers like bacteria or colloidal Janus particles self-propelling through a surrounding liquid. We develop a corresponding particle-scale statistical description in terms of a classical dynamical density functional theory, including all basic ingredients: self-propulsion, hydrodynamic and steric interactions, thermal fluctuations, and confinement. Starting from the individual swimmer properties, we investigate the collective behaviour of many interacting microswimmers, both in monodisperse suspensions and in binary mixtures. Emergent dynamical states comprise spontaneous self-organization in effective fluid pumps or overall alignment associated with directed collective motion. |
| Debora Monego | University of Sydney | Size-dependent sedimentation of nanocrystals: the role the ligand shell structure |
| Abstract: The chemical, electronic, optical, magnetic and catalytic properties, as well as the self-assembly of hybrid inorganic-organic core-shell nanoparticles depend strongly on their size and composition. Hence, there is a pressing need for a reliable method that can provide full characterisation of these particles. Although transmission electron microscopy (TEM) provides high resolution detail of particle size, shape and structure, discerning the organic materials bound on the nanocrystal surface is still a challenge, due to low atomic contrast. Yet, parameters related to the overall hybrid particle (inorganic core and organic shell) properties influence particles’ solubility, electronic properties, assembly and reactivity. In this context, analytical ultracentrifugation (AUC) appears as an alternative to characterise these particles in solution, being sensitive to changes in the density and size of both core and shell components of nanoparticles [1]. We use molecular dynamics simulations (MD) to test the validity of the Stokes-Einstein-Sutherland (SES) equation for systems of CdSe quantum dots coated with alkanethiol ligands in chloroform and explain inconsistencies between the sedimentation trends observed in AUC experiments and the ones predicted for these particles in a Stokes flow. We find that varying the size of the particle changes the structure of the ligand shell and the way it interacts with the solvent and will dictate the diffusion and sedimentation behaviours of the nanocrystals.
1. Cölfen, H., Analysis of nanoparticles <10 nm by analytical ultracentrifugation. In Particle Sizing and Characterization, Provder, T., Texter, J., Eds.; ACS Symposium Series 881; American Chemical Society: Washington, DC, 2004; pp 119−137. |
| Huong Thi Lan Nguyen | The University of Adelaide | Coarse-graining of anisotropic molecules for simplified and fast molecular dynamic simulations |
| Abstract: Atomistic simulations are not generally feasible for studying dynamics of molecular systems in realistic space and time scales. A solution to this is to use coarse-grained (CG) models, which increases the simulation efficiency by replacing a collection of atoms as a single interacting site. In this talk, a new systematic methodology to generate CG models for molecular dynamic (MD) simulations using force matching is introduced and validated. The algorithm of this method is developed so that MD simulations can be simplified but still accurately represent the physical and thermodynamic properties of the simulated materials. More importantly, this method can produce models that capture the anisotropy of molecules, which is especially useful for theoretical studies of organic materials and has not previously been achieved via a systematic algorithm. To validate the method, CG models of typical anisotropic organic molecules are produced. Simulations using these models accurately describe the structural and thermodynamic properties of the atomistic models and show improvement over previous CG models for these materials. |
| Jordan Orchard | Swinburne University of Technology | |
| Charlotte Petersen | University of Innsbruck | Understanding confined liquids: Confinement by periodic boundaries |
| Abstract: Confined liquids have interesting and counter-intuitive phase and transport properties, whose relationship to the liquid's structure is not obvious. For liquids confined to a slit, this structure can be characterised by two particularly apparent features. The first is a non-uniform density profile caused by layering at the walls. The second is a non-monotonic change in the structural correlations with the wall spacing.
Here, we disentangle the effects of layering and local packing by confining a fluid to the surface of a torus or long cylinder. This is equivalent to periodic boundary conditions, where one dimension of the system is very small. We quantify correlations in this quasi-confined fluid by calculating the static structure factor with both hard-sphere computer simulations and the Percus-Yevick closure in liquid state theory. We observe that the particle correlations exhibit a similar non-monotonic behaviour to a liquid confined between two walls, even though the density is homogeneous. The dynamic properties of the fluid also reflect the non-monotonicity observed in the static properties. These results indicate that the correlations in real confined liquids may not be intrinsically related to their oscillating density profile.
|
| Isaac Pincus | Monash University Department of Chemical Engineering | Viscometric Functions and Rheo-optical Properties of Dilute Polymer Solutions: Comparison of FENE-Fraenkel Dumbbells with Rodlike Models |
| Abstract: Rigid macromolecules or polymer chains with persistence length on the order of or greater than the contour length have traditionally been modelled as rods or very stiff springs. The bead-FENE-Fraenkel-spring dumbbell, which is finitely extensible about a non-zero natural length with tunable harmonic stiffness, is one such model which has previously been shown by other authors to reproduce bead-rod behaviour in the absence of hydrodynamic interaction. However, it is unclear whether these bead-rod and bead-spring models are equivalent in the presence of hydrodynamic interactions. It is also unclear how predicted
solution properties such as viscometric functions or rheo-optical observables change in the crossover between a bead-rod and more extensible bead-spring model. Using a semi-implicit predictor-corrector Brownian dynamics algorithm, the FENE-Fraenkel spring is shown to imitate a rod with hydrodynamic interactions when spring stiffness, extensibility and simulation timestep are chosen carefully. Furthermore, comparisons with experimental data on the viscosity and Linear Dichroism of high aspect ratio, rigid macromolecules shows that the extensibility and stiffness of the FENE-Fraenkel spring allows for equal or improved accuracy in modelling inflexible molecules compared to rodlike models. |
| Matthew Pinson | University of Divinity | |
| Paolo Raiteri | Curtin University | |
| Madhuranga Rathnayake | The University of Sydney | Evaluating classical force fields to study dissolution and crystallisation of hybrid organometallic halide perovskites |
| Abstract: Abstract:
Organometallic halide perovskites show a promising future as an efficient and low-cost alternative for conventional photovoltaics. Devices with power conversion efficiencies over 20% have already been reported. Solution processability, high absorption coefficient and tunable bandgap make hybrid perovskites a promising alternative to silicon-based photovoltaics. However, further development is necessary before perovskite solar cells can be widely adopted by industry. For instance, they cannot currently be printed at scale. Even though a wide variety of specific fabrication methods exist to make hybrid perovskites with desired properties and composition, a molecular-level understanding of how they crystallise from solution is still lacking. Ab initio molecular dynamics simulations have been used to obtain atomistic insight into some of these processes, but are limited to timescales of the order of 10s of ps. In contrast, processes on much longer timescales have been successfully studied using classical force fields for both organic and inorganic systems. A couple of attempts have been made recently to parameterise force fields for studying organometallic halide perovskites, with some success at reproducing solid-state properties. However, a force field that can accurately describe dissolution and crystallisation is still lacking.
In this work, we compare existing non-polarizable force fields developed to study the solid or solution phases of organometallic halide perovskites with the AMOEBA polarizable force field. The aim is to test whether computationally expensive polarizable force fields like AMOEBA offer better transferability between liquid and solid phases, the ultimate goal being the study of pre-critical clusters, nucleation and interfaces of organometallic halide perovskites.
Acknowledgement:
Authors acknowledge the ARC Centre of Excellence in Exciton Science for financial assistance and the Artemis HPC facility at The University of Sydney for providing computational power. |
| Kannan Ridings | The University of Auckland | Nanowire Stability during Solid-Liquid Phase Coexistence |
| Abstract: The stability of nanowires during solid-liquid coexistence is considered by solving an Allen-Cahn-type dispersion relation, and by conducting molecular dynamics simulations of copper nanowires. Contrary to the stability of liquid jets as predicted by Plateau-Rayleigh theories, for a surface melted nanowire, there is no preferred wavelength for which the solid will become unstable and breakup. The derived model suggests that for a given wire radii, as its length is increased, the spectrum of its solid-liquid profile will shift to longer wavelengths. We compare the interface spectrum of the liquid-vapour and solid-liquid interfaces with molecular dynamics, which infer the former obey Plateau-Rayleigh theory while the latter does not. |
| Michael Rinaudo | University of Sydney | Packing and Phase Behaviour of Nanorods |
| Abstract: Solar power in highly urbanised settings is limited due to the pervasiveness of high-rise buildings with limited roof space available for solar power generation. However the facades of these buildings are often mainly comprised of windows which could be utilised in a Luminescent Solar Concentrator (LSC). LSC's utilise total internal reflection to direct light to their edges for solar harvesting. These systems can also be built as nearly transparent allowing windows to be still used. A limitation on LSC's is the low efficiency of these systems. But, by controlling the assembly of nano-luminophores in the wave guide it is possible to increase their efficiency.
In this paper the packing of rounded rods in 3D space is examined for the purpose of designing an improved LSC wave guide. This is achieved through simulating Brownian dynamics with the LAMMPS molecular dynamics package. Several orderings of binary rods are observed both with entirely attractive and mixed interaction regimes. Some of these orderings are as predicted from hard disc packings, but new pentagonal orderings are observed as well. These results are also examined through calculating coordination. From these simulations it is observed that the correct number ratio is important to form an extended lattice. Additionally it is determined that mixed interactions are required to prevent separation of the two rod types within a lattice into separate phases. From these discoveries goals for future experimental studies in this area have been more clearly defined |
| Dominic Robe | Monash University | Physical Aging in a Colloidal Glass as Transitions Between Metastable States |
| Abstract: Soft matter systems near their glass transition can undergo a slow change in material properties. This process called “physical aging” is qualitatively distinct from normal equilibration, and can cause materials to continue changing for years after manufacture. We present a model which makes general assumptions about the series of metastable states through which an aging system passes and arrives at a simple prediction for the rate at which such transitions should occur. The generality of our model’s assumptions provides a common ground for the variety of microscopic mechanisms that seem to produce similar aging effects. We perform simulations of a 2D colloidal system to test this prediciton. In our colloidal system, transition events are observed as irreversible rearrangements, detected through Voronoi neighbour analysis. The decaying rate of these rearrangement events agrees with the predictions of our model. |
| Tony Roberts | University of Adelaide | Multiscale computation of microscale systems |
| Abstract: Suppose that in some problem we have a trustworthy
microscale simulator, and a 'spatial' domain so large the
microscale code is not feasible, but we do not know and/or
cannot derive a macroscale closure. Answer: use the
microscale simulator on small patches of space, with the
patches craftily coupled over unsimulated space so that we
make accurate macroscale predictions. Such predictions are
computed relatively quickly when the patches are a small
fraction of the whole spatial domain. We are developing a
flexible toolbox for users, and have established the scheme
is accurate in some macroscale homogenisation problems. |
| Richard Sadus | Swinburne University of Technology | Ab Initio Potentials in Molecular Simulation |
| Abstract: In the context of molecular simulation, the calculation of both phase equilibria and thermodynamic properties has been largely confined to using either empirical or semi-empirical intermolecular potentials. The Lennard-Jones potential arguably forms some part of the overwhelming majority of molecular simulations either as a representation of the interaction between atoms or to obtain the non-bonded contributions in commonly used molecular force fields In parallel with the widespread use of empirical potentials, advances in quantum chemistry, most notably the CCSD(T) method, has witnessed the development of accurate two-body potentials for both atomic systems and some simple molecules.
In this talk, we demonstrate that ab initio potentials can be used to predict both vapor-liquid equilibria and thermodynamic properties more accurately than commonly used empirical potentials. Furthermore, the calculations can be performed without the significant computational cost of traditional ab initio calculations.
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| Aritra Santra | Monash University | Universal Behaviour of Associative Polymer Solutions |
| Abstract: Associative polymers are macromolecules consisting of sticky groups distributed along the backbone of the polymer chain that can form reversible physical bonds. A key feature of the solutions of associative polymers is the formation of transient gels and networks via inter-chain association between the sticky groups, and they find use in a wide range of applications including paints and coatings, drug delivery, tissue engineering and so on. To understand the statistical mechanics of associative polymer solutions, scaling relations have been proposed initially by Rubinstein and Semenov (Macromolecules 1998, 31, 1373-1385) and subsequently developed by Dobrynin (Macromolecules 2004, 37, 3881-3893) based on mean field theory. These relations correlate the intra-chain and inter-chain associations between the stickers with the chain topology (such as number of stickers per chain and the spacing between the stickers) and system parameters (such as monomer concentration, solution temperature, sticker strength etc.). There are currently no systematic simulations or experiments to validate these scaling predictions. In the present study we have developed a multi-chain Brownian dynamics (BD) algorithm to simulate coarse-grained models of associative polymers using a novel interaction potential and scrutinised the predictions of the scaling theory. By appropriate combination and rescaling of variables we have shown the universality of the scaling relations over a range of values of chain lengths, spacer lengths and other system parameters. In this study we have also identified the different signatures of gelation transition in these systems and determined the relationship between the gelation concentration and chain topology at constant sticker strength and solution temperature. |
| Gerd Schroeder-Turk | Murdoch University | Morphometry.org : Minkowski Functionals: Robust and Versatile Shape Descriptors |
| Abstract: Integral geometry provides some useful metrics to quantitatively characterise the shape, structure and form of disordered materials. This has led to the Minkowski functional concept and to the Minkowski tensor shape metrics, both concepts that were developed over the last two decades by Klaus Mecke, and co-workers, and many others. Fabian Schaller, Sebastian Kapfer and Michael Klatt have now put together an interactive educational and concept testing website called morphometry.org which graphically and interactively allows to explore these concepts, and will hopefully help to disseminate these techniques to a broader range of disciplines. |
| Zakiya Shireen | University of Melbourne | Modeling and simulation of aggregation of binary colloids |
| Abstract:
We proposed a new model to study the complex systems of binary colloids. The simulation method is Brownian Cluster Dynamics (BCD) by accommodating the binary spherical colloids and their selective interaction between intra-species. The model was tested by considering the irreversible diffusion limited cluster aggregation (DLCA) of binary colloids, which serves as a model for chemical gels. To explore the wide adaptability of the model, the new algorithm was also tested by considering aggregation process in both the species and in only one species with another species being mobile/immobile hard spheres. The size and shape of the randomly distributed colloids of both the species are identical and undergo Brownian diffusion.
Two cases were investigated in details: (i) kinetics and structure of the system with both species aggregating (ii) dynamics of the mobile hard-spheres, when only one species is aggregating and forming system spanning cluster.
In case (i) Irreversible aggregation of binary colloidal particles lead to the formation of system spanning cluster of one species or both the species also called “bigels”. An inequality relation was proposed which predicts the appearance of bigels in irreversible aggregation of binary DLCA. The structures generated by binary aggregation was compared with one component system and was shown to be different with fractal dimension 2.0, which is slightly higher than that of the monomeric system (1.8). The structures with fractal dimension 2.0 in flocculation limit were identified as branched polymers (Lattice animals). As chemical distance of a cluster in flocculation limit follows the same scaling law as predicted for lattice animals. For the fist time it was shown that irreversible binary DLCA comes under the universality class of percolation theory as it was validated that the binary DLCA follows the scaling laws proposed by percolation theory [1].
In case (ii) the dynamical study of the binary DLCA system unraveled an unknown aspect of anomalous diffusion of hard spheres: the cage dynamics of the clusters. The dynamics of the non interacting hard-spheres inside the percolating cluster was shown to be similar to the glass like dynamics in gels. Two aspects were considered, one where the percolated particles moved within the bonds or cage dynamics was allowed and other where the movement within the bonds are restricted or cage dynamics was not allowed. The hard spheres showed anomalous diffusion in both the cases as randomly diffusing hard spheres were stuck inside the cages of percolating species. The intermediate scattering function of the hard-spheres revealed that for a volume fraction of φ < 0.49 system shows double relaxation similar to colloidal glass for smaller fraction of hard-sphere particles. For higher fraction binary system is showing only a single stretched exponential for hard spheres. In case of the static cage hard spheres showed only a single relaxation to a constant value which was indicative of the fact that the particles were permanently stuck inside the cages created by the percolating cluster. The probability distribution of particles revealed that the distribution of hard spheres in the double relaxation regime is given by a broad distribution while for the case, where system has only a single stretched relaxation it was proposed that system is possibly having a fraction as slow and fast particles [2].
References:
[1] Shireen, Z. and Babu, S.B., 2017. Lattice animals in diffusion limited binary colloidal system. The Journal of chemical physics, 147(5), p.054904.
[2] Shireen, Z. and Babu, S.B., 2018. Cage dynamics leads to double relaxation of the intermediate scattering function in a binary colloidal system. Soft matter, 14(45), pp.9271-9281.
PS: This work (PhD work) was carried out at Indian Institute of Technology (IIT) Delhi, India. The author is currently working at University of Melbourne as Research Fellow.
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| Mirella Simoes Santos | The University of Queensland | Local self-diffusion coefficients of confined fluids through local dissipation theorem |
| Abstract: Confined fluids are fluids under a geometric constraint in the nanometric scale. They are present in nature and different processes, from living cells, reverse micelles, oil and gas reservoirs to nanofluidic devices. Thermodynamic and transport properties of confined fluids largely differ from the ones observed in bulk conditions. This is caused by the inhomogeneity that arises from the magnitude of the interactions between the fluid and the confining walls. Molecular simulations can be a great tool for determining the properties of confined fluids in a variety of systems but at the same time, proper methodologies for calculating local transport properties in inhomogeneous fluids are still limited. Here, we propose the use of local dissipation theorem for the calculation of local self-diffusion coefficients of confined fluids. Two case studies are consider for the validation of the methodology, the first one being a carbon nanotube filled with argon, and the second one an ionic liquid confined between graphene electrodes. The former is a simplified representation of a supercapacitor, which is the main interest of this project. Diffusivity of electrolytes is a direct indicator of power densities of supercapacitors and this kind of information can be used as a tool for guiding the enhancement of the performance of supercapacitors. |
| Gang Sun | University of Sydney | Structure-dynamics connection in glass forming liquids |
| Andrew Tarzia | Imperial College London | |
| Billy Todd | Swinburne University of Technology | Heat Flux Beyond Fourier's Law |
| Abstract: It has been known for some time, though not widely so, that the heat flux for a liquid under shear involves additional couplings apart from the well-known Fourier law which is proportional to the temperature gradient. This is true in the linear regime, arbitrarily close to equilibrium [1-3]. In this work, we use nonequilibrium molecular dynamics (NEMD) to explore the effect of shear flow on heat flux [4]. By simulating a simple fluid in a channel bounded by tethered atoms, the heat flux is computed for two systems: a temperature driven one with no flow and a wall driven, Couette flow system. The results for the temperature driven system give the Fourier’s law thermal conductivity, which is shown to agree well with experiments.
Through comparison of the two systems, we quantify the additional components of the heat flux parallel and normal to the walls due to shear flow. To compute the heat flux in the flow direction, the Irving-Kirkwood equations [5] are integrated over a volume, giving the so-called volume average form, and they are also manipulated to get expressions for the surface averaged and method of planes forms [6-12]. The method of planes and volume average forms are shown to give equivalent results for the heat flux when using small volumes.
The heat flux in the flow direction is obtained consistently over a range of simulations, and it is shown to vary linearly with strain rate, as predicted by theory. The additional strain rate dependent component of the heat flux normal to the wall is obtained by fitting the strain rate dependence of the heat flux to the expected form [12].
As a result, the additional terms in the thermal conductivity tensor quantified in this work should be experimentally testable.
References
[1] A. Baranyai, D.J. Evans and P.J. Daivis, Phys. Rev. A, 46, 7593 (1992).
[2] D. Risso and P. Cordero, Phys. Rev. E, 56(1), 489 (1997).
[3] P.J. Daivis and J.L. Khayyam Coelho, Phys. Rev. E, 61(5), 6003 (2000).
[4] B.D. Todd and P.J. Daivis, Nonequilibrium Molecular Dynamics: Theory, Algorithms and Applications. Cambridge University Press, 2017.
[5] J.H. Irving and J.G. Kirkwood, J. Chem. Phys., 18, 817 (1950).
[6] R. J. Hardy, J. Chem. Phys., 76, 622 (1982).
[7] J. Cormier, J. M. Rickman, and T. J. Delph, J. Appl. Phys., 89, 99 (2001).
[8] B.D. Todd, P.J. Daivis, and D.J. Evans, Phys. Rev. E, 51, 4362, (1995).
[9] B. D. Todd, D.J. Evans, and P.J. Daivis, Phys. Rev. E, 52, 1627 (1995).
[10] M. Han and J. S. Lee. Phys. Rev. E, 70, 061205 (2004).
[11] D. M. Heyes, E. R. Smith, D. Dini, and T. A. Zaki. J. Chem. Phys., 135, 024512 (2011).
[12] E. R. Smith, P.J. Daivis and B.D. Todd. J. Chem. Phys., 150, 064103 (2019).
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| Alf Uhlherr | AeRO | |
| Rahil Valani | Monash University | Superwalking droplets |
| Abstract: A walker is a droplet of liquid that can self-propel on the free surface of a vibrating bath of the same liquid through feedback between the droplet and its wave field. We have studied walking droplets in the presence of two driving frequencies and have observed a new class of walking droplets, which we coin superwalkers. Superwalkers may be more than double the size of the largest walkers, may travel at more than triple the speed of the fastest ones, and enable a plethora of novel multi-droplet behaviors. Physical insights from numerical simulations into the emergence of the superwalking behavior are also discussed.
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| Sleeba Varghese | Swinburne University of Technology | Effect of Hydrogen Bonds on the Dielectric Properties of Interfacial Water |
| Abstract: The dielectric constant for water is reduced under confinement. Although this phenomenon is well known, the underlying physical mechanism for the reduction is still in debate. In this work, we investigate the effect of the orientation of hydrogen bonds on the dielectric properties of confined water using molecular dynamics simulations. We find a reduced rotational diffusion coefficient for water molecules close to the solid surface. The reduced rotational diffusion arises due to the hindered rotation away from the plane parallel to the channel walls. The suppressed rotation in turn affects the orientational polarization of water, leading to a low value for the dielectric constant at the interface. We attribute the constrained out-of-plane rotation to originate from a higher density of planar hydrogen bonds formed by the interfacial water molecules. |
| Asaph Widmer-Cooper | University of Sydney | Twisting of nano-platelets: a tale of stress and strain |
| Abstract: Nanoplatelets (NPLs) are very thin crystals, typically 5-20 atomic layers in thickness and 10-100 nm in the lateral dimensions. Experiments have shown that CdSe NPLs can be made to twist and untwist in solution in response to the addition of surface-active molecules. This presentation will explain the physical and chemical driving forces for this behaviour and what it has in common with twisting of seed pods, using results from molecular dynamics simulations. |
| Marltan Wilson | The University of Adelaide | |
| Jared Wood | Univerisity of Sydney | The behavior of Nanorod assemblies, examined with biased sampling methods |
| Abstract: Nanorods are an interesting material exhibiting both confined size effects, but also having a defined orientation, which can give materials anisotropic behaviour when interacting with light.
We look at the nucleation pathway of nanorods transitioning from an isotropic state to a more ordered state, using umbrella sampling to observe the free energy of a growing ordered cluster in the system. From this we are able to view the structure of the growing cluster and what the favoured growth pathway is.
We also look at the phase behaviour of janus nanorods with an attractive and a hard repulsive segment. We examine the free energy changes with changing density using a hybrid of grand canonical monte carlo and molecular dynamics with sequential umbrella sampling. This allows us to determine the entropic contribution to any phase changes. |