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Fall 2017 Seminars

August 28, 2017Methods for cyclic coating test protocol developmentAaron Feickert
September 4, 2017

 <Labor day>

September 11, 2017Molecular Dynamics Lattice Gas: a New Tool to Understand Lattice Gases and Lattice BoltzmannAlexander Wagner
September 18, 2017Differential Capacitance of Ionic Liquids with and without Added SolventSylvio May
September 25, 2017Probing student reasoning approaches through the lens of dual-process theoriesCody Gette
October 2, 2017Lattice Gas with a Molecular Dynamics collision operatorReza Parsa
October 9, 2017Rejection-free cluster Monte Carlo algorithms in the presence of symmetry-breaking fieldsYen Lee Loh
October 16, 2017
October 27, 2017<special day>Electrostatic Interactions at Dielectric Interfaces: From Membranes to ColloidsGuilherme Bossa
October 30, 2017Superintegrability in classical and quantum mechanicsBjorn Berntson
November 6, 2017Multi-functional liquid crystalline epoxy networksMike Kessler
November  13, 2017The Full Spectrum Boost project in nanoparticle solar cells: Downconversion, Upconversion, Transport.Gergely Zimanyi
November 15, 2017<Special Day>Entangled Granular MaterialsScott Franklin
November 20, 2017Crowding in Polymer-Nanoparticle MixturesWyatt Davis
November 27, 2017The role of adhesion in crumpled polymer filmsAndrew Croll
December 4, 2017Carrier Multiplication in Chiral Single-Walled Carbon Nanotubes and Silicon Nanowires: DFT-Based StudyDeyan Mihaylov
December 11, 2017<Final Exam Week>


Carrier Multiplication in Chiral Single-Walled Carbon Nanotubes and Silicon Nanowires: DFT-Based Study
Deyan Mihaylov

Ph.D. Candidate,
Department of Physics,
North Dakota State University


Monday December 4, 3:00-4:00pm,

221 South Engineering

The conclusion about multiple exciton generation (MEG) efficiency in a nanoparticle can only be made by comprehensive study of different relaxation channels, such as phonon-mediated carrier thermalization, carrier multiplication and recombination, etc. Here, we study time evolution of a photo-excited state using Boltzmann transport equation (BE) that includes phonon emission/absorption together with the exciton multiplication and recombination. BE rates are computed using non-equilibrium finite-temperature many-body perturbation theory (MBPT) based on DFT simulations. Exciton effects are included by solving Bethe-Salpeter equation with RPA-screened Coulomb interaction (with additional simplifying approximations). We compute rates for both all-singlet MEG and Singlet Fission channels, which are of order 1014 s-1. For all-singlet MEG we calculate internal quantum efficiency (QE), the number of excitons generated from a single absorbed photon. Efficient MEG in chiral single-wall carbon nanotubes (SWCNTs), such as (6,2), both pristine and Cl-doped, and (6,5) is present within the solar spectrum range. We predict QE = 1.3-1.6 at the excitation energy of 3 times the optical gap in (6,2) and (6,5). However, QE = 1 is found in CNT (10,5) which suggests strong chirality dependence of MEG. MEG efficiency in functionalized SWCNTs is enhanced compared to the pristine case. Also, we calculate corrections to the bi-exciton state energy due to exciton-exciton interactions.

Theory and simulation of dispersions with competing interactions applied to protein solutions
Gerhard Nägele

Institute of Complex Systems (ICS-3),
Forschungszentrum Jülich GmbH


Monday February 12, 3:00-4:00pm,

221 South Engineering

Dynamic clustering of globular particles in dispersions exhibiting competing short-range attraction and long-range repulsion (SALR) such as in low-salinity protein solutions has gained a lot of interest over the past years. We investigate the influence of clustering on the dynamics and structure of globular particle dispersions. For this purpose, we combine a semianalytic hybrid method where hydrodynamic interactions (HIs) are approximately included [1] with multi-particle collision (MPC) simulations accounting for the full many-particles HIs [2]. By this simulation-theory comparison, we establish the high accuracy of the hybrid method for calculating diffusion and viscosity properties of SALR systems in the dispersed fluid phase region. We show that a cluster peak is present also in the hydrodynamic function characterizing the short-time dynamics, in accord with neutron spin echo results on lysozyme solutions [3]. Enhanced short-range attraction leads to a smaller self-diffusion coefficient and a larger dispersion viscosity. The behavior of the (generalized) sedimentation coefficient is more intricate showing, e.g., non-monotonic interaction strength dependence. Inter- and intraclusters dynamics in the equilibrium cluster phase region is analyzed using MPC simulations [2]. Simulation results for the mean cluster lifetime, and the comparison with the dissociation time of an isolated particle pair reveal quantitative differences, pointing to the importance of many-particle HIs for the cluster dynamics. The cluster lifetime in the cluster-fluid phase increases far stronger with increasing attraction strength than in the dispersed-fluid phase.  Significant changes in cluster shapes are observed in the course of time.

[1] J. Riest and G. Nägele, Short-time dynamics in dispersions with competing short-range attraction and long-range repulsion, Soft Matter 11, 9273 (2015).
[2] S. Das, J. Riest, R.G. Winkler, G. Gompper, J.K.G. Dhont and G. Nägele, Clustering and dynamics of particles in dispersions with competing interaction: Theory and simulation, submitted (2017).
[3] J. Riest, G. Nägele, N.J. Wagner, Y. Liu (NIST), and D. Godfrin, Short-time dynamics of Lysozyme solutions with competing short-range attraction and long-range repulsion: Experiment and theory, submitted (2017).

Methods for calculating effective interactions and pressure in charge-stabilized dispersions with application to filtration
Mariano Brito

Candidate for Ph.D.,
Institute of Complex Systems
Forschungszentrum Jülich GmbH


<Special Day>Tuesday February 13, Time TBA,

221 South Engineering

Charge-stabilized suspensions have interesting static features, reflected in properties such as the suspension osmotic pressure and ionic microstructure. These properties are determined by electro-steric interactions between all ionic species. Due to the large size asymmetry between colloidal macroions and small microions, the degrees of freedom of the latter can be integrated out, resulting in an effective one-component interaction potential describing microion-dressed colloidal quasi-particles.

We present a comparison, and partial extension, of various methods of calculating effective colloidal interaction parameters including effective charges and screening constants as functions of concentration and ionic strength [1]. We discuss osmotic suspension pressure calculations for dispersions in Donnan equilibrium with a salt ion reservoir. Methods are discussed including cell-models, renormalized jellium models, and multi-colloid-centered mean-field models. The pros and cons of the various methods are assessed by comparison with primitive model based computer simulations.

As an application to a technologically relevant process, a parameter-free model for cross-flow ultrafiltration is presented [2]. In this process, a dilute charge-stabilized dispersion is concentrated and purefied by continouosly pumping it though an array of cylindrical membranes having nano-sized pores.

[1] M. Brito, J. Riest, A. Denton and G. Nägele, to be submitted (2017).
[2] M. Brito, J. Riest, O. Nir, M. Wessling and G. Nägele, work in progress.

Spring 2018 Seminars

Jan 8, 2018<TBA>Timothy Twohig
Jan 15, 2018

 <Martin Luther King, Jr. Day>

January 22, 2018
January 29, 2018<TBA>Alan Denton
February 5, 2018
February 12, 2018Theory and simulation of dispersions with competing interactions applied to protein solutionsGerhard Nägele
February 13, 2018Methods for calculating effective interactions and pressure in charge-stabilized dispersions with application to filtrationMariano Brito
February 19, 2018<President's Day>
February 26, 2018Time-Dependent Excited State Molecular Dynamics: from photo-fragmentation to photo-polymerizationYulun Han
March 5, 2018<APS March Meeting>
March 12, 2018<Spring Break>
March 19, 2018
March  26, 2018
April 2, 2018<Holiday>
April 9, 2018
April 16, 2018
April 23, 2018
April 30, 2018
May 7, 2018<Final Exam Week>


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