Physics

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Research from the Department of Physics. The department website may be found at https://www.ndsu.edu/physics/

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Applying theoretical frameworks from cognitive psychology to assess faculty professional development and student reasoning in physics
(North Dakota State University, 2024) McInerny, Alistair
Understanding human behavior and reasoning is essential for developing successful instruction. Discipline-based education researchers have examined how students learn, informing the development of successful instructional strategies. Researchers have also identified barriers to the successful implementation of such strategies. This work utilizes two theoretical frameworks from psychology to further examine: 1) efforts to enact instructional change and 2) the effectiveness of instructional approaches to improve students' reasoning in physics. The Theory of Planned Behavior (TPB) is used to assess professional development supporting the successful implementation of evidence-based instructional strategies. The Dual Process Theories of Reasoning and Decision-making (DPToR) are used to model human reasoning and explain persistent inconsistencies in student responses. Guided by the TPB, an assessment instrument was created, validated, and implemented to evaluate instructor’s beliefs and intentions about active-learning methodologies. A semi-novel research methodology was also applied to address response-shift bias, a phenomenon common in professional development self-reported assessments. The validation of the instrument and the utility of the retrospective pretest methodology are reported, together with initial assessment results, demonstrating the value of both the TPB and the retrospective pretest in the context of professional development. The second half of this work discusses inconsistent student reasoning, where students correctly apply conceptual understanding in one context but fail to do so in similar situations. This phenomenon is examined using the Dual Process Theories of Reasoning, which describes reasoning in terms of two processes: a fast, automatic process 1 and a slow, resource-intensive process 2. Process 1 is quick but frequently inaccurate. Process 2 is analytical but time-consuming and effortful. Four reasoning hazards are identified and examined through the lens of DPToR. Three different types of interventions are implemented to help students develop skills to navigate reasoning hazards: 1) Collaborative exams are used to trigger socially-mediated-metacognition in a high-stakes environment, modeling process 2 activation through group reasoning, 2) a multi-stage guided individual intervention followed by a classroom discussion, and 3) explicit discussion of human reasoning modeled by DPToR. The impacts of these interventions are assessed by comparing results from the treatment (intervention) and controlled (alternative intervention) groups.
Biophysical Characterization of Living Cells and Membrane Receptors by Atomic Force Spectroscopy
(North Dakota State University, 2021) Alhalhooly, Lina
Cellular biomechanics and cellular communication via receptor-ligand interactions play an important role in controlling cell development and maintaining cellular functions. Atomic force spectroscopy (AFM) technique has been widely used to characterize the changes in cellular biomechanics and quantify the receptor-ligand interactions. In this dissertation, we introduce working principles of AFM-based force spectroscopy, visualize cross-communications between membrane mechanics and cellular signaling, and identify quantitative relationship between receptor-ligand binding dynamics and multivalent interactions. First, by exploiting force spectroscopy methods, we probed biomechanical kinetics (stiffness, morphology, roughness, adhesion) of the brain, breast, prostate, and pancreatic cancer cells with standard chemotherapeutic drugs in normoxia and hypoxia over 12 – 24 hours. After exposure to the drugs, we found that brain, breast, and pancreatic cancer cells became approximately 20 – 50% less stiff, while prostate cancer cells became more stiff, due to either drug-induced disruption or reinforcement of cytoskeletal structure. However, the rate of the stiffness change decreased up to 2-folds in hypoxia, suggesting a correlation between cellular stiffness and drug resistance of cancer cells in hypoxic tumor microenvironment. Our results show that a degree of chemotherapeutic drug effects on biomechanical and biophysical properties of cancer cells is distinguishable in normoxia and hypoxia, which are correlated with alteration of cytoskeletal structure and integrity during a drug-induced apoptotic process. Second, we probed the binding strength of ligand-receptor interactions on live pancreatic cancer cells using single-molecule force spectroscopy methods, in which the peptides (cyclic arginine-glycine-aspartic acid: cRGD) was functionalized on a force probe tip through the polyethylene glycol-based bifunctional linker molecules. Although the density of integrin heterodimer receptors on the cell surface of each cell differs from cell to cell, the individual cRGD-integrin complexes exhibited a cell type-independent, monovalent bond strength. The load-dependent, bond strength of multivalent cRGD-integrin interactions scaled sublinearly with increasing bond number, consistent with the noncooperative, parallel bond model. Comparison of energy landscapes of the bond number revealed a substantial decrease of kinetic off rates for multivalent bonds, along with the widened width of the potential well and the increased potential barrier height between bound and unbound state, enhancing the stability of multivalent bonds between them.
Buckling Instabilities of Nanoscale Polymer Films and Colloidal Particle Layers
(North Dakota State University, 2015) Gurmessa, Bekele Jemama
Nanoscale polymer films have numerous potential applications such as protective coatings, flexible electronics, energy harvesting devices, and drug delivery systems. For realization of these potential applications, the mechanical properties of these materials and the underlying physics need to be understood. This dissertation focuses on understanding the responses of nanoscale films to mechanical deformations. In this regard, an elastic instability was exploited to locally bend and impart a local tensile stress in a nanoscale polystyrene film, and directly measure the resulting residual stress caused by the bending. Our results indicate that the onset of permanent deformation for thin polystyrene films is an order of magnitude smaller than what has been reported for the bulk value. In addition, not only is the onset of failure strain found to be small but also it increases with increased confinement. Using similar processing techniques, the yield strain of a more complex material - poly(styrene-b-divinylpyridine) - was studied. Similar to the polystyrene films, failure in polystyrene-b-poly(2-vinylpyridine) is also initiated at extremely low strain and is influenced by thin film confinement effects. In addition, we have demonstrated that internal nanostructure of self-assembled polystyreneb- poly(2-vinylpyridine) affects the onset of failure strain. Having introduced an idealized heterogeneity to a sample through ultraviolet/ozone treatment, we have created samples ranging from continuous thin films to sets of isolated plates. We demonstrated that, when subjected to mechanical deformation, the unbounded plates form isotropic undulations that persist even beyond high strain. In contrast, isolated plates undergo non-isotropic undulations in the range of high strains. The non-isotropic undulation shape has been described through a simple numerical modeling subjected to controlled boundary conditions. The agreement between experiment and numerical modeling is remarkable. Finally, through an integrated experimental methods and theoretical modeling, the response of discrete colloidal layers to mechanical deformations have been exploited. The buckling profiles measured experimentally demonstrate a great insight that the continuum model may not be able to predict the response of discrete systems. Theoretically, a granular model was constructed and structural stability analysis was investigated to predict the experimental observations. The overall agreement of the experiment and the modeling was good.
Computational Modeling of Polymer Crowding: Influence of Solvent Quality and Dimensionality on Conformations
(North Dakota State University, 2018) Davis, Wyatt Julian
The structure and function of polymers in confined environments, e.g., biopolymers in the cytoplasm, are affected by macromolecular crowding. To explore the influence of solvent quality and dimensionality on conformations of crowded polymers, polymers are modeled as penetrable ellipsoids/ellipses, whose shapes are governed by the statistics of random walks. Within this coarse-grained model, Monte Carlo simulations of two and three-dimensional polymer-nanoparticle mixtures, including trial changes in polymer size and shape, are performed. Penetration of polymers by nanoparticles is incorporated via a free energy cost predicted by polymer field theory. Simulation results of polymer conformation are compared with predictions of free-volume/area theory for polymers in good and theta solvents. Results indicate that dimensionality and solvent quality significantly affect crowded conformation, especially in the limit of small crowders. This approach may help to motivate future experimental studies of polymers in crowded environments, with relevance for drug delivery.
A Density Functional Theory and Many Body Perturbation Theory Based Study of Photo-Excited Charge Separation in Doped Silicon Nanowires with Gold Leads: Toy Models for the Photovoltaic Effect
(North Dakota State University, 2020) Walker, Nathan Thomas
We analyze a toy model for p-n junction photovoltaic devices by simulating photoexcited state dynamics in silicon nanowires. One nanowire is approximately circular in cross section with a diameter of d = 1.17 nm. The other has an approximately rhombic cross-section with d1 = 1.16 nm and d2 = 1.71 nm. Both nanowires have been doped with aluminum and phosphorus atoms and capped with gold leads. We use Boltzmann transport equation (BE) that includes phonon emission, carrier multiplication (CM), and exciton transfer. BE rates are computed using non-equilibrium finite-temperature many-body perturbation theory (MBPT) based on Density Functional Theory (DFT) simulations, including excitonic effects from Bethe-Salpeter Equation. We compute total charge transfer amount generated from the initial photoexcitation and find an enhancement when CM is included. In particular, we see between 78% and 79% enhancement in the smaller wire, while we see 116% enhancement in the larger nanowire
Electrostatic Interactions at Dielectric Interfaces: From Colloids to Membranes
(North Dakota State University, 2017) Volpe Bossa, Guilherme
In this thesis we have investigated electrostatic interactions at dielectric interfaces using theoretical models based on the non-linear Poisson-Boltzmann theory and its extensions. We have focused on three major topics: (1) modeling the energetics and interactions of charged nanoparticles trapped at the air-water interface; (2) calculation of the line tension between domains in charged lipid membranes, lipid-lipid correlations, and how membrane curvature is influenced by charged peptides; and (3) extensions of the classical Poisson-Boltzmann theory by accounting for the influence of ion-specific solvent-mediated interactions. More precisely, ion-specificity has been accounted for using the Poisson-Helmholtz-Boltzmann formalism, which adds to the bare Coulombic interactions a Yukawa-like potential that accounts for the interacting hydration shells of ions. Motivated by recent experimental and computational results, all projects present here aim to provide a deeper understanding of fundamental physical properties of charged dielectric interfaces.
Excited State Dynamics in 1D Thermoelectric Materials
(North Dakota State University, 2020) Gima, Kevin Victor
Here, nonadiabatic computations are used to study the thermoelectric effect and evaluate electron relaxation rates in lead telluride nanowires. κ_e = 1/τ_el is defined as the electron relaxation rate. It is directly connected to the thermoelectric figure of merit in a material. This work provides computational evidence in support of the proceeding hypothesis. The hypothesis is the electron relaxation rates will comply with the following band gap law: Ke = Aexp(-αΔE), where Ke is the electronic relaxation rate, A and α are constants, and ΔE is the energy difference between the initial and final states. This work reports results on PbTe (lead telluride) atomistic models doped with sodium and iodine that contain approximately 300 atoms in simulation cells with periodic boundary conditions.
Fluctuations in the Lattice Boltzmann Method
(North Dakota State University, 2012) Kaehler, Goetz August
The implementation of fluctuations in the lattice Boltzmann method has made significant progress in the last 10 years. The significance of incorporating noise to all non-conserved degrees of freedom was a significant recent discovery that was based on a simplified Langevin treatment of the linarized Boltzmann equation. However, for non-vanishing mean velocities significant deviations in the correlation functions were observed. In this thesis we show how we can largely alleviate these deviations by incorporating fully velocity dependent moment transforms and thus recover a fluctuation dissipation theorem that is valid for a larger range of velocities. Furthermore we show that the remaining deviations can be attributed to the collision operator of the linearized Boltzmann equation not being identical to the one of the BGK collision which forms the basis of most modern lattice Boltzmann applications. Finally we show that the locally velocity dependent transforms significantly improve the stability of fluctuating lattice Boltzmann simulations at low particle densities.
Free Energy Minimization and Multicomponent, Multi-Phase Lattice Boltzmann Simulations of Van Der Waals Fluid Mixtures
(North Dakota State University, 2018) Ridl, Kent Stephen
In this thesis, we develop a general framework for the lattice Boltzmann method to simulate multiphase systems with an arbitrary number of components. Theoretical expectations are easily visualized for binary mixtures, so we focus on characterizing the performance of the method by numerically minimizing the free energy of a binary van der Waals mixture to generate phase diagrams. Our phase diagrams contain very intriguing features that are not well-known in today’s physics community but were understood by van der Waals and his colleagues at the turn of the 20th century. Phase diagrams and lattice Boltzmann simulation results are presented in a density-density plane, which best matches with LB simulations performed at constant volume and temperature. We also demonstrate that the algorithm provides thermodynamically consistent results for mixtures with larger numbers of components and high density ratios. All of the theoretical phase diagrams are recovered well by our lattice Boltzmann method.
Fundamental Studies of Interfacial Forces Acting on Thin Films
(North Dakota State University, 2021) Twohig, Timothy John
A thin film is a material that is many orders of magnitude thinner than it is long or wide. They are commonly found in many forms and have been adapted to a wide variety of uses. The art of origami uses thin films(sheets of paper) and precise folding to create complex, three-dimensional shapes out of flat, quasi two-dimensional sheets, and has emerged as a unique way to solve problems in engineering and science. As technology and devices are scaled to smaller sizes new understanding of origami methods are required to work at these small scales. The interactions between thin films and liquids, substrates that films exist on, and other thin films is the focus of this dissertation. Capillary interactions are used to manipulate and fold thin films that are too thin to be actuated with hands or everyday tools. The relation between the macroscopic and the microscopic interactions at the point where the capillary liquid and the film meet is explored. We show how films can be manipulated by capillary drops and how exactly the force is applied to the film. The adhesive interactions of the film were studied as a method of precisely placing folds for elastic film origami. The capillary peel of a film from a substrate drove folds to desired locations. Adhesion of a film to itself was used to lock these bends in place in lieu of the permanent creases commonly used in plastic systems such as paper. The combination of these two methods enabled the creation of stable, multi-step origami systems from reusable elastic films. This research culminates in the discussion of fundamentally new origami-like designs that rely only on adhesion of the film to itself, which we call kuttsukugami (sticky+paper from Japanese). This new form allows for the creation of shapes that are nearly impossible to create with traditional origami methods such as loops, tubes, and cones. Advances made in capillary and adhesive thin film studies allow for kuttsukugami shapes to be scaled down to microscopic sizes for a huge array of applications including drug delivery, thin electronics, encapsulation, and more.
Guiding Self-Assembly of Functionalized Nanoparticles by Computational Modeling of Effective Interactions
(North Dakota State University, 2018) Shah, Vijay
Nanoparticles have attracted much attention because of their unusual physical properties, which allow them to be used in many practical applications. The self-assembly of nanocrystals into crystalline arrays can be facilitated by functionalizing the nanocrystals with ligand brushes, allowing for bulk dispersions to be sterically stabilized against aggregation. Studies have been conducted to study the clustering of gold nanoparticle dispersions. To study the self-assembly of gold nanoparticle dispersions based on nanocrystal volume fraction and ligand coverage, we performed Monte Carlo simulations and characterized the ability of the nanoparticle dispersions to self-assemble into crystalline arrays. Experiments have shown that silver nanoparticles can self- assemble into equilibrium superlattices in the presence of free ligands. To better understand the role of adsorbed and free ligands in self-assembly, we extracted the effective pressure between two flat, ligated plates through molecular dynamics simulations. Our results are compared to the theoretical prediction and discrepancies are discussed.
Hot Electron Effect in Ultrathin Photovoltaic Junctions
(North Dakota State University, 2012) Mihaylov, Deyan Ivov
The focus of the research work described in the following thesis is increasing the efficiency of photovoltaic devices by reducing hot carrier thermalization losses. In principle this can be achieved by reducing the size of the absorber down to lengths comparable to the thermalization length for hot carriers. With the use of ultrathin absorbers hot carrier can be collected before they have reached thermal equilibrium with the lattice. The theoretical work on the subject is comprised of improving the empirical relationship developed in the most recent publication on the topic by. By making the assumption that the energy loss rate fits the exponential decay model, an expression for the energy as a function of absorber thickness was developed. The experimental work consist of fabricating devices with different absorber thicknesses and testing their ability to show change in performance due to collection of hot electrons.
Inspection of Excited State Properties in Defected Carbon Nanotubes from Multiple Exciton Generation to Defect-Defect Interactions
(North Dakota State University, 2020) Weight, Braden Michael
Covalent SP3-hybridization defects in single-walled carbon nanotubes (CNTs) have been prevalent in recent experimental and theoretical studies for their interesting photophysical properties. These systems are able to act as excellent sources of single, infrared photons, even at room temperature, making them marketable for applications to sensing, telecommunications, and quantum information. This work was motivated by recent experimental studies on controllable defect placement and concentration as well as investigating carrier multiplication (CM) using DFT-based many-body perturbation theory (MBPT) methods to describe excitonic relaxation processes. We find that pristine CNTs do not yield appreciable MEG at the minimum threshold of twice the optical gap 2Eg, but covalent functionalization allows for improved MEG at the threshold. Finally, we see that defect-defect interactions within CNT systems can be modeled simply as HJ-aggregates in an effective Hamiltonian model, which is shown to be valid for certain, highly-redshifted defect configurations at low defect-defect separation lengths.
Investigation of 2D polymer thin films in 3D structures involving Crumpling, Origami, Kirigami and Adhesion
(North Dakota State University, 2024) Amadoru Jayawardana, Wathsala
This dissertation delves into the fascinating world of thin films and their ability to transform into three-dimensional (3D) structures. One can simply crush a sheet of paper into a ball to make a 3D object a faster, yet more complex structure or by following a more organized folding technique (Origami) to create 3D objects with predictable structures. This dissertation mainly focuses on such 3D structures and introduces cuts (Kirigami) and adhesion to those 3D structures to understand more of their mechanical behavior. The investigation of polymer crumple mechanics by introducing the Kirigami approach, and exploring how the sheet's topology influences crumpling, is discussed in this dissertation, which employs confocal microscopy, force experiments, and molecular dynamics simulations to investigate the effects of cutting on the behavior of crumpled sheets. The findings reveal that cutting does not significantly alter the compressive behavior; force scales according to a power law regardless of cuts, with only minor reductions in magnitudes. The second chapter of this dissertation studies Origami-inspired adhesive capable of securely holding objects on a wall yet easily removable without damage, which should withstand significant forces when attached, and then swiftly transition to a low-adhesion state for removal. Bi-stability of the proposed Origami designs with different compliance to achieve noticeable switching ratios has been investigated. These devices demonstrate moderate switching ratios and scalability, offering the potential for arrayed applications through repetition of the Origami pattern. Crumples combined with adhesion create stable 3D structures made from elastic thin sheets. The last chapter discusses the impact mitigation of such crumpling systems by observing sticky crumpled matter subjected to simple ball drop tests. These findings highlight the potential utility of sticky crumples as replacements for intricate engineered structures in protective layers.
Lattice Gas and Lattice Boltzmann Methods for Fluctuating Systems, Barrier Coatings, and Overrelaxation
(North Dakota State University, 2022) Strand, Kyle
In the field of computational fluid dynamics, lattice gas and lattice Boltzmann methods are powerful simulation methods derived from kinetic theory. These methods are renowned for their simplicity of implementation and computational speed. In recent years, lattice Boltzmann has risen in popularity for modeling hydrodynamic flows, diffusion, and more. However, a limitation of these methods is the lack of fluctuations due to the continuous nature of the model. Fluctuations arise from the discreteness found in nature, so including fluctuations presents difficulties. This dissertation explores new and novel ways of improving lattice Boltzmann and lattice gas methods. First, we present a new derivation for a fluctuating lattice Boltzmann method in a diffusive system. Fluctuations are absent lattice Boltzmann since they were derived as a Boltzmann average of discrete lattice gases. This lattice Boltzmann method is exact and includes density dependent noise which models fluctuations to high accuracy. Second, we extend diffusive lattice Boltzmann methods to apply to physical systems for diffusion through barrier coatings. We found that these models were able to reproduce the behavior from previous experiments and provided a simple tool for analyzing such systems. Higher order corrections to lattice Boltzmann methods are explored for extending the range for successful lattice Boltzmann implementations. Recently, the implementation of an integer lattice gas with a Monte Carlo collision operator by Blommel et al. provided a template for incorporating fluctuations through the discrete nature of lattice gases. A sampling collision operator for integer lattice gases by Seekins et al. was able to reproduce the fluctuating diffusion equation in the Boltzmann limit similar to the diffusive fluctuating lattice Boltzmann. However, lattice gases have a more limited range of transport coefficients than lattice Boltzmann methods, since lattice Boltzmann collisions are deterministic and allow for the implementation of over-relaxation and lattice gas collisions are probabilistic and overrelaxation in a lattice gas requires a probability greater than 1. The final section of this dissertation presents a simple method for including overrelaxation into an integer lattice gas using the sampling collision operator. It will be shown that this is possible through a permutation of occupation numbers.
Multi-Scale Simulation Methods of Crosslinked Polymer Networks and Degradation
(North Dakota State University, 2018) Feickert, Aaron James
Crosslinked thermoset polymers are used heavily in industrial and consumer products, as well as in infrastructure. When used as a protective coating, a thermoset's net-like structure can act as a barrier to protect an underlying substrate from permeation of moisture, salt, or other chemicals that otherwise weaken the coating or lead to substrate corrosion. Understanding how such coatings degrade, both at microscopic and macroscopic scales, is essential for the development and testing of materials for optimal service life. Several numerical and computational techniques are used to analyze the behavior of model crosslinked polymer networks under changing conditions at a succession of scales. Molecular dynamics is used to show the effects of cooling and constraints on cavitation behavior in coarse-grained bulk thermosets, as well as to investigate dynamical behavior under varying degradation conditions. Finite-element analysis is applied to examine strain distributions and loci of failure in several macroscopic coated test panel designs, discussing the effects of flexure and coating stack moduli. Finally, the transport of moisture through model coatings under cycled conditions is examined by lattice Boltzmann numerical techniques, considering several common concentration-dependent diffusivity models used in the literature and suggesting an optimal behavior regime for non-constant diffusivity.
The Nature of Single-Wall Carbon Nanotube-Silicon Heterojunction Solar Cells
(North Dakota State University, 2015) Harris, John Michael
Since their inception in 2007, nanotube-silicon heterojunction solar cells have experienced rapid improvement due to the diligent work of several research groups. These devices have quickly reached a point where they might begin to possibly compete with current well-established silicon solar technologies; however the development of industrial-scale nanotube synthesis and purification capabilities remains problematic. Although there has been significant recent progress in improving performance, the precise classification of nanotube-silicon heterojunctions has remained ambiguous. In this thesis, I use type, chirality and length purified single-wall carbon nanotubes to clarify the nature of this particular class of solar cell. The junctions that I assembled were made from freestanding nanotube sheets that showed remarkable stability in response to repeated crumpling and folding during fluid processing, which suggests that the films could be well suited to flexible device platforms. Despite modest ideality factors, the best diodes created in this study met or exceeded state-of-the-art device characteristics, but with a surprising lack of any significant dependence on sample type. The data further suggest that these devices might be simultaneously categorized as either Schottky or p-n junctions. More importantly, the results of this study demonstrate the manner in which band-gap engineering can optimize these devices while emphasizing the important role of the junction morphology.
A Novel Macroscopic Technique to Measure the Nanomechanics of Durable Multifunctional Nanosheets
(North Dakota State University, 2017) Taufique, Abu Md Niamul
In the recent advancement of nanotechnology, carbon nanotubes (CNTs) have shown promise and potential for a wide variety of applications due to their excellent mechanical, electrical, and optical properties. A network of single wall carbon nanotubes (SWCNTs) is of interest due to its potential applications in flexible electronics, composites, constructional materials, and so on. Characterizing the mechanical properties of these thin films will be critical to achieving a full understanding of their behavior, yet relatively few experimental methods exist for querying the deformation mechanics of these films. To provide additional insight into the large-deformation mechanics of these films, we propose a novel method for evaluating the mechanical properties of thin SWCNT films. We provide theoretical background, describe the experimental approach, and use a MATLAB based analysis to extract the film modulus. Finally, we compare our results to existing wrinkling-based measurements, where we find reasonable agreement between the two techniques.
A Numerical and Analytical Analysis of the Physics of Phase-Separation Fronts
(North Dakota State University, 2012) Foard, Eric Merlin
My dissertation is an investigation into the basic Physics of phase separation fronts. Such phase-separation fronts occur in many practical applications, like the formation of immersion precipitation membranes, Temperature induced phase-separation of polymeric blends, or the formation of steel. Despite the fact that these phenomena are ubiquitous no generally acceptable theory of phase-separation front exists. I believe the reason lies in the complexity of many of these material systems where a large number of physical effects (like phase-separation, crystallization, hydrodynamics, etc) cooperate to generate these structures. As a Physicist, I was driven to develop an understanding of these systems, and we choose to start our investigation with the simplest system that would incorporate a phase-separation front. So we initially limited our study to systems with a purely diffusive dynamics. The phase-separation front is induced by a control-parameter front that is a simple step function advancing with a prescribed velocity. We investigated these systems numerically using a lattice Boltzmann method and also investigated them analytically as much as possible. Starting from a one-dimensional front moving with a constant velocity we then extended the complexity of the systems by increasing the number of dimensions, examining a variable front velocity, and finally by including hydrodynamics.
Osmotic Swelling Behavior of Ionic Microgels
(North Dakota State University, 2020) Alziyadi, Mohammed Obaid
This dissertation studies the thermodynamic and structural properties of aqueous dispersions of ionic microgels – soft colloidal particles composed of cross-linked polymer gels that swell in a good solvent. Starting from a coarse-grained model of microgel particles, we perform computer simulations and theoretical calculations using two complementary implementations of Poisson- Boltzmann (PB) theory. Within the framework of a cell model, the nonlinear PB equation is exactly solved and used to compute counterion distributions and osmotic pressures. By varying the free energy with respect to microgel size, we obtain exact statistical mechanical relations for the electrostatic component of the single-particle osmotic pressure. Explicit results are presented for equilibrium swelling ratios of charged microcapsules and of charged cylindrical and spherical microgels with fixed charge uniformly distributed over the surface or volume of the particle. Molecular dynamics simulations validate the theoretical findings. In the second method, within a one-component model framework, based on a linear-response approximation for effective electro- static interactions, we develop Monte Carlo (MC) simulations to compute the equilibrium swelling ratio, bulk osmotic pressure, radial distribution function, and static structure factor. Results presented in this dissertation demonstrate that swelling of ionic microgels increases with increasing microgel charge and decreases with increasing concentration of salt and microgels. In addition, results demonstrate that the microion distributions and osmotic pressure determine equilibrium swelling of microgels. Cell model predictions for bulk osmotic pressure agree well with data from MC simulations of the one-component model. The MC simulations also provide access to structural properties and to swelling behavior of microgels in highly concentrated suspensions. Taken together, results obtained in this work provide insight into factors of importance for practical use of microgels as drug delivery systems, in tissue engineering, and for other biomedical applications.

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