North Dakota State University
|Research specialty||Degree type|
(Final degree/Enroute to PhD)
|Condensed Matter Physics||Both||Both|
|Nano Science and Technology||Both||Both|
|Physics Education Research||Both||Both|
|Soft Matter Physics||Both||Both|
|Solid State Physics||Both||Both|
|Statistical & Thermal Physics||Theoretical||Both|
Dynamics of Fluid Mixtures
Theory and computer simulation of equilibrium and non equilibrium phenomena using Lattice-Boltzmann simulation and other multi-particle collision methods.
Electronic Structure of Nanoparticles from First Principles
The ability to control properties of nanomaterials via size, shape, composition, surface structure, and self-assembly has opened new degrees of freedom inaccessible in conventional device design. At the same time, computational studies of nanostructures have become an attractive alternative to actual experiments, since the ability to explore the vast set of all possible configurations experimentally is limited. In recent years, advances in ab initio electronic structure techniques, such as Density Functional Theory, combined with new computational capabilities have enabled accurate calculations for atomistic models of nanoparticles. The results of these studies often serve as a unique source of insight into nanomaterial properties.
We study properties of photoexcited semiconductor nanoparticles, such as quantum dots, nanowires, nanofilms, and carbon nanotubes. This requires description of electrons, photons, and atomic vibrations (phonons), all of which are interacting quantum mechanical particles. In our work methods of modern quantum field theory, which have been mostly used in theoretical nuclear and particle physics, are combined with advanced computational electronic structure capabilities.
Macromolecular Crowding and Depletion
The conformations of flexible polymer coils, such as biopolymers in biological cells, are strongly affected by crowded environments. Correspondingly, depletion-induced interactions between crowders (nanoparticles) in polymer-nanoparticle mixtures depend in range and strength on size, shape, and concentration of depletants. By simulating hard-sphere nanoparticles and random-walk polymers, modeled as fluctuating ellipsoids, we compute polymer shape distributions and depletion-induced potentials. Comparisons with theory, simulations, and experiments show that polymer shape fluctuations play an important role in depletion and crowding phenomena.
Multiscale Modeling of Soft Nanomaterials
Soft nanomaterials -- multicomponent mixtures of macromolecules and nanoparticles -- have attracted much attention recently for their rich physical properties and technological applications. For example, colloidal crystals are widely explored for photovoltaic and photonic applications, with practical importance for solar cells, optical switches, and (potentially) quantum computers. In charged colloidal suspensions, microion screening of electrostatic forces between macroions influences the structure and stability of many common materials, from foods to pharmaceuticals. Addition of nanoparticles can modify electrostatic screening, enriching the tunability of interparticle forces and phase stability. In mixtures of polymers and nanoparticles, macromolecular crowding and polymer depletion can induce demixing and modify conformations of soft polymer coils, with biological relevance for protein and RNA folding and phase separation in the cell nucleus. To better understand screening, crowding, and depletion phenomena in these complex systems, we are developing Monte Carlo and molecular dynamics simulations of coarse-grained models. While surmounting computational challenges posed by diverse length and time scales, our multiscale modeling approaches also provide physical insight to guide experiments and help interpret observations.
Physics Education Research
We conduct discipline-based research in Physics Education. The objective is to examine student understanding, and identify and analyze conceptual and reasoning difficulties that students encounter in studying physics. The next step is to design instructional strategies that target specific student difficulties identified by the research, and to assess the effectiveness of these strategies. Therefore, research, curriculum development, and instruction are all integral parts of our investigation.
John Buncher, Warren Christensen, Mila Kryjevskaia
Soft Colloids and Biomembranes
Polyelectrolyte microgels and microcapsules are microscopic gel particles that are swollen in a solvent. Composed of porous, elastic networks of cross-linked polymers, microgels are soft colloids that can encapsulate dye molecules or drugs. Their sensitive response to environmental conditions (e.g., temperature and pH) and influence on flow properties suit microgels to widespread applications in the chemical, pharmaceutical, food, and consumer care industries. We model microgels and microcapsules using molecular simulations and Poisson-Boltzmann theory.
Lipid membranes surround all living cells, forming a barrier that ensures integrity and function. We are interested in understanding the physical properties of membranes and relating them to biological functions. Among the interesting questions are how different lipids influence the lateral organization of a lipid bilayer and what the role of membrane-associated proteins is. Some of our work also addresses bending of lipid membranes and electrostatic interactions between the lipid bilayer and adsorbed macroions.
Alan Denton, Sylvio May
Soft Matter Physics
Colloids, polymers, surfactants, and liquid crystals are widespread in daily life and nature and have practical importance for many technological applications, including paints, cosmetics, emulsions, foods, and pharmaceuticals. These systems contain macromolecules, whose complex intermolecular interactions determine the stability and unusual properties of soft materials. We study soft matter systems collaboratively using a variety of theoretical, computational, and experimental methods.
Yongki Choi, Andrew Croll, Stuart Croll, Alan Denton, Erik Hobbie, Sylvio May, Alexander Wagner
Flexible Nanotube Networks:
Using single-wall carbon nanotubes (SWCNTs) that have been purified by length and/or electronic type (metallic or semiconducting), we are assembling thin flexible nanotube films on soft polymer substrates and characterizing the coupling between mechanical flexibility and electronic performance.
Self-Assembly and Photoluminescent Stability of Silicon Nanocrystals:
In this work, we are purifying silicon nanocrystals (quantum dots) in an effort to reduce size polydispersity to the point where super-lattice assembly becomes viable over large (macroscopic) length scales.
High Performance Polymer Nanocomposites:
By mixing silicon nanocrystals (SiNCs) with different polymers, for example, we can make photoluminescent coatings with optical sensitivity to changes in temperature, stress or chemical environment.
Transmembrane Nanoparticle Transport:
We use fluorescent imaging and microscale time-resolved spectroscopy to measure the specific cellular uptake mechanisms of PEGylated nanoparticles, quantum dots, and cargo carrying copolymer micelles.
Alan Denton, Erik Hobbie, Sylvio May, Alexander Wagner
Thin Block Copolymer Films:
With modern techniques, creating and characterizing polymer films of nanoscopic thickness has become commonplace. What has quickly become clear is that there are many reasons that thin films differ in behavior from thick films. Here we study these issues by using films with anisotropic internal structure (diblock copolymers) using other forms of confinement (droplets, or cylinders) and by manipulating the stress found within the materials.
As anyone who has seen microscopic pictures of tissue can attest, there are many differences between a real tissue and our naive continuum approximation. Inspired by these differences, we examine an idealized system – block copolymer vesicles. Polymersomes (as they are known) form analogously to lipid vesicles, but are much stronger and allow for chemical manipulation, making them ideal for examining questions of adhesion, vitrification, and failure.
When a sheet of paper is bent, it is simple to see that the new shape is accommodated easily by the material. However, if the paper is next bent in an orthogonal direction, it collapses into sharp points and folds. The stress has been focused to a point, rather than spread over the material simply because the sheet is stretched into a geometry of non-zero Gaussian curvature. We examine what the general rules of such a transition might be using an idealized buckling system – wrinkles!
Single-Molecule Electronics and Science
Nanoscale electronic devices like field-effect transistors have long promised to provide sensitive, label-free detection of biomolecules. In particular, single-walled carbon nanotubes have the requisite sensitivity to detect single molecule events, and have sufficient bandwidth to directly monitor single molecule dynamics in real time. With this tool, we investigate the unknown molecular mechanism and complex kinetics of protein activities at a single molecule level.