North Dakota State University

We have incorporated the very lipophilic photosensitizer temoporfin into liposomes of varying membrane composition, cholesterol content, vesicle size and life-prolonging liposome modifications.  We measured the resulting pharmacokinetic profile of the liposomal carrier and the incorporated temoporfin in a rat model. A novel pharmacokinetic model allowed distinguishing between temoporfin eliminated together with the liposomal carrier and temoporfin that is first transferred to other blood components (e. g. plasma proteins) before being eliminated from the blood. Our analysis using this model demonstrates that a fraction of temoporfin is released from the liposomes prior to being eliminated from the blood. In case of unmodified liposomes this temoporfin release was observed to increase with decreasing bilayer fluidity, indicating an accelerated temoporfin transfer from gel-phase liposomes to e.g. plasma proteins. This was predicted by earlier in vitro experiments using a liposome-liposome transfer system. The advantages of such a “leaky” carrier system for therapy will be discussed.

Monte Carlo simulations are employed to study a fluid vesicle that contains a single worm-like polymer chain. We vary the degree of polymer confinement in our simulations by increasing the persistence length of the polymer. The vesicle is represented by a randomly triangulated self-avoiding network that can undergo bending deformations. Upon increasing the persistence length of the polymer beyond the size of the vesicle, we observe a transition of the polymer from an isotropic disordered random conformation to an ordered toroidal coil. Concomitantly, the vesicle adopts an oblate shape to allow for some expansion of the polymer coil inside the vesicle. It is convenient to characterize both polymer and vesicle in terms of the asphericity, a quantity derived from the gyration tensor. At the onset of the polymer's ordering transition, the asphericity passes through a minimum for both polymer and vesicle. The increase in vesicle asphericity for a semi-flexible polymer can be understood in terms of ground state energy calculations, either for a simplified representation of the vesicle shape (we specifically discuss a disk shape with a semi-toroidal rim) or involving a full vesicle shape optimization. The asphericity of the polymer's coil results from conformational fluctuations and can be rationalized using Odijk's deflection length of strongly curved semi-flexible polymers.

Soft materials are dynamical by nature and the study of the dynamics of soft materials is an exciting, rich area of current interest. During macromolecular self-assembly, as occurs in block copolymers, long structural relaxation timescales due to collective molecular motion are often seen. How microstructure influences the dynamics, the existence and lifetime of metastable states, and the dynamics of long-lived non-equilibrium structures are all poorly-understood issues. Our large-scale dynamical simulations address these questions in the context of the nucleation of one microstructured phase out of another in a block copolymer melt. Specifically, we simulate the model-B dynamics of the conserved monomer density driven by a Landau-Brazovskii free-energy, which is appropriate for diblock copolymer melts. I will discuss our simulations of nucleation at the cylinder-to-lamellar transition, and our recent study of the kinetics of crystallization of the body-centered cubic phase of spherical micelles from disorder.

Biological macromolecules and colloidal aggregates such as micelles, liposomes, etc. in aqueous dispersion exhibit a relatively large interface to the water. Structural changes of these particles change their particle volume due to variations of (i) their internal packing (i.e. the void volume between the atoms) and (ii) their hydration (altered packing of water at the interface). The relatively new method of pressure perturbation calorimetry (PPC) permits one to measure thermotropic changes in partial volume and expansivity at a new level of precision and convenience. The talk presents PPC experiments to study the unfolding of globular proteins, DNA helicies and tetraplexes, the melting of lipids, self-association of surfactants, and micellar shape transitions. The overall aim is to better dissect packing and hydration effects and utilize the structural and thermodynamic information from volumetric experiments to its full capacity.

The Surface Plasmon Resonance (SPR) phenomenon is routinely exploited to qualitatively probe changes to the optical properties of nanoscale coatings on thin metallic surfaces, for use in probes and sensors. Unfortunately, extracting truly quantitative information is usually limited to a select few cases -- uniform absorption/desorption of small biomolecules and films, in which a continuous ``slab'' model is a good approximation. I will present advancements in the SPR technique that expand the number of cases for which the technique can provide meaningful results. Use of a custom, angle-scanning SPR imaging system, together with a refined data analysis method, allow for quantitative kinetic measurements of laterally heterogeneous systems. I will first demonstrate the directionally heterogeneous nature of the SPR phenomenon using a directionally ordered sample, then show how this allows for the calculation of the average coverage of a heterogeneous sample. Finally, the degradation of cellulose microfibrils and bundles of microfibrils due to the action of cellulolytic enzymes will be presented as an excellent example of the capabilities of the SPR imaging system.

Thin membranes of single-wall carbon nanotubes (SWCNTs) on elastic polymer substrates show considerable promise for flexible electronics applications, but the modulus and conductivity of these films decrease dramatically in response to applied strains. This softening arises from the strong van der Waals interactions between contacted nanotubes, which favor the parallel coarsening of SWCNT bundles in response to even very small external forces. By capping the SWCNT membranes with a thin layer of glassy polymer, we demonstrate a dramatic improvement in the mechanical response of the strained films. We link this behavior to the stabilizing influence of excluded-volume interactions mediated by the glassy polymer layer.

The coupling between mechanical flexibility and electronic performance is evaluated for thin coatings of electronically type-sorted metallic and semiconducting single-wall carbon nanotubes (SWCNTs) deposited on both soft-polymer and noncompliant supports. The microstructure, transparency, and electronic properties of the films are independently characterized using both optical and electron microscopy, as well as optical-absorption and impedance spectroscopy. Uniaxial and cyclic compression experiments suggest that thin films made from metallic SWCNTs show better durability as flexible transparent conductive coatings, which we attribute to a combination of superior mechanical performance and higher interfacial conductivity. In current research we investigate the use of both electronically type-sorted and chirality-sorted semiconducting films as the chemically functionalized p-type layer in photovoltaic pn-junctions on crystalline silica.

Diblock copolymers form an interesting class of synthetic surfactant molecules, finding uses ranging from compatibilizing polymer blends (alloys) to drug delivery schemes.  Remarkably, aside from analogy to small molecular surfactants, little has been done to understand (and exploit) many of the unique properties of block copolymers at fluid interfaces.  Here we present some of the results of our ongoing studies of the polystyrene-polyethylene oxide (PS-PEO) system in water/toluene emulsions.  In particular, we focus on using buoyancy as a convenient experimental handle on the system.  We show a simple and very sensitive measurement of the surface tension, some observations of adhesive properties as well as some unique hydrodynamic instability.

In suspensions of charged macroions, such as charge-stabilized colloids and polyelectrolyte microgels, the electrostatic interactions between macroions are relatively easily controlled by changing the sizes and charges of the macroions, as well as the concentration of salt.  This tunability of interactions can be exploited to stabilize various structures that self-assemble under appropriate conditions. In this talk, a statistical mechanical coarse-graining approach to modeling mixtures of charged spherical macroions will be discussed and results presented for effective electrostatic interactions in binary mixtures of charged colloids and microgels.  Finally, applications to charge renormalization, structure, and phase behavior of charged colloidal mixtures will be discussed.

In my talk I will give an introduction to the mathematical approaches to model biological evolution. In particular, I will formulate the celebrated Eigen's quasispecies model and state the basic results concerning the quasispecies concept and the error threshold. A number of results in this area were discovered and rediscovered by physicists using methods of statistical physics. I will conclude my presentation with a discussion of open problems.
All are welcome!

When sufficiently concentrated, colloidal particles, suspended by Brownian motion in a solvent, may self-assemble into crystals with lattice constants comparable to wavelengths of visible light. Colloidal crystals can serve as templates for fabricating "inverse opals" -- highly porous materials, whose micron-sized cavities affect the propagation of (and can even trap) light.  This year's Nobel Prize in Physics was awarded for fundamental research on controlling trapped photons and ions while preserving their quantum states.  Colloidal  crystals may provide the delicate matrix needed to enable potential applications to quantum computers.  Mixing colloids with nanoparticles, and independently varying concentrations, size and charge ratios, vastly expands the possibilities for tuning interparticle forces and stabilizing new crystal structures.  In modeling such complex materials, multiscale methods often prove essential to surmount computational challenges posed by multiple length and time scales.  I will summarize our recent efforts to develop a hierarchical approach to modeling effective interactions, with the goal of predicting structural and thermodynamic properties of colloidal and other soft materials. 

Graphite, the parent compound of graphene, contains the characteristics of both monolayer and bilayer graphene. The massless holes at graphite's H-point behave like Dirac fermions in monolayer graphene, while the massive electrons at the K-point behave like Schrodinger fermions in bilayer graphene but with an adjusted interlayer coupling constant. Understanding graphite helps to learn more about the nature of monolayer, bilayer and multilayer graphene, while the later two hold promise for commercial applications. Using the magneto-reflectance study, we determine the tight-binding band parameters from the SWM band theory. Moreover, the result implies that not only should the H-point fermions regarded as massless relativistic particles, the K-point fermions can also be regarded as massive relativistic particles at magnetic field. As a result, graphite may be an excellent platform to study some phenomena which were considered inaccessible in a laboratory setting. In the magneto-transmittance study, the electron-phonon couplings between H-point fermions and two zone boundary phonon modes are manifested as two kinds of resonant phenomena. We discovered a strong anticrossing resonance due to the interactions between the large momentum K-point phonons and the H-point Dirac-like fermions, as well as a Fano resonance, resulting from the coupling of the -point phonons with the cyclotron resonance of charged carriers. The uniqueness and implication of these resonances will be discussed.

Gender differences in student learning in the introductory, calculus-based electricity and magnetism course were assessed by administering the Conceptual Survey of Electricity and Magnetism pre- and postcourse. As expected, male students outgained females in traditionally taught sections as well as sections that incorporated interactive engagement (IE) techniques. In two of the IE course sections, however, the gains of female students were comparable to those of male students. Classroom observations of the course sections involved were made over an extended period. In this paper, we characterize the observed instructor-student interactions using a framework from educational psychology referred to as wise schooling. Results suggest that instructor practices affect differential learning, and that wise schooling techniques may constitute an effective strategy for promoting gender equity in the physics classroom. 

When a drop of colloidal suspension evaporates we usually find that a large portion of the colloids is deposited at the edges of the drop, which we are all familiar with because this is also known as the coffee ring effect. The underlying reason for this deposition is that the edges of the drop is pinned and this causes a hydrodynamic flow of material towards the edges which drags the small particles towards the edge of the drop.

A separate phenomenon is that suspended particles typically like to aggregate at an interface. The timescale for this is related to the time it takes a particle undergoing Brownian motion to reach the interface. Once the interface is coated with particles, this will reduce the evaporation from the covered surface. This raises the possibility that a particle suspension where a noticeable fraction of the surface is covered with particles near the edges will show a much reduced hydrodynamic flow and thereby a reduction of deposition of particles at the rim of the drop.

In this talk we will analyze the effects of evaporation and present our analytical predictions of the difference in evaporation rates as a function of the covering of the surface with particles and validate our analytical predictions with numerical simulations and speculate on the feasibility of utilizing this mechanism to generate (more) uniform particle distributions as a result of the evaporation process.

Lipid bilayers are a few nanometers thin sheets that form the basis of biological membranes. Due to their softness lipid bilayers are subject to thermal fluctuations. In the seminar we will present an experimental method for direct measurement of their bending elasticity through the analysis of thermally induced shape fluctuations of lipid vesicles. Theoretical and experimental aspects of the method will be addressed and the connection with Monte-Carlo simulations, where the stochastic Metropolis-Hastings algorithm allows us to sample thermal fluctuations of a system in thermodynamical equilibrium, will be  discussed.

Large viral shells or fullerenes exhibit molecular crystalline structures with icosahedral shapes. Other faceted shapes, including Platonic and Archimedean geometries, arise spontaneously in shells formed by more than one component (1). We describe buckling of a crystalline shell with two co-existing elastic components into various polyhedra. Our work explains the principles to design hollow polyhedra and the existence of regular and irregular polyhedral shells observed in organelles and in halophilic organisms wall envelopes, as well as viral capsids made of various proteins. We provide experimental evidence of the spontaneous buckling phenomena in shells made of mixtures of cationic and anionic amphiphiles  (2), where their co-assembly is driven by electrostatics, which orders the assembly into faceted ionic structures with various crystalline domains. These shells are stable at high monovalent salt concentrations. Their crystalline structure and shape, however, are modified by the pH value of the solution. Our work provides guidelines for the fabrication of robust nanocontainers with specific shapes and may aid to elucidate paradigms that relate shape and composition of cellular shells.

(1) G. Vernizzi, R. Sknepnek, and M. Olvera de la Cruz "Platonic and Archimedean geometries in multi-component elastic membranes" Proc. Natl.Acad. Sci. USA, 118, 4292–4296 (2011).

(2) M. A. Greenfield, L. C. Palmer, G. Vernizzi, M. Olvera de la Cruz, and S. I. Stupp “Buckled Membranes in Mixed-Valence Ionic Amphiphile Vesicles” J. Am. Chem. Soc., 131, 12030–12031 (2009).

Photoelectron current and nonlinear optical effects are considered in terms of quantum correlation functions, i.e., nonequilibrium Green's functions (NEGFs). In this approach photoelectric current is expressed in terms of NEGFs. To find NEFs we numerically solve two-time Kadanoff-Baym equations for a model system where electron correlation is due to the e-e interaction between QD and semiconductor electrons. The results are compared with the results obtained from the often used approximation based on the Markovian rate equations for a density matrix. The found discrepancy appeared to be up to one order of magnitude. Nonlinear optical effects for a pump-probe experiments are theoretically studied for the same system. We have found that nonlinear polarization can differ by a factor of five for current states ( the current is on) and for isolated QD (the current is off). There is a discrepancy between the Kadanoff-Baym and Markovian approaches either.

3M is famous for its innovations - ranging from the world's first masking and transparent tapes to the latest in high tension power lines and plastic films that achieve 99% reflectance of the visible spectrum by packing hundreds of layers into a 50 micron thickness. More important to 3M than these individual technological advances is the culture of open innovation that made these and thousands of other advances possible. This talk will cover some of the 'how' behind 3M's reputation for innovation and some of the history that enabled a sandpaper company to evolve into a world leader in dozens of technologies, now employing more than 10,000 technical employees worldwide and spending $1.6 billion on R&D in 2011. A good deal of time will be spent on the Tech Forum, the organization of technical employees that keeps everyone in 39 labs talking to each other so silos can't form and technologies flow freely between businesses. The speaker will also share a story of his own experience with an adhesion challenge in a corporate product development lab. Come hear the not-always-fun story, complete with instances of 'old-timers' saying, "We've tried that; it doesn't work." and a 'simple' addition that worked in hundreds of lab experiments but failed totally in its first two tries in the factory.

Recent advances have made it possible to prepare clouds of atoms at temperatures of about 0.0000000004 Kelvin – less than a billionth of a degree above absolute zero.  At such low temperatures the atoms reveal their quantum mechanical wave nature, marching in lockstep to produce novel phases of matter.  The density, composition, disorder, interactions, and lattice potential can be all be controlled independently using lasers and magnetic fields.  Ultracold atomic gases hold great promise for precision measurement (clocks), quantum computing, and the emulation of condensed matter systems such as the Fermi Hubbard model, which may apply to high-temperature superconductors.

The Fermi Hubbard model has now been realized using potassium-40 atoms in optical lattices.  However, the average entropy is rather high (S/N ≈ 1 kB/atom).  Our quantum Monte Carlo simulations suggest that antiferromagnetic order begins to develop below S/N ≈ 0.65 kB/atom, which may be achievable with current techniques.  However, lower entropies are required for robust antiferromagnetism.  I have proposed a way to attain very low temperatures and entropies (≾ 0.03 kB/atom) by trapping fermions in a corral formed from another species of atoms.  This Fermi system can then be evolved into an antiferromagnet by morphing the lattice into a set of double wells, quasi-adiabatically.  Quantum dynamics simulations have, so far, given promising results.

[1] Thereza Paiva, Yen Lee Loh, Mohit Randeria, Richard T. Scalettar, and Nandini Trivedi, “ Fermions in 3D optical lattices: Cooling protocol to obtain antiferromagnetism,” Phys. Rev. Lett. 107, 086401 (2011)
[2] Yen Lee Loh, “Proposal for achieving very low entropies in optical lattice systems,” arxiv:1108.0628
All are welcome!

Statistical mechanics (SM) for systems in thermal equilibrium, founded over a century ago, forms part of the current physics core curriculum. However, like most homework problems do not directly apply to real situations, textbook equilibrium SM falls short of characterization of systems in non-equilibrium (NE), such as biological systems. An overarching theoretical framework of NESM remains elusive and has been attracting increasing interest from physicists.

In this talk, I first contrast the key features of NESM with the familiar ESM. To illustrate how tools developed in NESM help untangle the complex biological process, we then zoom in on modeling protein synthesis in bacteria through a particle transport model (the totally asymmetric simple exclusion process, or TASEP). We discuss the quantitative effect of different elongation rates, associated with different codons, on the overall protein production rate. We conclude with some ongoing projects and open questions that nestle on the interface of physics and biology to set the stage for further investigation in this field.

Spontaneous formation of ordered structures from amphiphilic molecules has attracted tremendous attention in the last decades.  Among the many different amphiphilic systems, block copolymers, with their rich phase behavior and ordering transitions, have become a paradigm for the study of structural self-assembly in soft materials.  Understanding the structures and phase transitions in block copolymers has been one of the most active research areas in polymer science in the past two decades. Theoretical studies of block copolymers focuses on their phase behavior and the phase transition pathways between different ordered phases.  I will present a strategy to discover complex ordered phases of block copolymers and a method to construct phase transition pathways connecting different ordered phases.  Applications of these methods will be illustrated by the phase behavior of ABC triblock copolymers and the phase transition pathways of AB diblock copolymers.

As a part of a multi-year investigation of student understanding of mechanical wave in introductory courses, a set of tutorials on wave behavior at a boundary has been developed.  However, even after the targeted instruction many students still are not able to systematically analyze complex unfamiliar situations. We hypothesized that poor performance on some post-tests may be attributable to difficulties in visualizing and reasoning spatially about transformations in the shape of a spring that occur over time as a complex pulse reflects from a boundary.  We probed the extent to which student performance hinges on their ability to visualize and reason spatially by examining the degree of association between student performance on the post-tests and on a spatial visualization test - paper folding test.

Models of self-driven particles similar to the Vicsek model [Phys. Rev. Lett. 75 (1995) 1226] are studied by means of kinetic theory. In these non-equilibrium models, particles try to align their travel directions with the average direction of their neighbors. At strong alignment a global flocking state forms. The alignment is defined by a stochastic rule, not by a Hamiltonian. The corresponding interactions are non-additive and are typically of genuine multi-body nature. The theory [1] is based on a Master equation in 3N-dimensional phase space, which is made tractable by means of the molecular chaos approximation. The phase diagram for the transition to collective motion is calculated and compared to direct numerical simulations. A stability analysis of a homogeneous ordered state is performed, which reveals a long wave length insta- bility for some of the considered models. The mean-field calculations of one of the models show a tricritical point where the flocking transition changes its character from continuous to discontinuous.

[1] T. Ihle, Phys. Rev. E 83 (2011) 030901

When a thin rigid plate is adhered to a soft substrate and compressed, the plate will buckle out of plane to accommodate the applied stress. The out of plane buckling results in a sinusoidal topography (wrinkles) due to the interplay of rubber stretching and film bending. When the plate is replaced by a collection of closely packed particles similar phenomena results – the positions of the particles move out of plane and follow a roughly sinusoidal curve. Due to the similarity of the end state of each system, the same continuum theory is often applied to model both films. Here, we use a carefully constructed experimental system consisting of micron-scale polymer and silica spheres on a PDMS elastomer substrate to demonstrate the physical differences between a continuum plate and a discrete set of particles.  In particular, because we can easily track the position of each particle in three dimensions with confocal microscopy, we have access to all aspects of the particles motion. We note that the wrinkling is independent of particle modulus, and highly dependent on particle packing. This leads us to suggest that the underlying physics is granular (and not continuum) in nature. This result may have implications in biology, where elastic continua are often made of discrete building blocks (e.g. cells).

The bending stiffness of a lipid bilayer generally depends on the presence of membrane additives such as sterols, cosurfactants, peptides, and drugs. As a consequence, the partitioning of such membrane additives into liposomes becomes selective with respect to liposome size; i.e., membrane rigidification depletes the membrane additives in the smaller (more strongly curved) liposomes. We have measured this liposome size-selective partitioning for two membrane additives, namely cholesterol and the porphyrin-based photosensitizer temoporfin, using asymmetrical flow field-flow fractionation (AF4) of liposomes and radioactive labeling of the membrane additive and lipid. The method yields either the molar cholesterol-to-lipid or the temoporfin-to-lipid ratio as a function of liposome size, from which we calculate the corresponding change of the membrane bending stiffness. For small unilamellar fluid liposomes composed of palmitoyloleoylphosphocholine (POPC) and palmitoyloleoylphosphoglycerol (POPG), we find that cholesterol rigidifies the host membrane in a manner consistent with previous findings. In contrast, temoporfin softens this membrane. Partitioning results for gel-phase liposomes composed of dipalmitoylphosphocholine (DPPC) and dipalmitoylphosphoglycerol (DPPG) are also curvature-sensitive but cannot be interpreted on the basis of the bending stiffness alone.

Undergraduates in the Solid State Nuclear Magnetic Resonance (NMR) Research Group at Minnesota State University Moorhead are currently studying a series of materials that have potential application as proton exchange membranes in hydrogen-oxygen fuel cells. These solids, called sodium thio-hydroxogermanates, are electronic insulators but allow hydrogen ions to diffuse through them with a mobility comparable to that of liquids! Using NMR, and principles of quantum and thermal physics, we will illustrate how fundamental physical principles allow data to be interpreted. We present arguments that the hydrogen dynamics involves coordinated water rotation and jump across hydrogen bonds within the material.  Understanding how the hydrogen dynamics depends on chemical composition could lead to improvements in fuel cells.

Daphnia magna (D.magna) is a freshwater crustacean that is an emerging model organism in biological research. Historically, this organism has been studied by ecologists and environmental toxicologists. Our work is focused on studies of the neural mechanism of movement in Daphnia magna. We have developed low-cost methods utilizing custom-made recording chambers, video-rate cameras, and a variety of free and commercial software to record and analyze Daphnia movement. We are using these methods to study the effects of drugs which perturb the dopaminergic nervous system, a neurotransmitter system involved in movement in many organisms. Data will be presented on three primary areas of this work. First, I will show that two dimensional swimming behavior of Daphnia magna can be reliably quantified using a multi-well chamber amenable to drug-screening studies. Using these imaging methods we have quantified the effect of drugs that perturb the dopamine nervous system in Daphnia. Second, I will describe experiments aimed at recording and modeling Daphnia swimming in three dimensions. This work involves labeling the daphnids with fluorescent quantum dots followed by imaging the animal’s trajectory over time. Preliminary experiments utilizing this quantum dot labeling method have enabled us to track the movement of fluorescent daphnids freely swimming in a 3D chamber. Finally, I will describe our efforts to clone and characterize genes involved in dopamine neuron function. The complete genome sequence of Daphnia pulex, a relative of Daphnia magna, was recently reported giving us access to genomic sequence information. Using bioinformatics tools to design DNA primers and a polymerase chain reaction (PCR) cloning strategy we have retained a partial clone of a dopamine decarboxylase gene from Daphnia magna.  We intend to use these molecular tools to identify dopamine neurons using microscopic fluorescent in situ hybridization methods. Taken together, these studies are beginning to shed light new light on Daphnia movement.

An investigation of the transfer kinetics of drugs from different liposomal formulations to their targets is inevitable for a successful development and optimization of liposomal drug delivery systems. Therefore, during the last years a lot of efforts have been made for the establishment of methods allowing the in vitro measurement of drug transfer from donor to acceptor vesicles [1]. A simple and fast technique is represented by fluorescence spectroscopy, which is based on excitation of fluorophores and following detection of emission light.  Within my prospective work the transfer kinetics of the lipophilic fluorescent photosensitizer 5,10,15,20-tetrakis(m-hydroxyphenyl)chlorine (mTHPC, Foscan®) between liposomal membranes will be investigated by using the so called fluorescence dequenching effect. For this purpose different drug-loaded liposomes (donor vesicles) as well as empty liposomes (acceptor vesicles) are prepared, whereby the latter serve as a model for biological membranes. The mTHPC concentration of donor liposomes is chosen in such a way, that the fluorescence intensity of the drug is self-quenched. Hence, the temporally transfer of the drug to acceptor liposomes can be analyzed via an increase in fluorescence intensity [2].

References

[1]

Hefesha, H., Loew, S., Liu, X., May, S., Fahr, A.: Transfer mechanism of temoporfin between liposomal membranes. J. Control. Release, 2011. doi:10.1016/j.jconrel.2010.09.021

[2]

Zhu, X.: Transfer of lipophilic drugs between liposomal membranes by using the ion-exchange micro-column technique and the fluorescence dequenching effect. Dissertation, 2008.  http://d-nb.info/993360122/34 (accessed: 22.03.2011)

Over the past decade, biology and pre-health care students have grown into the largest population served by physics service courses.  With the recent explosion of high-tech biology and medicine, both the character of the students in this class and their educational needs have changed dramatically from when the current course was designed decades ago.  What can physics departments do to provide a better education for our students from biology? In order to answer this question, we have to understand not only the physics required but also the context and needs of the students.

The Discipline-Based Education Research Groups in Physics and Biology (PERG/BERG) at the University of Maryland have been studying this question for a decade. We have developed an understanding of the context and needs of biology students both through extensive observations (interviews, surveys, quizzes, exams, and videotaped observations of labs and tutorials) and through detailed negotiations with biology faculty. In this talk, I will discuss some of our previous reforms, our insights into interdisciplinary challenges, and our current work on developing a new biology-oriented introductory physics class.

References

E. F. Redish and D. Hammer, “Reinventing College Physics for Biologists: Explicating an Epistemological Curriculum,” Am. J. Phys., 77, 629-642 (2009). [http://arxiv.org/abs/0807.4436]

“Collaboration seeks to create interdisciplinary undergraduate curriculum”, www.hhmi.org/news/ nexus20110608.html

(Seminar Announcement.)

Block copolymers are comprised of two or more homopolymer subunits linked by a covalent bond, yet their mutual immiscibility results in microphase separation leading to the formation of useful periodic nanostructures (spheres, cylinders, lamellar, etc). Nanolithographic techniques based on self-assembled block copolymer templates offer exceptional potential for fabrication of large-area nanostructure arrays. Applying these techniques to magnetic materials will allow for the study of fundamental magnetic properties at the nanometer scale. In addition, the block copolymer approach to nanolithography has huge implications for the progress of bit-patterned magnetic recording media for ultra high-density storage (>1Tb/in^2). In the case of pattern transfer, the production of well-ordered, extremely small (<50nm) features, has been plagued with complications. In this presentation I will describe these issues and report our recent approaches to overcoming these problems. The process we developed can be summarized as an overfill/planarize/etch-back scheme, exploiting the large Ar ion beam etch rate contrast between polymers and typical metals. The process is demonstrated via formation of a large-area array of 24 1.5 nm diameter Ni80Fe20 nanodots (~0.4 x 10^12 dots/in^2) with exceptional long-range, hexagonally-close-packed order. Extensive microscopy, magnetometry, and electrical measurements provide detailed characterization of the pattern formation and fidelity. We argue that this generic approach can be applied to a wide variety of materials and is scalable to even smaller feature sizes. This work is funded by the NSF MRSEC.

Recent theoretical advances in statistical mechanics have enabled difficult yet informative free energy calculations to be carried out for complex biological systems using molecular dynamics simulations. An intuitive explanation of the important aspects of the theory will be used to introduce some recent generalizations made by my group. Results we have obtained from the application of these techniques to several systems (NaCl disassociation, stretching energy of a short peptide, penetration of a trace amine through a neuronal membrane) will be presented and compared to experimental results.

Moore’s law predicts that the number of transistors on an integrated circuit doubles every eighteen months. This is possible only if the size of individual components can be continually reduced in a cost effective way. The most commonly used approach to build the components of an integrated circuit is the top-down approach, where externally-controlled devices, such as conventional lithographic techniques, are used to shape materials into the desired forms. This approach is reaching fundamental (diffraction) size limits, and is not useful for the precise positioning and interconnecting of molecular sized components. Therefore the second approach, a so called bottom-up approach, which utilizes the concepts of self-assembly and molecular recognition of individual molecules to build such devices, may facilitate the field of molecular electronics. Scanning tunneling microscopy/spectroscopy (STM/STS) is a unique technique with enough resolution to study the physical and chemical properties of surfaces and interfaces in molecular electronic devices. I will present STM/STS studies of various self-assembled molecular films and nano-structured Pt modified Ge(001) surfaces, relevant to the design and
implementation of molecular electronic devices.

The mechanical performance of living cells relies on the remarkable (visco)elastic properties of the cytoskeleton which consists of a dense meshwork of semiflexible protein filaments. I will discuss minimal theoretical descriptions of those networks, starting from the wormlike chain model for a single polymer. The first part of my talk concerns entangled solutions of semiflexible polymers, where interactions between impenetrable neighboring chains lead to the chains being confined into effective tube-like cages. One can calculate the average tube diameter using a Binary Collision approximation (BCA). I will show that within a systematic generalization of the BCA (the ”segment-fluid” model) it is possible to calculate not only the mean value of the tube radius R but also the distribution function P(R) of the tube radius [1]. In the second part, I will discuss the dynamics of entangled polymers by means of a phenomenological model for sticky polymer solutions, the ”glassy wormlike chain” The predictions of this model are compared to in-vitro and in-vivo experimental data.

[1] J. Glaser et al., Phys. Rev. Lett., 105 (2010) 037801

Nanotechnology represents a nexus of materials science, chemistry, physics, and engineering and is vital for the development of revolutionary applications ranging from electronics and photovoltaics to medicine. However, before advances in next generation technologies can come to fruition, understanding and control of the structure-property relationship of nanomaterials are required. Computational predictions based on atomistic modeling could provide valuable insight into these issues. In this talk, I first briefly overview the quantum-chemical approaches that we use to describe morphologies, optoelectronic properties, and photoexcited dynamics in novel nano-structured materials, such as semiconductor quantum dots (QDs). Based on these methods, we simulate phonon-assisted dynamics in ligated CdSe QDs with an ultimate goal to understand the role of surface ligands in fast relaxation of photoexcitation in QDs. Our simulations reveal that ligands passivating the QD surface introduce many hybridized states, which electronic density is spread over the QD and ligand atoms. Hybridized orbitals increase the overall electron-phonon couplings through their strong interaction both with the high frequency vibrations of the ligands and low energy phonons of the QD, thus, open new channels for relaxation. These results open a new prospective for understanding of fast energy relaxation mechanisms in QDs – a topic of general interest due to the recent focus on QD-based solar cells, light-emitting diodes, field-effect transistor, etc.

A new method combining ab initio electronic structure and density matrix approaches has been developed to simulate photo-excited dynamics in silicon-based energy materials. The interaction of electrons with thermalized lattice vibrations provides the dissipative terms in the equation of motion (EOM) for the reduced density matrix of the silicon surface and describes line broadening of optical excitations, dephasing, and population relaxation from the photoexcited state towards thermalized electronic state. The steady state solutions of the EOM in a basis of Kohn-Sham orbitals provide the electronic charge density for excited states responsible for the induction of a photovoltage at the surfaces [1], while time dependent solutions of the EOM provide rates for carrier relaxation induced by lattice vibrations [2]. Our simulations predict that absorption and photovoltage spectra of the silicon surfaces are drastically affected by presence of adsorbates on the surface or by p- or n- doping. The results obtained by our atomistic approach provide insight on trends relevant to the absorption of near IR, visible, and near UV light, which is of interest in measurements of photovoltages and in the utilization of solar energy.

The name Casimir effect is generally applied to a number of phenomena associated with the change in the zero-point energy of a quantized field due to the presence of external constraints. The simplest of such effects, and the one originally predicted by Casimir, is the force between two uncharged conducting parallel plates in vacuum. After introducing some of the basic concepts of the field we will discuss the difficulties of dealing with the Casimir effect in rectangular geometries and present some recent results for the double piston geometry.

One of the most interesting and simple examples of pattern formation in Nature can be found as one squeezes together tissue on an arm – a set of equally spaced bends appear in the skin; it wrinkles! In fact any system that is composed of a rigid layer (here the skin) on a much softer substrate (the tissue below the skin) and placed under compressive stress will show the same behavior. The simple sinusoidal wrinkle pattern, however, can break down and the stress which is spread evenly throughout the pattern can localize – in other words the system can be pushed to failure. Here I show how localization can occur due to an appropriately chosen perturbation, notably at a vanishingly small applied stress. I will then show how the stresses present in the wrinkled film itself have, in fact, already broken the energetic symmetry. This effect is most dramatically realized when a disordered block copolymer film is wrinkled and then allowed to microphase separate. This experiment show how a macroscopic stimulus (the applied stress) is easily translated into local variations in molecular mobility.

Although various organisms have long been able to biochemically produce uniform crystalline magnetic nanoparticles and leverage their unique properties, laboratory syntheses and applications of such particles are relatively recent developments. Scientists have devised synthetic schemes to generate batches of magnetic nanoparticles with very narrow size distributions and exceptional magnetic properties using precursors containing transition metals. Particles composed of nickel, cobalt, and their oxides generally have more favorable magnetic characteristics than particles of comparable size made of iron and its oxides. However, the toxicity of nickel and cobalt and the instability of pristine transition metal nanoparticles, make iron oxide-based materials a better choice for biomedical applications such as hyperthermia, targeting therapeutics, and MRI contrast agents.

Major histocompatability complex (MHC) class I proteins present pathogen peptides to effector T lymphocytes to trigger the destruction of infected cells.  The assembly of MHC I proteins in the endoplasmic reticulum (ER) involves their binding to a number of ER chaperones and accessory proteins, which supply the proteasome-generated peptides in the cytosol to the nascent MHC I.  Peptide-loaded MHC I proteins then dissociate from transporter associated with antigen processing (TAP) complex.  The mechanisms by which MHC proteins are transiently retained in the ER are not understood.  Conventaional biochemical analyses are inherently incapable of characterizing the association dynamics of MHC I with the TAP complex.  In the contribution, I will present our recent single-molecule studies of GFP-encoded MHC I diffusion in living mouse fibroblast cells.  Using multimodal and noninvasiv fluorescence micro-spectroscopy methods, we also quantified the intracellular distribution, lateral heterogeneity, and conformational dynamics of MHC I proteins in living cells under different conditions of proteasome inhibition and peptide loading.

 (Seminar Announcement)

Abstract: We have performed molecular dynamics simulations of vesicle fusion using coarse-grained lipid models. Using the weighted histogram method, we calculate the free energy barrier for two vesicles to come into contact and the initial fusion event to occur, i.e. mixing of lipids between the two outer leaflets. The structural dynamics of the initial fusion event is imaged and connected to the free energy differences. These calculations have been performed for four different vesicles. The lipids in the four vesicle types are DPPC, DOPC, a 3:1 mixture of DPPC/DPPE and an asymmetric lipid tail system in which one tail length was reduced to half the length. We find the free energy barrier to be about the same magnitude for all except the system with asymmetric tail lengths. In addition, the free energy curves can be overlaid on a single curve by plotting versus the surface separation. The asymmetric tailed system has a barrier about 3 times larger than the others. Examination of the initial lipid mixing shows that the initial lipids crossing from one vesicle to another often have splayed tails, and the tails span the gap between the two vesicles. Thus the splayed lipid provides the initial connection between the vesicles that then promotes successive lipids to cross between the vesicles leading to fusion of the outer leaflets and ultimately full fusion. The reason for the asymmetric tail system having a larger barrier is that the asymmetric tail requires a smaller separation for fusion, because its shorter tail can only span a smaller gap. Consequently, this system must go further up the free energy curve (i.e. more water must be removed from between the vesicles and more deformation). We compare our results with experimental data, but since our vesicle size is very small, direct comparison is not possible, but there are interesting issues that arise.

 (Seminar Announcement)

All eukaryotic cells including those of our body contain a large variety of molecular machines that convert the chemical energy released from nucleotide hydrolysis into mechanical work. This talk will focus on cytoskeletal motors which walk with two motor heads and have been intensely studied by single molecule experiments. One such motor is conventional kinesin, for which each motor head contains a single domain for ATP binding and hydrolysis. Our theory for these motors starts with a network representation based on the different nucleotide states of the motor heads. The properties of single motors are then used to describe the cooperative behavior of many motors. The latter behavior includes uni-directional and bi-directional transport of cargo particles by small teams of motors as well as pattern formation and phase transitions in motor traffic.

 (Seminar Announcement)

^{Supported by the Departments of Physics, Mechanical Engineering, Coatings & Polymeric Materials, and the Cooperative Sponsorship Committee.}

In 1998, two independent teams of astronomers announced that the Universe was in a period of accelerated expansion. This unexpected discovery signifies either a breakdown in our understanding of gravity on large scales or the existence of an ”anti-gravitational” force (dark energy) that permeates the Universe.
Due to the potential paradigm shift in our understanding of physics at a fundamental level, major funding sources (i.e., NASA, DOE, and NSF) have expressed interest in supporting research initiatives that seek to uncover the nature of dark energy. One of these projects, the Dark Energy Survey (DES), will observe 5000 square degrees of the southern hemisphere using a new wide-field imaging camera mounted on a 4-meter telescope in Chile. The DES will allow astronomers to characterize dark energy with high precision and provide important observational constraints on its nature. As a member of the DES, I will describe how this project will help to shed light on dark energy.

 (Seminar Announcement)

We review recent work on the molecular simulation of the crystallization process.  The aim of this work is to obtain a complete understanding of the molecular mechanisms underlying crystal nucleation and growth, and, in particular, to shed light on the polymorph selection process.  For this purpose, we carry out three different types of molecular simulation: (i) to determine the phase diagram of the simulated system, (ii) to simulate the crystal nucleation event and (iii) to gain a direct access to the crystal growth mechanism.  We present results obtained on a variety of systems, ranging from model systems to colloidal systems and metal nanopoarticles and discuss new leads to improve the accuracy of simulation methods for the determination of phase diagrams.

 (Seminar Announcement)

I will discuss several models of (first-failure) dielectric breakdown and polycrystal plasticity, with a focus on their mathematical derivation from more flexible power-law models via Γ-convergence, and on the characterization of the effective yield sets by means of variational principles associated to the limiting supremal functionals. This is based on joint with with Enzo Nesi (Universita di Roma, "La Sapienza"), and with Cristina Popovici (NDSU).

 (Seminar Announcement)

The ability of an applied strain to distort the nanoscale structure of thin polymer films has profound scientific and technological implications. The mechanical characteristics of such membranes are dictated by the arrangement and strength of molecular contacts, and structural changes at these small length scales can have a significant impact on film performance. Membranes comprised of single-wall carbon nanotubes (SWNTs) represent an intriguing example of this. The mechanical properties of the individual SWNTs can be outstanding while their high aspect ratio enables 2D network formation at remarkably low surface density. The mechanics of inter-SWNT contacts, however, can ultimately limit the response of the film. Recent advances in the separation of nanotubes by length and type make ultra-pure SWNT membranes a reality, creating the need to better understand the deformation mechanics of this technologically important class of film. I will describe our recent work on measuring the microscale wrinkling of thin SWNT films under compressive strains and demonstrate how existing models fail to capture the essential physics occurring at the nanoscale.

In this talk we will discuss mathematical modeling and computer simulation, and illustrate their power in understanding and predicting the behavior of polyester thin films when subjected to heat treatment. The motivation of this problem comes from applying thin transparent films on car windshields. Experimental data will be discussed, a mathematical model will be developed, and numerical results will be presented that predict how such films might deform when heated.

Monte Carlo simulations are employed to investigate the ability of a charged fluid-like vesicle to adhere to and encapsulate an oppositely charged spherical colloidal particle. The vesicle contains mobile charges that interact with the colloid and among themselves through a screened electrostatic potential. Both migration of charges on the vesicle surface and elastic deformations of the vesicle contribute to the optimization of the vesicle-colloid interaction. Our Monte Carlo simulations reveal a discontinuous wrapping transition of the colloid as a function of the number of charges on the vesicle. Upon reducing the bending stiffness of the vesicle, the transition terminates in a critical point. At large electrostatic screening length we find a re-entrant wrapping-unwrapping behavior upon increasing the total number of charges on the vesicle. We present a simple phenomenological model that qualitatively captures some features of the wrapping transition.

Block copolymers are long chain molecules made of segments of more than one polymer variety covalently joined together. This molecular architecture leads to many technologically important phenomena which are useful in applications that range from the semiconductor to the commodity polymer industries. Many of the most important properties of these systems result from the nanoscopic structures that form due to the chemical incompatibility of the blocks. In this talk I will present our progress towards a more detailed understanding of the physics of these systems. I will focus on the simplest system, that of symmetric diblock copolymers (where the chain consists of two distinct blocks of equal size) in a variety of different experimental confining geometries. I will show how the micro-phase separated structures lead to conically shaped fluid droplets, how the thin film geometry can be adapted into a very simple measurement of the Flory-Huggins interaction parameter and how the structured surface of a thin film can be used to drive a simple wrinkle into a state of stress localization.

A hybrid Lattice Boltzmann method is used to simulate binary fluids where also the temperature evolves following its dynamical equation.  The system is quenched by contact with cold walls at temperatures below the critical value and different morphologies are observed for different thermal diffusivities and viscosities.  Lamellar patterns are favoured at high viscosity and with slight asymmeteric concentrations.

^{Sponsored by NDSU Geosciences and Physics Department, the Department of Coatings and Polymeric Materials, and by the Cooperative Sponsorship program.}

There is little doubt that nanotechnology, the exploitation of nanometer sized entities in technology, is among the most rapidly expanding areas of materials science and technology.  It is also true that our understanding of such objects or of bulk materials on the same nm scale has not expanded at the same rate, and that knowledge of bulk material properties does not necessarily lead to reasonable predictions of the properties on the nanoscale. It is crucial to increase our understanding of how materials behave on the nm scale to provide a strong underpinning for emerging nanotechnologies. In this talk I will give two recent examples we have studied showing how soft materials in the “nanoworld” behave very differently than we may have guessed. Both of these examples have immediate technological implications.  The first example is the properties of the first few nm of a glassy polymer surface- a problem highly relevant for some recent proposals for high density information storage.  While this question has been hotly debated for about 15 years, only recently has a clear picture emerged.  We have shown that for glassy polymer systems there is strong evidence for a surface layer that in many ways behaves like a liquid (rather than a softer solid). We have also provided bounds on the length scale of this near surface effect.  The second example involves the interaction of proteins with nanoparticles.  This interaction is fundamental in the rapidly growing area of nanobiosensing, and in understanding potential toxicity of nanoparticles. We have shown that the adsorption of protein, as well as both reversible and irreversible structural changes of proteins on nanoparticles are surprisingly sensitive to the size of the nanoparticle. These examples highlight the importance of having basic knowledge keep pace with technological developments on the nanoscale. Finally, I will discuss how the Materials Science and Nanotechnology graduate program at NDSU can be a prominent player on the national and perhaps international stage in advancing nanotechnology and nanoscience particularly, but not exclusively, in areas traditionally denoted as soft materials.

To realize electronic applications of carbon nanotubes, such as quantum wires, ballistic conductors, microchip interconnects and transistors, reproducible fabrication of joints between individual nano-tubes and -electrodes has been identified as a major impediment. Previous studies have shown that electrically conductive connection between nano building blocks is not straightforward. Instead of desired ohmic contacts, tunnel junctions or weak links of a high contact resistance typically at 200 kΩ for contact regimes on the order of 1 nm², are often generated. Obviously, the gap-sensitive contact resistance makes it difficult to join nano building blocks with repeatable performance in nano-devices [1, 2]. 

The combination of biological molecules and nanostructures offers exciting possibilities for the design of new applications. In particular, DNA and RNA, two classes of extremely versatile bio-polymers may be connected non-covalently to carbon nanotubes to form a novel hybrid system with a number of interesting properties.

In this talk, we present the results of our first-principles study of the interaction of nucleic acid bases with a metallic carbon nanotube as a significant step towards an understanding of the fundamental physics and the mechanism of this sequence-dependent interaction of ssDNA with CNTs.

The second part of the talk will describe the results of calculations on the functionalized nanopore-embedded gold electrodes with the aim of improving nanopore-based DNA sequencing method.  The results of our study indicate that our proposed scheme could allow DNA sequencing with a robust and reliable yield, producing current signals that differ by at least one order of magnitude for the different bases.  Hydrogen bonds formed between the molecular probe and target bases appear to stabilize the scanned DNA unit against thermal fluctuations and thus greatly reduce noise in the current signal.

(in collaboration with  groups at Trinity College, Uppsala University, and Army Research Lab)

 

The plasma cell membrane is a dynamic biological structure which separates the cell from its surroundings, localizes proteins at an interface, and plays an active role in many cellular processes.  The multi-functionality of the membrane is provided by a diverse array of molecules.  For example, in the presence of water, amphiphilic molecules called lipids self-assemble into bilayers which provide a structural backbone for the membrane.  Of these lipids, cholesterol has been identified as unique.  I will describe work in my lab to understand how structural features of the cholesterol molecule confer important biophysical properties to the lipid membrane.  Two areas of focus will be on the transbilayer diffusion rates for phospholipids in the presence of sterols and line tension measurements between coexisting liquid phases.  Materials scientists and engineers seek to exploit the biophysical properties of these systems in the development of biomimetic surfaces and drug delivery systems. Potential applications of this work as a tool in biotechnology, experimental techniques, and future research directions will be described.

 

_{* Dr. Christensen is a candidate for a faculty position in the Department of Physics and the School of Education.}

_{Sponsored by NDSU Geosciences and Physics Department, the Department of Coatings and Polymeric Materials, and by the Cooperative Sponsorship program.}