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.}