Browsing by Author "Robert, Marc A."
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Item Interfacial properties of fluid phases in two dimensions and of three-component surfactant systems(1989) Chen, Li-Jen; Robert, Marc A.In this thesis, we study two fundamental problems in the theory and experiment of interfaces in simple and complex fluids. The first problem concerns the critical behavior of the thickness of interface of simple fluids in two dimensions of space. We study this problem using both theory and computer simulations. In our theoretical study of this problem, the equilibrium interface is assumed, in accord with current ideas, to consist of an intrinsic interface of the non-classical van der Waals type broadened by capillary wave fluctuations. It is shown that the interfacial thickness exhibits a crossover with change of critical exponent from capillary wave behavior at low temperatures to intrinsic structure behavior at temperatures extremely close to the critical temperature. The location of the crossover temperature is determined by a critical amplitude ratio. In our computer simulation study of this problem, the interfacial thickness between two two-dimensional Lennard-Jones fluid phases is determined by using the method of molecular dynamics. Within statistical uncertainty, the results for the interfacial thickness are found to be consistent with the prediction of the non-classical van der Waals theory of the intrinsic interface. The second problem studied in this thesis concerns the wetting behavior of three-component surfactant systems. In a three-phase equilibrium system, the middle phase either wets or does not wet the interface between the upper and lower phases. In this work, we perform a systematic experimental search for wetting transitions between wetting and nonwetting behaviors in three-component systems water/n-alkane/C$\sb{\rm i}$E$\sb{\rm j}$, where C$\sb{\rm i}$E$\sb{\rm j}$ denotes the surfactant polyoxyethylene alcohol C$\sb{\rm i}$H$\sb{\rm 2i+1}$(OC$\sb2$H$\sb4$)$\sb{\rm j}$OH. It is found that two systems, water/n-hexadecane/C$\sb6$E$\sb2$ and water/n-octadecane/C$\sb6$E$\sb2$, exhibit a wetting transition lying at least 10$\sp\circ$C below the upper critical temperature, and one system, water/n-tetradecane/C$\sb6$E$\sb2$, exhibits a wetting transition lying 4.3$\sp\circ$C below the upper critical temperature.Item Long-range order of magnetic nanocluster lattices and surface acoustic wave applications of lithium niobate thin films(2002) Lee, Tai-Chou; Robert, Marc A.; Rabson, Thomas A.Part I. We have investigated theoretically the interactions and phase transitions of arrays of magnetic nanoclusters embedded in nonmagnetic metals and semiconductors. Two nanoclusters of ferromagnetic elements embedded in a nonmagnetic semiconducting material are expected to be coupled magnetically if the nanoclusters are large enough and the semiconductor is sufficiently doped to provide enough electrons, since the latter mediate the indirect magnetic interactions between the nanoclusters. Apart from its potential technological applications, such as giant magnetoresistance for non-volatile memory applications, and DMS (diluted magnetic semiconductors) for combined process and storage devices, this study is of fundamental interest. As a consequence of the temperature dependence of the indirect interactions, it is shown that for semiconductors several ferromagnetic phase transitions are possible depending on the size of the nanoclusters, whereas for metals a single ferromagnetic phase transition occurs independently of the size of the nanoclusters. The critical temperature and spontaneous magnetization of the network of nanoclusters are evaluated in all cases. Part II. We have found evidence for surface acoustic wave (SAW) propagation of a lithium niobate (LiNbO3)/diamond/silicon multilayer structure. Devices with higher-frequency response and low insertion loss as well as smaller sizes and weight are required for telecommunications. Theoretical calculations had predicted that LiNbO3 thin films on diamond/silicon substrates have both high SAW propagation velocities and high electromechanical coupling coefficients. Metallo-organic decomposition (MOD) and rf-sputtering techniques are used here to grow LiNbO3 thin films on diamond/silicon substrates. These thin films are characterized by XRD, AFM, and interferometer. SAW filters are fabricated by depositing interdigital transducers (IDTs) onto the LiNbO3/diamond/silicon films. Microwave characterizations, such as frequency response, are done by using network analyzer test sets. The impulse response in the time domain is calculated by fast Fourier transforms. Evidence for SAW activity is found for this multilayer structure. Integrating a composite structure of a piezoelectric layer and a high acoustic-velocity layer onto silicon substrates thus shows promise as a way to increase the operating frequency for wireless-telecommunication applications.Item Monte Carlo simulation studies of DNA hybridization and DNA-directed nanoparticle assembly(2009) Araque, Juan Carlos; Robert, Marc A.A coarse-grained lattice model of DNA oligonucleotides is proposed to investigate how fundamental thermodynamic processes are encoded by the nucleobase sequence at the microscopic level, and to elucidate the general mechanisms by which single-stranded oligonucleotides hybridize to their complements either in solution or when tethered to nanoparticles. Molecular simulations based on a high-coordination cubic lattice are performed using the Monte Carlo method. The dependence of the model's thermal stability on sequence complementarity is shown to be qualitatively consistent with experiment and statistical mechanical models. From the analysis of the statistical distribution of base-paired states and of the associated free-energy landscapes, two general hybridization scenarios are found. For sequences that do not follow a two-state process, hybridization is weakly cooperative and proceeds in multiple sequential steps involving stable intermediates with increasing number of paired bases. In contrast, sequences that conform to two-state thermodynamics exhibit moderately rough landscapes, in which multiple metastable intermediates appear over broad free-energy barriers. These intermediates correspond to duplex species that bridge the configurational and energetic gaps between duplex and denatured states with minimal loss of conformational entropy, and lead to a strongly cooperative hybridization. Remarkably, two-state thermodynamic signatures are generally observed in both scenarios. The role of cooperativity in the assembly of nanoparticles tethered with model DNA oligonucleotides is similarly addressed with the Monte Carlo method, where nanoparticles are represented as finely discretized hard-core spheres on a cubic lattice. The energetic and structural mechanisms of self-assembling are investigated by simulating the aggregation of small "satellite" particles from the bulk onto a large "core" particle. A remarkable enhancement of the system's thermal stability is attained by increasing the number of strands per satellite particle available to hybridize with those on the core particle. This cooperative process is driven by the formation of multiple bridging duplexes under favorable conditions of reduced translational entropy and the resultant energetic compensation; this behavior rapidly weakens above a certain threshold of linker strands per satellite particle. Cooperativity also enhances the structural organization of the assemblies by systematically narrowing the radial distribution of the satellite particles bound the core.Item On the diffusion and phase transitions of confined colloid-polymer mixtures(2010) Amini, Amir; Robert, Marc A.Diffusion and phase transitions of confined neutral colloid-polymer mixtures are studied theoretically in one dimension, and theoretically and experimentally in two dimensions. For colloids in a channel, their short-time self- and collective diffusion coefficients and their long-time mobility are calculated, assuming the colloid-polymer interactions to be of depletion origin and described by the Asakura-Oosawa model. The colloid-polymer mixture is mapped onto an effective one-component system in which the size of the colloids, the hydrodynamic interactions, and the wall effects are taken into account. It is found that depletion interactions reduce the diffusion of colloids for short times and enhance their mobility for long times. For a single polymer in a colloidal suspension confined to a channel, the self-diffusion coefficient of the polymer center-of-mass is calculated in the ground-state dominance regime as function of suspension density, degree of confinement, and quality solvent quality. The scaling exponents describing the variations of the self-diffusion coefficient with the degree of polymerization and the radius of the channel are computed. These exponents are found to have higher values than those of a polymer in the absence of colloids. It is also shown that the influence of colloids on polymer diffusion under theta and good solvent conditions is much more pronounced for the latter case. Monolayers of mixtures of poly(lactic acid) (PLA) and two types of particles, magnetic colloids and Cd-Se nanoparticles, are prepared using the Langmuir-Blodgett technique. Pressure-area isotherms show that the transition from the isotropic phase to the liquid-crystalline smectic-A phase, observed for pure PLA, is suppressed at a critical concentration of the magnetic colloids, whereas it persists in the presence of nanoparticles, even at high concentrations. The theory developed by McMillan for the smectic-A phase in three dimensions is extended to the case of two dimensions, and its predictions are compared to those of the latter as well as to experiment.Item Phase equilibria, microstructure, and transport properties of confined colloid-polymer systems(2005) Chou, Cheng-Ying; Robert, Marc A.For two-dimensional geometries, computer simulations on a lattice based on the grand canonical Monte Carlo method, in combination with histogram reweighting and finite-size scaling, are used to determine the phase diagrams of colloid-polymer systems in which the colloids are modeled as hard spheres and the polymers as hard chains, and where an effective attractive interaction arises due to depletion effects. In contrast to the predictions of previous mean-field and other approximate theories, the nature of the coexistence phases is found to depend not solely on the polymer-to-colloid size ratio, but on the colloid diameter, the polymer radius of gyration, and the polymer monomer size. The threshold values of the polymer-to-colloid size ratio for the onset of liquid-liquid phase separation differ significantly from earlier predictions and from those of the corresponding three-dimensional systems. Extrapolation to the "protein limit" of very small colloid and very long polymer indicates that, in contrast to the case of three dimensions, immiscibility does not persist at this limit. The pair correlation functions, both positional and orientational, of the liquid and solid phases are determined experimentally by video microscopy and image analysis for aqueous suspensions of colloids with nonadsorbing polymer. Finally the diffusion of colloids and polymer (bacteriophage lambda-DNA) is studied experimentally by the same technique. The diffusivity of colloids as a function of polymer concentration exhibits a change of slope in the neighborhood of the overlap polymer concentrations. For systems confined in one-dimensional channels, the colloid pair correlation function is determined experimentally as above. The diffusion of the colloidal particles is obtained by tracking individual colloidal particle and by determining their mean square displacement. For short times, the diffusion is of Brownian motion, Fickian type, with mean square displacement varying linearly with time. For long times, however, the mean square displacement is found to increase more slowly with time than linearly, in agreement with the theoretical prediction that diffusion in one dimension, in which mutual crossing of the particles is not possible, is non-Fickian and the mean square displacement increases as the square root of time. A crossover between short-time and long-time diffusion is observed, and is found to depend on the colloid and polymer concentrations.Item Phase transitions and diffusion of colloid-polymer systems and thin ferroelectric films for device applications(2000) Lee, Jiun-Ting; Robert, Marc A.This thesis is divided into two independent parts: the first part deals with the phase transitions and diffusion of colloid-polymer systems, while the second part concerns the growth of thin ferroelectric films, their physical and chemical characterizations, and theoretical predictions for surface-acoustic-wave (SAW) propagation. In the first part, the phase transitions of colloid-polymer systems are studied by experiment and theory in both two and three dimensions. For a colloid-polymer system consisting of polystyrene spheres in aqueous solutions of hydroxyethylcellulose, the transitions are examined experimentally using enhanced videomicroscopy with particle resolution, and the results are compared with statistical mechanical predictions based on the Percus-Yevick and mean-field approximations. The Brownian motion of colloidal particles in polymer solutions is also investigated for the system of colloid (polystyrene)-polymer (polyethyleneoxide), and the diffusivity is determined as a function of polymer concentration. In the second part, thin films of the ferroelectric lithium niobate are deposited by radio-frequency magnetron sputtering on diamond-coated silicon substrates for high-frequency SAW-device applications. The SAW velocity and electromechanical coupling coefficient are predicted theoretically for various film orientations and thicknesses. Moreover, thin films of the ferroelectric barium titanate are deposited on silicon substrates for random-access-memory applications, and their crystal and electrical properties are investigated.Item Renormalization group theory for percolation and application to transport properties of random media(1991) Knackstedt, Mark; Robert, Marc A.A renormalization group method in real space is applied to the study of transport properties of random media. In particular, the method is applied to the study of the electrical properties of disordered materials and the study of the elastic properties of random materials. The theoretical predictions for the conductivity of disordered materials are in excellent agreement with both computer simulations and experimental results. Two microscopic models of elasticity of random materials are studied, the bond-bending model and the central-force model. The critical behavior of the elastic moduli of the bond-bending model is in good agreement with experimental and computer simulation results. The rigidity threshold of the central-force model is found not to correspond to that of pure percolation, and the predicted critical behavior of this model is found to differ markedly from that of the bond-bending model. A method to probe the universality of these two models by studying renormalization group flow diagrams, and an extension of the present method to the study of electrical breakdown and mechanical failure are proposed.Item Theory and computational studies of magnetic carbon nanotubes and of depletion effects in colloid-polymer systems(2004) Vo, Trinh Thi My; Robert, Marc A.Part I. ab initio Molecular Dynamics of Interaction of Fe Atoms with Single-Wall Carbon Nanotubes. The interaction of Fe atoms with a single-wall carbon nanotube is investigated using the ab initio molecular dynamics method of Car and Parrinello. The variations in stability, band gap, Fermi energy, and total magnetic moment of the Fe-single wall carbon nanotube systems are found to depend on the location of the Fe atoms relative to the carbon nanotube surface. Noteworthy is that the Fe atoms in the Fe-carbon nanotubes systems are coupled ferromagnetically. The curvature effects on the interaction of Fe atoms with carbon nanotubes are also studied by comparing with the Fe-graphite systems. Part II. Phase Transitions and Long-Range Order of Magnetic Carbon Nanotubes. The magnetic coupling between single-wall carbon nanotubes filled with magnetic transition metals, which is assumed to be of the indirect type, is shown to lead to long-ranged ferromagnetic order for arrays of both metallic and semiconducting carbon nanotubes. The critical temperature and spontaneous magnetization are determined. It is found that metallic and semiconducting carbon nanotubes filled with magnetic elements can be turned into magnetic materials. Part III. Computer Simulation of Depletion Effects in Three-Dimensional Colloid-Polymer Systems. The phase behavior of three-dimensional colloid-polymer systems with purely depletion-induced attractions (hard chain polymer and hard sphere colloid) is studied using finite-size scaling and histogram-reweighting Monte Carlo simulations. The nature of the coexisting phases and the phase diagrams are found to depend on the polymer-to-colloid size ratio q. The threshold values of q where liquid-liquid coexistence disappears are found to differ significantly from the value predicted by mean-field theories. Phase separation is found to occur at the "protein limit" of very large polymer and small colloids, in contrast to de Gennes' prediction. Part IV. Depletion Interaction in One Dimension: Short-Range Order. For one-dimensional systems in the continuum in which the particles interact with nearest-neighbor forces, the pair correlation function at short distances can be expressed exactly in a simple form. Results are given for the hard-core interaction potential in which the attractive potential is linear, which represents the depletion interaction potential in one dimension.Item Thermodynamics of confined colloid-polymer mixtures(2012) Lo, Jonathan N.; Robert, Marc A.Recent advances have elucidated the behavior of colloids and polymer systems in the archety pal three dimensions. However, these systems are ill-understood when confined to two dimensions. Using experimental techniques such as Langmuir-Blodgett and ellipsometry, we attempt to explain the behavior of two dimensional colloid/polymer systems via micron-scale imaging as well as measuring their surface-pressure versus area isotherms. Possible phase transition behaviors and mechanisms are uncovered and discussed. We explore the importance of proper colloidal stabilizing ligands in terms of hydrophobic forces and their impact on ellipsometry as well as Langmuir-Blodgett experiments. Lastly, we give insight for future work that still remains to be done in this area.Item Van der Waals model of liquid wetting layers(1989) McCrary, Jeffrey Kirk; Robert, Marc A.A phenomenological theory of wetting layer systems based on the van der Waals theory of interfaces is developed. For classical critical exponents the local composition profile and the wetting layer thickness are determined analytically and their behavior near a critical point is examined. The model is then extended to the case of nonclassical critical exponents, where numerical solutions for the composition profile and the wetting layer thickness are determined. The nonclassical model of the wetting layer is then applied to a well-studied experimental system and the results for the coefficient of ellipticity are compared with experimental measurements.