R-3 Repository :: Browsing by Author "Chapman, Walter G."

Browsing by Author "Chapman, Walter G."

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A density functional theory for patchy colloids based on Wertheim's association theory: Beyond the single bonding condition
(American Institute of Physics, 2013) Marshall, Bennett D.; Chapman, Walter G.
In the framework of Wertheimﾒs theory, we develop the first classical density functional theory for patchy colloids where the patch can bond more than once. To test the theory we perform new Monte Carlo simulations for the model system of patchy colloids in a planar slit pore. The theory is shown to be in excellent agreement with simulation for the density profiles and bonding fractions. It is also shown that the theory obeys the wall contact rule by accurately predicting bulk pressures from the wall contact density.
A Monte Carlo density functional theory for the competition between inter and intramolecular association in inhomogeneous fluids
(AIP Publishing LLC, 2013) Marshall, Bennett D.; García-Cuéllar, Alejandro J.; Chapman, Walter G.
A Monte Carlo density functional theory is developed for chain molecules which both intra and intermolecularly associate. The approach can be applied over a range of chain lengths. The theory is validated for the case of an associating 4-mer fluid in a planar hard slit pore. Once validated, the new theory is used to study the effect of chain length and temperature on the competition between intra and intermolecular association near a hard wall. We show that this competition enhances intramolecular association near wall contact and inverts the chain length dependence of the fraction bonded intermolecularly in the inhomogeneous region.
A perturbation density functional theory for the competition between inter and intramolecular association
(AIP Publishing LLC, 2012) Marshall, Bennett D.; García-Cuéllar, Alejandro J.; Chapman, Walter G.
Using the framework of Wertheim's thermodynamic perturbation theory we develop the first density functional theory which accounts for intramolecular association in chain molecules. To test the theory new Monte Carlo simulations are performed at a fluid solid interface for a 4 segment chain which can both intra and intermolecularly associate. The theory and simulation results are found to be in excellent agreement. It is shown that the inclusion of intramolecular association can have profound effects on interfacial properties such as interfacial tension and the partition coefficient.
Application of molecular modeling to the vapor–liquid equilibrium of alkyl esters (biodiesel) and alcohols systems
(Elsevier, 2015) Corazza, Marcos L.; Fouad, Wael A.; Chapman, Walter G.
This work is focused on the thermodynamic modeling of systems related to biodiesel processing using the Perturbed Chain form of the Statistical Associating Fluid Theory (PC-SAFT). Different binary ester + ester and methanol or ethanol + ester systems were investigated. The PC-SAFT equation of state was able to predict the vapor–liquid equilibrium and solid–liquid equilibrium of different esters + esters binary systems. Furthermore, using a transferable cross-association parameters approach, the phase behavior of alcohols + alkyl esters of biodiesel was successfully predicted for a wide range of pressure and temperature conditions. Polar interactions seem to play a minor role in determining the phase behavior of mixtures considered in this work.
Asphaltene Behavior in Crude Oil Systems
(2013-10-31) Panuganti, Sai; Chapman, Walter G.; Vargas, Francisco M; Hirasaki, George J.; Tomson, Mason B.
Asphaltene, the heaviest and most polarizable fraction of crude oil, has a potential to precipitate, deposit and plug pipelines, causing considerable production costs. The main objective of this study is to contribute to the thermodynamic and transport modeling of asphaltene in order to predict its precipitation, segregation and deposition. Potential calculation of some thermophysical properties of asphaltene is also explored. Predicting the flow assurance issues caused by asphaltene requires the ability to model the phase behavior of asphaltene as a function of pressure, temperature and composition. It has been previously demonstrated that the Perturbed Chain form of Statistical Association Fluid Theory (PC-SAFT) equation of state can accurately predict the phase behavior of high molecular weight compounds including that of asphaltene. Thus, a PC-SAFT crude oil characterization methodology is proposed to examine the asphaltene phase behavior under different operating conditions. With the fluid being well characterized at a particular reservoir depth, a compositional grading algorithm can be used to analyze the compositional grading related to asphaltene using PC-SAFT equation of state. The asphaltene compositional grading that can lead in some cases to the formation of a tar mat is studied using the same thermodynamic model. Quartz crystal microbalance experiments are performed to study the depositional tendency of asphaltene in different depositing environments. The possibility of simulating asphaltene deposition in a well bore is discussed by modeling the capillary data, which simultaneously accounts for asphaltene precipitation, aggregation and deposition. The work presented is expected to contribute to the calculation of thermophysical properties of hydrocarbons and in particular of asphaltene, characterization of crude oils, improve tools to model asphaltene phase behavior, check the quality of fluid samples collected and the accuracy of (pressure, volume and temperature) PVT tests, reduce the uncertainties related to reservoir compartmentalization, optimize the logging during data acquisition, prediction of tar mat occurrence depths, improved understanding of the asphaltene deposition process, and finally optimize the wellbore operating conditions to reduce the asphaltene deposit.
Associating fluids near a hard planar wall
(1994) Segura, Chad James; Chapman, Walter G.
The properties of fluids near a fluid-solid interface are important in many processes, such as: wettability as related to oil recovery and environmental cleanup, biochemical separation, bio-compatibility of materials, membrane separation, adsorption in porous solids and micro- or nanomanufacturing of thin films. However, little of the past simulation and theoretical work in the field has considered associating fluids. This work is a Metropolis Monte Carlo study of a simple model of water against a hard plate. The potential model is a spherical hard core with four highly anisotropic square well sites placed in tetrahedral symmetry. Reported are density of spheres and fraction of unbonded spheres as a function of distance from the plate. Computed wall densities are compared against those calculated by the wall density law. Also, the beginning of an outline of a suitable density functional theory for this fluid is presented.
Associating fluids near solid surfaces
(1998) Segura, Chad James; Chapman, Walter G.
The properties of fluids near interfaces, in particular, the fluid-solid interfaces on which this work is concentrated, are important in many processes, such as: wettability as related to oil recovery and environmental cleanup, biochemical separation, bio-compatibility of materials, membrane separation, adsorption in porous solids and micro- or nanomanufacturing of thin films. However, little of the past simulation and theoretical work in the field has considered associating fluids. In this work we perform Metropolis Monte Carlo computer simulations for one-sited (dimerizing), two-sited (linear chain forming), and four-sited (cluster forming) hard spheres against hard, smooth walls. Reported are results for density and fraction of monomers (which determines the change in Helmholtz free energy due to association according to Wertheim's theory) versus distance from the surface. Also computed are adsorption and for the four-site fluid, orientation, cluster size, and fraction of sites bonded as functions of distance from the surfaces. We also consider binary mixtures and an associating fluid near active surfaces. Except for orientation and cluster size, results are compared (favorably, in general) against a new density functional theory, which combines elements of the Tarazona density functional for hard spheres and Wertheim's theory of association. This dissertation concludes with ideas for further work in the area.
Carbon Sequestration through Biochar Soil Amendment: Experimental studies and mathematical modeling
(2012-09-05) Sun, Hao; Zygourakis, Kyriacos; Masiello, Caroline A.; Chapman, Walter G.; Hockaday, William C.
Intentional amendment of soil with charcoal (called biochar) is a promising new approach to sequester atmospheric carbon dioxide and increase soil fertility. However, the environmental properties of biochars can vary with production conditions, making it challenging to engineer biochars that are simultaneously optimized for carbon sequestration, nutrient storage, and water-holding capacity. For this reason, I have undertaken a systematic study to (a) determine the pyrolysis conditions that lead to biochars with desired chemical and physical properties, and (b) find how these properties affect the water-holding capacity and nutrient adsorption in biochar-soil mixtures. First, a library of biochars was produced in a custom-built pyrolysis reactor under precisely controlled conditions. The chemical and physical structures of the produced biochars were characterized with various analytical techniques including 13C NMR, XPS, EA and BET pore surface analysis. My results suggest that the chemical composition and pore structure of biochars are determined not just by the maximum heat treatment temperature, but also by several other factors that include the pyrolysis heating rate, treatment time at the maximum temperature and particle size. I also tested a new approach that combines thermogravimetric reactivity measurements, diffusion-reaction theory and structural models to achieve a better characterization of the complicated multi-scale pore structure of biochars. The structural models treat biochars as porous solids having micro- and macropores of different shapes and exhibiting widely ranging pore-size distributions. Simulations results are then compared to experimental data to identify the presence of ordered or random pore networks and test their size distributions and connectivity. I then developed a multi-solid one-dimensional model that can use experimentally determined biochar properties to predict their field performance in beds packed with soil/biochar mixtures. The model used a system of coupled partial differential equations to describe the dynamic adsorption/elution of ammonium nitrate, a model fertilizer, in columns packed with biochar/soil mixtures and perfused with aqueous solutions of the fertilizer. The PDE system was solved using orthogonal collocation on finite elements. My chromatographic model accounted for all the important processes occurring in this system, including external mass transfer between the fluid phase and the solid particles, as well as intraparticle diffusion and adsorption of the solute on the pore surface area of the sorbents. To our knowledge, this is the first chromatographic model that accounted explicitly for the presence of two solid phases with widely different pore structures and adsorption capacities. A systematic parametric study was carried out to determine the importance of each system parameter. The adsorption equilibrium parameters and the intraparticle effective diffusivity of ammonium had the most significant effect on environmental performance. To complete the theoretical analysis, I also developed a model to describe the saturation and drainage of water from the packed column. The model accounted for all the important processes occurring in this system: (a) water exchange between the interstitial pore region and two different smaller pore regions and (b) water flow inside the larger pore region and the two different smaller pore regions. The transient mass balances led to a system of partial differential equations that was solved using block centered finite difference.
Characterizing water-in-oil emulsions with application to gas hydrate formation
(2009) Aichele, Clint Philip; Chapman, Walter G.
This thesis implements nuclear magnetic resonance (NMR) techniques to directly measure water-in-oil emulsion properties and gas hydrate formation. This thesis introduces a novel application of the pulsed field gradient with diffusion editing (PFG-DE) NMR technique to measure drop size distributions of emulsions. The PFG-DE technique agrees with the standard pulsed field gradient (PFG) technique for a variety of emulsions. For the first time, this thesis utilizes the PFG-DE technique, coupled with the Carr-Purcell-Meiboom-Gill (CPMG) technique, to directly measure and quantify gas hydrate formation in emulsified systems. These unique data for black oil emulsions aid in developing effective flow assurance strategies. To elucidate emulsion formation mechanisms in well defined shear fields, this thesis implements Taylor-Couette flow to form water-in-oil emulsions. A range of oil viscosities is considered by selecting two crude oils that differ in viscosity, and each crude oil is matched with a model oil of similar viscosity. For the low viscosity crude/model oil systems, the computational fluid dynamics (CFD) simulations show that the intensity of Taylor vortices increases at higher rotational speeds, and this leads to multimodal drop size distributions. For the high viscosity crude/model oil systems, the CFD simulations show that the flow field is simple shear for all rotational speeds. The high viscosity crude oil emulsions exhibited multimodality for all rotational speeds investigated, while the corresponding model oil emulsions exhibited broad, smooth drop size distributions. In contrast to Taylor-Couette flow, this thesis also examined emulsification in complex flow conditions with inhomogeneous shear using a six bladed Rushton turbine. This work supplies transient drop size distributions for two crude oils. This work provides emulsion formation and stability characteristics for both high and low mixing speeds, as well as comparisons to established models that predict emulsion drop size in turbulent flow. Recent evidence suggests a relationship between water-in-oil emulsion morphology and gas hydrate blockage formation. An experimental setup to measure emulsion properties during gas hydrate formation was constructed, and the resulting NMR measurements indicate that for three of the four oils investigated, gas hydrate shells form around the water drops with thickness approximately equal to 1 mum.
Density functional study of dendrimer molecules in solvents of varying quality
(AIP Publishing, 2018) Zhang, Yuchong; Parambathu, Arjun Valiya; Chapman, Walter G.
Modified inhomogeneous statistical associating fluid theory (iSAFT) density functional theory is extended to dendrimer molecules in solvents of varying quality. The detailed structures of isolated dendrimers in implicit solvent are calculated and have a semi-quantitative agreement with simulation results available in the literature. The dendrimers form dense-core structures under all conditions, while their radius of gyration follows different scaling laws. Factors that affect the quality of the solvent are systematically studied in the explicit solvent case. It is found that the solvent size, density, chemical affinity and temperature all play a role in determining a solvent to be good or poor. New molecular dynamics simulations are performed to validate the iSAFT results. Our results provide insight into the phase behavior of dendrimer solutions as well as guidance in practical applications.
Density Functional Theory Study of Microstructure and Phase Behavior of Stimuli-Responsive Polymer Brushes
(2013-12-04) Gong, Kai; Chapman, Walter G.; Verduzco, Rafael; Riviere, Beatrice M.
Stimuli-responsive polymer materials can change their structure and physical properties drastically on external signals like a change in temperature, solvent properties (pH, ionic strength), the magnetic or electrical field etc. Such "smart" polymer materials play an important role in various fields such as biology, medicine, and soft materials. However, it is a great challenge to investigate such "smart" polymer materials due to highly inhomogeneous structure at the molecular scale and the complex interactions. In this thesis, we have systematically studied three common types of stimuli-responsive polymer brushes such as temperature responsive polymer brushes, copolymer brushes, and mixed polymer brushes by using classical density functional theory. We find a surface outer layer switch for both copolymer brushes and mixed polymer brushes with a selective solvent. Without using any temperature-dependent parameter, our theory successfully captures the lower critical solution temperature behavior of the associating polymer brushes. Related parameters such as molecular weight, grafting density, and solvent properties that affect the phase behavior of these stimuli-responsive polymer brushes have been also investigated. Qualitatively consistent with experimental observations, our results provide physical insight and helpful guidance for the experimental design of such stimuli-responsive polymer materials.
Dynamic Self-Stiffening in Liquid Crystal Elastomers
(Nature Publishing Group, 2013) Agrawal, Aditya; Chipara, Alin C.; Shamoo, Yousif; Patra, Prabir K.; Carey, Brent J.; Ajayan, Pulickel M.; Chapman, Walter G.; Verduzco, Rafael
Biological tissues have the remarkable ability to remodel and repair in response to disease, injury and mechanical stresses. Synthetic materials lack the complexity of biological tissues, and man-made materials that respond to external stresses through a permanent increase in stiffness are uncommon. Here we report that polydomain nematic liquid crystal elastomers increase in stiffness by up to 90% when subjected to a low-amplitude (5%), repetitive (dynamic) compression. Elastomer stiffening is influenced by liquid crystal content, the presence of a nematic liquid crystal phase and the use of a dynamic as opposed to static deformation. Through rheological and X-ray diffraction measurements, stiffening can be attributed to a mobile nematic director, which rotates in response to dynamic compression. Stiffening under dynamic compression has not been previously observed in liquid crystal elastomers and may be useful for the development of self-healing materials or for the development of biocompatible, adaptive materials for tissue replacement.
Effect of polar functional groups on the phase behavior of amino acids, small peptides, solvents, and polymers
(2002) Sauer, Sharon Gail; Chapman, Walter G.
Dipolar interactions significantly influence the phase behavior of many systems of interest to the biochemical, chemical, petroleum and polymer industries. For example, the solution behavior of amino acids, small peptides, polar solvents and co-polymers have potential applications for biochemicals, water-soluble polymers for paints and coatings, and surfactants. By considering molecular-level interactions, the phase behavior of a large range of systems can be predicted. Using structural analysis and thermodynamics, the essential role of polar functional groups on solubility of small biochemicals is established. An accurate model for fluid mixtures with multiple polar functional groups is developed. Results from a systematic experimental study on the aqueous solubility of amino acids and dipeptides as a function of temperature, salt type, and salt concentration are analyzed. Changes in temperature and residue sequence have the most substantial effect on solubility. Structural analysis shows that intra and intermolecular association largely influence the behavior. For small molecules, end effects dominate the behavior but should be less important for many biochemicals. Values for enthalpic changes from the solid to the infinitely dilute liquid state for the dipeptides of asp and gly are reported. An accurate model for mixtures of polar fluids, in which any number of groups and/or any component of the mixture may be polar, is developed by applying the u-expansion to a reference fluid mixture of polar and non-polar spheres. An in-depth parameter study of this model, named Polar SAFT, for a homologous series of ketones indicates that the model parameters have physically reasonable values. A methodology is proposed for developing a group-contribution approach for the model. The ability of Polar SAFT to accurately predict the effect of multiple dipolar groups and molecular shape on the phase behavior of binary mixtures of polar and non-polar components is exemplified by application to a series of ketone/alkane mixtures. Using only pure component parameters, Polar SAFT accurately represents these systems, indicating the predictive capability of the model and the importance of explicitly accounting for polar interactions. For alkane/copolymer (poly(ethylene-co-methyl acrylate)) solutions, Polar SAFT accurately predicts the polar co-monomer content and solvent effects on cloud point behavior.
Formation and dissociation mechanisms of clathrate hydrates
(2006) Gao, Shuqiang; Chapman, Walter G.
To better understand hydrate formation and dissociation mechanisms, Nuclear Magnetic Resonance (NMR), Magnetic Resonance Imaging (MRI), and viscosity measurements were employed to examine the hydrate transition processes of tetrahydrofuran (THF) - water (D2O or H2O) solution. Specifically, Spin-Lattice Relaxation Time (T1) and Spin-Spin Relaxation Time (T2) of THF in D2O were measured before hydrate formation, during hydrate formation, during hydrate dissociation, and after hydrate dissociation to probe the local molecular ordering changes around THF molecules. Hydrate formation and dissociation patterns were imaged using MRI. The viscosity of THF/H2O solution was monitored before hydrate formation and after hydrate dissociation using Champion Technologies Hydrate Rocking Cell (CTHRC) to investigate the residual viscosity phenomenon. NMR relaxation time results demonstrated that the presence of hydrate phase strongly influences the fluid structure of the coexisting liquid phase. T2 distribution technique was proven to be an effective tool in measuring the dynamic behavior of THF molecules in the hydrate phase and the liquid phase independently and concurrently. Comparison of T1's of THF in D2O solution during hydrate formation with that during dissociation revealed evidence of residual hydrate structures remaining in the liquid phase. Residual viscosity (as measured by CTHRC) was absent after THF hydrate dissociation. It was suggested that the residual viscosity observed by other groups after natural gas hydrate dissociation was more likely due to higher than equilibrium gas concentration than residual hydrate clathrate structures. To enable direct and accurate measurements of gas hydrate behavior in black oil, liquid-state proton NMR spectroscopy was innovatively applied to monitor the water peak area change in the NMR spectrum of water-in-oil emulsion during hydrate formation and dissociation. Because water in the hydrate phase does not contribute to the water peak area in such a spectrum, as water is being converted into hydrate, the water peak area would decrease. Results validated that it is feasible to directly and accurately monitor hydrate behavior in black oil using this technique.
High pressure measurements and molecular modeling of the water content of acid gas containing mixtures
(Wiley, 2015) Fouad, Wael A.; Yarrison, Matt; Song, Kyoo Y.; Cox, Kenneth R.; Chapman, Walter G.
Water content of three carbon dioxide containing natural gas mixtures in equilibrium with an aqueous phase was measured using a dynamic saturation method. Measurements were performed up to high temperatures (477.6 K = 400°F) and pressures (103.4 MPa = 15,000 psia). The perturbed chain form of the statistical associating fluid theory was applied to predict water content of pure carbon dioxide (CO2), hydrogen sulfide (H2S), nitrous oxide (N2O), nitrogen (N2), and argon (Ar) systems. The theory application was also extended to model water content of acid gas mixtures containing methane (CH4). To model accurately the liquid-liquid equilibrium at subcritical conditions, cross association between CO2, H2S, and water was included. The agreement between the model predictions and experimental data measured in this work was found to be good up to high temperatures and pressures.
Hydrophobic and hydrophilic interactions in aqueous mixtures of alcohols at a hydrophobic surface
(AIP Publishing LLC, 2013) Ballal, Deepti; Chapman, Walter G.
Aqueous solutions of alcohols are interesting because of their anomalous behavior that is believed to be due to the molecular structuring of water and alcohol around each other in solution. The interfacial structuring and properties are significant for application in alcohol purification processes and biomolecular structure. Here we study aqueous mixtures of short alcohols (methanol, ethanol, 1-propanol, and 2-propanol) at a hydrophobic surface using interfacial statistical associating fluid theory which is a perturbation density functional theory. The addition of a small amount of alcohol decreases the interfacial tension of water drastically. This trend in interfacial tension can be explained by the structure of water and alcohol next to the surface. The hydrophobic group of an added alcohol preferentially goes to the surface preserving the structure of water in the bulk. For a given bulk alcohol concentration, water mixed with the different alcohols has different interfacial tensions with propanol having a lower interfacial tension than methanol and ethanol. 2-propanol is not as effective in decreasing the interfacial tension as 1-propanol because it partitions poorly to the surface due to its larger excluded volume. But for a given surface alcohol mole fraction, all the alcohol mixtures give similar values for interfacial tension. For separation of alcohol from water, methods that take advantage of the high surface mole fraction of alcohol have advantages compared to separation using the vapor in equilibrium with a water-alcohol liquid.
Identification and calculation of reference fluid properties of electrolyte solutions
(1997) Jain, Pallav; Chapman, Walter G.
A new method to estimate the properties of electrolyte solutions has been outlined in this work. The models for electrolyte solutions at the Bonn-Oppenheimer level have always been perceived to be very complicated to solve by either integral equation theories or Perturbation theories. The present work identifies simpler BO level models called Square Well Simple Civilized model (SQWSCM) and Square Well Water Simple Civilized Model (SWSCM). The properties of SWSCM model have been obtained by simulations and hydration numbers have been found to be comparable to experimental results. In order to obtain an analytical solution for these models a Thermodynamic Perturbation type approach has been outlined. The appropriate reference fluid for the TPT approach has been identified. The properties of this reference fluid have been obtained by simulations.
The impact of lithologic heterogeneity and focused fluid flow upon gas hydrate distribution in marine sediments
(American Geophysical Union, 2014) Chatterjee, Sayantan; Bhatnagar, Gaurav; Dugan, Brandon; Dickens, Gerald R.; Chapman, Walter G.; Hirasaki, George J.
Gas hydrate and free gas accumulation in heterogeneous marine sediment is simulated using a two-dimensional (2-D) numerical model that accounts for mass transfer over geological timescales. The model extends a previously documented one-dimensional (1-D) model such that lateral variations in permeability (k) become important. Various simulations quantitatively demonstrate how focused fluid flow through high-permeability zones affects local hydrate accumulation and saturation. Simulations that approximate a vertical fracture network isolated in a lower permeability shale (kfracture >> kshale) show that focused fluid flow through the gas hydrate stability zone (GHSZ) produces higher saturations of gas hydrate (25–70%) and free gas (30–60%) within the fracture network compared to surrounding shale. Simulations with a dipping, high-permeability sand layer also result in elevated saturations of gas hydrate (60%) and free gas (40%) within the sand because of focused fluid flow through the GHSZ. Increased fluid flux, a deep methane source, or both together increase the effect of flow focusing upon hydrate and free gas distribution and enhance hydrate and free gas concentrations along the high-permeability zones. Permeability anisotropy, with a vertical to horizontal permeability ratio on the order of 10−2, enhances transport of methane-charged fluid to high-permeability conduits. As a result, gas hydrate concentrations are enhanced within these high-permeability zones. The dip angle of these high-permeability structures affects hydrate distribution because the vertical component of fluid flux dominates focusing effects. Hydrate and free gas saturations can be characterized by a local Peclet number (localized, vertical, focused, and advective flux relative to diffusion) relative to the methane solubility gradient, somewhat analogous to such characterization in 1-D systems. Even in lithologically complex systems, local hydrate and free gas saturations might be characterized by basic parameters (local flux and diffusivity).
Isolating the non-polar contributions to the intermolecular potential for water-alkane interactions
(AIP Publishing LLC., 2014) Ballal, Deepti; Venkataraman, Pradeep; Fouad, Wael A.; Cox, Kenneth R.; Chapman, Walter G.
Intermolecular potential models for water and alkanes describe pure component properties fairly well, but fail to reproduce properties of water-alkane mixtures. Understanding interactions between water and non-polar molecules like alkanes is important not only for the hydrocarbon industry but has implications to biological processes as well. Although non-polar solutes in water have been widely studied, much less work has focused on water in non-polar solvents. In this study we calculate the solubility of water in different alkanes (methane to dodecane) at ambient conditions where the water content in alkanes is very low so that the non-polar water-alkane interactions determine solubility. Only the alkane-rich phase is simulated since the fugacity of water in the water rich phase is calculated from an accurate equation of state. Using the SPC/E model for water and TraPPE model for alkanes along with Lorentz-Berthelot mixing rules for the cross parameters produces a water solubility that is an order of magnitude lower than the experimental value. It is found that an effective water Lennard-Jones energy εW/k = 220 K is required to match the experimental water solubility in TraPPE alkanes. This number is much higher than used in most simulation water models (SPC/E—εW/k = 78.2 K). It is surprising that the interaction energy obtained here is also higher than the water-alkane interaction energy predicted by studies on solubility of alkanes in water. The reason for this high water-alkane interaction energy is not completely understood. Some factors that might contribute to the large interaction energy, such as polarizability of alkanes, octupole moment of methane, and clustering of water at low concentrations in alkanes, are examined. It is found that, though important, these factors do not completely explain the anomalously strong attraction between alkanes and water observed experimentally.
Liquid Crystal Elastomer Based Novel Functional Materials
(2014-06-10) Agrawal, Aditya; Verduzco, Rafael; Chapman, Walter G.; Biswal, Sibani Lisa; Ajayan, Pulickel M.
Liquid crystal elastomers (LCEs) are fascinating materials in which the elasticity of polymer network is coupled to liquid crystalline (LC) order and exhibits rich behavior under various stimuli such as heat, light, electric and magnetic fields. Previous studies were helpful in understanding isolated properties of bulk LCEs under static conditions and large-strain deformation. In this thesis we focused on the development of new functional materials based on LCEs and gaining new insight into their mechanical properties under dynamic deformations. In chapter-2&3, LCE based buckling instability in thin films is discovered and found to be useful for nanoscale thin film metrology (down to 30nm). This overcomes the limitations of earlier known techniques that require clamping and mechanically straining of films and present challenges with small samples. For thick PS films (over 500nm), well-defined self-folding 3D dynamic structures (like lotus or helix) are achieved just by engineering film patterning on LCEs. The phenomena are quantitatively captured through FEM simulations and could be used to predict the film patterning on LCE to achieve desired 3D deformed structure. In chapter-4 a LCE based dynamic wrinkling instability is demonstrated by synthesizing an electrically conductive LCE nanocomposites (LCE-NCs) using a new two-step approach. LCE-NCs exhibit rapid (response times as fast as 0.6 s), large-amplitude (up to 30 %), and fully reversible shape changes (stable to over 5000 cycles) under externally applied voltages (5 – 40V) via joule heating. Neonatal rat ventricular myocytes were cultured on LCE-NCs substrates, and good cell attachment and viability is observed. LCE-NCs provide a straightforward and scalable route to investigate cell response to a dynamically changing surface pattern. In chapter-5 a novel self-stiffening behavior in LCEs is discovered, a dramatic 90% increase in stiffness is observed under low-amplitude, repetitive (dynamic) compression. Such stiffening behavior is common in biological tissues but rare in synthetic elastomers. Combination of rheological measurements, optical microscopy, 2D- WAXD and FEM simulations, demonstrates dynamic stiffening is due to rotation of the nematic director under repetitive compression. The use of low-strain, repetitive compression represents a facile method to prepare uniformly aligned LCEs and finds applications in biocompatible, adaptive materials for tissue replacement.

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