DRUM Community: Chemical & Biomolecular Engineering

Hybrid Polymer Hydrogels with Regions of Distinct Properties

2012-01-01T00:00:00Z

Title: Hybrid Polymer Hydrogels with Regions of Distinct Properties Authors: Banik, Stephen John Abstract: This thesis investigates a new approach to create hybrid polymer hydrogels that comprise multiple gel types juxtaposed in predetermined zones, with the unique properties of each gel being retained. The key is to ensure that the viscosities of pre-gel mixtures are sufficiently high when brought into contact and subsequently polymerized, preventing convective mixing at gel/gel interfaces. The final gel appears as a single, homogeneous material with robust interfaces between the dissimilar zones. By modifying the pre-gel viscosity, we construct hybrid hydrogels by a procedure that is quick, simple, and has fewer limitations than alternate methods. By varying the components of each gel, we have produced a vast array of hybrid hydrogels with regions of distinct chemical, optical, and mechanical properties. This has enabled the creation of strong, highly-extensible soft materials (e.g. a spinal disc mimic), and of gels bearing hidden patterns that can be revealed with a variety of stimuli.

Hot and Cold Water as a Supercritical Solvent

2012-01-01T00:00:00Z

Title: Hot and Cold Water as a Supercritical Solvent Authors: Fuentevilla, Daphne Anne Abstract: This dissertation addresses the anomalous properties of water at high temperatures near the vapor-liquid critical point and at low temperatures in the supercooled liquid region. The first part of the dissertation is concerned with the concentration dependence of the critical temperature, density, and pressure of an aqueous sodium chloride solution. Because of the practical importance of an accurate knowledge of critical parameters for industrial, geochemical, and biological applications, an empirical equation for the critical locus of aqueous sodium chloride solutions was adopted in 1999 by the International Association for the Properties of Water and Steam (IAPWS) as a guideline. However, since this original Guideline on the Critical Locus of Aqueous Solutions of Sodium Chloride was developed, two new theoretical developments occurred, motivating the first part of this dissertation. Here, I present a theory-based formulation for the critical parameters of aqueous sodium chloride solutions as a proposed replacement for the empirical formulation currently in use. This formulation has been published in the International Journal of Thermophysics and recommended by the Executive Committee of IAPWS for adoption as a Revised Guideline on the Critical Locus of Aqueous Solutions of Sodium Chloride. The second part of the dissertation addresses a new concept, considering cold water as a supercritical solvent. Based on the idea of a second, liquid-liquid, critical point in supercooled water, we explore the possibility of supercooled water as a novel supercooled solvent through the thermodynamics of critical phenomena. In 2006, I published a Physical Review letter presenting a parametric scaled equation of state for supercooled-water. Further developments based on this work led to a phenomenological mean-field "two-state" model, clarifying the nature of the phase separation in a polyamorphic single-component liquid. In this dissertation, I modify this two-state model to incorporate solutes. Critical lines emanating from the pure-water critical point show how even small additions of solute may significantly affect the thermodynamic properties and phase behavior of supercooled aqueous solutions. Some solutes, such as glycerol, can prevent spontaneous crystallization, thus making liquid-liquid separation in supercooled water experimentally accessible. This work will help in resolving the question on liquid polyamorphism in supercooled water.

Droplet Dynamics in Microfluidic Junctions

2012-01-01T00:00:00Z

Title: Droplet Dynamics in Microfluidic Junctions Authors: Boruah, Navadeep Abstract: The dynamics of droplets in confined microfluidic geometries is a problem of fundamental interest as such flow conditions occur in multiphase flows in porous media, biological systems, microfluidics and material science applications. In this thesis, we investigate computationally the dynamics of naturally buoyant droplets, with constant surface tension, in cross-junctions and T-junctions constructed from square microfluidic channels. A three-dimensional fully-implicit interfacial spectral boundary element method is employed to compute the interfacial dynamics of the droplets in the junctions and investigate the problem physics for a wide range of flow rates, viscosity ratios and droplet sizes. Our investigation reveals that as the flow rate or the droplet size increases, the droplets show a rich deformation behavior as they move inside the microfluidic devices. In the cross-junction, after obtaining a bullet-like shape before the flow intersection, the droplet become very slender inside the junction (to accommodate the intersecting flows), then it obtains an inverse-bullet shape as it exits the junction which reverts to a more pointed bullet-like shape far downstream. In the T-junction, the droplet obtains a skewed-bullet shape and a highly deformed slipper shape after entering the flows intersection. The viscosity ratio also has strong effects on the droplet deformation especially for high-viscosity droplets which do not have the time to accommodate the much slower deformation rate during their channel motion. Our results are in agreement with experimental findings, and provide physical insight on the confined droplet deformation.

Mapping metabolic fluxes in plant cells to understand carbon-nitrogen interactions and nitrogen storage and cycling

2012-01-01T00:00:00Z

Title: Mapping metabolic fluxes in plant cells to understand carbon-nitrogen interactions and nitrogen storage and cycling Authors: Nargund, Shilpa Abstract: Plants provide commodities like food, fiber, fuel and chemicals. Understanding plant metabolism will help find genetic engineering targets that enhance production of these commodities. Interactions between the macronutrients - carbon (C) and nitrogen (N) determine growth and developmental functions in plants (Nunes-Nesi, Fernie, and Stitt 2010; Sakakibara, Takei, and Hirose 2006) and are regulated by complex mechanisms that need systems-level analyses. Metabolic fluxes, the rates of C flow in metabolic pathways, provide a system-wide view of metabolism and are quantified by steady state metabolic flux analysis (MFA) wherein isotopic tracers (13C, 15N) are fed to the cells and the resulting labeling patterns of biomass components are used to fit the fluxes. In this study we i) statistically designed isotope labeling experiments (ILEs) in silico to enhance accuracy of flux estimates through the pentose phosphate pathway (PPP) ii) conducted MFA on heterotrophic cell suspensions of Arabidopsis thaliana (Arabidopsis), a model plant, to investigate regulatory role of light in cell metabolism and iii) conducted MFA on cell suspensions of poplar (Populus tremula × Populus alba; clone N 717-B4), a potential biofuel crop, to understand C-N interactions. In silico label design studies determined that accuracy of flux estimates in the PPP improves by ILEs with 1,2-13C glucose and measuring labeling patterns of sugars, especially ribose. Metabolic fluxes, estimated by the designed ILEs on Arabidopsis cells, under continuous light or dark, showed negligible changes between treatments indicating that light does not regulate central carbon metabolism in heterotrophic Arabidopsis cells. The designed ILEs improved confidences of non-oxidative PPP flux estimates by 40-80% from previous studies (Masakapalli et al. 2009a). ILEs on poplar cell suspensions, grown in batch cultures, displayed unexpected back-mixing between unlabeled seed biomass and newly synthesized labeled biomass. Novel metabolic network models were developed that successfully account for observed back-mixing. ILEs on poplar cells, subjected to different C-N supply treatments to understand C-N interactions showed significant differences in phenylalanine labeling which may implicate role of flavonoid biosynthesis pathway in C-N interactions. Design of ILEs and subsequent improvement in flux estimates and the improvements in modeling metabolic networks are the novel contributions of this work.