Exact solutions for chemical concentration waves of self-propelling camphor particles racing on a ring: A novel potential dynamics perspective

A potential dynamics approach is developed to determine the periodic standing and traveling wave patterns associated with self-propelling camphor objects floating on ring-shaped water channels. Exact solutions of the wave patterns are derived. The bifurcation diagram describing the transition between the immobile and self-propelling modes of camphor objects is derived semi-analytically. The bifurcation is of a pitchfork type which is consistent with earlier theoretical work in which natural boundary conditions have been considered.Comment: 12 pages, 4 figure

Spectroscopic characterization of iron-oxygen intermediates in human aromatase (CYP19A1)

CYP19A1 or aromatase, is a human steroidogenic P450 important for estrogen biosynthesis in humans. Over activation of aromatase results in malignancies of the breast tissue, especially in post menopausal women. In fact, aromatase inhibitors constitute the front line therapy for estrogen receptor positive (ER+) breast cancer in post-menopausal women which accounts for over 70% of all breast cancer cases in the United States. Starting with its androgenic substrates, testosterone and androstenedione, CYP19A1 forms estradiol and estrone utilizing one molecule of atmospheric oxygen and two reducing equivalents in the form of NADPH. This is accomplished in a three-step process one of which involves a carbon-carbon bond scission and aromatization. The catalytic mechanism of P450s has been long studied and it is well known that an oxo-ferryl π-cation radical, known as “Compound 1” in P450 chemistry is the reactive intermediate that catalyzes most of the reactions of P450s. The identity of the reaction intermediate that catalyzes the terminal step estrogen biosynthesis by CYP19A1 is still a mystery. There is evidence in the literature suggesting the involvement of Compound 1 via a hydrogen abstraction that initiates deformylation and subsequent aromatization. There is also suggestion of the peroxo-anion or “Compound 0” acting as a nucleophile, attacking the electrophilic carbonyl carbon of 19-oxo-androstenedione forming a peroxide adduct that then fragments to produce acyl-carbon cleavage. Owing to the interesting chemistry CYP19A1 catalyzes and its role in human health I focused my attention towards elucidating the mechanism of this critical enzyme with the hope that a detailed picture of the workings of CYP19A1 will help guide efforts to make more specific inhibitors and improve breast cancer prognosis. CYP19A1 is a membrane-bound hemeprotein with a rich spectroscopic landscape thus affording an opportunity to apply a repertoire of biophysical approaches to help piece together a reaction mechanism. I used the Nanodisc technology to stabilize CYP19A1 in its native membrane-like environment to obtain a mono-disperse, stable and homogenous enzyme preparation that is amicable to the optical, resonance Raman (rR) and electron paramagnetic resonance (EPR) spectroscopy and also, cryoradiolysis and cryospectroscopy. The approach I have applied in this project has been that of characterizing the individual fate of reaction intermediates on their way from substrates to products thereby catching them ‘in action’. My cryospectroscopy, EPR, rR and steady state kinetics efforts outlined in this doctoral thesis all implicate “Compound 1” as the reactive intermediate that is responsible for the carbon-carbon scission reactivity of CYP19A1

Elucidating the Electronic Origins of Intermolecular Forces in Crystalline Solids

It is not possible to study almost any physical system without considering intermolecular forces (IMFs), no matter how insignificant they may appear relative to other energetic factors. Countless studies have shown that IMFs are responsible for governing a wide variety of physical properties, but often the atomic-origins of such interactions elude experimental detection. A considerable amount of work throughout the course of this research was therefore placed on using quantum mechanical simulations, specifically density functional theory (DFT), to calculate the electronic properties of solid-materials. The goal of these calculations was a better understanding of the precise origins of interatomic energies, down to the single-electron level. Furthermore, experimental X-ray diffraction and terahertz spectroscopy were both utilized because they are able to broadly probe the potential energy surfaces of molecular crystals, enhancing the theoretical data. Combining DFT calculations with experimental measurements enabled in-depth studies into the nature of specific non-covalent interactions, with results that were often unexpected based on conventional descriptions of IMFs. Overall, this work represents a significant advancement in understanding how subtle changes in characteristics like orbital occupation or electron density can have profound effects on bulk properties, highlighting the fragile relationship that exists between the numerous energetic parameters occurring within condensed phase systems

Surface Driven Flows : Liquid Bridges, Drops and Marangoni Propulsion

Molecules sitting at a free liquid surface against vacuum or gas have weaker binding than molecules in the bulk. The missing (negative) binding energy can therefore be viewed as a positive energy added to the surface itself. Since a larger area of the surface contains larger surface energy, external forces must perform positive work against internal surface forces to increase the total area of the surface. Mathematically, the internal surface forces are represented by surface tension, defined as the normal force per unit of length. One common manifestation of surface tension is the difference in pressure it causes across a curved surface. This is the main principle behind capillary breakup extensional rheometry (CaBER). The other manifestation is the Marangoni flow which drives the interface towards the direction of the increasing surface tension gradient. The surface tension gradient can be caused by concentration gradient or by a temperature gradient (surface tension is a function of temperature). Both of these phenomenon will be investigated through various experimental techniques. Predicting and controlling the rheology of polymeric fluids as a function of molecular chemistry has been of great interest in both academia and industry. While extensional rheology measurements of polymer melts have been performed in the past, those experiments were performed under nitrogen and at temperatures chosen to avoid polymer degradation and reaction. In this work we will explore the effect that oxygen at high temperatures can have on both the shear and extensional rheology of a series of polymer melts. We will demonstrate the high temperature evolution of extensional viscosity of three selected commercially available polycarbonates – one linear, one branched and one hyper-branched. The measurements were performed using a custom built high temperature capillary extensional break up rheometer (CABER). The experiments were performed in the temperature range of T= 250C and 370C both in air and nitrogen. We will present a stark difference in the extensional behavior of the three grades of polycarbonate and demonstrate an obvious differentiation between random chain scission which is the first phase of polymer degradation and repolymerization or crosslinking which takes place at higher temperatures. In a number of recent studies, the large extensional viscosity of dilute polymer solutions has been shown to dramatically delay the breakup of jets into drops. For the low shear viscosity solutions, the jet breakup is initially governed by a balance of inertial and capillary stresses before transitioning to a balance of viscoelastic and capillary stresses at later times. This transition occurs at a critical time when the radius decay dynamics shift from a 2/3 power law to an exponential decay as the increasing deformation rate imposed on the fluid filament results in large molecular deformations and rapid crossover into the elastocapillary regime. By experimental fits of the elasto-capillary thinning diameter data, relaxation time as low as 40 microseconds have been successfully measured. In this work, we will show that with better understand of the transition from the inertia-capillary to the elasto-capillary breakup regimes that relaxation times close to a single microsecond can be measured with the relaxation time resolution limited only by the frame rate and spatial resolution of the high speed camera. In this work, the dynamics of drop formation and pinch-off will be presented using Dripping onto Substrate Extensional Rheometry (DoS) for a series of dilute solutions Polyethylene Oxide in water and in a water and glycerin mixture. Four different molecular weights between 100k and 1M g/mol will be shown with varying solvent viscosities between 1mPa-s and 22mPa-s and at concentrations between 0.004 and 0.5c*. We will show the dependence of the relaxation time and extensional viscosity on these varying parameters while searching for the lower limit in solution elasticity that can be detected. We will also show that this approach is a powerful technique for characterizing a notoriously difficult material, namely low-viscosity printing inks. In this last project we have investigated the flow dynamics around a cylindrical disk propelled by Marangoni propulsion. Self-propulsion was achieved by coating one quarter of the disk with either soap or isopropyl alcohol in order to generate and then maintain a surface tension gradient across the surfer. As the propulsion strength and the resulting disk velocity were increased, a transition from a straight-line translational motion to a rotational motion was observed. Although spinning has been observed before for asymmetric objects, these are the first observations of spinning of a symmetric Marangoni surfer. Particle tracking and Particle Image Velocimetry (PIV) measurements were used to interrogate the resulting flow field and understand the origin of the rotational motion of the disk. These measurements showed that as the Reynolds number was increased, interfacial vortices attached to sides of the disk were formed and intensified. Beyond a critical Reynolds number of Re \u3e 120, a single vortex was observed to shed resulting in an unbalanced torque on the disk that caused it to rotate. The interaction between the disk and the confining wall of the Petri dish was also studied. Upon approaching the bounding wall, a transition from straight-line motion to rotational motion was observed at significantly lower Reynolds numbers than on an unconfined interface. Interfacial curvature was found to either enhance or eliminate rotation motion depending on whether the curvature was repulsive (concave) or attractive (convex). Along with investigations done for the case of a disk shaped Marangoni surfer, we have also look at the effect of shape and orientation of motion on the on the stability of these surfers. We have looked at spherical shaped Marangoni surfers and also at elliptical disks. The stability of the elliptical disks were found to be strongly linked to their aspect ratio and their orientation. While looking at shape effects on Marangoni propulsion we have also investigated the effect of reverse flow underneath the surfer and its effect on the motion. We were experimentally able to observe the phenomenon of reverse Marangoni propulsion of surfers which was found to be a function of water depth underneath the surfer. We performed a parametric study on the effect of depth of water on the mode of motion of a cylindrical disk shaped and a spherical shaped surfer. Along with critical depth, it was found that the Reynolds number also played a critical role in flow reversal. At higher Reynolds number, no reverse phenomenon was observed as inertia dominated the motion of the disk whereas at low Reynolds numbers and at the critical Depth, flow reversal was observed

Plastic deformation of polycrystalline sodium nitrite

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Thin film composite membranes by interfacial polymerization for organic solvent nanofiltration

One of the challenges of current organic solvent nanofiltration (OSN) membranes is to improve permeability in polar and non-polar solvents without compromising selectivity. Here, the development of a new generation of OSN membranes: High flux Thin Film Composite membranes (TFC) via interfacial polymerization (IP), is proposed. This thesis offers a comprehensive study that analyses the relationship of OSN high flux TFC membrane formation and post-formation parameters, morphology, structure and surface polarity, to membrane functional performance in both polar and non polar solvents. The dissertation starts with the development of novel high flux TFC membranes for polar aprotic solvents to address the trade-off between permeability and selectivity. This is accomplished by using two different approaches: (a) incorporation of polyethylene glycol inside the pores of the support prior to the IP reaction, and; (b) post-treatment of the TFC membranes with an “activating solvent”. Subsequently, a detailed analysis of membrane performance and morphology, considering the aforementioned approaches was conducted, resulting in dramatically increased solvent fluxes without compromising rejection. Additionally, a detailed study to manipulate molecular weight cut-off (MWCO) of these TFC membranes was carried out and successfully achieved by using different amines in the IP reaction. Next, novel high flux hydrophobic TFC membranes via IP with tuned MWCO for non-polar solvents were developed, elucidated and studied. The surface properties of hydrophilic TFC OSN membranes were modified by capping the free acyl chloride groups on their surface with different monomers containing hydrophobic groups. A detailed study on surface polarity and membrane performance was undertaken, suggesting that surface chemistry plays an important role in solvent permeation. The membrane performance was compared to commercial OSN integrally skinned asymmetric (ISA) and TFC rubber-coated membranes. In the next stage of this thesis, the effects of different support membranes on TFC membrane formation and functional performance were studied for both polar and non-polar solvents. It was found that support membranes have an effect on TFC membrane formation and solvent permeation. Finally, to increase permeability even further without a requirement for treating the TFC membrane with an activating solvent, highly porous TFC membranes have been developed via IP by controlling the structure of the top layer at a molecular level. This was achieved by incorporating a monomer with a contorted structure during the IP reaction, resulting in a highly porous polymer network. It is believed high flux TFC OSN membranes prepared by interfacial polymerization may offer new degrees of freedom in membrane design, which could lead to the next generation of high performance OSN membranes