An Experimental Study of Dispersion in Oscillating Flows in Cylindrical Tubes

The cochlea of the inner ear has fluid filled spaces. Drugs are delivered to the cochlea via transtympanic injections to the base of the cochlea, at a membrane called the Round Window Membrane (RWM). Drugs diffuse through the RWM into the cochlear fluids. This method relies on the mechanism of pure diffusion. Hence drug delivery is slow and treatment efficacy is affected. The cochlear fluid oscillates when stimulated by sound. This thesis experimentally investigates if drug dispersion in the cochlea can be enhanced by oscillating the cochlear fluids for amplitudes as small as that of the cochlea. To answer this question, empirical dye dispersion experiments for oscillating flows were performed in a cylindrical tube over a range of frequencies and amplitudes of oscillation. An experimental apparatus was designed and assembled to conduct experiments on the dispersion of a dye in water. Experiments were conducted for 3 sets of frequencies for amplitudes varying from 0 (pure diffusion) to 2.5 times the radius of the cylindrical tube. A time series of images of the dye were used to measure concentration and ultimately used to calculate an effective diffusion coefficient and to quantify the enhancement in diffusion. Dispersion coefficients obtained for a constant frequency increase linearly as a function of the square of the amplitude, and for a constant amplitude dispersion coefficients increase with frequency. These trends agree with previously established results from literature. The results of the experiments, when scaled to the cochlea and feasible magnitudes of oscillation, predict that oscillation does not substantially enhance diffusion

The Hessian biased singular value decomposition method for optimization and analysis of force fields

We present methodology (HBFF/SVD) for optimizing the form and parameters of force fields (FF) for molecular dynamics simulations through utilizing information about properties such as the geometry, Hessian, polarizability, stress (crystals), and elastic constants (crystals). This method is based on singular value decomposition (SVD) of the Jacobian describing the partial derivatives in various properties with respect to FF parameters. HBFF/SVD is effective for optimizing the parameters for accurate FFs of organic, inorganic, and transition metal compounds. In addition it provides information on the validity of the functional form of the FF for describing the properties of interest. This method is illustrated by application to organic molecules (CH2O, C2H4, C4H6, C6H8, C6H6, and naphthalene) and inorganic molecules (Cl2CrO2 and Cl2MoO2)

Hessian-biased force fields from combining theory and experiment

We describe a new approach of combining experimental and theoretical information to develop accurate valence force fields. We combine the Hessian from ab initio calculations with the structural and spectroscopic data from experiment to generate a new Hessian that is used for extracting force fields. Emphasis is placed on obtaining accurate geometries and accurate frequencies. This technique is illustrated by calculations on formaldehyde, formate anion, thioformaldehyde, carbonyl chloride, and carbonyl fluoride

First Principles Study of the Ignition Mechanism for Hypergolic Bipropellants: N,N,N′,N′-Tetramethylethylenediamine (TMEDA) and N,N,N′,N′-Tetramethylmethylenediamine (TMMDA) with Nitric Acid

We report quantum mechanics calculations (B3LYP flavor of density functional theory) to determine the chemical reaction mechanism underlying the hypergolic reaction of pure HNO_3 with N,N,N′,N′-tetramethylethylenediamine (TMEDA) and N,N,N′,N′-tetramethylmethylenediamine (TMMDA). TMEDA and TMMDA are dimethyl amines linked by two CH_2 groups or one CH_2 group, respectively, but ignite very differently with HNO_3. We explain this dramatic difference in terms of the role that N lone-pair electrons play in activating adjacent chemical bonds. We identify two key atomistic level factors that affect the ignition delay: (1) The exothermicity for formation of the dinitrate salt from TMEDA or TMMDA. With only a single CH_2 group between basic amines, the diprotonation of TMMDA results in much stronger electrostatic repulsion, reducing the heat of dinitrate salt formation by 6.3 kcal/mol. (2) The reaction of NO_2 with TMEDA or TMMDA, which is the step that releases the heat and reactive species required to propagate the reaction. Two factors of TMEDA promote the kinetics by providing routes with low barriers to oxidize the C: (a) formation of a stable intermediate with a C–C double bond and (b) the lower bond energy for breaking the C–C single bond (by 18 kcal/mol comparing to alkane) between two amines. Both factors would decrease the ignition delay for TMEDA versus TMMDA. The same factors also explain the shorter ignition delay of 1,4-dimethylpiperazine (DMPipZ) versus 1,3,5-trimethylhexahydro-1,3,5-triazine (TMTZ). These results indicate that TMEDA and DMPipZ are excellent green replacements for hydrazines as the fuel in bipropellants

Initiation mechanisms and kinetics of pyrolysis and combustion of JP-10 hydrocarbon jet fuel

In order to investigate the initiation mechanisms and kinetics associated with the pyrolysis of JP-10 (exo-tricyclo[5.2.1.0^2,6]decane), a single-component hydrocarbon jet fuel, we carried out molecular dynamics (MD) simulations employing the ReaxFF reactive force field. We found that the primary decomposition reactions involve either (1) dissociation of ethylene from JP-10, resulting in the formation of a C8 hydrocarbon intermediate, or (2) the production of two C5 hydrocarbons. ReaxFF MD leads to good agreement with experiment for the product distribution as a function of temperature. On the basis of the rate of consumption of JP-10, we calculate an activation energy of 58.4 kcal/mol for the thermal decomposition of this material, which is consistent with a strain-facilitated C−C bond cleavage mechanism in JP-10. This compares well with the experimental value of 62.4 kcal/mol. In addition, we carried out ReaxFF MD studies of the reactive events responsible for oxidation of JP-10. Here we found overall agreement between the thermodynamic energies obtained from ReaxFF and quantum-mechanical calculations, illustrating the usefulness of ReaxFF for studying oxidation of hydrocarbons. The agreement of these results with available experimental observations demonstrates that ReaxFF can provide useful insights into the complicated thermal decomposition and oxidation processes of important hydrocarbon fuels

Explanation of the colossal detonation sensitivity of silicon pentaerythritol tetranitrate (Si-PETN) explosive

DFT calculations have identified the novel rearrangement shown here for decomposition of the Si derivative of the PETN explosive [pentaerythritol tetranitrate (PETN), C(CH_2ONO_2)_4] that explains the very dramatic increase in sensitivity observed experimentally. The critical difference is that Si-PETN allows a favorable five-coordinate transition state in which the new Si−O and C−O bonds form simultaneously, leading to a transition state barrier of 33 kcal/mol (it is 80 kcal/mol for PETN) and much lower than the normal O−NO_2 bond fission observed in other energetic materials (40 kcal/mol). In addition this new mechanism is very exothermic (45 kcal/mol) leading to a large net energy release at the very early stages of Si-PETN decomposition that facilitates a rapid temperature increase and expansion of the reaction zone

Thermal Decomposition of Energetic Materials by ReaxFF Reactive Molecular Dynamics

We report the study of thermal decomposition of 1,3,5-trinitrohexahydro-s-triazine (RDX) bonded with polyurethane (Estane) and of the bulk hydrazine by molecular dynamics (MD) simulations equipped with the reactive force field (ReaxFF). For the polymer binder explosive, the simulation results show that the thermal decomposition of RDX is affected by the presence of the polymer binder Estane. Generally, with addition of Estane the decomposition of RDX slows down. Final products including N2, H2O, CO, CO2 and intermediates NO2, NO and HONO have been identified from the thermal decomposition processes. For the bulk hydrazine, it is found that with the increase of temperature, its decomposition increases and more N2 and H2 are generated, but NH3 molecules are consumed much faster at higher temperatures. This simulation work provides us an approach to quickly test the response of various energetic materials to thermal conditions

Solar fuels editorial

Every major change in the living standards for humans on our planet has had an energy revolution at its heart – the advent of the industrial age with the steam engine and use of coal, the internal combustion engine and large-scale electricity generation. The energy demand, primarily from emerging economies, will double by 2050. The countervailing urgency of the threat of climate change requires a major shift in our energy sourcing, creating four new trends that will shape the current century: electrification, decarbonization, localization, and optimization

Electronic Structures of Group 9 Metallocorroles with Axial Ammines

The electronic structures of metallocorroles (tpfc)M(NH_3)_2 and (tfc)M(NH_3)_2 (tpfc is the trianion of 5,10,15-(tris)pentafluorophenylcorrole, tfc is the trianion of 5,10,15-trifluorocorrole, and M = Co, Rh, Ir) have been computed using first principles quantum mechanics [B3LYP flavor of Density Functional Theory (DFT) with Poisson−Boltzmann continuum solvation]. The geometry was optimized for both the neutral systems (formal M^(III) oxidation state) and the one-electron oxidized systems (formally M^(IV)). As expected, the M^(III) systems have a closed shell d^6 configuration; for all three metals, the one-electron oxidation was calculated to occur from a ligand-based orbital (highest occupied molecular orbital (HOMO) of B_1 symmetry). The ground state of the formal M^(IV) system has M^(III)-Cπ character, indicating that the metal remains d^6, with the hole in the corrole π system. As a result the calculated M^(IV/III) reduction potentials are quite similar (0.64, 0.67, and 0.56 V vs SCE for M = Ir, Rh and Co, respectively), whereas the differences would have been large for purely metal-based oxidations. Vertically excited states with substantial metal character are well separated from the ground state in one-electron-oxidized cobalt (0.27 eV) and rhodium (0.24 eV) corroles, but become closer in energy in the iridium (0.15 eV) analogues. The exact splittings depend on the chosen functional and basis set combination and vary by ~0.1 eV