Fingering is a hydrodynamic instability that occurs when a more mobile fluid displaces a fluid of lower mobility. When the primary source of the mobility difference is viscosity, the instability is termed viscous fingering. Viscous fingering is often, though not always, undesirable in industrial processes, particularly secondary petroleum recovery. Linear stability analysis by Hejazi et al. has indicated that the production of a non-monotonic viscosity profile can stabilize the interface. Herein, we use step-growth polymerization at the interface between two miscible monomers as a model system. In particular, a dithiol monomer displaced a diacrylate that reacted to form a linear polymer that behaves as a Newtonian fluid. Viscous fingering was imaged in a horizontal Hele-Shaw cell via Schlieren, which is sensitive to changes in index of refraction, and therefore polymer conversion. By varying reaction rate via initiator concentration along with flow rate via a syringe pump, we were able to demonstrate increasing stabilization of the flow with increasing Damköhler number.

Viscous fingering can occur in fluid motion whenever a high mobility fluid displaces a low mobility fluid in a Darcy type flow. When the mobility difference is primarily attributable to viscosity (e.g., flow between the two horizontal plates of a Hele-Shaw cell), viscous fingering (VF) occurs, which is sometimes termed the Saffman-Taylor instability. Alternatively, in the presence of differences in density in a gravity field, buoyancy-driven convection can occur. These instabilities have been studied for decades, in part because of their many applications in pollutant dispersal, ocean currents, enhanced petroleum recovery, and so on. More recent interest has emerged regarding the effects of chemical reactions on fingering instabilities. As chemical reactions change the key flow parameters (densities, viscosities, and concentrations), they may have either a destabilizing or stabilizing effect on the flow. Hence, new flow patterns can emerge; moreover, one can then hope to gain some control over flow instabilities through reaction rates, flow rates, and reaction products. We report effects of chemical reactions on VF in a Hele-Shaw cell for a reactive step-growth cross-linking polymerization system. The cross-linked reaction product results in a non-monotonic viscosity profile at the interface, which affects flow stability. Furthermore, three-dimensional internal flows influence the long-term pattern that results.

Inexpensive in situ monitoring of a conformational change in a macro object over long periods of time in a chemical reactor is challenging. One research area which would benefit from improvements in screening methods is the study of smart hydrogels, particularly when they are intended as oral forms for drug delivery or as multifunctional scaffolds replacing surgically removed tissues. Smart materials have the ability to alter their volume by swelling and/or collapsing in response to a specific stimulus in their environment. Conventional methods used to record this change such as gravimetric analysis, are invasive, require manpower for time-consuming hydrogel handling and often result in material fragmentation leading to inaccuracy. In this work, a novel reactor design is implemented in combination with inexpensive optics to achieve a non-invasive method that can be used reliably over long periods of time. Optical quality flat glass windows are incorporated in a jacketed reactor vessel design to enable undistorted imaging. The reactor was made from a chemical engineering viewpoint to enable temperature control, continuous stirring and sampling while preventing evaporative loss of solvent. Image resolution was measured using a USAF 1951 resolution test target. The setup was validated using pH responsive PVP-Chitosan hydrogels to demonstrate the capabilities of the method in monitoring the change in volume of the responsive hydrogel with time.

Interfaces between different fluids can be unstable with regard to hydrodynamic instabilities such as viscous fingering or buoyancy-driven convection. To study such instabilities experimentally for transparent fluids, dyes or chemical indicators are most often used to track the dynamics. While the interfacial deformation can easily be tracked by color changes, it is difficult to have access to the internal flow structure for comparison with theoretical predictions. To overcome this problem, a modification of a Schlieren technique is introduced to image 3D flows during viscously driven instabilities in a horizontal Hele-Shaw cell without using any dye or chemical indicator. The method is exquisitely sensitive, readily yielding information about 3D flows in gaps under a millimeter and allowing imaging of the flow structure internal to the fingers, rather than merely imaging the flow boundary. Following a description of the technique, visualization of dynamics for nonreactive water–glycerol and reactive displacements is presented revealing previously unobserved internal flows. These flows are tentatively interpreted in terms of known theoretical predictions.

We study the spatio-temporal evolution of the viscosity field during stable and unstable radial flows of glycerol-water solutions in a horizontal Hele-Shaw cell where a localized temperature gradient is imposed. The viscosity field is reconstructed from the measurement of the fluorescence emitted by a viscosity-sensitive molecular probe (Auramine O). For an immiscible flow, the viscosity and temperature fields are obtained accurately. For miscible displacements, we show how the interplay between the viscosity changes of both fluids and the variation of the fluid thickness in the gap prevents obtaining strict quantitative reconstruction of the viscosity field. We explain how the reconstructed viscosity field can nevertheless be interpreted to obtain information about the fluid thickness and the local viscosity and temperature.

For the first time, electron paramagnetic resonance spectroscopy was used to compare numbers and distributions of radicals produced in frontal free-radical polymerization of multifunctional acrylates and methacrylates to those produced by bulk free-radical polymerization. A comparison of radical concentrations was performed for individual polymers and selected copolymers of trimethylolpropane trimethacrylate (TMPTMA), 1,6-hexanediol diacrylate, trimethylolpropane triacrylate, and pentaerythritol tetraacrylate (PETA). Frontally polymerized samples showed a large spike in intensity at the point of initiation. Within a few centimeters, the radical signal diminished to a steady state. The radical concentration remained almost constant over 3 months under helium in flame-sealed tubes. The types of radicals were similar to those in bulk polymerization. For both TMPTMA and PETA, frontally polymerized samples had significantly higher numbers of radicals than did the bulk-prepared samples, achieving concentrations as high as 8.7 x 10(-3) mol/kg in the frontally prepared samples of TMPTMA. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 4261-4266, 2011

Experimental EPR spectra in several modified vanadate glass systems reveal hyperfine structure (hfs) lines whose widths vary with the molar ratio of modifier to vanadium pentoxide, R. In the RNa2O.V2O5 system, for example, hfs lines show no resolution at low R values (near 0.1); by contrast, these lines exhibit dramatic narrowing as R approaches 0.5. In the model proposed here, this narrowing is due to an increase in hopping time for small polarons associated with V4+ ions in these systems. Increases in polaron hopping times are accompanied by increases in electron spin-spin relaxation times T2's, and, an associated narrowing of EPR linewidths. Experiments confirm that spectral widths are limited by electron T2's due to the fact that EPR linewidths do not vary with temperature down to 4.2K. Resolved spectra in RNa2O.V2O5 at R=0.5 reveal a hyperfine coupling parameter of 0.0177±0.0008T, corresponding to an upper-limit polaron hopping frequency of 487±20MHz. By similar analyses, the systems of RCaO.V2O5, RBaO.V2O5, and RLi2O.V2O5 exhibit comparable polaron hopping frequencies limits of 480±20MHz, 469±20MHz, and 468±20MHz, respectively, when R is near 1.0. In addition to the relaxation effects discussed here, results of modeling of resolved spectra to obtain hyperfine coupling constants A|| and A┴, and g┴ values g|| and g┴ are presented and discussed.

Electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) are used to identify and characterize V 4+ ions in a bulk single crystal of vanadium dioxide (V O2). These S=1/2 defects are observed in the as-grown crystal because an adjacent nonmagnetic M4+ impurity, e.g., a Si4+ ion, has destroyed the normal antiferromagnetic coupling associated with the close pairs of V 4+ ions that occur in the low-temperature monoclinic phase of V O2. EPR spectra taken near 5 K with the magnetic field along the [110] and [001] axes show resolved hyperfine patterns due to one 51V nucleus. ENDOR spectra taken at 5 K with the magnetic field parallel to the [001] axis have large nuclear electric quadrupole splittings as a result of a significant electric field gradient at the 51V nucleus. Spin-Hamiltonian parameters describing the electron Zeeman, hyperfine, and nuclear electric quadrupole interactions are reported.

Electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) are used to identify and characterize V 4+ ions in a bulk single crystal of vanadium dioxide (V O2). These S=1/2 defects are observed in the as-grown crystal because an adjacent nonmagnetic M4+ impurity, e.g., a Si4+ ion, has destroyed the normal antiferromagnetic coupling associated with the close pairs of V 4+ ions that occur in the low-temperature monoclinic phase of V O2. EPR spectra taken near 5 K with the magnetic field along the [110] and [001] axes show resolved hyperfine patterns due to one 51V nucleus. ENDOR spectra taken at 5 K with the magnetic field parallel to the [001] axis have large nuclear electric quadrupole splittings as a result of a significant electric field gradient at the 51V nucleus. Spin-Hamiltonian parameters describing the electron Zeeman, hyperfine, and nuclear electric quadrupole interactions are reported.

Electron paramagnetic resonance (EPR) is used to identify photoinduced titanium-associated electron traps in TiO2 crystals (rutile). Defect production occurs at low temperature with 442 nm laser light. Spectra with S=1/2 and S=1 are assigned to singly ionized and neutral oxygen vacancies, respectively. These oxygen vacancies have their unpaired spins localized on the two neighboring titanium ions aligned along the c axis. A Ti3+ ion next to a Si4+ ion, a Ti3+ self-trapped electron, and a self-trapped hole shared by two adjacent oxygen ions are also observed. Isolated substitutional Fe3+ and Cr3+ ions serve as hole traps.

A simple technique for determining the diffusion coefficient between two miscible liquids is presented based on observing concentration-dependent ultraviolet-excited fluorescence using a digital camera. The ultraviolet-excited visible fluorescence of corn syrup is proportional to the concentration of the syrup. The variation of fluorescence with distance from the transition zone between the fluids is fit by the Fick’s law solution to the diffusion equation. By monitoring the concentration at successive times, the diffusion coefficient can be determined in otherwise transparent materials. The technique is quantitative and makes measurement of diffusion accessible in the advanced undergraduate physics laboratory.