Tag: Fall 2010

Competing Timescales for Surfactant Transport and Flow at Microscale Fluid-Fluid Interfaces

Friday December 3
Chem. Sci. & Engineering Room 211
10:00 a.m.

Presenter: Dr. Shelley L. Anna, Carnegie Mellon UniversityDepartment of Chemical Engineering and Mechanical Engineering

Abstract: The presence of surface-active species dramatically alters interfacial processes such as droplet breakup and coalescence, and coating and film flows. Tipstreaming is an example of a unique surfactant-mediated effect that occurs when strong convection past a stationary droplet drives surfactant to the pole and draws a high curvature thread from the pointed end of the drop.  We use tipstreaming in a microfluidic device to generate micron-scale droplets.  To gain control of this droplet generation method, we recognize that there are several competing processes that give rise to tipstreaming.  These include: bulk diffusion of soluble surfactants to the interface, kinetics of adsorption and desorption, convection, and relaxation of surface concentration gradients.  To properly determine the magnitudes of the timescales involved in tipstreaming, we first need to know the kinetic rate constants for each surfactant-fluid-fluid system of interest.  Kinetic constants are not readily available for oil-water interfaces due to small fluid density differences.  We address this limitation by developing a new method to obtain dynamic surface tension for oil-water interfaces. We then develop a scaling argument showing that kinetics become rate limiting for micron-scale pendant drops, and we use this idea to demonstrate a method for obtaining reliable values for the kinetic rate constants.  Using these measured rate constants combined with tipstreaming experiments, scaling arguments, and numerical simulations, we show that tipstreaming occurs when convection is fast enough to maintain a surface tension gradient, and when adsorption is too slow to quench the gradient.  These conditions are met only when the surface coverage is very small, due to the intimate coupling of fluid velocity and surface concentration.  Finally, we use these observations for three different nonionic surfactants to suggest ways to increase the operating window for tipstreaming and control the production of tiny droplets.  These two problems are examples of ways that we can use competing timescales for surfactant transport and flow to engineer soft materials.

Biosignature of Oxidative Stress in the Eye: A New Approach to Treat Retinal Diseases

Friday November 19
Chem. Sci. & Engineering Room 211
10:00 a.m.

Presenter: Dr. Wan Jin Jahng, Michigan Tech UniversityDept. of Biological Sciences

Abstract: The regeneration of the 11-cis-retinyl imine chromophore of rhodopsin during the visual cycle and mechanisms that control this process are central questions in the field of vision research. The retinal pigment epithelium (RPE)-specific protein RPE65 is centrally involved in the isomerization and hydrolysis of all-trans-retinyl esters. We investigated that RPE65 cleavage and potential regulatory mechanisms under oxidative stress conditions. Our results indicate that oxidative stress during the visual cycle results in cleavage of RPE65. RPE is essential for retinoid recycling and phagocytosis of photoreceptors.  Understanding of proteome changes that mediate oxidative stress-induced degeneration of RPE cells may provide further insight into the molecular mechanisms of retinal diseases. In the current study, comparative proteomics has been applied to investigate global changes of RPE proteins under oxidative stress.Proteomic techniques including two dimensional electrophoresis, differential gel electrophoresis (DIGE), tandem time-of-flight (TOF-TOF) mass spectrometry, and fluorescent microscopy were used to identify early protein markers of oxidative stress in the RPE. Two biological models of RPE cells revealed several differentially-expressed proteins that are involved in key cellular processes such as energy metabolism, protein folding, redox homeostasis, cell differentiation, and retinoid metabolism. Our results provide a new perspective on early signaling molecules of redox imbalance in the RPE and putative therapeutic target proteins of eye diseases caused by oxidative stress.

Liquid Water Percolation in PEM Fuel Cell Porous Transport Layers – Comparison of Numerical Predictions with Experimental Result

Friday October 29
Chem. Sci & Engineering Room 211
10:00 a.m.

Presenter: Dr. Jeffrey S. Allen, Michigan Technological University Mechanical Engineering—Engineering Mechanics

Abstract: The standard proton exchange membrane (PEM) fuel cell consist of a series of nonwetting porous layers compressed between the bipolar plates.  The layers can be categorized as anode and cathode porous transport layers (PTL), also known as gas diffusion layers, each with a catalyst layer and a proton exchange membrane. The cathode PTL has the dual role of facilitating the access of the reactants to the catalyst layer while removing the water produced by the electrochemical reaction.  In a PEM fuel cell, liquid water may percolate through the non-wetting PTL forming conduits or fingers that are influenced by capillary pressure and the PTL morphology. Using a specially designed ex-situ experimental setup, images of the water percolation and the pressure required to inject the water in a PTL were simultaneously recorded. The time evolution of projected area occupied by water and the percolation pressure are indicative of the drainage flow pattern taking place. Scaling of key parameters in conjunction with the capillary number and the viscosity ratio has resulted in a non-dimensional number correlating the pressure-area data; two variables which are usually analyzed separately in two-phase porous media flow. Using this non-dimensional number a simple logarithmic dependence for all injection flow rates was obtained for a given PTL sample; effectively collapsing the drainage phase diagram to a single curve. When a PTL sample with different morphological and wetting properties was tested, a new linear relationship was obtained. Each type of PTL has a unique curve and, based on preliminary results, the slope of this curve can be used to characterize PTLs with respect to water percolation. This scaling technique can also be used as a validation method for numerical simulation of drainage in porous media.rous media.

Aberrant hydrophobicity of mutant SODls: Implication for toxicity in Amyotrophic Lateral Sclerosis

Friday October 15
Chem. Sci. & Engineering Room 211
10:00 a.m.

Presenter: Dr. Ashutosh Tiwari, Michigan Technological Univ.Dept. of Chemistry

Abstract: More than 100 different mutations in the gene encoding Cu/Znsuperoxide dismutase (SODl) cause preferential motor neuron degeneration in familial amyotrophic lateral  sclerosis (ALS).  Although the cellular target(s) of mutant SODl toxicity have not been precisely specified, evidence to date suggests that altered conformations of mutant SODls trigger perturbations of cellular homeostasis that ultimately cause motor neuron degeneration.  My efforts have focused on identifying the underlying mechanism(s) by which mutant SODl proteins misfold or aggregate to produce toxicity.  My studies show that SODl proteins upon mutation are predisposed to loss of metal ion binding, destabilization, disulfide reduction, partial unfolding, and monomerization.  Overall, our findings support the notion that misfolding associated with metal deficiency may facilitate aberrant interactions of SODl with itself or with other cellular constituents and may thereby contribute to neuronal toxicity.

Solid nanoparticles to stabilize water/oil emulsions and catalyze reactions at the liquid/liquid interface

Friday October 8
Chem. Sci. & Engineering Room 211
10:00 a.m.

Presenter: Dr. Daniel Resasco, Univ. of OklahomaSchool of ChemicalBiological & MaterialsEngineering

Abstract: Pyrolysis oil is a complex mixture of oxygenated compounds with up to ~30-40% of water. Depending on the cooling process used in the condensation of the pyrolysis vapors, the crude bio-oil can generate a biphasic system, in which molecules are distributed, between the two phases, depending on their solubility. It is desirable to conduct reactions at the water/oil interface that can both enhance the fuel value of the molecules and effect phase migration, based on solubility, avoiding fractionation by heating,  which is known to negatively impact bio-oil.We have developed a new reaction/separation concept based on a family of recoverable nano-hybrid catalysts that simultaneously stabilize emulsions in biphasic bio-oil. These nanostructured solid particles exhibit a unique advantage in streamlining biomass refining, where the immiscibility and thermal instability of crude bio-oil greatly complicates purification procedures. These novel  catalyst/emulsifier hybrids can catalyze reactions with high “phase-selectivity” either in the aqueous or organic phases.These catalysts are obtained by fusing carbon nanotubes with metal-oxide particles, which results in twofaced nanohybrid solids that are able to stabilize water/oil emulsions by forming a rigid film at liquidliquid interface of the droplets and increasing the apparent viscosity of the system (1).The metal oxide in the nanohybrid acts both, as the hydrophylic side of the emulsifier, and as a catalyst for condensation reactions in the aqueous phase. Accordingly, small oxygenates soluble in water, with low fuel value, condense via aldol-condensation, ketonization, or etherification resulting in products, which are no longer water-soluble molecules and therefore migrate to the organic phase. The oxide used can vary in acid/base characteristics. Some of the metal-oxides tested are; MgO, SiO2, TiO2 and ZnO.In the organic phase, transition metals such as Pd, Ni and Cu have been deposited onto the hydrophobic carbon nanotube of the nanohybrids to catalyze deoxygenation reactions including hydrogenation, hydrogenolysis, or decarbonylation occurring on the oil side of the emulsion.We have accomplished biphasic hydrodeoxygenation and condensation catalysis in high yields, using these nono-hybrid catalysts for several systems of interest in biomass refining (2). Reactions were conducted in a semi-batch reactor with the liquid composed of three layers oil/emulsion/water and with continuous flow of hydrogen, at temperatures in the range 80- -900 psi) high enough to maintain the water in the liquid state.