Emilio J. Tozzi

A simulation study on the combined effects of nanotube shape and shear flow on the electrical percolation thresholds of carbon nanotube/polymer composites

Here we investigate the combined effects of carbon nanotube (CNT) properties such as aspect ratio, curvature, and tunneling length and shear rate on the microstructure and electrical conductivities of CNT/polymer composites using fiber-level simulations. Electrical conductivities are calculated using a resistor network algorithm. Results for percolation thresholds in static systems agree with predictions and experimental measurements. We show that imposed shear flow can decrease the electrical percolation threshold by facilitating the formation of conductive aggregates. In agreement with previous research, we find that lower percolation thresholds are obtained for nanotubes with high aspect ratio. Our results also show that an increase in the curvature of nanotubes can make more agglomeration and reduce the percolation threshold in sheared suspensions.

The effect of mixing on the liquefaction and saccharification of cellulosic fibers

The enzymatic hydrolysis of cellulosic material is a key step in the biochemical routes for production of renewable fuels and chemicals. This must be performed at high solids to be economically viable. High solids operations creates numerous processing challenges, most importantly the limitations due to mass transfer and poor mixing of enzymes in the cellulose suspensions. We use magnetic resonance imaging (MRI), a cylindrical penetrometer, and HPLC to demonstrate the importance of spatial homogeneity in the distribution of enzyme on the rates of liquefaction of the substrate and in the suspension mechanical strength. Our results show that the largest mechanical strength changes occur in a narrow interval of time during the initial stages of conversion. Differences in enzyme concentration distribution occurring at the centimeter-scale produced order of magnitude differences in liquefaction and saccharification rates, supporting the hypothesis that mixing quality has a major influence in both liquefaction and saccharification rates.

Yield stress of pretreated corn stover suspensions using magnetic resonance imaging

Cellulose fibers in water form networks that give rise to an apparent yield stress, especially at high solids contents. Measuring the yield stress and correlating it with fiber concentration is important for the biomass and pulp industries. Understanding how the yield stress behaves at high solids concentrations is critical to optimize enzymatic hydrolysis of biomass in the production of biofuels. Rheological studies on pretreated corn stover and various pulp fibers have shown that yield stress values correlate with fiber mass concentration through a power‐law relationship. We use magnetic resonance imaging (MRI) as an in‐line rheometer to measure velocity profiles during pipe flow. If coupled with pressure drop measurements, these allow yield stress values to be determined. We compare our results with literature values and discuss the accuracy and precision of the rheo‐MRI measurement, along with the effects of fiber characteristics on the power‐law coefficients. Biotechnol. Bioeng. 2011;108: 2312–2319.

The role of endoglucanase and endoxylanase in liquefaction of hydrothermally pretreated wheat straw.

The role of endocellulases and endoxylanase during liquefaction and saccharification of hydrothermally pretreated wheat straw was studied. The use of a flow‐loop setup with in‐line magnetic resonance imaging enabled frequent measurements of viscosity at 55°C during saccharification at 6% total solids content. Viscosity data were complemented with off‐line measurements of fiber lengths and release of soluble sugars. A clear correlation between fiber attrition and a decrease in viscosity was found. Fiber lengths and viscosity dropped quickly within the first hour and then stagnated, while sugar yields increased substantially thereafter, illustrating that liquefaction and saccharification are separate mechanisms. Both endoglucanase and endoxylanase were shown to have a significant effect on viscosity during liquefaction while the addition of endoxylanase also increased sugar yield.

A process for energy-efficient high-solids fed-batch enzymatic liquefaction of cellulosic biomass.

The enzymatic hydrolysis of cellulosic biomass is a key step in the biochemical production of fuels and chemicals. Economically feasible large-scale implementation of the process requires operation at high solids loadings, i.e., biomass concentrations >15% (w/w). At increasing solids loadings, however, biomass forms a high viscosity slurry that becomes increasingly challenging to mix and severely mass transfer limited, which limits further addition of solids. To overcome these limitations, we developed a fed-batch process controlled by the yield stress and its changes during liquefaction of the reaction mixture. The process control relies on an in-line, non-invasive magnetic resonance imaging (MRI) rheometer to monitor real-time evolution of yield stress during liquefaction. Additionally, we demonstrate that timing of enzyme addition relative to biomass addition influences process efficiency, and the upper limit of solids loading is ultimately limited by end-product inhibition as soluble glucose and cellobiose accumulate in the liquid phase.

Flipping, scooping, and spinning: Drift of rigid curved nonchiral fibers in simple shear flow

The motion of isolated, rigid, neutrally-buoyant, non-Brownian, curved, nonchiral fibers in simple shear flow of an incompressible Newtonian fluid at low Reynolds number is studied by computer simulation. For some initial orientations, fibers with small curvature drift steadily in the gradient direction without external forces or torques. The average drift velocity and direction depend on the fiber aspect ratio, curvature, and initial orientation. The drift results from the coupling of rotational and translational dynamics, and the combined effects of flipping, scooping, and spinning motions of the fiber.

Quantifying Mixing using Magnetic Resonance Imaging

Mixing is a unit operation that combines two or more components into a homogeneous mixture. This work involves mixing two viscous liquid streams using an in-line static mixer. The mixer is a split-and-recombine design that employs shear and extensional flow to increase the interfacial contact between the components. A prototype split-and-recombine (SAR) mixer was constructed by aligning a series of thin laser-cut Poly (methyl methacrylate) (PMMA) plates held in place in a PVC pipe. Mixing in this device is illustrated in the photograph in Fig. 1. Red dye was added to a portion of the test fluid and used as the minor component being mixed into the major (undyed) component. At the inlet of the mixer, the injected layer of tracer fluid is split into two layers as it flows through the mixing section. On each subsequent mixing section, the number of horizontal layers is duplicated. Ultimately, the single stream of dye is uniformly dispersed throughout the cross section of the device. Using a non-Newtonian test fluid of 0.2% Carbopol and a doped tracer fluid of similar composition, mixing in the unit is visualized using magnetic resonance imaging (MRI). MRI is a very powerful experimental probe of molecular chemical and physical environment as well as sample structure on the length scales from microns to centimeters. This sensitivity has resulted in broad application of these techniques to characterize physical, chemical and/or biological properties of materials ranging from humans to foods to porous media 1, 2. The equipment and conditions used here are suitable for imaging liquids containing substantial amounts of NMR mobile 1H such as ordinary water and organic liquids including oils. Traditionally MRI has utilized super conducting magnets which are not suitable for industrial environments and not portable within a laboratory (Fig. 2). Recent advances in magnet technology have permitted the construction of large volume industrially compatible magnets suitable for imaging process flows. Here, MRI provides spatially resolved component concentrations at different axial locations during the mixing process. This work documents real-time mixing of highly viscous fluids via distributive mixing with an application to personal care products.

Effective diffusivities of BSA in cellulosic fiber beds measured with magnetic resonance imaging

The temporal and spatial evolution of concentration profiles of bovine serum albumin (BSA) in various cellulosic fiber beds is measured using magnetic resonance imaging. Effective diffusivities are calculated using a numerical one dimensional Fickian model to match experimental concentration profiles. Experimental values of the diffusivities are compared with predictions from a simple diffusion-adsorption model which accounts for porosity, tortuosity, and surface adsorption. BSA was found to have negligible adsorption in the concentration range studied, resulting in a simplified diffusion model based on fiber characteristics and geometry. Effective diffusivities agreed well with the predicted values and were within an order of magnitude of the estimated bulk diffusivity of BSA.

Settling dynamics of asymmetric rigid fibers

The three-dimensional motion of asymmetric rigid fibers settling under gravity in a quiescent fluid was experimentally measured using a pair of cameras located on a movable platform. The particle motion typically consisted of an initial transient after which the particle approached a steady rate of rotation about an axis parallel to the acceleration of gravity, with its center of mass following a helical trajectory. Numerical and analytical methods were used to predict translational and angular velocities as well as the evolution of the fiber orientation as a function of time. A comparison of calculated and measured values shows that it is possible to quantitatively predict complex motions of particles that have highly asymmetric shape. The relations between particle shape and settling trajectory have potential applications for hydrodynamic characterization of fiber shapes and fiber separation.

Robust processing of capillary velocimetry data via stress-rescaled velocity functions

A method is presented to obtain rheological information from capillary velocimetry experiments coupled with pressure drop measurements. The method is based on rescaling the velocity profile with the wall shear stress. The stress-rescaled velocity curve generated depends only on the rheological properties of the fluid, and not on other experimental variables such as tube dimensions, flow rates or pressure drops employed, thus providing a direct means for rheological characterization. We also present transformed functions of the rescaled velocity that facilitate the interpretation of the rheological information via plots that resemble conventional rheograms. In contrast with previous data processing methods that require differentiation, model fitting, or smoothing, the proposed rescaling approach does not require the introduction of additional data processing parameters, such as smoothing factors, that may affect reproducibility of the results. The rescaling method should be useful for robust measurements i...