Droplet coalescence and phase separation in a topical ointment: Effects of fluid shear and temperature.

The physical stability of a prototypical pharmaceutical topical ointment, consisting primarily of an emulsion of propylene glycol droplets dispersed in a continuous white petrolatum medium, was studied with regard to droplet size growth and phase separation when the ointment undergoes heating or fluid shear. To investigate the effects of shear, the ointment at 32 °C was sheared using a transparent, narrow-gap, temperature-controlled Taylor-Couette flow apparatus operated under laminar flow conditions which provided approximately uniform shear rates. Optical methods based on microscopy were used to obtain in-situ, time-dependent propylene glycol droplet size distributions, while a wide-field lens and camera were simultaneously used to detect gross phase separation as the ointment was sheared.

Simulation of algal photobioreactors: recent developments and challenges.

Widespread cultivation of phototrophic microalgae for sustainable production of a variety of renewable products, for wastewater treatment, and for atmospheric carbon mitigation requires not only improved microorganisms but also significant improvements to process design and scaleup. The development of simulation tools capable of providing quantitative predictions for photobioreactor performance could contribute to improved reactor designs and it could also support process scaleup, as it has in the traditional petro-chemical industries. However, the complicated dependence of cell function on conditions in the microenvironment, such as light availability, temperature, nutrient concentration, and shear strain rate render simulation of photobioreactors much more difficult than chemical reactors.

Multiphysics simulation of algal growth in an airlift photobioreactor: Effects of fluid mixing and shear stress.

A multiphysics model has been developed to predict the effects of fluid mixing and shear stress on microalgal growth in an airlift photobioreactor. The model integrates multiphase flow dynamics, radiation transport, shear stress, and algal growth kinetics using an Eulerian approach. The model is first validated by comparing its predictions with experimental data, and then the radiation transport and algal growth kinetics submodels are added to predict biomass accumulation under different flow conditions.

Comprehensive computational model for combining fluid hydrodynamics, light transport and biomass growth in a Taylor vortex algal photobioreactor: Lagrangian approach.

A comprehensive quantitative model incorporating the effects of fluid flow patterns, light distribution, and algal growth kinetics on biomass growth rate is developed in order to predict the performance of a Taylor vortex algal photobioreactor for culturing Chlorella vulgaris. A commonly used Lagrangian strategy for coupling the various factors influencing algal growth was employed whereby results from computational fluid dynamics and radiation transport simulations were used to compute numerous microorganism light exposure histories, and this information in turn was used to estimate the global biomass specific growth rate. The simulations provide good quantitative agreement with experimental data and correctly predict the trend in reactor performance as a key reactor operating parameter is varied (inner cylinder rotation speed).

Characteristic time scales of mixing, mass transfer and biomass growth in a Taylor vortex algal photobioreactor.

Recently it has been demonstrated that algal biomass yield can be enhanced using fluid flow patterns known as Taylor vortices. It has been suggested that these growth rate improvements can be attributed to improved light delivery as a result of rapid transport of microorganisms between light and dark regions of the reactor. However, Taylor vortices also strongly impact fluid mixing and interphase (gas-liquid) mass transport, and these in turn may also explain improvements in biomass productivity.

Simulation of photosynthetically active radiation distribution in algal photobioreactors using a multidimensional spectral radiation model.

A numerical method for simulating the spectral light distribution in algal photobioreactors is developed by adapting the discrete ordinate method for solving the radiative transport equation. The technique, which was developed for two and three spatial dimensions, provides a detailed accounting for light absorption and scattering by algae in the culture medium. In particular, the optical properties of the algal cells and the radiative properties of the turbid culture medium were calculated using a method based on Mie theory and that makes use of information concerning algal pigmentation, shape, and size distribution.

Enhanced algal growth rate in a Taylor vortex reactor.

The rate of production of algal biomass in optically dense photobioreactors depends crucially on the temporal light exposure of microorganisms, which in turn is determined by fluid flow patterns and the quantity and spatial distribution of photosynthetically active radiation. In this report it is demonstrated that highly organized and robust toroidal flow structures known as Taylor vortices cause significant increases in the rate of biomass production, efficiency of light utilization, and CO2 uptake, and these effects become more pronounced at higher Reynolds numbers. In light of these findings and previously reported experiments using Taylor vortex flow to culture algae, it is argued that the flashing light effect, rather than mass transport effects, is responsible for the observed increases in the rate of photosynthesis.

Thickness of residual wetting film in liquid-liquid displacement.

Core-annular flow is common in nature, representing, for example, how streams of oil, surrounded by water, move in petroleum reservoirs. Oil, typically a nonwetting fluid, tends to occupy the middle (core) part of a channel, while water forms a surrounding wall-wetting film. What is the thickness of the wetting film? A classic theory has been in existence for nearly 50 years offering a solution, although in a controversial manner, for moving gas bubbles.

Molecular dynamics simulation of fractal aggregate diffusion.

The diffusion of fractal aggregates constructed with the method by Thouy and Jullien [J. Phys. A 27, 2953 (1994)] comprised of N(p) spherical primary particles was studied as a function of the aggregate mass and fractal dimension using molecular dynamics simulations.

On equilibrium solutions of aggregation-fragmentation problems.

A characteristic feature of particulate systems that evolve due to competition between aggregation and breakage is that they sometimes produce non-trivial steady-state particle size distributions. If such solutions satisfy detailed balance conditions, then they are equilibrium solutions. The conditions that must be satisfied by aggregation and fragmentation rate kernels in order for equilibrium solutions to be produced are elaborated, and it is shown that the rate kernels are uniquely determined by the aggregation and breakage rate constants for the reactions involving monomers.

The effect of shear on colloidal aggregation and gelation studied using small-angle light scattering.

In situ light scattering measurements were performed to investigate the effect of low shear rates (0.13-3.56 s(-1)) on an aggregating colloidal system made of 20 nm polystyrene particles.

Destructive aggregation: aggregation with collision-induced breakage.

Mean-field population balance equations are used to describe the evolution of particle size distributions in a wide variety of systems undergoing simultaneous aggregation and breakage. In this paper we develop a population balance that includes aggregation combined with collision-induced particle breakage for arbitrary fragment distribution functions, provided that this distribution function depends only on the total mass of the particles undergoing a collision. We then develop a specific distribution function for arbitrary two-body collisions by postulating that each collision produces a transition-state aggregate having the morphology of a linear polymer.

Pore-scale study of nonaqueous phase liquid dissolution in porous media using laser-induced fluorescence.

Little quantitative, experimental pore-scale information exists regarding nonaqueous phase liquid (NAPL) ganglia undergoing dissolution in porous media. By using refractive index matched fluids and porous media, we have been able to nonintrusively visualize NAPL dissolution (at constant capillary numbers) in three dimensions using laser-induced fluorescence. The results provide dynamic, quantitative information regarding ganglia surface area, volume, position, and shape.

Vibration-induced mobilization of trapped oil ganglia in porous media: experimental validation of a capillary-physics mechanism.

The development of methods for mobilizing residual organic liquids trapped in porous media is becoming increasingly important as world demand for oil increases and because of the need to remediate aquifers degraded by slow-dissolving organic contaminants. Low-frequency elastic wave stimulation is one such technique, but until recently the lack of a mechanistic understanding of the effects of vibration on mobilization of oil ganglia has prevented the method from being applied predictably in the field. Recently, a simple capillary-physics mechanism has been developed to explain vibration-induced mobilization of a trapped non-wetting organic phase in porous media.

CFD simulation of shear-induced aggregation and breakage in turbulent Taylor-Couette flow.

An experimental and computational investigation of the effects of local fluid shear rate on the aggregation and breakage of approximately 10 microm latex spheres suspended in an aqueous solution undergoing turbulent Taylor-Couette flow was carried out. First, computational fluid dynamics (CFD) simulations were performed and the flow field predictions were validated with data from particle image velocimetry experiments. Subsequently, the quadrature method of moments (QMOM) was implemented into the CFD code to obtain predictions for mean particle size that account for the effects of local shear rate on the aggregation and breakage.

Quadrature method of moments for aggregation-breakage processes.

Investigation of particulate systems often requires the solution of a population balance, which is a continuity statement written in terms of the number density function. In turn, the number density function is defined in terms of an internal coordinate (e.g.