Publications

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This document summarizes a three year Laboratory Directed Research and Development (LDRD) program effort to improve our understanding of algal flocculation with a key to overcoming harvesting as a techno-economic barrier to algal biofuels. Flocculation is limited by the concentrations of deprotonated functional groups on the algal cell surface. Favorable charged groups on the surfaces of precipitates that form in solution and the interaction of both with ions in the water can favor flocculation. Measurements of algae cell-surface functional groups are reported and related to the quantity of flocculant required. Deprotonation of surface groups and complexation of surface groups with ions from the growth media are predicted in the context of PHREEQC. The understanding of surface chemistry is linked to boundaries of effective flocculation. We show that the phase-space of effective flocculation can be expanded by more frequent alga-alga or floc-floc collisions. The collision frequency is dependent on the floc structure, described in the fractal sense. The fractal floc structure is shown to depend on the rate of shear mixing. We present both experimental measurements of the floc structure variation and simulations using LAMMPS (Large-scale Atomic/Molecular Massively Parallel Simulator). Both show a densification of the flocs with increasing shear. The LAMMPS results show a combined change in the fractal dimension and a change in the coordination number leading to stronger flocs.

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A thin flow-focusing microfluidic channel is evaluated for generating monodisperse liquid droplets. The microfluidic device is used in its native state, which is hydrophilic, or treated with OTS to make it hydrophobic. Having both hydrophilic and hydrophobic surfaces allows for creation of both oil-in-water and water-in-oil emulsions, facilitating a large parameter study of viscosity ratios (droplet fluid/continuous fluid) ranging from 0.05 to 96 and flow rate ratios (droplet fluid/continuous fluid) ranging from 0.01 to 2 in one geometry. The hydrophilic chip provides a partially-wetting surface (contact angle less than 90°) for the inner fluid. This surface, combined with the unusually thin channel height, promotes a flow regime where the inner fluid wets the top and bottom of the channel in the orifice and a stable jet is formed. Through confocal microscopy, this fluid stabilization is shown to be highly influenced by the contact angle of the liquids in the channel. Non-wetting jets undergo breakup and produce drops when the jet is comparable to or smaller than the channel thickness. In contrast, partially-wetting jets undergo breakup only when they are much smaller than the channel thickness. Drop sizes are found to scale with a modified capillary number based on the total flow rate regardless of wetting behavior. © The Royal Society of Chemistry.

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The effects of algae concentration, ferric chloride dose, and pH on the flocculation efficiency of the freshwater algae Chlorella zofingiensis can be understood by considering the nature of the electrostatic charges on the algae and precipitate surfaces. Two critical conditions are identified which, when met, result in flocculation efficiencies in excess of 90% for freshwater algae. First, a minimum concentration of ferric chloride is required to overcome the electrostatic stabilization of the algae and promote bridging of algae cells by hydroxide precipitates. At low algae concentrations, the minimum amount of ferric chloride required increases linearly with algae concentration, characteristic of flocculation primarily through electrostatic bridging by hydroxide precipitates. At higher algae concentrations, the minimum required concentration of ferric chloride for flocculation is independent of algae concentration, suggesting a change in the primary flocculation mechanism from bridging to sweep flocculation. Second, the algae must have a negative surface charge. Experiments and surface complexation modeling show that the surface charge of C. zofingiensis is negative above a pH of 4.0±0.3 which agrees well with the minimum pH required for effective flocculation. These critical flocculation criteria can be extended to other freshwater algae to design effective flocculation systems. © 2011 Wiley Periodicals, Inc.

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We examine several methods to create a sheet of magnesium oxide (MgO) macroporous ceramic material via tape casting. These methods include the approach pioneered by Akartuna et al. in which an oil/water emulsion is stabilized by surface-modified metal oxide particles at the droplet interfaces. Upon drying, a scaffold of the self-assembled particles is strong enough to be removed from the substrate material and sintered. We find that this method can be used with MgO particles surface modified by short amphiphilic molecules. This approach is compared with two more traditional methods to induce structure into a green ceramic: 1) creation of an MgO ceramic slip with added pore formers, and 2) sponge impregnation of a reticulated foam with the MgO slip. Green and sintered samples made using each method are hardness tested and results compared for several densities of the final ceramics. Optical and SEM images of the materials are shown.

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