Adam Raegen

Nanoparticle Flotation Collectors: Mechanisms Behind a New Technology

This is the first report describing a new technology where hydrophobic nanoparticles adsorb onto much larger, hydrophilic mineral particle surfaces to facilitate attachment to air bubbles in flotation. The adsorption of 46 nm cationic polystyrene nanoparticles onto 43 μm diameter glass beads, a mineral model, facilitates virtually complete removal of the beads by flotation. As little as 5% coverage of the bead surfaces with nanoparticles promotes high flotation efficiencies. The maximum force required to pull a glass bead from an air bubble interface into the aqueous phase was measured by micromechanics. The pull-off force was 1.9 μN for glass beads coated with nanoparticles, compared to 0.0086 μN for clean beads. The pull-off forces were modeled using Scheludkos classical expression. We propose that the bubble/bead contact area may not be dry (completely dewetted). Instead, for hydrophobic nanoparticles sitting on a hydrophilic surface, it is possible that only the nanoparticles penetrate the air/water interface to form a three-phase contact line. We present a new model for pull-off forces for such a wet contact patch between the bead and the air bubble. Contact angle measurements of both nanoparticle coated glass and smooth films from dissolved nanoparticles were performed to support the modeling.

Surface plasmon resonance imaging of the enzymatic degradation of cellulose microfibrils

We present the first study of the interaction of a cellulase enzyme mixture with cellulose microfibrils using surface plasmon resonance (SPR) imaging. The cellulose microfibrils, obtained from the bacterium Acetobacter xylinum, were heterogeneously distributed on a thin layer of thioglucose deposited onto a gold film. SPR images collected as a function of time allowed us to observe the adsorption of the enzymes onto both the cellulose microfibrils and the bare surface, and the subsequent degradation of the cellulose microfibrils in real time. In particular, we were able to characterize the decrease in the thickness and variations in thickness of the cellulose microfibril-coated regions with time, and to define a characteristic time for the removal of cellulose from the surface. These results demonstrate the distinct advantage of the SPR imaging technique for measuring the effectiveness of enzymes on cellulose microfibrils and provide useful metrics of enzyme activity that are of relevance to the cellulosic ethanol industry.

Advances in Surface Plasmon Resonance Imaging Enable Quantitative Tracking of Nanoscale Changes in Thickness and Roughness

To date, detailed studies of the thickness of coatings using surface plasmon resonance have been limited to samples that are very uniform in thickness, and this technique has not been applied quantitatively to samples that are inherently rough or undergo instabilities with time. Our manuscript describes a significant improvement to surface plasmon resonance imaging (SPRi) that allows this sensitive technique to be used for quantitative tracking of the thickness and roughness of surface coatings that are rough on the scale of tens of nanometers. We tested this approach by studying samples with an idealized, one-dimensional roughness: patterned channels in a thin polymer film. We find that a novel analysis of the SPRi data collected with the plane of incidence parallel to the patterned channels allows the determination of the thickness profile of the channels in the polymer film, which is in agreement with that measured using atomic force microscopy. We have further validated our approach by performing SPRi measurements perpendicular to the patterned channels, for which the measured SPR curve agrees well with the single SPR curve calculated using the average thickness determined from the thickness profile as determined using AFM. We applied this analysis technique to track the average thickness and RMS roughness of cellulose microfibrils upon exposure to cellulolytic enzymes, providing quantitative determinations of the times of action of the enzymes that are of direct interest to the cellulosic ethanol industry.