Application of droplet migration scaling behavior to microchannel flow measurements.

In confined channels in low Reynolds number flow, droplets drift perpendicular to the flow, moving across streamlines. The phenomenon has proven useful for understanding microfluidic droplet separation, drug delivery vehicle optimization, and single-cell genomic amplification. Particles or droplets undergo several migration mechanisms including wall migration, hydrodynamic diffusion, and migration down gradients of shear. In simple shear flow only wall migration and hydrodynamic diffusion are present. In parabolic flow, droplets also move down gradients of shear. The resulting separation depends on parameters including particle size and stiffness, concentration, and flow rate. Computational methods can incorporate these effects in an exact manner to predict margination phenomena for specific systems, but do not generate a descriptive parametric dependence. In this paper, we present a scaling model that elucidates the parametric dependence of margination on emulsion droplet size, volume fraction, shear rate and suspending fluid viscosity. We experimentally measure the droplet depletion layer of silicone oil droplets and compare the results to theoretical scaling behavior that includes hydrodynamic diffusion and wall migration with and without an added shear-gradient migration. Results demonstrate the viability and limitations of applying a simple scaling behavior to experimental systems to describe parametric dependence. Our conclusions open the possibility for parametric descriptions of migration with broad applicability to particle and droplet systems.

Investigation to Minimize Electrochemical Impedance Spectroscopy Drift.

Electrochemical impedance spectroscopy (EIS) is a technique used to characterize physiochemical processes, especially in the field of biosensors. However, EIS has been known to have reproducibility issues due to an inherent drift. When taking repeated measurements of the same exact solution using EIS, impedance measurements have an increasing trend which can be detailed by a linear slope. The reported EIS drift ranges from 0.11 to 5.5 Ω/min. We studied the drift to assist with future data interpretation and model fitting. We discovered the cleanliness and treatment of the working electrode effects EIS drift, and the minimization of the drift can occur by rinsing the working electrode in-between repeated runs.

Evaluation of Metal Oxide Surface Catalysts for the Electrochemical Activation of Amino Acids.

Electrochemical detection of amino acids is important due to their correlation with certain diseases; however, most amino acids require a catalyst to electrochemically activate. One common catalyst for electrochemical detection of amino acids are metal oxides. Metal oxide nanoparticles were electrodeposited onto glassy carbon and platinum working electrodes. Cyclic voltammetry (CV) experiments in a flow cell were performed to evaluate the sensors' ability to detect arginine, alanine, serine, and valine at micromolar and nanomolar concentrations as high as 4 mM. Solutions were prepared in phosphate buffer saline (PBS) and then 100 mM NaOH. Specifically, NiO surfaces were responsive to amino acids but variable, especially when exposed to arginine. Polarization resistance experiments and scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) data indicated that arginine accelerated the corrosion of the NiO catalyst through the formation of a Schiff base complex.

Proper Controls to Electrochemically Evaluate Carotenoids using ß-Cyclodextrin Modified Surfaces.

We initially tested the electrochemical activity of beta-carotene and lutein at unmodified glassy carbon electrodes. We found good sensitivity (1 nA/µM) at high, micromolar concentrations, but serum levels are at nanomolar concentrations. To enhance the electrochemical activity, we modified the sensor surface with ß-cyclodextrin, which has a hydrophobic core. Our goal was that the beta-carotene will be attracted to the ß-cyclodextrin core, increasing surface interaction and sensitivity. Instead we saw a decrease in electrochemical activity. Further investigation with a methylene blue mediator indicated two results. First, it is unlikely the beta-carotene strongly interacts with the ß-cyclodextrin surface. And, second, the presence of a co-solvent or surfactant can greatly disrupt the surface ß-cyclodextrin activity.