Anthony Yeung

Stabilization of Oil-Water Emulsions by Hydrophobic Bacteria

ABSTRACT Formation of oil-water emulsions during bacterial growth on hydrocarbons is often attributed to biosurfactants. Here we report the ability of certain intact bacterial cells to stabilize oil-in-water and water-in-oil emulsions without changing the interfacial tension, by inhibition of droplet coalescence as observed in emulsion stabilization by solid particles like silica.

Micropipette: a new technique in emulsion research

Abstract Micron-scale studies on emulsions have, to date, been largely limited to the imaging of colloidal structures. In this communication, micropipette techniques are introduced as a progression beyond mere visualization: using small suction pipettes, mechanical experiments are performed on individual emulsion drops, from which interfacial properties can be deduced. To demonstrate this technique, the interfacial tension, emulsion stability and adsorption characteristics are directly assessed at the surfaces of micron-sized water droplets that are dispersed in crude oil.

Efficient Micromixing Using Induced-Charge Electroosmosis

Lab-on-a-chip (LOC) devices which utilize electrokinetics for fluid transport are invariably poor mixers due to the nature of low-Reynolds-number flows. For many such devices, efficient mixing is needed for fast analysis, but the predominant mechanism of equalizing concentration differences is often diffusion-a relatively slow form of mass transfer. In this numerical study, we propose a novel micromixer design which utilizes the recent concept of induced-charge electroosmosis for enhancing fluid mixing. As validation, it is shown that numerical simulations of fluid flow in the proposed system are in good agreement with analytical solutions available for electrokinetic flow and electrokinetic mixing in traditional microchannels. The conventional mixing performance index is modified so that it accounts for the length required for desired mixing as well as the concentration gradients across the channel width. With the help of the modified mixing index, the proposed mixer is compared with the traditional mixer design and found to be superior in performance. Furthermore, the effects of design parameters on mixing performance are analyzed for possible device implementation.

Induced charge electro osmotic mixer: Obstacle shape optimization.

Efficient mixing is difficult to achieve in miniaturized devices due to the nature of low Reynolds number flow. Mixing can be intentionally induced, however, if conducting or nonconducting obstacles are embedded within the microchannel. In the case of conducting obstacles, vortices can be generated in the vicinity of the obstacle due to induced charge electro-osmosis (ICEO) which enhances mixing of different streams: the obstacle shape affects the induced zeta potential on the conducting surface, which in turn influences the flow profile near the obstacle. This study deals with optimization of the geometric shape of a conducting obstacle for the purpose of micromixing. The obstacle boundary is parametrically represented by nonuniform rational B-spline curves. The optimal obstacle shape, which maximizes the mixing for given operating conditions, is found using genetic algorithms. Various case studies at different operating conditions demonstrated that the near right triangle shape provides optimal mixing in the ICEO flow dominant regime, whereas rectangular shape is the optimal shape in diffusion dominant regime. The tradeoff between mixing and transport is examined for symmetric and nonsymmetric obstacle shapes.

Immobilization of aminoglycosidic aminocyclitols antibiotic onto soap-free poly(MMA-EA-AA) latex particles

Monodispersed soap-free poly(MMA-EA-AA) latex particles with surface carboxyl groups were synthesized by emulsion polymerization of methyl methacrylate (MMA), ethyl acrylate (EA) and acrylic acid (AA) in aqueous medium, and streptomycin sulfate (SMS) was immobilized onto these particles using three different methods. A model experiment was designed to test the feasibility of the reaction between the carboxyl groups of polymer and the amino groups of the medicine. The covalent coupling between the latex particles and the medicine was confirmed by XPS. Results showed that the medicine molecules were located on the particle surface after immobilization, and the coupling efficiency of SMS in pre-adsorption method was higher than that in direct method. The highest coupling efficiency of this medicine was achieved using the spacer-arm method. It was demonstrated that the immobilized medicine had similar antimicrobial activity as the free form using Escherichia coli as an evaluating organism.

Analysis of Electrokinetic Mixing Techniques Using Comparative Mixing Index

Abstract: The performance of micro-mixers is evaluated in terms of deviations from perfectly mixed state and mixing length ( i.e. , device length required to achieve perfect mixing). Different variations of T-mixer are reported for improved mixing performance, including geometric constrictions/obstacles embedded in the channel wall, heterogeneously charged walls, grooves on channel base, cet . Most of the reported designs provide improved mixing at the expense of reduced flow rate; there exists therefore a tradeoff between mixing and transport. The reduced flow rate, which affects species residence time, is unfortunately not taken into account in most micro-mixing performance analyses. This issue is addressed by the comparative mixing index (CMI), which evaluates mixing performance more appropriately by normalizing the effect of residence time among different designs. In this study, the performance of several mixing strategies are evaluated based on the CMI; these are mixer designs that incorporate (a) physical constrictions, (b) induced charge electro-osmotic (ICEO) effects, and (c) heterogeneously charged walls. The present analysis clearly identifies conditions under which a given mixer design is superior to a T-mixer.

A small point regarding DLVO coagulation conditions.

Criteria for colloidal destabilization that we had imposed in earlier studies (Yeung et al., 2003 [1] and Esmaeili et al., 2012 [2]) are re-examined. Those conditions are shown here to be incorrect, but with only negligible error. We also note a minor issue with the widely-accepted condition of coagulation that is seen in almost all colloids literature.

Induced charge electro-osmotic concentration gradient generator

Biomolecule gradients play an important role in the understanding of various biological processes. Typically, biological cells are exposed to linear and nonlinear concentration gradients and their response is studied for understanding cell growth, cell migration, and cell differentiation mechanisms. Recent studies have demonstrated the use of microfluidic devices for precise and stable concentration gradient generation. However, most of the reported devices are geometrically complex and lack dynamic controllability. In this work, a novel microfluidic gradient generator is presented which utilizes the induced charge electro-osmosis (ICEO) by introducing conducting obstacle in the microchannel. With the ICEO flow component, significant transverse convection can be generated within the microchannel, which can, in turn, be used to create nonlinear as well as asymmetric gradients. The characteristics of the developed concentration gradient are dependent on the interplay between fixed charge electro-osmotic and ICEO flows. It is shown that the proposed device can switch between linear and nonlinear gradients by just altering the applied electric field. Finally, the formation of user-defined concentration profiles (linear, convex, and concave) is demonstrated by varying the conducting obstacle size.