Dynamic Flow Characteristics and Design Principles of Laminar Flow Microbial Fuel Cells.

Laminar flow microbial fuel cells (MFCs) are used to understand the role of microorganisms, and their interactions with electrodes in microbial bioelectrochemical systems. In this study, we reported the flow characteristics of laminar flow in a typical MFC configuration in a non-dimensional form, which can serve as a guideline in the design of such microfluidic systems. Computational fluid dynamics simulations were performed to examine the effects of channel geometries, surface characteristics, and fluid velocity on the mixing dynamics in microchannels with a rectangular cross-section. The results showed that decreasing the fluid velocity enhances mixing but changing the angle between the inlet channels, only had strong effects when the angle was larger than 135°. Furthermore, different mixing behaviors were observed depending on the angle of the channels, when the microchannel aspect ratio was reduced. Asymmetric growth of microbial biofilm on the anode side skewed the mixing zone and wall roughness due to the bacterial attachment, which accelerated the mixing process and reduced the efficiency of the laminar flow MFC. Finally, the magnitude of mass diffusivity had a substantial effect on mixing behavior. The results shown here provided both design guidelines, as well as better understandings of the MFCs due to microbial growth.

Prediction of Microdroplet Breakup Regime in Asymmetric T-Junction Microchannels.

Splitting droplets is becoming a major functional component in increasing number of droplet microfluidic applications, and there is an increasing interest in splitting droplets into two daughter droplets with different volumes. However, designing an asymmetric droplet splitter and predicting how a droplet splits in such designs is not trivial. In this study, numerical simulations were conducted to study droplet breakup in asymmetric T-junctions of square cross-sections having different pressure gradient ratios (i.e. T-junctions with outlet branches of different lengths). The goal of the simulation is to identify the conditions where a parent droplet breaks or does not break into two smaller droplets of different sizes (so called critical condition) and to identify the important fluid and microchannel parameters in this process. Four modes of droplet breakup (primary-, transition-, bubble-, and non-breakups) are identified and an empirical correlation is introduced that can predict the breakup/non-breakup of the droplet based on the parent droplet size and the capillary number. The simulation results are then compared with experimental data to verify its accuracy and the effect of fluids properties on the proposed correlation are studied. Two major asymmetric breakup mechanisms are determined, namely "breakup with permanent obstruction" and "unstable breakup". The numerical results show that the splitting ratio for the asymmetric breakup mechanisms depends on flow conditions and dwell time of the droplet at the junction prior to splitting. Finally, the results from two-dimensional and three-dimensional simulations were compared. It is shown that two-dimensional simulation may not accurately predict the breakup behavior for asymmetric droplet breakup and viscosity ration has a greater effect on the prediction critical condition.