Active control of the depletion boundary layers in microfluidic electrochemical reactors.

In this paper, we describe three methods to improve the performance of pressure-driven laminar flow-based microreactors by manipulating reaction-depletion boundary layers to overcome mass transfer limitations at reactive surfaces on the walls, such as electrodes. The transport rate of the reactants to the reactive surfaces is enhanced by (i) removing the depleted zone through multiple periodically-placed outlets; (ii) adding fresh reactants through multiple periodically-placed inlets along the reactive surface; or (iii) producing a spiraling, transverse flow through the integration of herringbone ridges along the channel walls. For approaches (i) and (ii), the network of microfluidic channels needs to be designed such that under the operating conditions used the right amount of boundary layer at each outlet or inlet is removed or replenished, respectively. Here, we report a set of design rules, derived with the help of a fluidic resistance circuit model, to aid in the design of appropriate microfluidic networks. Also, the actual enhancement of the performance of the electrochemical microreactor, i.e. chemical conversion efficiency, using multiple inlets, multiple outlets, or herringbone ridges is reported.

Gravity-induced reorientation of the interface between two liquids of different densities flowing laminarly through a microchannel.

This paper experimentally quantifies the reorientation of the liquid-liquid interface between fluids of different densities flowing side-by-side in pressure-driven laminar flow in microchannels. A gravity-induced pressure mismatch at the interface will gradually drive the denser fluid to occupy the lower portion of the microchannel. The rate of this process is expected to depend on the interplay of viscous forces--which tend to dominate at the microscale-and inertial and gravitational forces. A correlation that relates the position of such a liquid-liquid interface to physical variables and channel dimensions was derived. The extent of reorientation of the streams was then related to two dimensionless numbers: Fr, the square root of the ratio of inertial to gravitational forces; and Re/Fr2, the ratio of gravitational to viscous forces. Further analysis showed that the reorientation of the streams depends only on the gravitational and viscous forces, but not inertia. The quantitative description of the position of the interface between liquids of different densities described in this paper aids in the rational design of the rapidly growing number of microchemical systems that utilize multistream laminar flow for performing spatially resolved chemistry and biology inside microfluidic channels.

Laminar flow-based electrochemical microreactor for efficient regeneration of nicotinamide cofactors for biocatalysis.

One of the long-standing challenges in biocatalysis is the search for methods to continuously regenerate essential cofactors such as NADH that would enable a wide range of enzymes to be used in the more environmentally friendly synthesis of chiral fine chemicals including pharmaceuticals, cosmetics, and food additives. This communication reports a microreactor-based cofactor regeneration method that exploits the microfluidic phenomenon of laminar flow: a reactant stream and a buffer stream are introduced in a microchannel and continue to flow side by side without turbulent mixing between two electrodes that cover opposing channel walls. Adjustment of the flow rate ratio of the two streams in laminar flow enables focusing of the reactant stream close to the cathode, thereby reversing a normally unfavorable reaction equilibrium essential for cofactor regeneration. The absence of a bulk phase in these microreactors prevents the undesired reverse reaction to take place, which has prevented the use of electrochemical cofactor regeneration in macroscale processes. Here, we demonstrate the regeneration of NADH with conversion efficiencies as high as 31%. We also show the subsequent in situ conversion of an achiral substrate, pyruvate, into a chiral product, l-lactate, within this microreactor.