Some recent advances in the design and the use of miniaturized droplet-based continuous process: applications in chemistry and high-pressure microflows.

This mini-review focuses on two different miniaturizing approaches: the first one describes the generation and use of droplets flowing within a millifluidic tool as individual batch microreactors. The second one reports the use of high pressure microflows in chemistry. Millifluidics is an inexpensive, versatile and easy to use approach which is upscaled from microfluidics. It enables one to produce hierarchically organized multiple emulsions or particles with a good control over sizes and shapes, as well as to provide a convenient data acquisition platform dedicated to slow or rather fast chemical reactions, i.e., from hours to a few minutes. High-pressure resistant devices were recently fabricated and used to generate stable droplets from pressurized fluids such as supercritical fluid-liquid systems. We believe that supercritical microfluidics is a promising tool to develop sustainable processes in chemistry.

Chemical reaction imaging within microfluidic devices using confocal raman spectroscopy: the case of water and deuterium oxide as a model system.

Microfluidic devices face presently a tremendous interest, especially for the development of labs-on-a-chip systems. One of the primary challenges for such applications is the ability to perform local chemical detection and analysis from various species. In this paper, we investigate the use of confocal Raman spectroscopy from both qualitative and quantitative sides, to obtain spatially resolved concentration maps of chemically reactive fluids flowing in different channels networks. As a model chemical reaction, we used the isotopic exchange reaction between D(2)O and H(2)O, which is diffusion-controlled and whose equilibrium states exhibit distinct Raman signatures depending on the composition. Two types of chip technologies were studied, which are typical of those used for chemical kinetics investigations. In the first one, reagent mixing occurs by molecular interdiffusion of the two streams (H(2)O and D(2)O) flowing side by side in the same channel; in the second one, reagents are hosted in droplets moving in winding channels that enhance the mixing. In the first series of experiments, we were able to extract Raman images of H(2)O, D(2)O, and HOD concentrations in the main channel together with an estimate of an interdiffusion coefficient, and in the second one, we evidenced the influence of channel wiggles on mixing efficiency.

On-line laser Raman spectroscopic probing of droplets engineered in microfluidic devices.

Sub-nanolitre droplets engineered in microfluidic devices constitute ideal microreactors to investigate the kinetics of chemical reactions on the millisecond time scale. Up to date, fluorescence detection has been extensively used in chemistry and biology to probe reactants and resultant products within such nanodroplets. However, although fluorescence is a very sensitive technique, it lacks intrinsic specificity as frequently fluorescent labels need to be attached to the species of interest. This weakness can be overcome by using vibrational spectroscopy analysis. As an illustrative example, we use confocal Raman microspectroscopy in order to probe the concentration profiles of two interdiffusing solutes within nanolitre droplets transported through a straight microchannel. We establish the feasibility of the experimental method and discuss various aspects related to the space-time resolution and the quantitativeness of the Raman measurements. Finally, we demonstrate that the droplet internal molecular mixing is strongly affected by the droplet internal flow.