Microfluidic mixing and jetting devices based on SU8 and glass for time-resolved molecular imaging experiments

The development of ultrafast X-ray free-electron laser (XFEL) sources and third-generation synchrotrons has opened many new horizons for the study of complex molecular structures and their reaction kinetics. An essential element of these types of experiment is the method used for sample delivery. Microfluidics technology provides the ideal platform for performing these types of measurements since it enables control, manipulation and delivery of small volumes of fluid inside microchannels. Several key functions including mixing, particle separation, and injection, can be integrated on a single chip making the technology very attractive for use in Xray characterisation of molecular dynamics. Key challenges however, in using microfluidics to both mix and deliver samples, include chemical inertness and mechanical stability of the devices, particularly at micron length scales. Here we report a repeatable method for fabricating microfluidic mixer-jet devices based on photolithography and SU8 with a glass substrate. In experiments we have shown that these devices can withstand the high gas pressures required to produce stable, long-range, liquid jets. Coupled with their chemical inertness and reproducibility this makes them promising candidates for time-resolved X-ray diffraction measurements of molecular dynamics. Incorporating an integrated serpentine micromixer capable of homogeneous mixing prior to the liquid jet the devices presented here can be applied to the study of the dynamics of chemically driven biomolecular reactions. The focus of the current work is on the experimental characterization of the mixer through analysis of the concentration profiles along the length of the serpentineshaped microchannel.