Stagnation point flows in analytical chemistry and life sciences

Isolated microfluidic stagnation point flows-stagnation point flows formed within microfluidic interfaces-have come a long way as a tool for characterizing materials, trapping/manipulating micro particles, and generating confined flows and localized chemistries. Early applications of these fluidic stagnation flows focused on characterizing emulsions and polymers. However, in recent times, these flows have taken microfluidic forms to expand the range of applications to single-cell analysis, substrate patterning, and chip-integrated devices, amongst others. This article focuses on reviewing the microfluidics literature to highlight the concept behind isolated microfluidic stagnation point flows, underline different approaches of experimentally resolving them, and then look into their applications in chemistry and life sciences; with a specific focus on micro-total-analysis systems. The literature on this topic is approached historically; from the isolated fluidic stagnation point flow generated by the early four-roll mill to the currently predominating microfluidic stagnation point flows of the cross-slot device. Finally, isolated microfluidic stagnation flows produced by the relatively recent open space microfluidics-opposed jets, microfluidic probes and micropipettes-are critically analyzed and their applications are discussed. Ultimately, the goal of this article is to inform the scientific community on the constructive aspect of microfluidic stagnation flows by demonstrating their past, present, and future applicability in analytical chemistry and life sciences.