Pressure Drop Measurements in Microfluidic Devices: A Review on the Accurate Quantification of Interfacial Slip

The correct theoretical definition of boundary conditions of flow underpins all fluid dynamics studies, and is particularly important in situations in which the flow is confined on the nano- and micro-scale. Microfluidic devices are an excellent platform to measure boundary flow conditions, and the pressure drop versus flow rate method is particularly useful in detecting evidence of microscale interfacial slip and drag reduction. This review focuses on the pressure drop method, identifying the main experimental parameters affecting the accuracy and reproducibility of microfluidic experiments of slip, quantifying the magnitude and source of common errors, and providing practical solutions and guidelines. A summary of literature results of interfacial slip obtained with pressure drop measurements in microfluidic devices is also provided, and the slip results are directly compared to expected slip models. This review serves as an introduction for new researchers moving into the field of interfacial slip, and as reminder for established researchers of the need to create highly controlled experimental procedures in order to obtain reproducible and reliable measurements of boundary flow conditions. A direct comparison of accurate experiments with theoretical models is bound to bring about clarity about the mechanisms of slip on smooth and structured surfaces.