Taylor flow is a strategy-aimed flow to transfer conventional single-phase into a more efficient two-phase flow resulting in an enhanced momentum/heat/mass transfer rate, as well as a multitude of other advantages. To date, Taylor flow has focused on the processes involving gas–liquid and liquid–liquid two-phase systems in microchannels over a wide range of applications in biomedical, pharmaceutical, industrial, and commercial sectors. Appropriately micro-structured design is, therefore, a key consideration for equipment dealing with transport phenomena. This review paper highlights the hydrodynamic aspects of gas–liquid and liquid–liquid two-phase flows in microchannels. It covers state-of-the-art experimental and numerical methods in the literature for analyzing and simulating slug flows in circular and non-circular microchannels. The review’s main objective is to identify the considerable opportunity for further development of microflows and provide suggestions for researchers in the field. Available correlations proposed for the transition of flow patterns are presented. A review of the literature of flow regime, slug length, and pressure drop is also carried out.

Experimental flow visualization is a valuable tool for analyzing microfluidics and nanofluidics in a wide variety of applications. Since the late 1990s, considerable advances in optical methods and image postprocessing techniques have improved direct optical measurements, resulting in an accurate qualitative and quantitative understanding of transport phenomena in lab-on-a-chip capillaries. In this study, a comparison of different optical measurement techniques is presented. The state-of-the-art development of particle image velocimetry (PIV) to date, particularly in microscale applications, is reviewed here in detail. This study reviews novel approaches for estimating velocity field measurements with high precision within interrogation windows. Different regularization terms are discussed to demonstrate their capability for particle displacement optimization. The discussion shows how single- and multi-camera optical techniques provide two-dimensional and three-component velocity fields. The performance of each method is compared by highlighting its advantages and limitations. Finally, the feasibility of micro resolution PIV in bioapplications is overviewed.

This study experimentally and numerically investigates the hydrodynamic characteristics and heat transfer of developing and fully developed laminar liquid–liquid Taylor flows. The problem is conducted in circular mini-channels with different diameters subjected to a constant wall temperature boundary condition. An experimental setup is designed employing an open-loop water/oil two-phase nonboiling flow at mini-scale tubing sizes of 1.42, 1.52, and 1.65 mm. Two silicone oils with the dynamic viscosities of 1 and 5 cSt at several volumetric flow rates are used to establish segmented flow. The impacts of the channel diameter, viscosity, and flow rate ratio on the flow pattern, pressure drop, film thickness, and heat transfer rate are discussed. In good agreement with the literature, it is found that the pressure drop generated by the interface increases the total pressure loss by up to 200% compared to the single-phase flow. The results also explain how recirculating regions within the slugs influence the film region and the physics of backflow. Furthermore, introducing segmented water slugs significantly enhances the heat transfer rate as the dimensionless thermal length decreases. A significant relation between the recirculating regions and heat transfer has been demonstrated for the first time.