Capillarity is commonly used for fluid transport in microfluidic devices. The capillary flow can be divided into three different flow regimes: entry regime, Poiseuille regime, and surface tension regime as shown in Fig. 1[1]. Generally, it is anticipated that at the entrance of any narrow confinement, the flow goes through entrance flow regime. For capillary flow, this entrance regime has generally been neglected in the literature. Beyond this entrance regime, the flow attains the fully developed velocity profile across the channel, which is termed as a Poiseuille flow. Moreover, in the capillary flow, the interface is always under traction — due to the capillary forces and hence, a third flow regime needs to be considered behind the interface which is referred as the surface tension regime. These regimes are yet to be experimentally explored and analyzed. An “in-house” developed μ-PIV system is used to quantify the flow field at the liquid/air interface (surface tension regime) in a rectangular glass microchannel of dimension 1.5 mm (width) × 500 μm (depth). The magnitude of velocity and the flow front evolution along the microchannel is calculated utilizing commercially available image processing software. Figure 2 shows the μ-PIV experimental setup used here. The main components of the experimental setup include an imaging device, magnification optics, and a continuous laser source (473 nm) in back illumination mode. The fluorescent particle of 1.9 m in diameter with DI water is used as a working fluid. The concentration of the microparticles is very less which is approximately 1%, therefore the effect of microbead concentration on the wetting properties is considered to be negligible for the present study. Moreover, it is assumed that the surface properties of the particles also do not affect the fluid flow. The capillary flow interface is captured and the corresponding processed images are presented to depict the velocity field at the liquid/air interface. The enlarged view of the microchannel cross section is shown in Fig. 2. The section A-A is the location at which the images are captured for the analysis.

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