Abstract
In microchannel fluid flow, understanding of the liquid-gas interface behavior is vital for developing a wide range of microfluidic devices. The dynamic contact angle of the liquid-air meniscus varies with its velocity and the ensuing meniscus shape has profound effect on the local transport characteristics in its vicinity. Depending on the application, dynamic menisci shapes eventually control the momentum, heat and mass transport coefficients in two-phase microchannel flow geometries, where such conditions are often encountered. To better understand the effect of dynamic contact angle on meniscus shape, high speed visualization of menisci of four different liquids (water, ethanol, glycerin and silicon oil) has been undertaken at different Capillary numbers. Quantitative information of the velocity field and its distribution near the moving liquid-air interface has been done using micro-PIV measurements in a 1 mm × 1 mm dry square capillary having deionized water as the working fluid. This provides vital information on the local flow transport characteristics (two-dimensional velocity fields on a longitudinal plane) in the wake of the meniscus. To augment and complement the study, three-dimensional simulation of the flow field near the liquid-air meniscus has also been performed on Comsol ® , applying the two-phase flow level-set method. The results clearly demonstrate that, while the u-velocity profile in the liquid domain is parabolic (Poiseuille-type flow) in nature away from the interface, it drastically changes (become flatter) as we approach the meniscus. Close to the meniscus the flow becomes three-dimensional with both v and w velocities showing a double-vortex, the strength of the latter being lower than the former. This observation is clearly noted both by PIV data as well as the simulations. The characteristic of the flow field in the meniscus wake is the most important parameter which affects the viscous stress generated due to the meniscus motion. The study reveals that controlling the wettability of the liquid can be an effective tool to control the overall transport behavior of the moving confined meniscus.
Highlights
Transport phenomenon at liquid-air interface inside microscale devices has emerged as an important research field due to its relevance in many upcoming systems and devices
The simulations reveal that controlling the wettability of the liquid can be an effective tool to control the viscous shear and the pressure drop required to move the meniscus; the meniscus region of liquids with comparatively higher wettability manifests a stronger shear stress as compared to a non-wetting liquid meniscus moving at the same Capillary number
Results obtained both experimentally as well as by simulation, are reported hereunder in the following sequence: (i) Shape of liquid-air meniscus formed inside the tube at different flow Capillary numbers when dynamic steady-state motion of the meniscus is achieved inside the capillary tube, (ii) Fluid flow field obtained by micro-PIV close to meniscus at different flow Capillary numbers, and, (iii) Parametric study highlighting the effect of wettability of the liquidtube material combination on the near-meniscus hydrodynamics
Summary
Transport phenomenon at liquid-air interface inside microscale devices has emerged as an important research field due to its relevance in many upcoming systems and devices. The three-dimensionality of the flow in close proximity of the meniscus needs to be understood for proper design equations for the viscocapillary dissipation and resulting local transport In this background, μ-PIV study has been carried in the liquid domain (D.I. water) of a moving liquid-air meniscus inside a dry square glass capillary tube of size 1 mm x 1 mm. It must be noted that the dynamic contact angle is a function of the Capillary number of the meniscus and depending on the flow condition, which includes the effect of wettability, the shape of the meniscus changes, affecting the flow field close to it This has been scrutinized by high speed visualization in conjunction with the PIV flow field measurements, to understand the mechanism of enhancement of local and average transport coefficient. For validation and benchmarking of the data away from the interface, the following analytical solution for fully developed velocity profile inside a square channel (Eq 1), as given by Bruus (2008), has been used, Fig. 1 Schematic diagram of the videography and μ-PIV setup for fluid flow velocity measurement inside glass capillary tubes of size 1 mm × 1 mm
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