Marangoni effect on compound droplet inside bifurcated micro-confinement

Abstract

The combination of pressure-driven flow and thermocapillary forces greatly affects droplet dynamics in microfluidic environments, which has substantial implications for droplet-based technologies. In this work, we have investigated the thermocapillary migration and deformation of a single droplet subjected to a continuous pressure-driven flow in a bifurcated Y-shaped microchannel. In this study, the combined impacts of surface wettability, viscosity ratio, surface tension fluctuation, and externally imposed temperature gradient have been the main focus. Using energy equations and numerical simulations based on Navier–Stokes, a comprehensive parametric analysis has been conducted, taking into account all appropriate interfacial boundary conditions. The outcome demonstrates that a greater temperature gradient exacerbates thermocapillary-driven motion, causing droplets to visibly migrate toward areas with less surface tension. The ratio of the droplet's viscosity to that of the surrounding fluid is discovered to significantly change the droplet's deformation and velocity. The high-viscosity droplets have been found to prevent thermocapillary-induced stretching. The study shows that surface wettability has a significant effect on the contact angle dynamics and the droplet's attachment to the channel walls. The research highlights how the variation in temperature gradient impacts the timing and position of droplet pinch-off. The effect of Marangoni flow has been studied here, and it has been found that as the Marangoni number (Ma = 0–0.4) increases, the droplet migrates toward the warmer areas with the pinch-off point moving toward the region of reduced surface tension. It alters the droplet's separation dynamics completely. Generally, at low capillary numbers (Ca = 0.01–0.15), the deformation of the droplet is minimum, but beyond a critical value, the droplet experiences an asymmetric deformation and displacement. This is due to the effect of Marangoni forces. This work examines the hydrodynamics of compound droplets in a bifurcated microchannel in a novel manner. This includes the combined impacts of capillary number, viscosity ratio, Marangoni number, and pressure-driven flow. The complex interactions between core and shell interfacial tensions are captured simultaneously. Marangoni-driven surface tension gradients and bifurcation-induced asymmetry have been studied primarily on single droplets or straight-channel configurations. By exposing previously unknown regimes where Marangoni stresses may either enhance or suppress droplet deformation and splitting, the results offer novel design insights for precise droplet manipulation in microfluidic applications. These findings may prove beneficial in targeted material delivery, biological diagnostics, and lab-on-a-chip devices.

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