Coupled Thermocapillary–Wettability Effects on Droplet Splitting in a Y-Shaped Microchannel

Abstract This study numerically examines the coupled influence of surface wettability and thermocapillary forces on droplet splitting in a symmetric Y-shaped microchannel using a finite element-based level-set method. A temperature field is applied along the top daughter branch, generating interfacial tension gradients that induce Marangoni stresses, driving asymmetric deformation and controlled migration. The effects of wall temperature, temperature field configuration, surface wettability, and initial droplet parameters on breakup behavior are systematically evaluated. Under the reference conditions of a droplet surface area of 0.67 mm2, an initial position of 1.0 mm measured from the channel inlet, and a hydrophilic bottom wall with a contact angle of 78°, complete migration occurs at 31.1 °C, 31.8 °C, and 47.2 °C for hydrophilic, neutral, and hydrophobic top walls having contact angles of 78°, 90°, and 118°, respectively, when the entire top daughter branch is heated. Moving the heated region downstream increases the corresponding critical migration temperatures (wall temperature required for complete migration into the heated branch without splitting) to 47.1–49.4 °C, demonstrating the importance of the spatial location of thermal forcing. For a hydrophilic top wall with a contact angle of 78° and an initial position of 1.0 mm, the critical migration temperature varies from 25.7 to 75 °C as the initial droplet surface area increases from 0.62 to 0.75 mm2, highlighting the strong dependence of thermocapillary responsiveness on droplet size. Similarly, for a droplet surface area of 0.67 mm2 and a hydrophilic top wall with a contact angle of 78°, increasing the initial droplet position from 0.8 to 1.5 mm increases the critical migration temperature from 29.9 to 36.2 °C. The results demonstrate that thermocapillary forces, surface wettability, heating configuration, droplet size, and initial position collectively govern the transition between splitting and complete migration, providing useful guidelines for thermally actuated droplet manipulation in microfluidic systems.