Measurement of droplet evaporation, migration and thermal convection in non-uniform temperature fields

The evaporation and migration of tiny droplets in non-uniform temperature fields involve the coupling of multi-physical fields such as heat and flow. These processes are crucial for applications in microfluidics, microscale heat dissipation, and biomedical engineering. To explore the interaction between contact line migration and internal thermal flow instability under temperature gradients, an experimental system was developed. The system includes an experimental substrate to control local temperature and combines high-speed microscopy, infrared thermography, and micro-PIV. Two droplets with distinct wettability, anhydrous ethanol and HFE-7200, were selected as study subjects. Their evaporation behaviors were investigated in both uniform and non-uniform temperature fields. Results show that under a temperature gradient of 0.41 K/mm, the HFE-7200 droplet, with a low contact angle hysteresis, migrates from the hot to the cold side. Its migration speed follows a trend consistent with classical lubrication theory. In contrast, the anhydrous ethanol droplet, which has a high contact angle hysteresis, only shows asymmetric retraction of the contact line, with no significant overall movement. Internal heat convection patterns reveal that the HFE-7200 droplet's flow structure shifts toward the cold side in the non-uniform field. The ethanol droplet displays hydrothermal waves propagating along the contact line and the range of these waves contracts toward the cold end in the non-uniform field. Analysis based on the Marangoni number, Rayleigh number, and dynamic and static Bond numbers shows that during the evaporation of HFE-7200, buoyancy effects and thermocapillary effects compete significantly. For the ethanol droplet, internal flow is primarily driven by thermocapillarity. And the evolution of dimensionless numbers correlates well with changes in internal flow patterns. This study clarifies the critical role of wettability and contact line dynamics in thermocapillary migration. It also provides insights into how temperature gradients influence internal flow modes and instability, offering a basis for predicting and controlling droplet behavior in non-uniform thermal environments.