Illustrating flow patterns for hydrothermal waves in sessile droplets

The sessile droplet phase change, occurring in many engineering applications, has been reported with showing the complex temperature patterns. The hydrothermal waves (HTWs) are one of the observed phenomena; however, they are not well explained through experiments. We thus designed the tests to illustrate the sessile droplet lifetime though the infrared (IR) camera and particle image velocimetry (PIV) measurements. The droplets were maintained with the constant contact radius in the most lifetime on the heated substrate during the measurement. The agreements are found between the IR images and the micro-PIV measurements for linking HTWs to the convective cells for the sessile droplets. The convective cells are generated by the capillary flow and thermocapillary flow with the interfacial complex temperature gradients near the droplet contact line. The contact angles are critical for forming of the HTWs. Three stages of the evolution of the instability in the phase-change droplets are identified. The splitting and merging of the HTW patterns in the IR measurement can be explained with the flow field from the micro-PIV as well.