THERMAL PATTERNS AND INTERNAL FLOW MECHANISMS IN EVAPORATING INVERTED SESSILE DROPS OF PURE WATER

We report an experimental investigation on contact line dynamics, thermal patterns, and internal fluid flow during the evaporation of inverted sessile drops of pure water. This configuration of sessile drop when placed on a heated substrate should lead to thermal stratification and any internal convective flow will be governed by surface tension driven Marangoni flow. First, we report contact line dynamics and thermal patterns recorded using an optical camera and infrared camera, respectively. An interesting outcome from the present study is the resemblance observed between the evolution of contact angle and interfacial temperature difference during evaporation. By performing Particle Image Velocimetry to delineate the internal flow characteristics, we report an axisymmetric counter-rotating flow inside the drop. This flow is directed towards the substrate from the apex at the centerline of the drop. In literature, a similar directional flow is reported to be due to Marangoni flow albeit for a normal sessile drop. Further, by extracting the magnitude of velocity, we report a maximum velocity in the flow occurring at the center of drop which in turn increases with substrate temperature. The results reported in the present study shed light on the presence of Marangoni flow in pure water drops whose understanding is of paramount importance in many academic and industrial applications.