Hydrothermal waves in evaporating sessile drops (APS 2009)
HHydrothermal waves in evaporating sessile drops(APS 2009)
D. Brutin, F. Rigollet, C. LeNiliotAix-Marseille University, IUSTI Laboratory13013 Marseille, FRANCENovember 7, 2018
Abstract
This fluid dynamics video was submitted to the Gallery of FluidMotion for the 2009 APS Division of Fluid Dynamics Meeting in Min-neapolis, Minnesota. Drop evaporation is a simple phenomena but stillunclear concerning the mechanisms of evaporation. A common agree-ment of the scientific community based on experimental and numericalwork evidences that most of the evaporation occurs at the triple line.However, the rate of evaporation is still empirically predicted due tothe lack of knowledge on the convection cells which develop inside thedrop under evaporation. The evaporation of sessile drop is more com-plicated than it appears due to the coupling by conduction with theheating substrate, the convection and conduction inside the drop andthe convection and diffusion with the vapour phase. The coupling ofheat transfer in the three phases induces complicated cases to solveeven for numerical simulations. We present recent experimental fluiddynamics videos obtained using a FLIR SC-6000 coupled with a micro-scopic lens of 10 µ m of resolution to observe the evaporation of sessiledrops in infrared wavelengths. The range of 3 to 5 µ m is adapted tothe fluids observed which are ethanol, methanol and FC-72 since theyare all half-transparent to the infrared. The movie sample presented is dealing with methanol sessile drops underevaporation. Three different fluids are used Methanol, Ethanol and FC-72.The original movie presended here is recorded with a resolution of 640x5121 a r X i v : . [ phy s i c s . f l u - dyn ] O c t ixels. This movie is presented at real time speed (25 frames/sec). To fitthe web site request, the video quality have been sharply reduced. For thethree cases, the substrate is PFTE at constant surface temperature andwith a surface roughness of 400 nm and no specific direction. The surfacetemperature is regulated to be constant using a PID heating device coupledwith heating cartridges. The ambient temperature for all three cases is 25 ◦ Cand the atmosphere is air at 1 atm.Table 1: Physical properties of fluids at 25 ◦ C and 1 atm (cid:37) L Cp Lv µ σ T sat Prkg.m − J.kg − .K − kJ.kg − mPa.s mN.m − ◦ C -Water 997 4180 2449 0.890 72.7 100 6.14FC-72 1680 1100 88.0 0.638 12.0 56.0 12.3Methanol 791 2531 1165 0.560 22.7 64.7 6.98Ethanol 789 2845 841 1.095 22.0 78.0 22.3
Initially, the substrate is at a constant temperature. This is checked bythe first image of the video. The thermal homogeneity is less than 1 ◦ C at thebeginning the experiment. All fluids for these thickness are half-transparent.The different steps of a volatile drop evaporation can be distinguished asfollow:1. Phase 1 - Drop warming up - The start of the experiment is definedas the initial contact of the drop posed on the substrate. The firstseconds of the experiments are characterized by a transition phenom-ena, the drop which was initially at room temperature is posed on awarm surface, the drop first is heated to reach almost the substratetemperature. The first step of the phenomena is thus only driven bythe fluid heat capacity.2. Phase 2 - Drop evaporation - The heat flux transferred to the dropreach a maximum value which correspond the beginning of our evap-oration investigation. The drop is under evaporation, the heat fluxlinearly decrease during a first phase which correspond to the exis-tence of convective cell inside the drop. The flow motion inside thedrop depends on the fluid physical properties such as the fluid viscosityand latent heat of vaporization.2. Phase 3 - Film evaporation - The last step of evaporation is charac-terized by the sharp decrease of the heat flux when the convectioncells are vanished. No more fluid motion can be observed inside thedrop. An explanation can be provided based on the critical thicknessfor thermal flow instabilities to develop.Below, we present three experimental fluid dynamics videos for threedifferent liquids with one objective: show up the complexity of drop evapo-ration mechanism.1. In the case of Methanol, numerous hydrothermal waves can be ob-served at the first stage of the evaporation near the drop perimeter.In the center of the drop, the flow is much more unstable. The ther-mal flow motion inside the drop is fast, compared to the other fluids.The flow motion inside the drop stop when the drop reach 2 mm indiameter.2. In the case of Ethanol, it is possible to observe with a greyscale evi-dence the temperature isotherm which are located in the middle of thedrop. Near the drop perimeter, less hydrothermal waves are observed.Then during the evaporation, the thermal flow change very quicklyto big hydrothermal waves inside the drop. The time of evaporationis comparable to the methanol case since the heat of vaporization iscomparable.3. In the case of FC-72, the latent of vaporization is at least 10 timessmaller compared to methanol and ethanol. Consequently even witha substrate temperature at 29 ◦ C (slightly above the room tempera-ture), the drop evaporation time is fast compared to the two otherliquids. During the FC-72 evaporation, multiple internal convectivecell structures can be observed which is completely different in thecase of methanol and ethanol evaporation dynamics.To conclude, for fluids with even comparable latent heat of vaporiza-tion, the thermal flow motion inside the drop can be very different since thefluid viscosity is the only important difference in the fluid physical proper-ties. With the knowledge of the temperature scale, it is possible to relatethe temperature difference inside the drop up to 10 ◦ C for methanol to thequick flow motion; where as for the ethanol drop, the maximum tempera-ture difference is only 5 ◦◦