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Transferts de Chaleur et de Masse

Phase Change and Interfaces

Sessile drops evaporation on Earth

 

Recent publication:

  • Sobac B., Brutin D., Thermocapillary instabilities in an evaporating drop deposited onto a heated substrate, Physics of Fluids, Vol. 24, 032103, pp. 1-16, 2012, Download the publication.
  • Sobac B., Brutin D., Triple-line behavior and wettability controlled by nano-coated substrates: Influence on sessile drop evaporation, Langmuir, Vol. 27, pp. 14999-15007, 2011, Download the publication.
  • Brutin D., Sobac B., F. Rigollet, C. LeNiliot, Infrared visualization of thermal motion inside a sessile drop deposited onto a heated surface, Experimental Thermal and Fluid Science, Vol. 35, pp. 521-530, 2011, Download the publication.

 

Drop evaporation is a basic phenomenon but the mechanisms of evaporation are still not entirely clear. A common agreement of the scientific community based on experimental and numerical work is that most of the evaporation occurs at the triple line. However, the rate of evaporation is still predicted empirically due to the lack of knowledge of the governing parameters on the heat transfer mechanisms which develop inside the drop under evaporation. The evaporation of a sessile drop on a heated substrate is a complicated problem due to the coupling by conduction with the heating substrate, the convection/conduction inside the drop and the convection/diffusion in the vapor phase. The coupling of heat transfer in the three phases induces complicated cases to solve even for numerical simulations. We obtain experimental results using an infrared camera coupled with a microscopic lens giving a spatial resolution of 10 µm to observe the evaporation of sessile drops in infrared wavelengths.

Different fluids fully characterized, in the infrared wavelengths of the camera, are investigated: ethanol, methanol and FC-72... These liquids were chosen for their property of semi-transparency in infrared, notably in the range of the camera from 3 to 5 µm. Thus, it is possible to observe the thermal motion inside the drop. This visualization method allows us to underline the general existence of three steps during the evaporating process: first a warm-up phase, second (principal period) evaporation with thermal-convective instabilities, and finally evaporation without thermal patterns. The kind of instabilities observed can be different depending on the fluid. We are also interested in the evolution of these instabilities and the link with the temperature difference between the heating substrate and the room temperature.

Ethanol 4 mm-diameter drop under evaporation at the laboratory (Infrared Visualization with a SC6000 IR Camera - FLIR)



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