FTIR Calibration

This report describes what was done during a Master’s semester project lasting 14 weeks and concerns the calibration of Frustrated Total Internal Reflection (FTIR) and optimizing the setup for measuring the thin layer of air trapped between an impacting droplet and a solid surface before total wetting occurs. For this, a LED light source illuminating a prism is used, and a high speed camera reads the reflected image which can be analysed and can give the thickness of the layer or gap between the droplet and the solid surface, as the reflected light intensity read by the camera is directly a function of this gap thickness. To calibrate the setup a spherical lens is placed on top of the prism and its elastically deformed profile (under its own weight) is measured thanks to the FTIR setup, and compared to the analytical solution for the deformed profile provided by Hertzian solid contact theory. The measures are investigated with the light source being polarized and therefore the differences between the results of s− and p−polarized light are directly compared. It is found that s−polarized light gives a higher quality image with less error and noise and therefore s−polarized light is used for the rest of the calibration. A new intensity normalization method is investigated and appears to yield very promising results with low error considering the nanometer scale of deformation. After completion of the calibration, a droplet impact was tested and the image revealed that a pulsing LED would yield better results given that the very high velocity at which the layer of air disappears as the liquid completely wets the solid surface requires a very high framerate.