Eur. Phys. J. E (2017) 40: 54
https://doi.org/10.1140/epje/i2017-11542-4

Tips and Tricks

Measuring thin films using quantitative frustrated total internal reflection (FTIR)

¹ Faculty of Science and Technology, Hirosaki University, 0368561, Aomori, Japan
² Physics of Fluids Group, Mesa+ Institute, University of Twente, 7500, AE Enschede, The Netherlands
³ Center for Combustion Energy and Department of Thermal Engineering, Tsinghua University, 100084, Beijing, China
⁴ Max Planck Institute for Dynamics and Self-Organization, 37077, Göttingen, Germany

^* e-mail: mshirota@hirosaki-u.ac.jp

Received: 15 November 2016
Accepted: 5 April 2017
Published online: 9 May 2017

Abstract

In the study of interactions between liquids and solids, an accurate measurement of the film thickness between the two media is essential to study the dynamics. As interferometry is restricted by the wavelength of the light source used, recent studies of thinner films have prompted the use of frustrated total internal reflection (FTIR). In many studies the assumption of a simple exponential decay of the intensity with film thickness was used. In the present study we highlight that this model does not satisfy the Fresnel equations and thus gives an underestimation of the films. We show that the multiple reflections and transmissions at both the upper and the lower interfaces of the film must be taken into account to accurately describe the measured intensity. In order to quantitatively validate the FTIR technique, we measured the film thickness of the air gap between a convex lens of known geometry and a flat surface and obtain excellent agreement. Furthermore, we also found that we can accurately measure the elastic deformations of the lens under loads by comparing them with the results of the Herzian theory.