Investigation of fibre optic sensing techniques for the measurement of physical-chemical properties of liquids

Binetti, Leonardo

Abstract

Single-mode optical fibre (SMF) sensors have been extremely useful in biomedical applications because of their small size and chemical inertness. Many optical fibre techniques have been used to measure the surface tension (𝛾) of liquids. However, due to the cylindrical shape and small size of the optical fibre used, the contact angle (θ) was either ignored or supposed to be equal to zero by other researchers, affecting the measurement by ~20%. In this thesis, the contact angle is practically measured via two methods.
The first method is via the critical meniscus height (CMH) measurement with a SMF for liquid volumes up to 5 μL. In addition, the light used to enlighten (1550 nm) the optical fibre could be absorbed by some liquids, which would increase the temperature of the said liquids, and 𝛾 could be affected. Therefore, a halogen light source has been filtered with an optical fibre coupler (centred at 850 nm), and the liquids that do not absorb those wavelengths resulted in a CMH comparable with the results from the new setup. Differently, the CMH value was increased with the new setup for liquids that absorb light at 1550 nm. For example, the CMH of water increased by ~10 μm, and the overall sensitivity increased by ~15%. Moreover, the CMH varies with different 𝛾 and dynamic viscosity of the liquid.
The second method started with measuring the meniscus height (MH) from the liquid profile on the fibre and finished with the MH measurement on the fibre surface. In fact, the θ could be calculated once the MH is known, and by measuring the force on the fibre, the surface tension can be determined. The MH is measured using a cleaved and side polished SMF attached to an SMF with a known separation distance between their fibre-tips. The MH seems to increase as the surface tension decreases, and the variation of the mass measured by the scale increases with the increase of the surface tension. This sensor was also used to measure the interfacial surface tension at the liquid/liquid interface since the θ could be measured, which is useful for biomedical applications.
In conclusion, both methods could be used for measuring the surface chemistry properties of liquids. The advantage in comparison with other methods is the decrease in the amount of liquid needed for the measurement, i.e., up to 5 μL. In addition, using the cleaved and side polished fibre attached to an SMF gives contemporaneous knowledge about the force and the contact angle of different liquids on the fibre. Therefore, the proposed technique modifies the commonly used method to provide valuable information on surface tension.

• Thesis Type: Thesis
• Deposit Date: Mar 22, 2023
• Publicly Available Date: Mar 24, 2023
• DOI: https://doi.org/10.17869/enu.2022.3050846
• Award Date: Aug 2, 2022