Surface Light Scattering Under Considertation of Line Broadening Effects

Conclusion: In the present work, the instrumental line broadening of an existing SLS experiment was investigated by the reference fluid toluene and the results were applied to a semitransparent IL [BMIM][TCM].

During the measurement with toluene several problems like the impurity of the fluid due to the measurement cell and a contamination of the top window of the measurement cell due to the preparation procedure were faced. At the end, for relatively large incident angles of about 3°, the measured thermophysical properties of surface tension and dynamic viscosity were in very good agreement with the reference data. Based on this, an experimental analysis of the setup was performed with the focus on small incident angles where line broadening effects become dominant. It could be shown that the experimental setup is complex and cannot be easily treated theoretically. Therefore an experimental calibration with the reference fluid toluene, whose thermophysical properties σ and η are well-known, should be preferred.

In accordance with the SLS theory, a mathematical concept was developed and programmed to extract the instrumental line broadening effect from the measurements with toluene. These measurements were performed for the first time in a range of incident angles between 0.3° and 1°. The instrumental line broadening Δq was calculated and additionally an unexpected error in the wave number q0 was found. Both Δq and q0 depend on the adjusted wave number and have a large variance for individual measurements. This result shows that the experimental setup is not optimal for the use with small incident angles, but excellent results can be obtained at larger angles. The unexpected error in the wave number q0 and its variance might be related to the adjustment of the laser beam in horizontal direction by hand. 

Nevertheless a possible correction of the measured semitransparent IL [BMIM][TCM] was investigated but it turned out that a correction is not possible. The corresponding calculated thermophysical properties possess errors of more than 10% in σ and η. 

A major task for the future will be an analysis of the unexpected error in the wave number q0. The assumed origin is the horizontal movement of the laser by hand which should be verified by an experiment. In the next step a mechanism has to be invented which allows an exact positioning of the laser beam. Without such modifications the current setup is of limited use at small incident angles.

Another important point refers to the programming of an algorithm that allows the calculation of σ and η as suggested. The main problem here is the time consuming evaluation of the dispersion relation which could be possibly solved by a pre-calculation of the dispersion relation and the use of an interpolation function. I.e. the dispersion relation calculates ωq and Γq for many input combinations of σ, η and q a priori; an interpolation function uses this information and can answer function calls for all possible input combinations by interpolation; the algorithm of the minimization problem calls the interpolation function instead of the dispersion relation.