Surface light scattering for non-transparent fluids

• Applicant: Professor Dr.-Ing. Andreas Paul Fröba
• Subject Area: Technical Thermodynamics
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 540431975

Surface light scattering (SLS) probes the dynamics of thermal fluctuations on the phase boundary of fluids by analyzing the temporal behavior of the scattered light intensity. It is routinely used for measuring liquid viscosity and surface or interfacial tension of transparent fluids with typical expanded uncertainties (k = 2) of 2% and below in a non-invasive way. Also for non-transparent fluids, thermophysical properties of fluids under process-relevant thermodynamic states in terms of temperature and pressure play a crucial role in the design of various technical processes and apparatuses. Under extreme conditions including high temperatures, however, liquid viscosity and surface tension data are only available for a small selection of fluids. The application of SLS in reflection geometry is challenging since the resulting scattered light intensities are significantly lower than those obtained in transmission geometry. This makes it inevitable to use small scattering angles, i.e. to probe small wave vectors q. Here, so-called line-broadening effects are present due to a lack of definition in q. If not considered appropriately, erroneous results for viscosity and interfacial tension are obtained. For an on-line or in-line operation of SLS in the range of small q, physically solid evaluation approaches and a proper design of the experiment are still missing. The main goal of the proposed project is the further development of the SLS method for the simultaneous determination of liquid viscosity and surface tension with low uncertainties for non-transparent fluids. It will be investigated for which range of wave vectors a new Monte-Carlo-based data evaluation approach delivers satisfying results. In addition, it will be studied if macroscopic fluctuations of the fluid surface or the instrumental uncertainty of the apparatus dominates the line-broadening effects. This will be achieved through systematic changes in the optical arrangement as well as the integration of a camera-based beam profiler and an active beam stabilization system. By this, it is aimed to realize a setup that does not need any calibration. A further goal is the determination of the wave vector distribution in an absolute way, which would allow determining viscosity and surface tension even in the presence of pronounced line broadening. For proving the success of the developments and to show that the advantages of the SLS method can be transferred to the study of non-transparent fluids, a room-temperature metallic melt will finally be investigated. Overall, this research should provide the experimental and data evaluation foundations for future SLS studies under extreme conditions.

• DFG Programme: Research Grants