We report on the development of the optical digital interferometry (ODI) technique for measuring diffusion and Soret coefficients. The unique feature of this method is that it traces the transient path of the system in the entire two-dimensional cross section of a cell. In this way, it is applicable not only for measurements of Soret coefficients, but also for studying diffusive transport mechanism and convection. Presently, this method is not widely used and, in our view, not because of fundamental limitations but rather due to a lack of properly developed postexperimental procedures of the raw data analysis. Thus, in this paper our attention is focused on the successive analysis of different steps: the fringe analysis, the choice of reference images, the thermal design of the cell, and multiparameter fitting procedure. Using the ODI technique, we have measured the diffusion and the Soret coefficients for three binary mixtures composed of dodecane (\documentclass[12pt]{minimal}\begin{document}${\rm C}_{12}{\rm H}_{26}$\end{document}C12H26), isobutylbenzene, and 1,2,3,4-tetrahydronaphtalene at a mean temperature of \documentclass[12pt]{minimal}\begin{document}$25^{\circ }$\end{document}25∘C and 50 wt. % in each component. These measurements were compared with their benchmark values and show an agreement within less than 3%.

We report experimental evidence of convection caused by translational vibration of nonuniformly heated fluid in low gravity. The theory of vibrational convection in weightlessness is well developed but direct experimental proof has been missing. An innovative point of the experiment is the observation of a temperature field in the front and side views of the cubic cell. In addition, particle tracing is employed. The evolution of this field is studied systematically in a wide range of frequencies and amplitudes. The flow structures reported in previous numerical studies are confirmed. The transition from four-vortex flow to the pattern with three vortices is observed in the transient state.

We report on the measurement of diffusion (D), thermodiffusion (D_T), and Soret (S_T) coefficients in water-isopropanol mixtures by three different instrumental techniques: thermogravitational column in combination with sliding symmetric tubes, optical beam deflection, and optical digital interferometry. All the coefficients have been measured over the full concentration range. Results from different instruments are in excellent agreement over a broad overlapping composition (water mass fraction) range 0.2 < c < 0.7, providing new reliable benchmark data. Comparison with microgravity measurements (SODI/IVIDIL (Selected Optical Diagnostic Instrument/Influence of VIbration on DIffusion in Liquids)) onboard the International Space Station and with literature data (where available) generally gives a good agreement. Contrary to theoretical predictions and previous experimental expectations we have not observed a second sign change of S_T at low water concentrations.

Vibrations acting on a fluid with density gradient induced by temperature variations can cause relative flows. High-frequency vibration leads to the appearance of time-averaged (mean) flows (or streaming flows), which can essentially affect heat and mass transfer processes. This phenomenon is most pronounced in the absence of other external forces (in particular, static gravity). In this work, an extensive experimental and computational study of thermal vibrational convection in a reduced-gravity environment of a parabolic flight is performed. The transient evolution of the temperature field in a cubic cell subjected to translational vibration is investigated by optical digital interferometry. The mean flow structures previously reported in numerical studies are confirmed. The transition from four-vortex flow to a pattern with a large diagonal vortex and two small vortices is observed in the transient state. The experiments reveal a significant enhancement of heat transfer by vibrational mean flows with increasing the vibrational strength. Three-dimensional direct numerical simulation with real microgravity profile and two-dimensional numerical modelling based on averaging approach provide a very good agreement with the experimental results. The influence of residual gravity on heat transfer and bifurcation scenario is first investigated numerically and correlated with the experimental data. It is demonstrated that gravity effects on non-uniformly heated fluids can be reproduced in weightlessness by applying vibrations to the system.