Collaborative Research: ISS: Revealing interfacial stability, thermal transport and transient effects in film evaporation in microgravity

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Evaporation at the surface of a liquid film is critical to many industrial processes including coatings, semiconductor crystal growth, surface texturing in magnetic storage devices, surface finish during paper production, polymer processing, and electrode manufacturing for fuel cells. These applications depend on volatility-induced evaporation (no applied heat). Many other evaporation applications involve heating a solid substrate such as cooling, drying, fuel vaporization, food processing, chemical manufacturing and pharmaceuticals. Evaporation is also used for self-assembly of particles and morphological control of porous structures. Many space-based thermal management systems also depend on evaporation. Despite this ubiquity, the current understanding of film-evaporation processes is incomplete. The unsteady motion at the film surface and the motion of the liquid within the films drastically affect evaporation rates and heat transfer, particularly during transient, or unsteady, evaporation. The overarching goal of this research is to probe the fundamental mechanisms of these complex phenomena by conducting detailed experiments and numerical analysis of evaporating films in both normal- and microgravity conditions. The improved understanding of evaporation processes resulting from this effort may have broad impact in numerous practical applications. This project will use the unique capabilities of the ISS for long-duration microgravity testing to develop a more complete understanding of the behavior of evaporating films by revealing physical mechanisms normally masked under terrestrial conditions. Specific scientific objectives include determining: (i) the transitions between long-wave and short-wave surface instabilities in evaporating films, (ii) the evolution of the convective structures from the onset of evaporation to steady-state, and (iii) the corresponding impacts on thermal transport. Evaporation rates will be controlled using a combination of external heat addition and impulsively changing the system pressure. Diagnostic techniques include ultrasonic film thickness measurements, optical imaging, and thermal and pressure measurements. The stability of evaporating films will be examined by linear stability analysis and Recurrence Quantification Analysis. The latter is a relatively new diagnostic technique that provides metrics such as the rates and trapping-times that measure the recurrence of interfacial film states and their duration. The extraction of these quantitative signatures of film events can serve as triggers for “early warning systems” to predict and control emergent film behavior. The new, detailed information resulting from this investigation will be transformative in that it will lead to fundamental understanding of the origins and nature of the complex mechanisms that impact the liquid film behavior and the rate of heat transfer in evaporating films.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.