Collaborative Research: An Experimental Study of the Dynamics of Heated Contact Lines Using Combined High Resolution Thermography and Interfermometry

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An Experimental Study of the Dynamics of Heated Contact Lines Using Combined High ResolutionThermography and InterferometryUnderstanding of evaporating thin films is essential to the development of devices used in a wide variety of industries including those involved in coating, microelectronics fabrication and packaging, chemical processing, and materials development. Although there is a good theoretical understanding of how these films behave, direct measurements of both the heat transfer and the thin film thickness to verify the theoretical predictions have never been made due to the very small length scales involved. This investigation will simultaneously employ, for the first time, two very powerful and complementary experimental techniques: 1) Fluorescence techniques to measure the temperature and heat flux in the vicinity of the contact line; and 2) Multi-wavelength, image analyzing interferometry/reflectometry that enables us to determine the shape of the vapor-liquid interface, the curvature and curvature gradient of that surface, and the adsorbed film thickness ahead of the contact line. The results will contribute to making these processes more efficient, ultimately saving energy, materials, and labor costs, and will affect the design and development of many technologies that operate by controlling contact line dynamics using interfacial energy gradients (e.g., heat pipes, boiling, spreading and wetting on unheated and heated surfaces, fuel cells, evaporation induced self-assembly, micro-chemical laboratories, etc.). Many fundamental questions remain regarding the mechanisms by which energy is transferred in the interfacial region. With a completely wetting fluid, this region is characterized by a very thin adsorbed layer ahead of the contact line, by a region behind the contact line where the curvature of the vapor-liquid interface rapidly changes, and by a primary meniscus region where the curvature of the vapor-liquid interface is relatively constant. A partially wetting fluid may or may not have the adsorbed film. In addition, oscillations of the contact line have been observed in thin films on heated surfaces. The research will address fundamental phase-change heat and mass transfer questions by using interfacial energy gradients due to capillarity and disjoining pressure to naturally control the flow of simple fluids or dilute, ideal fluid mixtures. The validity of theoretical predictions regarding the structure and dynamics of the processes at the three-phase contact line will be investigated. The objectives of this work are: 1) To measure the shape of isothermal menisci using pure fluids and compare intermolecular forces obtained from those measurements with predicted values for various fluids on dielectric substrates; 2) To design, build, and operate an experiment to measure both the heat flux distribution and curvature as a function of position within the heated meniscus of pure fluids and to test current theories of interfacial transport in thin films; 3) To expand on the experiments with a pure fluid to include dilute, ideal mixtures and to test current theories of interfacial transport in thin films of mixtures; 4) To use these techniques to characterize contact line instability and oscillations for both the pure fluid and mixtures; and 5) To evaluate the degree to which molecular shape affects slip at the solid liquid interface and hence also affects transport processes in the contact line region. The fluorescence technique will allow direct measurement of the local heat flux and temperature. The interferometry/reflectometry technique will allow us to record what happens to the shape of the extended meniscus and the contact line as that energy gradient is perturbed, or to infer the local heat transfer from surface curvature measurements as the meniscus moves over the surface. Both techniques must be combined to obtain the data required to assess the current theories of interfacial transport.