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Evaporative cooling in the contact line region

University of British Columbia

Evaporative cooling of small, high power dissipating devices using thin liquid films is becoming an increasingly important field of research as the requirements for this tech- nology grow. Understanding the mechanisms which cause liquid motion in the film and evaporation from the liquid surface is essential to the practical designer attempting to create a truly effective cooling scheme. For liquid films of less than 1 μm, previous experiments have confirmed the existence of an adsorbed layer where no evaporation occurs, plus a region of slightly increased thickness where limited evaporation takes place. A theory has been proposed for the non-evaporating region which modifies the thin liquid film pressure due to the molecular attraction of the underlying substrate. The pressure adjustment is termed the disjoining pressure and is thought to be the dominant driving force for liquid motion in this thin film region In order to test the validity of the disjoining pressure concept for an evaporating film, an experiment was designed which uses a dielectric liquid, FC-72, and a highly polished silicon substrate inclined at a 5° angle from the horizontal to create an extended meniscus. A fluorescent light was used in an interferometer to provide increased film profile data and a specially designed focusing ellipsometer was used to measure film thicknesses in the adsorbed film region. Heat was supplied to the meniscus through a 400 μm wide boron diffused heater within the silicon substrate. Surface temperature and mass evaporation rates were also measured. It was concluded from the results that the disjoining pressure model which had orig- inally been developed for static non-evaporating thin films is equally applicable to evap- orating thin film environments. The model proposes an inverse cubic relationship (1/h 3) between the disjoining pressure and the adsorbed thickness. However, values of the Hamaker constant used in the relationship, inferred from the experiments were 4-5 times the theoretical value. The corresponding heat and mass transfer models which employ the disjoining pres- sure require further study, as evidenced by their prediction of total heat and mass trans- fer rates at least one order of magnitude lower than those measured in the experiments. Improvement of the heater design and of the thin film profiling method are two very important areas to be considered for future work in this field.

Thesis/Dissertation

application/pdf

2009-06-11

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http://hdl.handle.net/2429/8975

Mechanical Engineering

University of British Columbia

UBCV

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