The film thickness of seven seed oils and two petroleum-based oils of varying chemical structure was investigated by the method of optical interferometry. The measured film thickness (hmeas) was then compared to that calculated using the Hamrock–Dowson (H-D) equation. The result showed that the H-D equation adequately and without exception predicted hmeas in the high entrainment speed region but not in the low entrainment speed region. In order to understand these deviations further, the chemical structural variabilities of the oils were quantified using empirical parameters, and its effect on film thickness in the low entrainment speed region was examined. The result of this examination indicated that, in the low entrainment speed region, the film thickness of seed oils (i) rarely correlates with viscosity as proposed by the H-D equation, (ii) increases with decreasing polarity of the oil, and (iii) increases with decreasing degree of unsaturation of the oils.
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