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    Air entrainment in dip coating under reduced air pressures

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    Air entrainment in dip coating under reduced air pressures in ChemEngSci2008.pdf (483.5Kb)
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    Publication date
    2008
    Author
    Benkreira, Hadj
    Khan, M.I.
    Keyword
    Air Entrainment
    Dip Coating
    Coating Flows
    Dynamic Wetting
    Contact Angle
    Experiments
    Air Viscocity
    Vacuum Pressure
    Rights
    © 2008 Elsevier Ltd. Reproduced in accordance with the publisher's self-archiving policy.
    Peer-Reviewed
    Yes
    
    Metadata
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    Abstract
    This study examines experimentally and for the first time the effect of reduced air pressure on dynamic wetting. The purpose is to assess the role of air viscosity on dynamic wetting failure which hitherto has been speculated on but not measured. In this paper we used dip coating as the model experimental flow and report data on air entrainment velocity Vae we measured with a series of silicone oils in a range of viscosities in a vacuum chamber where the pressure can be reduced from atmospheric down to a few mbar when the mean molecular free path of air is large and air ceases to have a viscosity. To complement earlier work, we carried out the experiments with a range of substrates of varying roughness. The substrates were chosen so that for each one, their two sides differ in roughness. This enables simultaneous comparative observation of their wetting performance and reduces the experimental error in assessing the role of roughness. The data presented here capture the effects of viscosity, roughness and air pressure but the important result of this study is that Vae can be increased considerably (exponentially) when the pressure is reduced with the suggestion that Vae approaches infinity as pressure approaches zero. In other words, the role of the surrounding air viscosity is important in dynamic wetting. The data from this study have significant implication to the fundamental understanding of dynamic wetting. Indeed they form the missing data link to fully understand this phenomenon. The data presented in this work also confirm the complex role of roughness, in that it can increase or decrease the air entrainment speed depending on the value on the viscosity of the coating solution. The results presented in this paper are very useful in practice as they imply that if one chooses carefully roughness one can coat viscous formulation at unexpectedly very high speeds with a moderate vacuum (50 mbar typically).
    URI
    http://hdl.handle.net/10454/808
    Citation
    Benkreira, H. and Khan, M.I. (2008). Air entrainment in dip coating under reduced air pressures. Chemical Engineering Science. Vol. 63, No. 2, pp. 448-459.
    Link to publisher’s version
    http://dx.doi.org/10.1016/j.ces.2007.09.045
    Type
    Article
    Collections
    Engineering and Informatics Publications

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