Université Paris-Sud

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Role of surface rheology in liquid film formation

Benoit Scheid

Although the concepts of surface viscosity and elasticity date back to the Belgian scientist Plateau in the 19th century, they have only later been formalized mathematically and subsequently been used in quantitative descriptions of surface flows. The concepts of surface viscosity and elasticity (Marangoni) effects are intimately related, and are not always distinguishable in an experimental setting. One such setting is the Landau-Levich-Derjaguin dip-coating flow, wherein a film of liquid is ‘deposited’ onto a solid substrate as it is withdrawn from a bath. This process is so fundamental to coating flows that it is ubiquitous in today’s coating technology. It is commonly assumed that, as for pure liquids, there is no thickening for liquids containing large amounts of soluble surfactants, i.e. the substrate withdrawal does not induce Marangoni stresses. However, in some cases thickening does occur, as demonstrated experimentally by collaborators at LPS, Orsay [Delacotte et al. Langmuir, 2012]. On the theoretical front, we have demonstrated that thickening in the absence of Marangoni effects can be rationalized by surface viscosity [Scheid et al., EPL, 2010]. In a second part, we have considered the case of pure elasticity induced by surface tension gradient by considering the formation of contact-free films assisted by thermocapillary effects. This concept is at the origin of a new concept of fast production methods of silicon ribbons used in solar panels [Scheid et al., APL 2010]. We have demonstrated that the resulting film thickness is at leading order independent of the pulling speed and then show how it is influenced at next order by inertia, gravity, heat transfer and heat conduction [Scheid et al. PoF 2012].

Dernire modification : October 03 2012, 08:56:02.