Spreading laws for liquid polymer droplets : interpretation of the « foot »
Résumé
Depending upon the nature of the polymer-substrate system, it has been observed that the shape of a spreading polymer droplet on a flat surface could be either a spherical cap — as for low viscosity liquids — or a spherical cap with a projecting macroscopic « foot ». We interpret this « foot » by an effect of finite slip of the polymer liquid at the surface : the existence of a large extrapolation length (b) for the velocity field of polymeric liquids near a smooth surface was predicted long ago by one of us. We calculate the detailed shape of the foot, and show that the foot thickness is comparable to b ; this length may reach very high values (> 100 microns) for entangled polymers. On the other hand, if the surface is rough, or if some chains are strongly bound to it, b is drastically reduced : this explains why the existence of the « foot » depends on the precise conditions at the surface.
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