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Wetting and confined flows of yield stress fluids
Foams, emulsions, polymer gels are example of every day life yield stress fluids. What makes such systems particularly interesting for applications, but fundamentally difficult to describe, is their intermediate fluid / solid behaviour : at rest they behave like an elastic solid, but they are able to flow like a liquid above a critical stress, called yield stress.
This intermediate solid/liquid behaviour is well described macroscopically by phenomenological laws, such
as the Bingham or Herschel-Bulkley laws. These however fail to describe the dynamics of concentrated emulsion and granulars under strong confinement [Goyon2008, Jop2012, Kamrin2012, Geraud2013]. Reducing the scale also implies an increase of the surface to volume ratio and in many applications (coating, lab-onchip, jet-printing), a key role is played by the “boundary” phenomena.
In this seminar, I will consider the flows of a polymer microgels in two situations where surfaces are important (illustrated on the figure) :
1- Confined flows of polymer gels into micro-channels whose gap is of the same order of magnitude of the gel structure.
2- Capillary rise of the same polymer microgel.
These two studies highlight the strong influence of the hydrodynamic boundary conditions and of the existence of the yield stress. In the particular case of the capillary rise, the imbibition height strongly depends on the wall roughness and few depends on the gap contrary to simple fluids.
[Geraud2013] Géraud et al, Eur. Phys. J. E (2013) 36 : 30 .