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Lubricated thin sheets : from adhesion to elastocapillary rise
I will discuss two problems involving thin liquid films covered by flexible membranes :
1) A very primitive adhesion mechanism is encountered when trying to disjoin two parallel flat surfaces separated by a fluid layer. Drainage of the neighbouring fluid into the opening gap induces viscous resistance to traction. In the case of two stiff surfaces, this force was calculated more than a century ago by Stefan and Reynolds. We will present experimental and numerical results showing how elastic deformations of one of the membranes can greatly enhance this hydrodynamic adhesion.
2) The statics and dynamics of capillary rise of a wetting liquid in a porous medium have been widely documented in the past centuries. Here, I will describe experimental work on capillary rise between flexible plates. We show that the collapse of the imbibition cell under the negative hydrostatic pressure caused by the ascent enables liquid to rise without limit between the plates. The height of the rising front is found to increase with time as t^1/3 , a characteristic of capillary imbibition in a wedge. The mechanical and geometrical characteristics of the imbibition cell set the shape of the wetting front and enable to predict wether a cell behaves as rigid or flexible.
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