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The capillary bridge: a powerful test to characterize low-energy surfaces


The capillary bridge: a powerful test to characterize low-energy surfaces

One of the outstanding issues of modern materials science involves the development of antiadhesive coatings which have many practical applications. Examples are antifouling-boat paints, antiadhesive coatings for chirurgical implants avoiding the formation of hazardous biofilms, repellency product in the treatment of carpets, fabrics and leather against stains and acarids or self cleaning treatments for eye-glasses. In order to optimize the efficiency of the coatings, objective characterization through usual adhesion tests (like the peeling tests which measure the force needed to peel a adhesive tape from a surface) are not efficient since the measured adhesion forces on these surfaces are two low.

 

Figure 1: Experimental setup.
The two pictures are the vue of the capillary bridge from above and from the side.

 

An original test has been developed at the “Laboratoire de Physique des solides” thanks to the help of the engineers and technicians from the “Electronique and Instrumentation” group. In this, so-called “Capillary bridge test” the adhesion of the surface is balanced against the elasticity of a liquid interface, i.e. its surface tension. A curved surface, coated with the antiadhesive coating to be characterized, is put into contact with a liquid bath. The evolution of the shape of the capillary bridge connecting the surface and the bath when the bath – surface distance is varied allows characterizing the ability of the surface to retain a liquid. It gives access to a very precise benchmarking of the different surface treatments. Two withdraw velocity regimes have been identified. In the low velocity limit, the capillary bridge keeps a quasistatic shape and the bridge is stable if the distance is below the critical rupture distance. In this regime, it is possible to perform approach-withdraw cycles, for which the wetted area on the solid surface gives access to a precise measurement of the hysteresis of the contact angle on the solid surfaces with an accuracy 10 times better than with other contact angle measurements techniques. Moreover the measured hysteresis is averaged over the whole solid surface. This hysteresis has been shown to be responsible for the volume of the final drop staying on the solid surface at the end of the bridge rupture process. At larger withdraw velocity, a thin liquid film is entrained on the solid surface. This film was called the “pancake”. The central part of the bridge remains mainly quasistatic but the pancake recedes more slowly. A larger quantity of liquid is entrained on the solid surface showing that the dynamical adhesion is higher than the static one. The receding velocity of the pancake which behaves as a dewetting film appears very sensitive to tiny details of the coatings which affect friction of the liquid on the solid surface. This technique should allow us in the future to gain a better understanding of the correlation between adhesion and friction and of the molecular mechanisms of the triple line pinning due to chemical or physical defects.

 

Figure 2: Rupture of the capillary bridge in the case of a viscous liquid and for a high withdraw velocity.

 

This device has been developed in the framework of long term collaboration with the Essilor company, and an improved version of the apparatus has been especially modified by the “Electronique et Instrumentation” group for them.

 

Contact :

F. Restagno (restagno@lps.u-psud.fr)

L. Léger (leger@lps.u-psud.fr)

 

References :

[1] Contact angle and contact angle hysteresis measurements using the capillary bridge technique Frédéric Restagno, Christophe Poulard, Céline Cohen, Laurianne Vagharchkian, et Liliane Léger, Langmuir, Langmuir, DOI : 10.1021/la901616x.

[2] Capillary Bridge Formation and Breakage : A Test to Characterize Antiadhesive Surfaces Laurianne Vagharchakian, Frédéric Restagno, Liliane Léger The Journal of Physical Chemistry B 2009 113 (12), 3769-3775.