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How to apply a coating to an object? The immersion method is often used in the industry with a central question: how to predict the thickness of the layer that will cover the object? Far from being insignificant, this question raises important financial issues for manufacturers. A team from the CBI laboratory at ESPCI Paris has developed an experimentally verified theoretical framework to predict the coating thickness in the case of yield-stress fluids. The researchers just report their findings in the journal Physical Review Letter.
ESPCI Paris
Even with chocolate, it's possible to measure the coating thickness on a rod, or on a finger!
To cover an object with a liquid coating, the object is often immersed into a fluid reservoir containing the fluid of interest and then pulled out. This technique, known as dip-coating, has been previously studied both experimentally and numerically when the fluid has a yield-stress (such as paints, gels, glues, foams or creams), but a complete description of the behaviour has been lacking. The researchers addressed this problem by studying a model system in which axial symmetry plays an important role. It involves plunging a cylindrical rod into a cylindrical bath. They showed that the thickness of the deposited coating results from the fluid flow induced by the movement of the rod. However, this flow depends mainly on the geometry and dimensions of the reservoir! For yield-stress fluids, it is therefore the geometry of the bath that determines the thickness of the deposited film (the dimensions of the withdrawn object being also an important factor).
These experimental results are in line with numerical calculations of the fluid mechanics. This work will undoubtedly interest manufacturers who use the dip-coating process to apply coatings, whether they are paints, cements, gels or even chocolate (the researchers’ favourite).
Reference:
W. J. Smit, C. Kusina, J.-F. Joanny, A. Colin, Stress field inside the bath determines dip coating with yield-stress fluids in cylindrical geometry, Phys. Rev. Lett. 123,2019.
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.123.148002
Contact: wilbert.smit@espci.fr
Phone: 0140794670