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Coating Simulation Insights: Rivulet Formation

When manufacturers try to increase coating speeds, they often encounter instabilities in the coating process that destroy the uniformity of the coated product. One such instability is the formation of rivulets, which are stripes of wet and dry regions on the coating substrate. Rivulet formation in the slide coating is typically observed to start near an end wall and then propagate across the slide as additional stripes are formed.

CFD simulation modeling rivulet formation with FLOW-3D
Example of rivulet formation in
a FLOW-3D coating simulation

At the beginning of this simulation there is a coating liquid on the slide, but it does not extend to the tip. A constant input of liquid onto the slide surface is made at the upstream boundary of the computation.

Initially, liquid is pulled up the end wall by adhesion before it reaches across the gap and attaches to the coating surface. After about 26 ms liquid has reached the coating surface along the entire length of the slide. Uniform coating conditions are reached after approximately 40ms. The speed of the coating surface is impulsively accelerated from 20.0 cm/s to 30.0 cm/s at the 40 ms time point. After a few more milliseconds a tear opens in the coating bead near the stationary end wall. Once the tear appears, surface tension forces contract the liquid surfaces on either side of the tear causing a rivulet with vertical sides to be generated on the coating surface.