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Image by F.Weyer/ULg
The picture shows two compound droplets on a horizontal fiber. They are made of an oil drop containing a pure (blue, on the left) or a soapy (red, on the right) water droplet.
The compound droplets are of great importance in fiber-based microfluidics since they prevent evaporation and contamination. Depending on the nature of the inner droplet, the behavior of the system is different. For pure water, the inner droplet forms a spherical cap whereas, for soapy water, the droplet spreads inside the oil drop and a quadruple contact line is formed. The position of the droplets on the fiber depends on the balance between capillary and gravity forces. When the gravity forces prevail over the capillary forces, the droplets fall off the fiber. The picture shows the form adopted by the droplets when they are close to the detachment. The nature of the inner droplet influences the shape of the compound drop which is more elongated for pure water than for soapy water.
F.Weyer is financially supported by an FNRS grant. This work is also supported by the FRFC 2.4505.12.