Electric Field Induced Patterning of Thin Coatings on Fiber Surfaces

We explore the electric field induced instabilities of a thin liquid film coated on a fiber surface. Thin liquid films on the curved surfaces spontaneously self-organize into interesting patterns such as a string of beads/droplets when the destabilizing radial curvature force dominate over the stabilizing in plane curvature. Application of an external electrostatic field in such a situation opens up the added possibility of fabricating ridge like structures issuing outward from the film surface when electric field dominates over the curvature forces. With the help of a general linear stability analysis and long-wave nonlinear simulations, the study uncovers the conditions under which the ridges and beads form on the fiber surface. In particular, we show that the ridges are favored morphologies when electric field is stronger because of higher film to air filling ratio in between the electrodes, higher applied voltage, and larger fiber radius. The beads and the ridges are found to coexist when the destabilizing forces are equally dominant whereas only beads are formed when the radial curvature force is the dominant destabilizing force. The analysis also shows that tuning the voltage, fiber radius, and film thickness, length scales ranging from a few hundred nanometers to a few micrometers can be achieved for all these morphologies. The results reported can potentially be exploited in the applications related to the field-induced microfabrication on the curved surfaces.