Electrohydrodynamic coating flows

What makes coating processes unique is the presence of multiple free surfaces. Imposing an electric field produces electric stress only at free surfaces in most practical coating flows. There it is directed from the liquid to the gas. Properly deployed, this electric stress prevents or delays the onset of defect-generating phenomena of wavy wetting line, jagged wetting line, air fingers and air entrainment.; Electrostatically assisted slot coating was modeled with the Navier-Stokes system of equations in the liquid phase and a Laplace equation system describing the static electric field in the substrate and in the air and liquid phases. The two systems are coupled at the air-liquid interface through interfacial boundary conditions. Parameter space mapping by continuation in capillary number, Taylor number (the ratio of electric to viscous stress), Reynolds number, and parameters of die shape and placement reveals an operating window.; A slot coater that employs a cantilevered glass annulus as a backing roll was used to investigate electrostatically assisted slot coating. The effects of an electrostatic field on the apparent dynamic and static contact angles, wetting line shape, vee formation, air entrainment, and bead breakup into rivulets were studied. As expected, the coating window is enlarged by delay in the onset of vee formation, air entrainment, and rivulets. The minimum ratio of final wet layer thickness to the coating gap that can be coated is lower the greater the electrical potential difference across the coating gap.; An electric force acting on the free surface changes the apparent dynamic contact angle and pulls the upstream free surface farther upstream. The increased length of the coating bead and the change in contact angle delay the onset of coating bead instabilities such as wavy contact lines, air vees and air entrainment and finally complete breakdown of the coating bead into alternating dry and coated lanes.