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Abstract
We studied the deformation of thin liquid films induced by surface charge patterns at the solid–liquid interface quantitatively by experiments and numerical simulations. We deposited a surface charge distribution on dielectric substrates by applying potential differences between a conductive liquid droplet and a grounded metal plate underneath the substrate that was moved in a pre-defined trajectory. Subsequently, we coated a thin liquid film on the substrate and measured the film thickness profile as a function of time by interference microscopy. We developed a numerical model based on the lubrication approximation and an electrohydrodynamic model for a perfect dielectric liquid. We compared experiments and simulations of the film deformation as a function of time for different charge distributions and a good agreement was obtained. Furthermore, we investigated the influence of the width of the surface charge distribution and the initial film thickness on the dielectrophoretic deformation of the liquid film. We performed a scaling analysis of the experimental and numerical results and derived a self-similar solution describing the dynamics in the case of narrow charge distributions.
Highlights
Local build-up of electrostatic charges can potentially cause hazards, especially in high-speed processing of dielectric materials
We deposited electrostatic surface charge patterns on dielectric substrates by applying a voltage to a droplet of de-ionized water that was moved in a pre-de ned trajectory over the dielectric substrate
Synchronized control of the translation stages and high voltage power supply as well as acquisition of the signal from the electrostatic voltmeter described in Section II B was achieved by the Labview so ware (National Instruments)
Summary
Local build-up of electrostatic charges can potentially cause hazards, especially in high-speed processing of dielectric materials. Examples are dust attraction on solar cells[1] or explosions due to charge build-up in the oil industry.[2] In roll-toroll liquid coating processes, non-uniform static charge densities at the substrate may cause deformations and defects in the nal coating, whereas uniform charges are sometimes utilized to improve the coatability of partially wetting substrates.[3,4,5]. It is well known that liquid lms can deform due to an inhomogeneous electric eld.[6] Hat eld[7] recognized the application potential of this mechanism for separation processes, which since has found numerous applications in microbiology and colloid science and technology.[8,9] Chou et al studied lithographically induced self-assembly of periodic polymer micropillar arrays[10,11,12,13] and achieved feature sizes on the order of 50 nm.[14] They hypothesized that the lm deformations were governed by an electrohydrodynamic mechanism. Schaffer et al studied the pattern formation and replication in ultrathin polymer lms and bilayers induced by an electric eld applied perpendicular to the polymer lms.[15,16,17] Subsequently, many groups conducted experiments or numerical simulations of instabilities of single or multiple liquid layers sandwiched
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