The spreading of liquids on solids is of significant importance in everyday life, for instance, while painting, writing with ink on a sheet of paper and spreading of the oil film over the pan. Wetting and spreading also play basic role in many industrial applications [113-120], wetting of porous rocks in oil/water reservoirs [121], in agriculture [117,119], phase-change heat transfer [115, 116] as well as numerous other fields such as coating, deposition, soldering, and lubrication.
In all the above applications, detailed knowledge of the spreading dynamics is needed. In general, spreading is a complicated problem since many known or unknown parameters are involved. This is a common problem in the field of surface science. Even a small amount of impurity in the liquid phase or solid surface may significantly affect the macroscopic observed phenomena. The topography of the solid surface and even its crystallographic structure may affect the spreading dynamics [122].
Spreading of fluids belongs to the broad field of wetting phenomena. There is a macroscopic difference between the case where liquid completely spreads over substrate (oil over stainless steel) and partial wetting (water over stainless steel) [122]. Complete wetting corresponds to the situation, where the macroscopic contact angle tends to zero when the time goes to infinity [123]. Partial wetting corresponds to an equilibrium condition with a non-zero finite contact angle [124].
The key to the analysis of spreading can be found in the experimental studies presented by Hardy [125], who recognized that an ultra thin film always precedes a spreading droplet. The thickness of the film is normally below micron range. This ultra thin film has been called the precursor film. At the early stages of precursor film spreading, the molecular thickness (below 1 nm) develops progressively during the spreading process of a droplet as a tongue-like structure ahead of the nominal contact line of the main spreading film. From the experimental observations through detailed optical techniques at
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the Naval Research Laboratory, Bascom realized that the long-range forces should be considered to explain the behavior and dynamics of the ultra thin film precursors [126]. He noticed that in the case of non-ionic precursor films, the dynamics of the film is governed by a dispersive van der Waals forces, while for the charged films the dynamics of the thin film is governed by both van der Waals forces and charged layer interactions. This was only the beginning and led to more than a century of struggles to better understand different aspects of this complicated problem [125-134]. In spite of such extensive efforts, the mechanisms of the precursor film formation still remained one of the most challenging issues in the field of condensed matter. Moreover, some fundamental paradoxes, most famously, the Huh-Scriven paradox [135] regarding the no- slip boundary condition at the solid substrate that the precursor film spreads, remained unanswered. As Huh and Scriven wrote [135]: “Not even Hercules could sink a solid if the physical model (no-slip boundary condition) were entirely valid.” This has opened more than 40 years ongoing effort of physicists to understand seemingly one of the easiest fluid mechanics problems, coating flow, appearing in every undergraduate fluid mechanics textbooks.
Electrocapillarity, the basis of modern electrowetting, was first explained in detail in 1875 by an ingenious physicist Gabriel Lippmann, who won the Nobel prize in 1908 [136,137]. Lippmann found that the capillary depression of mercury in contact with electrolyte solutions could be varied by applying a voltage between the mercury and electrolyte. The original Lippmann paper has only been available in French, but the interested reader may find a translation of his work in [136]. The work of Lippmann and of those who followed him in the subsequent more than a hundred years was devoted to aqueous electrolytes in direct contact with mercury surfaces or mercury droplets in contact with insulators. A major obstacle to broader applications was electrolytic decomposition of water upon applying voltages beyond a few hundred millivolts. The recent developments were initiated in the early 1990s by Berge [138], who introduced the idea of using a thin insulating layer to separate the conductive liquid from the metallic electrode in order to eliminate the electrolysis phenomenon at the metallic interface. This is the concept that has also become known as electrowetting on dielectric (EWOD).
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Electrowetting on dielectric layer techniques are only limited to the case of controlling a droplet on a low energy surface. Gently deposited droplets over partial wetting substrates reach an equilibrium thermodynamic state with an equilibrium contact angle. The method is difficult or technically impossible to apply for manipulating droplets over a high energy surface since the droplet will completely wet the substrate. Therefore, an alternative method should be used to control the liquid film/droplet on high energy substrates.
The literature dealing with spreading of dielectric liquids over a solid surface is very sparse. Although several investigations were performed to investigate the EHD instabilities induced by corona discharge over a dielectric interface, no net spreading effect has been reported [139,140]. In a recent attempt, the alternative technique for droplet/film spreading over high energy substrates, such as a metallic substrate, was introduced by a joint research group at Harvard and Princeton [141] who demonstrated a selective spreading of a silicone oil film on a relatively high energy substrate. In their technique, the surface charge is accumulated over the dielectric interface and the tangential component of electric forces was created by an inclined planar electrode. A strong shear stress is developed over the interface, which expands the liquid from the low field region to the high field region. The spreading was accompanied by formation of Taylor cones at the liquid interface with several periodic jets ensuing during the film expansion process.