ABSTRACT
Phenomenon ............................................................................................495 19.2.1 Characteristic Timescales ................................................................. 495 19.2.2 Characteristic Length Scales ............................................................ 496
19.3 Electrowetting Theories ................................................................................ 498 19.3.1 Classical Electrowetting Theory: The Lippmann-Young
Equation ........................................................................................ 498 19.3.1.1 Thermodynamic Approach ................................................ 498 19.3.1.2 Energy Minimization Approach ........................................ 499 19.3.1.3 Electromechanical Approach ............................................. 501
19.3.2 Extended Lippmann-Young Equations ............................................504 19.3.2.1 Electrowetting on Rough Surfaces.....................................504 19.3.2.2 Electrowetting on Curved Surfaces ...................................507 19.3.2.3 Electrowetting under High Voltage....................................509 19.3.2.4 Electrowetting at Micro-and Nanoscales .......................... 511
19.3.3 Contact Angle Hysteresis and Droplet Actuation ............................. 513 19.3.3.1 Contact Angle Hysteresis ................................................... 513 19.3.3.2 Minimum Actuation Voltage ............................................. 515
19.3.4 Marangoni Convection ..................................................................... 516 19.3.5 Precursor Film .................................................................................. 517
19.3.5.1 Huh-Scriven Paradox ........................................................ 517 19.3.5.2 Possible Explanations for Huh-Scriven Paradox ............... 518 19.3.5.3 Molecular Kinetic Theory and Properties of Precursor
Film .................................................................................... 518 19.3.6 Spontaneous Electrowetting ............................................................. 521
19.4 Experiments and Molecular Dynamics Simulations .................................... 526 19.4.1 Electrowetting Experiments on Surfaces with
Various Congurations ..................................................................... 526
Electrowetting (EW) or electrowetting-on-dielectric (EWOD) has been widely used as a tool for the manipulation of microuidics in microelectromechanical systems, and there have been rapid developments in the last two decades. In EW, when an external voltage is applied, the contact line of the droplet on solid surface moves until the droplet reaches a new equilibrium. The moving contact line (MCL) phenomenon in EW is a matter of the solid-liquid interface, in which tribology works. This chapter focuses on the MCL problem for EW or EWOD, in which the “Huh-Scriven paradox” is also valid. As a matter of fact, the MCL problem has remained an issue of controversy and debate for more than 40 years since the famous paper by Huh and Scriven in 1971. The dif- culty stems partly from the fact that classical hydrodynamic equations coupled with the conventional no-slip boundary condition predict a singularity for the stress that results in a nonphysical logarithmically singular energy dissipation rate at the triple contact line.
