Droplet Actuation by Electrowetting-on-Dielectric (EWOD): A Review

This paper reviews publications that have fortified our understanding of the electrowetting-on-dielectric (EWOD) actuation mechanism. Over the last decade, growing interest in EWOD has led to a wide range of scientific and technological investigations motivated by its applicability in microfluidics, especially for droplet-based optical and lab-on-a-chip systems. At this point in time, we believe that it is helpful to summarize the observations, insights, and modeling techniques that have led to the current picture showing how forces act on liquid droplets and how droplets respond in EWOD microfluidic devices. We discuss the basic physics of EWOD and explain the mechanical response of a droplet using free-body diagrams. It is our hope that this review will inspire new research approaches and help design useful devices. © 2012 Copyright Taylor and Francis Group, LLC

Water in oil microemulsions : criteria for dilution at constant droplet size

The validity of the customary method for diluting water in oil (w/o) microemulsions is discussed. In concentrated systems and in the vicinity of critical consolute points, the method has deficiencies.On discute la validité de la procédure de dilution des microémulsions utilisée dans de nombreux laboratoires, notamment aux concentrations élevées et au voisinage de points critiques de démixtion

Water in oil microemulsions : transient electric birefringence response

The structure of water in oil microemulsions has been investigated using transient electric birefringence. At low water concentrations, transient coalescence of droplets is observed. At higher water concentrations, the Kerr signal probes density fluctuations. A fast negative response is attributed to the relaxation of interfacial layers. Further work is under way to make these preliminary results more quantitative.Nous avons étudié la structure de microémulsions eau dans huile au moyen de la biréfringence électrique transitoire. A faible concentration, on observe l'association des gouttelettes, à concentration moyenne des fluctuations de densité dans le système. Une composante négative, rapide, est attribuée aux fluctuations des interfaces. Une étude plus complète suivra ces résultats préliminaires

Dynamics of wetting on smooth and rough surfaces.

The rate of spreading of non-volatile liquids on smooth and on rough surfaces was investigated. The radius of the wetted spot was found to agree with recently proposed scaling laws (t 1/10 for capillarity driven andt 1/8 for gravity driven spreading) when the surface was smooth. However, the crossover between these regimes was not observed at a constant value of the radius. Rough surfaces exhibited at least four spreading regimes which could be rationalized in terms of a macroscopic contact angle and macroscopic deviations thereof. An interesting feature is the appearance of a wetted rim around the central drop. This rim follows a diffuce spreading law (t 1/2). Partially wetting liquids followed the same dynamics provided the spreading conditions was fulfilled

Dynamics of wetting on smooth and rough surfaces.

The rate of spreading of non-volatile liquids on smooth and on rough surfaces was investigated. The radius of the wetted spot was found to agree with recently proposed scaling laws (t 1/10 for capillarity driven andt 1/8 for gravity driven spreading) when the surface was smooth. However, the crossover between these regimes was not observed at a constant value of the radius. Rough surfaces exhibited at least four spreading regimes which could be rationalized in terms of a macroscopic contact angle and macroscopic deviations thereof. An interesting feature is the appearance of a wetted rim around the central drop. This rim follows a diffuce spreading law (t 1/2). Partially wetting liquids followed the same dynamics provided the spreading conditions was fulfilled

A moving contact line: Further studies of 'Haines' jumps'.

When a liquid containing surface active molecules advances on a hydrophilic solid surface, specific instabilities are observed. They are due to the diffusion of the surface active molecules which adsorb on the surface, increasing the contact angle. These instabilities appear as waves travelling along the contact line and are visible only in a limited range of liquid edge velocities

Anomalous viscosity of microemulsions near a critical point

We have observed an anomalous behaviour of the shear viscosity close to a critical point in microemulsion systems. Although there is a good qualitative agreement between experiments and theory, the quantitative comparison is less satisfactory.Nous avons observé une anomalie de la viscosité de cisaillement de phases microémulsions au voisinage d'un point critique. Malgré un bon accord qualitatif avec la théorie, l'interprétation quantitative est encore insuffisante