Electrophoretic motion of a liquid droplet in a cylindrical pore

Electrophoretic motion of a liquid droplet in a cylindrical pore is investigated theoretically at arbitrary levels of surface potential and double-layer thickness. The effect of double-layer polarization and the droplet viscosity are taken into account. We found, among other things, that the lower the viscosity of the droplet is, the greater the electrophoretic velocity is. On the other hand, an induced electric field opposite to the direction of the droplet motion slows down the droplet movement in general. This is referred to as the polarization effect here, which manifests itself for highly charged droplets. The presence of a nearby cylindrical pore further complicates this polarization effect both hydrodynamically and electrostatically. An intriguing phenomenon is observed when the fluidic channel becomes very narrow: the lowly charged droplet may move faster than the highly charged one. The reason behind it is thoroughly explained here. Moreover, a three-dimensional axisymmetric vortex flow is observed inside the droplet as a rheological response to the (electric) driving force. In addition, with the generation of an electroosmotic flow, the charged wall may either enhance the droplet motion or deter it, depending on the surface potential on the wall and the double layer thickness of the droplet. This study provides fundamental information about the electrophoretic motion of a droplet in micro/nanofluidic channels, which is essential to the successful design and operation in related practical applications, such as microreactors or microcapsules frequently utilized in chemical and biological fields. ? 2012 American Chemical Society.