Abstract
The drag of fluid flow at the solid–liquid interface in the micro/nanoscale is an important issue in micro/nanofluidic systems. Drag depends on the surface wetting, nanobubbles, surface charge and boundary slip. Some researchers have focused on the relationship between these interface properties. In this review, the influence of an applied voltage on the surface wettability, nanobubbles, surface charge density and slip length are discussed. The contact angle (CA) and contact angle hysteresis (CAH) of a droplet of deionized (DI) water on a hydrophobic polystyrene (PS) surface were measured with applied direct current (DC) and alternating current (AC) voltages. The nanobubbles in DI water and three kinds of saline solution on a PS surface were imaged when a voltage was applied. The influence of the surface charge density on the nanobubbles was analyzed. Then the slip length and the electrostatic force on the probe were measured on an octadecyltrichlorosilane (OTS) surface with applied voltage. The influence of the surface charge on the boundary slip and drag of fluid flow has been discussed. Finally, the influence of the applied voltage on the surface wetting, nanobubbles, surface charge, boundary slip and the drag of liquid flow are summarized. With a smaller surface charge density which could be achieved by applying a voltage on the surface, larger and fewer nanobubbles, a larger slip length and a smaller drag of liquid flow could be found.
Highlights
The interface of solid and liquid plays an important role in liquid flow in various fluidics based micro/nano-electromechanical systems (MEMS/NEMS), which have a large surface area to volume ratio [1,2]
We have reviewed the recent research on the surface wetting, nanobubbles and boundary slip with an applied voltage
We tried to find the relationships between the surface charge density, slip length, nanobubbles, surface wetting and the drag of fluid flow based on the experiments by applying voltage
Summary
The interface of solid and liquid plays an important role in liquid flow in various fluidics based micro/nano-electromechanical systems (MEMS/NEMS), which have a large surface area to volume ratio [1,2]. At the interface of solid and liquid, surface wetting, surface charge, nanobubbles and boundary slip are believed to affect the drag of liquid flow [3,4,5,6,7,8,9,10]. It can be inferred that when a voltage is applied, the surface wettability, surface charge, nanobubbles and boundary slip will be changed, which causes the change in the drag of liquid flow. The study of the influence of the applied voltage on the surface wettability, surface charge, nanobubbles and boundary slip is important and necessary for the minimization of the drag of liquid flow. Wettability, surface charge, nanobubbles and boundary slip with applied voltage may provide a method to reduce the drag of liquid flow in micro/nano scale. Bhushan and Pan found the CAH of DI water on a PS surface could be decreased by applying an AC voltage [76]
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