Abstract
Abstract Self-propelled droplet is important in the digital microfluidic systems for increasing mixing velocity, and directional water collection or quick drainage without the needs of external pumps and valves. However, one major challenge in moving discrete liquid droplets is to overcome the hysteresis force and viscous shear force. These two forces will lead to deceleration and stop of the droplet, unless other external forces are applied. Thus, conventionally, spontaneous motion efforts toward the use of surface tension gradient for liquid transport. To further increase this moving tendency, this paper reports the study on spontaneous and fast motion for a water droplet on nanotextured glass conical-shaped capillary surfaces with a wide range of curvature gradients. The surface is highly related to the surface tension gradient that is mainly formed by three distinct driving forces, including hydrophilic gradients, chemically patterned nanotexture, and curvature gradient capillaries. The experiments were performed by dispensing a small water droplet on a conical glass capillary surface which the radius of the curvature gradients are fixed to 15–750 um. The droplet moves towards the region of lower curvature with four different surface conditions, including A. original glass surface (CA~25°), B. oxygen plasma treated surface (CA~10°), C. oxygen plasma treated plus silane-modified surfaces (CA~30°), and D. oxygen plasma treated nanotextured surface (CA~0°). The initial position, the size of the droplet, and the cone half angle movement behavior on these four different surfaces were also compared in this study. The evolutions of droplet movements on different surfaces shows the droplet velocity increases with the decreasing of the surface contact angle, and the maximum speed appears on the D surface up to 28.4 cm/s. Also, the droplet velocity shows a dependency on the droplet initial position at the conical capillary curvature surfaces, and the initial position with larger curvature can induce higher droplet velocity. Besides, larger droplet size induces faster droplet moving velocity for gaining larger Laplace pressure on the conical surface. Finally, the moving velocity of a droplet will increase as the cone half angle increase. Therefore, we can conclude that the nanowetting accompanies curvature gradient actuation by the capillarity and Laplace pressure for manipulating water droplet.