Figure 5
From: Effective directional self-gathering of drops on spine of cactus with splayed capillary arrays
Illustration of splayed capillary micro-tubes induced self-transport of drop.
(a) Drop moving continuously on the spine with splayed capillary micro-tubes array composed of overlapping titled-up scales from the top to bottom of spine as an integrating regime for water self-gathering. (b) The effect of scale-by-scale array and formation of ultra low adhesion. As the drop is condensed on the initial scale defined as scale 0 at top of spine. The insets (Frames b1-b4) indicate the effect of capillary micro-tube array on spine for drop self-transport. Drop spreads asymmetrically as a result of the different contact angles on the left (θl) and right (θr) sides (Frame b1). The driving force (Fd) can be generated by the difference of advancing (left) contact angle and receding (right) contact angle. Drop moves directionally influenced by the splayed capillary micro-tubes. Liquid meniscus in scale 0 introduces Laplace pressure P0, which propels the drop liquid into scale 0 and spread over location of scale 1. P0 decreases with the growing liquid menisci in scale 0 (Frame b2). The liquid of the left side of the drop enters into the capillary micro-tube formed by scale 2. The Laplace pressure (P2 and P3) introduced by the curvatures of liquid in scale 2 drags the liquid tend to move to location of the scale 3 (Frame b3). P3 is growing larger with liquid spreading and finally will be larger than P0. When the Laplace pressure difference ΔP produced by cooperation of P0, P2 and P3 is large enough, the entire drop overcomes the hinge effect of scale 2 and move toward location of scale 3 (Frame b4). (c,d) Drop self-transport from scale-by-scale and formation of ultra-thin film over surface of spine. Drop moves directionally due to driving force resulted from the asymmetric contact at interface of scale-by-scale (c). Due to the cavities between splayed scales and groove base of spine, the drop remains the ultra thin liquid film along overlapping scales to form ultra low-adhesive surface on spine (d).
