Abstract
It has been reported that, in a foggy environment, water drops with either barrel or clam-shell shapes are capable of self-running on conical wire-like structures, such as cactus spines, spider silk, and water striders’ legs. On the other hand, the corresponding moving mechanisms are still not quite understood. For instance, it is unclear under what conditions clam-shell drops would move from the tip towards the root on a conical wire. In this work, based on the balance of forces, we derive conditions for a drop to self-transport towards or away from the root. We find that, although barrel and clam-shell drops have different shapes, these conditions are applicable to both of them, which thus provide good guidelines for developing artificial fog collectors. Furthermore, based on the derived conditions, we interpret drop movements on both hydrophilic and hydrophobic wires, with the support of experimental results on cactus spines. Finally, our results indicate that not all the cacti are able to harvest water from fog.
Highlights
When the drop transports a certain distance away from the tip, it may stop due to the violation of this inequality. The result of this case indicates that, if contact angle hysteresis is larger than conical angle, and if a conical wire is lyophilic, a large drop may still move towards the root on the conical wire
The contact angle hysteresis of either isopropyl alcohol (IPA) or oil was less than the corresponding conical angle on these spine samples
Given that contact angle hysteresis is zero and that conical angle is small, drops may move towards the root and tip, respectively, on lyophilic and lyophobic wires
Summary
It has been reported that, in a foggy environment, water drops with either barrel or clam-shell shapes are capable of self-running on conical wire-like structures, such as cactus spines, spider silk, and water striders’ legs. The corresponding moving mechanisms are still not quite understood It is unclear under what conditions clam-shell drops would move from the tip towards the root on a conical wire. Liquid drops may exhibit two basic shapes: barrel and clam-shell (Fig. 1). An inequality, which involves the drop size, contact angle, and wire radius, has been previously derived in ref 4 to identify whether a drop has a barrel or clam-shell shape. Using a similar line of reasoning, we considered the case that the apparent contact angle was non-zero[8] We found that these drops might move from the tip to root only on a lyophilic conical wire. A clam-shell drop at rest suffers three forces: gravity, supporting force of the wire, and surface tension-induced force (Fig. 2a).
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