Liquid Drop Runs Upward between Two Nonparallel Plates

We have recently observed an interesting phenomenon: even under gravity, a microliter-scaled silicone oil drop was still able to run upward between two nonparallel plates that were approximately vertically placed. We also saw the same phenomenon in the case of isopropyl alcohol (IPA) drops. In this work, we developed simple models to interpret this phenomenon, followed by experimental validation. We demonstrated that, by changing the locations of drops or tilt and opening angles of plates, the moving directions of silicone oil, IPA, and water drops could be controlled. In the cases of silicone oil and IPA, we also found that the speed of a drop had a linear relation with the square of the drop location when the drop was far away from the corner of two nonparallel plates and that the drop moved faster as it became closer to this corner

Liquid Drop Runs Upward between Two Nonparallel Plates

We have recently observed an interesting phenomenon: even under gravity, a microliter-scaled silicone oil drop was still able to run upward between two nonparallel plates that were approximately vertically placed. We also saw the same phenomenon in the case of isopropyl alcohol (IPA) drops. In this work, we developed simple models to interpret this phenomenon, followed by experimental validation. We demonstrated that, by changing the locations of drops or tilt and opening angles of plates, the moving directions of silicone oil, IPA, and water drops could be controlled. In the cases of silicone oil and IPA, we also found that the speed of a drop had a linear relation with the square of the drop location when the drop was far away from the corner of two nonparallel plates and that the drop moved faster as it became closer to this corner

Wetting States on Circular Micropillars with Convex Sidewalls after Liquids Contact Groove Base

It is considered that the Cassie–Baxter wetting state should be transitioned to that of the Wenzel state if a water drop has contact with the base of a roughness groove. However, in recent tests on the leaf surfaces of three lotus varieties, we found that the transition did not occur. To explore the reason behind this, in this work we model the corresponding surface structures as circular micropillars with convex sidewalls, and derive an angle inequality for judging transition from Cassie–Baxter to Wenzel States. We also consider the transition on spherical microparticles, as well as that on circular micropillars with straight sidewalls. Finally, we validate the angle inequality through pressing tests on three lotus varieties and spherical microballs

Angle Inequality for Judging the Transition from Cassie–Baxter to Wenzel States When a Water Drop Contacts Bottoms of Grooves between Micropillars

A specific criterion is often used to judge the transition from Cassie–Baxter wetting state to that of Wenzel. In this work, we examine the applicability of this specific criterion to the case of micropillars with circular or polygonal cross-sections. For this purpose, we derive an angle inequality. When this angle inequality is violated, the specific criterion holds for the corresponding micropillars. Otherwise, the specific criterion may not be applicable to such micropillars. These results are validated by pressing tests on six types of micropillars, which have circular, triangular, square, hexagonal, T-shaped, and star-like cross-sections

Bioinspired Plate-Based Fog Collectors

In a recent work, we explored the feeding mechanism of a shorebird to transport liquid drops by repeatedly opening and closing its beak. In this work, we apply the corresponding results to develop a new artificial fog collector. The collector includes two nonparallel plates. It has three advantages in comparison with existing artificial collectors: (i) easy fabrication, (ii) simple design to scale up, and (iii) active transport of condensed water drops. Two collectors have been built. A small one with dimensions of 4.2 × 2.1 × 0.05 cm³ (length × width × thickness) was first built and tested to examine (i) the time evolution of condensed drop sizes and (ii) the collection processes and efficiencies on the glass, SiO₂, and SU-8 plates. Under similar experimental conditions, the amount of water collected per unit area on the small collector is about 9.0, 4.7, and 3.7 times, respectively, as much as the ones reported for beetles, grasses, and metal wires, and the total amount of water collected is around 33, 18, and 15 times. On the basis of the understanding gained from the tests on the small collector, a large collector with dimensions of 26 × 10 × 0.2 cm³ was further built and tested, which was capable of collecting 15.8 mL of water during a period of 36 min. The amount of water collected, when it is scaled from 36 to 120 min, is about 878, 479, or 405 times more than what was collected by individual beetles, grasses, or metal wires

Separation of Oil from a Water/Oil Mixed Drop Using Two Nonparallel Plates

In this work, we have developed a simple approach to separate oil from a microliter-scaled water/oil mixture by squeezing the mixture using two nonparallel plates. Three pairs of plates with Teflon, SU-8, and SiO₂ coatings, respectively, are used in the tests, and all of these plates are capable of separating the water/oil mixed drops. 95.5% silicone oil and 97.0% light mineral oil have been collected from their corresponding mixtures with water through the pair of Teflon plates. Furthermore, on the basis of pressure difference inside a liquid drop, theoretical models have been developed to interpret the corresponding mechanisms of the separation process, as well as the observed phenomena. To judge whether two immiscible liquids could be separated using the developed approach, a sufficient condition has also been derived, which includes three theoretical relations. The sufficient condition is subsequently validated by experiments. This condition also provides criteria for choosing a good plate coating. Such a coating should ensure (i) the oil wets the plate surface with a relatively large contact angle, and has small contact angle hysteresis, and (ii) the advancing contact angle that the water/oil interface forms on the plate surface is larger than 90°

Bioinspired Plate-Based Fog Collectors

In a recent work, we explored the feeding mechanism of a shorebird to transport liquid drops by repeatedly opening and closing its beak. In this work, we apply the corresponding results to develop a new artificial fog collector. The collector includes two nonparallel plates. It has three advantages in comparison with existing artificial collectors: (i) easy fabrication, (ii) simple design to scale up, and (iii) active transport of condensed water drops. Two collectors have been built. A small one with dimensions of 4.2 × 2.1 × 0.05 cm³ (length × width × thickness) was first built and tested to examine (i) the time evolution of condensed drop sizes and (ii) the collection processes and efficiencies on the glass, SiO₂, and SU-8 plates. Under similar experimental conditions, the amount of water collected per unit area on the small collector is about 9.0, 4.7, and 3.7 times, respectively, as much as the ones reported for beetles, grasses, and metal wires, and the total amount of water collected is around 33, 18, and 15 times. On the basis of the understanding gained from the tests on the small collector, a large collector with dimensions of 26 × 10 × 0.2 cm³ was further built and tested, which was capable of collecting 15.8 mL of water during a period of 36 min. The amount of water collected, when it is scaled from 36 to 120 min, is about 878, 479, or 405 times more than what was collected by individual beetles, grasses, or metal wires

Attachment and Release of Water Fleas’ Ephippia on a Medium-Sized Waterfowl’s Leg for Migration

Planktonic crustaceans of the genus Daphnia live in aquatic environments. Although they lack walking and flying capabilities, they have developed adaptations that facilitate the dispersal of their dormant forms, ephippia, to cross terrestrial barriers and reach neighborhood water bodies. It increases the survival rate of their species. It is reported that one of the ways this spread occurs is the transport of their ephippia through waterfowls’ legs. Yet, little is known about how these ephippia are initially attached to the waterfowls’ legs. In this work, using the legs of American Pekin ducks as test samples, we found that a “coating” mechanism might play a significant role in this attachment and that surface tension-induced attraction might have a secondary effect on it. Furthermore, we demonstrated that, no matter whether a duck’s leg was inserted into water at a high or low speed, an ephippium could be released from the leg

Attachment and Release of Water Fleas’ Ephippia on a Medium-Sized Waterfowl’s Leg for Migration

Planktonic crustaceans of the genus Daphnia live in aquatic environments. Although they lack walking and flying capabilities, they have developed adaptations that facilitate the dispersal of their dormant forms, ephippia, to cross terrestrial barriers and reach neighborhood water bodies. It increases the survival rate of their species. It is reported that one of the ways this spread occurs is the transport of their ephippia through waterfowls’ legs. Yet, little is known about how these ephippia are initially attached to the waterfowls’ legs. In this work, using the legs of American Pekin ducks as test samples, we found that a “coating” mechanism might play a significant role in this attachment and that surface tension-induced attraction might have a secondary effect on it. Furthermore, we demonstrated that, no matter whether a duck’s leg was inserted into water at a high or low speed, an ephippium could be released from the leg

Behavior of a Liquid Drop between Two Nonparallel Plates

Liquid drops have shown interesting behaviors between two nonparallel plates. These plates may be fixed or movable relative to each other. In this work, we also explore these behaviors through a combination of theoretical and experimental investigations and obtain some new results. We show that when the two plates are fixed, different from the previous understanding, a lyophilic drop may not necessarily fill the corner of the two plates. We also demonstrate that it may fill the corner, when more liquid is added to the drop or when the top plate is lifted. Furthermore, we propose a physical model to interpret the shifting effect of a liquid drop. This effect appears when the drop is squeezed and relaxed between two nonparallel plates, and it has been used by some shorebirds to transport prey. On the basis of the proposed model, we have found three new phenomena related to the shifting effect