Directional Transport of a Liquid Drop between Parallel–Nonparallel Combinative Plates

Abstract

Liquids confined between two parallel plates can perform the function of transmission, support, or lubrication in many practical applications, due to which to maintain liquids stable within their working area is very important. However, instabilities may lead to the formation of leaking drops outside the bulk liquid, thus it is necessary to transport the detached drops back without overstepping the working area and causing destructive leakage to the system. In this study, we report a novel and facile method to solve this problem by introducing the wedgelike geometry into the parallel gap to form a parallel-nonparallel combinative construction. Transport performances of this structure were investigated. The criterion for self-propelled motion was established, which seemed more difficult to meet than that in the nonparallel gap. Then, we performed a more detailed investigation into the drop dynamics under squeezing and relaxing modes because the drops can surely return in hydrophilic combinative gaps, whereas uncertainties arose in gaps with a weak hydrophobic character. Therefore, through exploration of the transition mechanism of the drop motion state, a crucial factor named turning point was discovered and supposed to be directly related to the final state of the drops. On the basis of the theoretical model of turning point, the criterion to identify whether a liquid drop returns to the parallel part under squeezing and relaxing modes was achieved. These criteria can provide guidance on parameter selection and structural optimization for the combinative gap, so that the destructive leakage in practical productions can be avoided.

Motion of droplets into hydrophobic parallel plates

“…Finally, an optimized ratchet cycle was carried out to verify our analysis. Compared with previous studies, 25,30,31,37,38 the present strategy provides the following new advances.…”

“…This phenomenon has been speculated to be mainly caused by the asymmetric depinning (AD) of droplets, which means that the trailing edge remains pinned as the leading edge advances during the squeezing process and that the leading edge remains pinned as the trailing edge recedes during the relaxing process. 28,[30][31][32][33] Later research found that the droplet is transported not only by the AD but also by symmetric depinning (SD), which means that both the leading and trailing edges are advancing or receding. 31 In fact, the efficiency of the SD model is much lower than that of the AD model.…”

“…28,[30][31][32][33] Later research found that the droplet is transported not only by the AD but also by symmetric depinning (SD), which means that both the leading and trailing edges are advancing or receding. 31 In fact, the efficiency of the SD model is much lower than that of the AD model. If the SD model is included in the transport process, the movement efficiency of the liquid bridge will be reduced.…”

“…Wang et al 34 demonstrated that hydrophobic saw-tooth plates could improve the transport efficiency substantially by obstructing the inefficient SD model. In addition to the uniform structure, Huang et al 31 proposed a parallelnonparallel (P-NP) combinative structure for immersion lithography; however, it cannot be used in hydrophobic situations. How to transport droplets from an SPS into a PPS, which can be widely used in microuidics applications, has not been investigated.…”

Enhancing Spontaneous Droplet Motion on Structured Surfaces with Tailored Wedge Design

“…This can be achieved by creating a wettability gradient through chemical or thermal treatment, [ 22–25 ] roughness gradient through varying the spacial roughness ratio, [ 8,26–30 ] or surface structural gradient. [ 2,3,31–37 ] It has also been shown that spontaneous motion can be achieved between nonparallel [ 38–40 ] or flexible plates. [ 41 ] Besides, Chen et al [ 42 ] demonstrated the continuous unidirectional liquid spreading on the peristome surface of Nepenthes alata , which inspired many surface designs for liquid transport.…”