This article shows morphology-patterned stripes as a new platform for directing flow guidance of the fluid in microfluidic devices. Anisotropic (even unidirectional) spreading behavior due to anisotropic wetting of the underlying surface is observed after integrating morphology-patterned stripes with a Y-shaped microchannel. The anisotropic wetting flow of the fluid is influenced by the applied pressure, dimensions of the patterns, including the period and depth of the structure, and size of the channels. Fluids with different surface tensions show different flowing anisotropy in our microdevice. Moreover, the morphology-patterned surfaces could be used as a microvalve, and gas-water separation in the microchannel was realized using the unidirectional flow of water. Therefore, benefiting from their good performance and simple fabrication process, morphology-patterned surfaces are good candidates to be applied in controlling the fluid behavior in microfluidics.

Smart pH‐responsive surfaces that could autonomously induce unidirectional wetting of acid and base with reversed directions are fabricated. The smart surfaces, consisting of chemistry‐asymmetric “Janus” silicon cylinder arrays (Si‐CAs), are prepared by precise modification of functional groups on each cylinder unit. Herein, amino and carboxyl groups are chosen as typical pH‐responsive groups, owing to their protonation/deprotonation effect in response to pH of the contacted aqueous solution. One side of the Si‐CAs is modified by poly(2‐(dimethylamino)ethyl methacrylate), while the other side is modified by mixed self‐assembled monolayers of 1‐dodecanethiol and 11‐mercaptoundecanoic acid. On such surfaces, it is observed that acid and base wet in a unidirectional manner toward corresponding directions that are modified by amino or carboxyl groups, which is caused by asynchronous change of wetting property on two sides of the asymmetric structures. The as‐prepared Janus surfaces could regulate the wetting behavior of acid and base and could direct unidirectional wetting of water with reversed directions when the surfaces are treated by strong acid or base. Due to the excellent response capability, the smart surfaces are potential candidates to be applied in sensors, microfluidics, oil/water separation, and smart interfacial design.

We show morphology-patterned stripes modified by thermal-responsive polymer for smartly guiding flow motion of fluid in chips. With a two-step modification process, we fabricated PNIPAAm-modified Si stripes on silicon slides, which were employed as substrates for fluid manipulation in microchannels. When the system temperature switches between above and below the lower critical solution temperature (LCST) of PNIPAAm, the wettability of the substrates also switches between strong anisotropy and weak anisotropy, which resulted in anisotropic (even unidirectional) flow and isotropic flow behavior of liquid in microchannels. The thermal-responsive flow motion of fluid in the chip is influenced by the applied pressure, the thickness of PNIPAAm, and dimension of the microchannels. Moreover, we measured the feasible applied pressure scopes under different structure factors. Because of the excellent reversibility and quick switching speed, the chip could be used as a thermal-responsive microvalve. Through tuning the system temperature and adding the assistant gas, we realized successive "valve" function. We believe that the practical and simple chip could be widely utilized in medical detection, immunodetection, protein analysis, and cell cultures.

No abstract

liquid transportation, [3] self-cleaning, [4,5] and design of smart devices. [6][7][8][9] Some anisotropic wetting structures have long existed in nature, which help numerous plants or animals adapt to the living surroundings and realize diverse functions, for example, directional water repelling by butterflies, [10] fog-harvesting by cacti, some beetles, or spider silk, [11][12][13] and 1D water-directing by some plant leaves. [14] In these cases, the surfaces usually possess directional micro-nanoscale structures with asymmetric energy barrier in different directions, causing anisotropic wetting or liquid adhesion phenomena. Based on the principle of introducing asymmetric micro-or nanostructures, lots of artificial anisotropic wetting surfaces have been prepared. [15][16][17][18][19][20][21][22] Cai et al. prepared a polyacrylic acid-grafted cloth corduroy inspired by filefish, which owns oriented hook-like spines structure, and the surface possesses the ability of anisotropic underwater oleophobicity, which could be applied in directional transport of oil. [22] For morphology-based anisotropic wetting surface, apart from introducing asymmetric arrays, another strategy is focused on groove geometric surfaces. [23][24][25][26][27][28] In the case of surfaces with groove geometries, when water is deposited onto them, air-liquid-solid three contact line is discontinuous due to the groove structures, leading to different contact angle (CA) in different directions. Besides morphology-based anisotropic wetting surface, chemical heterogeneity of the surfaces could also induce water anisotropic wetting. [29][30][31][32][33] In the preparation of many chemical heterogeneity induced anisotropic wetting surfaces, patterning methods are employed to generate heterogeneous regions. The inhomogeneous surface tension dominates the motion of water, resulting in anisotropic wetting of water.Investigation of wetting behavior of water on inhomogeneous anisotropic wetting surfaces, such as surfaces integrate two anisotropic wetting structures, is expected. On the one hand, wetting behavior of water on patterned surfaces which integrate two isotropic patterns has already attracted wide scientific attention. Patterning methods can be utilized to integrate regions of different chemical compositions or substrate morphologies in a single surface. There are various technologies This paper reports the directional wetting behavior of water on a patterned heterogeneous surface, which is integrated by two asymmetric nanostructures with different directions. The asymmetric nanostructures are Janus silicon cylinder arrays (Si-CAs) modified by molecules with distinct surface energies on two sides. Through a photolithography process, the two asymmetric nanostructures are integrated in a single surface to form chessboard patterns with different directions. When water is injected onto the patterned surface, water droplets wet in a unidirectional manner, and the wetting direction is along resultant direction of the two wetting directions of water ...