A temperature controlled dual water/oil on-off switch is achieved by using a PMMA-b-PNIPAAm block-copolymer coated mesh, determined by the conformational change of the PNIPAAm chain around the lower critical solution temperature (LCST) and also the cooperation between PNIPAAm and PMMA. Water can permeate through the BCP-coated mesh, and oil cannot below the LCST, whereas oil can and water cannot above the LCST.
Since the water-collecting ability of the wetted cribellate spider capture silk is the result of a unique fiber structure, bioinspired fibers have been researched significantly so as to expose a new water-acquiring route in fogging-collection projects. However, the design of the geometry of bioinspired fiber is related to the ability of hanging drops, which has not been investigated in depth so far. Here, we fabricate bioinspired fibers to investigate the water collection behavior and the influence of geometry (i.e., periodicity of spindle knot) on the hanging-drop ability. We especially discuss water collection related to the periodicity of geometry on the bioinspired fiber. We reveal the length of the three phase contact line (TCL) at threshold conditions in conjunction with the maximal volume of a hanging drop at different modes. The study demonstrates that the geometrical structure of bioinspired fiber induces much stronger water hanging ability than that of uniform fiber, attributed to such special geometry that offers effectively an increasing TCL length or limits the contact length to be shorted. In addition, the geometry also improves the fog-collection efficiency by controlling tiny water drops to be collected in the large water drops at a given location.
Smart anisotropic-unidirectional spreading is displayed on a wettable-gradient-aligned fibrous surface due to a synergetic directing effect from the aligned structure and the ratio of hydrophilic components.
We designed a kind of smart bioinspired fiber using the N-isopropylacrylamide (NIPAAm) polymer, with roughness and curvature features similar to those of wetted spider silk. The motion of tiny water droplets can be manipulated reversibly in directions by the effective cooperation of multi-gradients such as roughness, curvature and temperature-responsive wettability.
Controlled directional spreading of a droplet on a smart high-adhesion surface was made possible by simply controlling anodic oxidation. The wettability gradient of the surface was controlled from 0.14 to 3.38° mm(-1) by adjusting the anodic oxidation conditions. When a water droplet made contact with the substrate, the droplet immediately spread in the direction of the wettability gradient but did not move in other directions, such as those perpendicular to the gradient direction, even when the surface was turned upside down. The spreading behavior was mainly controlled by the wettability gradient. Surfaces with a V- or inverse-V-shaped wettability gradient were also formed by the same method, and two droplets on these surfaces spread either toward or away from one another as designed. This method could be used to oxidize many conductive substrates (e.g., copper, aluminum) to form surfaces with variously shaped wettability gradients. It has potential for application in microfluidic devices.
Heterostructure rough spindle‐knot microfibers (HRSFs) are fabricated via a flexible parallel‐nozzle microfluidic method. In this method, the bioinspired HRSF with a roughness gradient between spindle‐knots and joints, can be manufactured in large‐scale, and with which the size of the spindle‐knots and joints can be precisely adjusted by regulating flow rates. The HRSFs, fabricated with chitosan and calcium alginate, have strong mechanical properties and corrosion resistance in acid environment (pH = 5) and alkaline environment (pH = 9), respectively. More attractively, under controlled treatment conditions, the morphology of the spindle‐knots on the HRSFs can be effectively managed by changing the composite content of calcium chloride in the fluid. During the water collection process, tiny droplets of moisture can be captured on the surface of the HRSFs, subsequently, the droplets can coalesce and be transported from joint to spindle‐knot sections. It is demonstrated that the surface morphology of spindle‐knots directly influences the water collection efficiency, where a higher roughness gradient generates higher water collection efficiency. This parallel‐nozzle microfluidic technology provides a low‐cost and flexible method to manufacture high biocompatibility bioinspired rough spindle‐knot microfibers, which has many potential applications in large‐scale water collection, sustained drug release, and directional water collection.
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