A Chinese research group, led by Jiang Lei who is a chemist at the Institute of Chemistry, Chinese Academy of Sciences (CAS) and also a CAS academician, made a breakthrough achievement on the mechanism of spider silk’s water collection. This significant work was published on Nature on Feb. 4 2010 (Nature 2010, 463, 640-643) with a cover story. The research revealed the directional water collection on wetted spider silk, based on the research of bio-surface special wettability (published on Nature, 2004, 432, 36; Adv. Mater. 2002, 14, 1857-1860) and the investigation on water-collection of natural bio-surface in the recent years. They observed in detail water collective behavior at micro-region of natural spider silk by optical microscope, and found that tens of micrometer-size drops move from one micro-region to another on spider silk, displaying distinctly the directionality of drop movement, and thereby revealed at micro- and nano-level the mechanism of “multi-cooperative effect” in driving drops responsible for water collection.
The study revealed that when the dry capture silk of cribellate spider (Uloborus Walckenaerius）was placed in fog, the puff of dry capture silk, composed of nanofibrils (Fig. 1) could be changed into alternative spindle-knot and slight joint between spindle-knots (Fig.2-3). Interestingly, the spindle-knot forms the freedom nanofibrils structure (Fig. 3c), and the joint forms the aligned nanofibrils structure (Fig.3e), in which the different structures between spindle-knot and joint would induce the surface energy gradient. In addition, the curvature gradient formed due to geometry of spindle-knot induces the difference in Laplace pressure (Fig.4). Two gradients acted cooperatively on micrometer-size drops, making a continuous condensing-drop collection and transportation from the joint to the spindle-knot. Spider silk achieved this task on effective condensing-drop collection and stable hang of larger water drops, displaying a strong ability of water collection. Inspired by this finding, the group designed artificial fibres that mimic the structural features of silk and exhibit its directional water collecting ability (Fig. 5).
This achievement will be hopeful to solve the bottle-neck problem of small drop in driving. For instance, in previous research, people found that when surface energy gradient or difference in Laplace pressure was designed on a surface, respectively, the drops in size of hundreds of micrometer moved easily, while a drop moved difficultly when its size is below 200 mm mostly due to the contact angle hysteresis. Jiang’s finding introduces a new approach to small size drop movement through understanding the mechanism of water collection on spider silk and further biomimetic researches.
This finding will inspire scientists to design the novel smart surfaces that can control the fluid in microfluidics, fiber-netty materials on a large scale for water collection in air and fog, serving for the needs at the places with the lack of water resource. The research finding will be of help in designing smart catalysts that realize the reaction of different chemical matter through fast gathering together due to effect of microstructure, and also the filtering materials used in the process of industry machining and so on.
The research has been supported by the State Basic Research Program of China (2007CB936403), the Key Program and Major International Cooperative Program of NSFC (119030601101), and the Knowledge Innovation Program from Chinese Academy of Sciences (2A200522222200301).
(Left) Cover caption：Caught in the web -- the structural flip that allows spiders' silk to collect water
(Right) Fig. 1 Dry capture silk of spider, a) Composed of puff and joint, along two-main-axis fibers; b) Puff is composed of nanofibrils.
Fig. 2 Silk structure is changed when water condensed on silk, puff becomes “spindle-knot” (a-d) and directional water collection of condensed drops (e-f); g-i）Directional movement of condensed drop on single spindle-knot.
Fig. 3 Microstructure on silk, b-c) Spindle-knot has freedom porous nanofibrils structure; d-e) Joint has aligned nanofibrils structure.
Fig. 4 Illustration of mechanism for condensed drop direction movement, a) Freedom nanofibrils structure forms the discontinuous three phase contact line (TCL) and aligned nanofibrils structure forms continuous TCL; b) Surface energy gradient and difference in Laplace pressure drive cooperatively the drop move directionally.
Fig. 5 The directional movement of drop was realized by fabricating the structure similar to that of spider silk.
The work was highlighted by worldwide medias as follows：
http://www.infox.ru/themes/science/, and so on.