SUPPLEMENTARY INFORMATION

Transcription

1 Supplementary Methods Natural spider silk. The orb-web of spider (Uloborus walckenaerius) was captured in Beijing, China. There are two types of silk in spider web, i.e., capture silk and radial silk. They are easily distinguished from the stickiness because capture silk is stickier. In this experiment, we selected capture silk to investigate water collection and used the term spider silk in main text and Supplementary Figures and Legends. The wet-rebuilt spider silks with periodic spindle-knot and joint formed when the fresh dry spider silk was explored in environment with the relative humidity of 90 ~ 99%. Thus, the directional water collection processes were observed on such wet-rebuilt spider silk. Characterization of microstructure. The structures of dry and wetted silk were observed by environmental scanning electron microscopy (ESEM, Quanta FEG 200, FEI) at accelerating voltage of ~ 4 kv. Water collection experiment and observation. In order to clearly observe the behaviors of water drops, the silks were carefully placed on a small U-shaped holder by means of micromanipulator. The chamber of sample can provide humidity within the range of 90 ~ 99% by an ultrasonic humidifier using Mill-Q water. In experiment, when the silks were placed in relative humidity of ~ 95%, larger water drops would form and hang on silks. The behaviors of water drops on wetted silks were recorded by the optical contact angle meter system (OCA 40, Dataphysics Instruments GmbH, Germany) with time scale. The starting time was set when the samples were exposed into the relative humidity of ~ 95 %. Control experiments about artificial spider silk, silkworm silk, nylon fiber and surfactant modified spider silk were done under the same experiment condition. Preparation of artificial spider silk. The artificial spider silk was prepared by immersing uniform nylon fiber into the poly(methylmethacrylate)/n,n-dimethylformamide ethanol (PMMA/DMF-EtOH) solution and then horizontally drawing out quickly. A thin polymer solution film formed on the fiber 1

2 and then the liquid film broke up to a series of tiny solution drops alternatively distributing on the fiber due to the Rayleigh instability. After these droplets dried, the periodic spindle-knots formed, similar to that of spider silk. Surfactant modified spider silk. Slight deformed spider silk was treated by relative dilute sodium dodecyl sulfate (SDS) surfactant solution (SDS: 0.05 wt.%) within short soaking time (1 s). Deformed spider silk was obtained through soaking silk into 0.1 wt.% SDS solution for 3 s. A partial damaged spider silk was obtained by treating silk with 0.1 wt.% SDS solution for 5 min. A damaged spider silk was obtained by treating silk with 0.2 wt.% SDS solution for 5 min. 2

3 Supplementary Figures and Legends Figure S1 Optical microscopy image of water drops chain on spider s web in mist. 3

4 Figure S2 a 0 s e s μm b s f s μm c s μm g μm s 2" d μm s 2 h 1" H s 2" 30 µm Detailed directional water drops movement of a single spindle-knot. a) The initial situation that the wetted spider silk is exposed in the relative humidity of ~95%. b) Small water drops randomly condensed on the spider silk. c) Condensed water drops (1, 2, 3 and 4 ) on silk. d) and e), With water drops growing, 3 and 4 on the spindle-knot coalesce into a water drop, while water drops 1 and 2 on the joint spontaneously move toward the spindle-knot (as red and blue arrows indicated). f) - h), The grown water drops 1 and 2 leave joints and coalesce into a larger water drop H covering the spindle-knot. Then, the joints refresh and a new water collection cycle that two regenerated drops 1" and 2" recondense on joints starts again. The black arrows indicate the positions of water drops condensing points at the joints that are favorable for the continuous collecting of water drops. In-situ observation of directional water collection on the capture silk of cribellate spider shows that the condensing drops move quickly from the joint to the spindle-knot (denoted with blue and red arrows). 4

5 Figure S3 a 0 s b s c s Silkworm silk d s e s f s 200 µm In-situ optical observation of water collection on silkworm silk. a) The dry silkworm silk. b) and c) Smaller water drops condense on the silk. d) - f) With the volume increasing, some water drops coalesce with the neighbor water drops and then form a bit larger drops, but there isn t the directional movement of water drops. 5

6 Figure S4 a 0 s b s c s d s e s f s 100 µm In-situ optical observation of water condensation on uniform nylon fiber with smooth surface. In mist, water drops randomly condense on nylon fiber. Except weak volume increasing, no evident directional movement of these drops is observed. 6

7 Figure S5 a d Spindle-knot 20 μm Joint Joint b e s f s 1 μm g s c h s 1 μm i s μm Slightly deformed spider silk shows directional water collection ability. a) c) ESEM images of surfactant-treated spider silk, which also shows spindle-knot/joint structure. a) The spindle-knot/joint structure retains. b) The spindle-knot is composed of random nanofibrils. c) The joint is composed of aligned nanofibrils. d) The optical image of surfactant-treated spider silk. e) When the dry treated silk is in mist, some tiny water drops condensed on the silk randomly. f) i) The condensed drops on joints grow and directionally move toward spindle-knot. Drop 1 on the left side of joint grows and moves to spindle-knot. At the mean time, drops 4, 3 and 2 successively coalesce to a drop when they move toward spindle-knot until they coalesce with drop 1. Ultimately, a larger water drop is collected on the spindle-knot. It indicated that the as-treated spider silk basically retained the multistructures and thus exhibited the directional water collection ability like natural spider silk. 7

8 Figure S6 a d 0 s 20 μm e s b f s g s 1 μm c h s i s 1 μm 20 μm Deformed spider silk retains directional water movement ability. a) ESEM image of the treated spider silk shows the deformed profile. b) and c) Magnified images of spindle-knot and joint that are of random and relative aligned surface structures. d) - e) When the treated silk is in mist, the water drops condensed on the silk. f) i) The growing drop directionally moves toward spindle-knot. It indicated that such a deformed spider silk showed the ability of directional movement of water drop. 8

9 Figure S7 a c 0 s Partially damaged silk d s 10 μm b e s f s 2 μm 50 μm Partial damaged spider silk without directional water collection ability. a) and b) Low- and high-magnification ESEM images of partially damaged spider silk without random and aligned nanofibrils structure. c) Optical image of the partially damaged silk. d) - f) The condensed drops grew up in place without directional movement. It indicated that the condensed drops merely grew on the remained spindle-knot without the directional water collection phenomenon. 9

10 Figure S8 a b 0 s c bared main-axis fiber s 5 μm d s e s 50 μm Damaged spider silk without directional water collection ability. ESEM a) and optical image b) of damaged spider silk, on which a relative uniform main-axis fiber is bared. c) e) Water condensation on bared main-axis fiber shows that water drops randomly condense on fiber without directional movement, which is similar to water condensation phenomenon on normal uniform fibers. It indicated that this water condensation behavior was more like normal uniform fibers (such as silkworm silk or nylon fiber etc.). The above-mentioned four control experiments (Figs. S5-8) demonstrated that the special multistructures played important roles in the directional water collection. As long as the random structure of spindle-knot and aligned structure of joint existed on the treated spider silk, the directional water collection ability would be retained. Otherwise, the water collection ability would be disabled once the multistructures of spider silk were damaged. These results verified that the special spindle-knot/joint structure of spider silk indeed played a vital important role in the directional water collection. 10

11 Supplementary Discussion Valuation of a minimum drop size in movement For a water drop less than capillary length (R cap = 2.7 mm for pure water), the gravity is usually negligible. The motion of such a water drop on a surface is a balance of driving force (which can be generated by wettability difference, Laplace pressure difference, chemical reaction, vibration, etc.) and contact angle hysteresis. If a water drop is placed on a surface with wettability gradient, the drop tends to be driven to move toward more hydrophilic region. Meanwhile, the force (F g ) originating from surface energy gradient must overcome a threshold hysteresis force (F h ) before the drop can move, as illustrated in Fig. S9. F h F g Ⅰ Ⅱ Ⅲ R < R c (not move) less wettable region R c R cap > R > R c (move) more wettable region Figure S9.Valuation of a minimum drop size in movement and the illustration of critical drop size for the movement of drop on surface with gradient wettability. When a drop (less than capillary length R cap ) is placed on a surface with gradient wettability, it suffers a non-equilibrium force due to the contact angle difference. This force [F g = πr 2 γ(dcosθ d /dx) ] tends to propel drop toward more wettable region. But the drop must overcome contact hysteresis [ F h = 2γR(cosθ r0 cosθ a0 ) ] before it moves, i.e. F g F h. It thereby exists a critical radius of drop [ R c = (cosθ r0 cosθ a0 ) / π (dcosθ d /dx) ] (drop Ⅱ). When the radius of drop R is lager than R c and less than R cap, the drop will move on the surface spontaneously as blue drop indicated (drop Ⅲ). Otherwise (R < R c ), the drop will be pinned on the surface instead of self-motion as green drop indicated (drop Ⅰ ). The net force (F) can be expressed as follows: F = F g F h = π R 2 γ (dcosθ d / dx) 2 γ R (cosθ r0 cosθ a0 ) 11

12 where θ d is the dynamic contact angle, cosθ d = (cosθ a + cosθ r ) / 2, and (cosθ r0 cosθ a0 ) is contact angle hysteresis at the central line of drop. In order to move the drop, the net force F must be more than zero. The size of drop thereby must be larger than a critical value (i.e., a minimum drop size), which could be valuated by R c = (cosθ r0 cosθ a0 ) / π (dcosθ d / dx). For example, in some representative researches on movement of liquid drops on various surfaces, the minimum drop size is usually limited to the scale of hundreds of micrometers due to resistance of the hysteresis. 7 9 Interestingly, spider silk could drive very tiny water drops of only several micrometers without additional driving force, which benefited from surface energy gradient and difference in Laplace pressure that cooperatively drove the movement of water drops. References: 6. Parker, A. R. & Lawrence, C. R. Water capture by a desert beetle. Nature 414, (2001). 7. Chaudhury, M. K. & Whitesides, G. M. How to make water run uphill. Science 256, (1992). 8. Daniel, S., Chaudhury, M. K. & Chen, J. C. Fast drop movements resulting from the phase change on a gradient surface. Science 291, (2001). 9. Daniel, S., Sircar, S., Gliem, J. & Chaudhury, M. K. Ratcheting Motion of Liquid Drops on Gradient Surfaces, Langmuir, 20, (2004). 12