Directional water collection on wetted spider silk
Entry by Sandeep Koshy, AP 225, Fall 2010
Title: Directional water collection on wetted spider silk
Authors: Yongmei Zheng, Hao Bai, Zhongbing Huang, Xuelin Tian, Fu-Qiang Nie, Yong Zhao, Jin Zhai & Lei Jiang
In this work, Zheng et. al analyzed the directional collection of water drops on spider silk using principles of capillarity and wetting. They showed that spider silk undergoes a conformational change when wetted to form bulged regions called “spindle-knots” that are connected by smooth regions called “joints”. They showed that water drops that originally nucleate on the joint region are driven to the spindle knot region by a combination of both a surface energy gradient, due to differences in the micro- and nanotopology of the two regions, and by a gradient of Laplace pressure, due to differences in the radii of curvature of the two regions. They used this newfound mechanistic understanding to design their own artificial spider silk that exhibited similar behavior. This work may provide valuable knowledge to the manufacturing industry in the area of water collection and liquid aerosol filtering.
Soft Matter Keywords: wetting, capillarity, surface energy, Laplace pressure
Collection and characterization of spider silk
Uloborus walckenaerius spiderwebs were collected and purified to contain only the “capture silk” portion of the web. Scanning electron microscopy (SEM) was then used on both dry and wetted silk for characterization of the surfaces.
Observation of water collection
To visualize water collection on silk fibers, the spider silk was held taught by micromanipulators within a chamber with humidity control and imaged using a optical contact angle meter system.
Artificial spider silk
Synthetic spider silk was created by immersing nylon fibers into a poly(methylmethacrylate)/N,N-dimethylformamide–ethanol (PMMA/DMF-EtOH) solution and withdrawing the fiber quickly. A thin polymer solution formed on the fiber and split into drops due to Rayleigh instability creating spindle-like variations as observed in natural spider silk.
Basic structure of spider silk
SEM images showed that dry spider silk was composed of two distinct periodically repeating regions termed “puffs” and “joints” (Fig.1-A). Magnified images (Fig. 1-B) showed that the puffs were composed of randomly oriented nanofibers. These fibers are known to be highly hydrophilic and provide high wettability of the spider silk.
Wetting of spider silk
When the spider silk was placed in a humidified chamber, it was observed that water drops (black dots with white arrows in Fig. 2-A) formed within the puffs. As the drops continue to grow and coalesce, the puffs contract into an opaque structure termed a “spindle-knot” (Fig. 2-B-C). Drops were seen to form on the “joint” regions between the spinders and then move directionally to the spindle knot (Fig.-2-D-F). By focusing on a single spindle knot, it could be seen that droplets formed on the adjacent joints would merge and coalesce on the spindle knot (Fig. 2-G-I).
Influence of fiber structure on the mechanism of directional water collection
SEM images showed that silk fiber consisted of periodic spindle-knot and joint regions (Fig. 3-A). Close examination of these two microarchitectural features showed that the spindle knot contained random fibrils (Fig. 3-B-C) while the joint regions contained highly aligned fibers (Fig. 3-D-E). These differences in the structures of the two regions are through to give rise to a surface energy gradient due to a difference in the surface roughness, driving the water to the more wettable (higher surface energy) region. A difference in the Laplace pressure in the two regions of different diameter was thought to create an even greater driving force for directional transport.
Mechanism of directional water collection
From Wenzel’s law, it is known for chemically identical surfaces that rougher surfaces will result in a lower contact angle. Thus the smoother joint region surface will be less wettable than that of the more rough spindle knot, creating a surface energy gradient that makes it more favorable for water to wet the spindle region (Fig. 4-A). This surface energy gradient from variations in roughness will drive water to collect at the more hydrophilic spindles, even when the drops are originally formed at the joints.
It can also be seen that the differences in geometry of the two regions will result in differences in Laplace pressure (Fig. 4-B). Specifically, the Laplace pressure on the highly curved joint regions will be higher than that on the less curved spindle regions and will drive water towards the spindle region.
The authors suggest both of these mechanisms are necessary to overcome contact hysteresis effects present at the microscale, which would hinder movement of droplets. In addition, the aligned structure of the joint region is ideal for water movement, owing to the creation of continuous vapor-liquid-solid three-phase contact lines that are continuous as opposed to discontinuous on the randomly arranged nanofibrils of the spindle-knobs. Water drops moving along the joint region will thus experience reduced hysteresis relative to if they had traveled on the spindles.
Creation of artificial spider silk with directional water collecting properties
Finally, the authors showed that they could use this mechanistic understanding of the directional flow capabilities of spider silk in order to generate an artificial spider silk exhibiting the same behavior. They used a technique whereby PMMA drops were formed on a nylon fiber, which closely resembled the wetted spider silk structure by SEM observation (Fig. 5 A-B). It was seen that the nanotopology of the pseudo-spindle regions were more randomly arranged relative to the nylon fiber pseudo-joint regions (Fig. 5 C-D). Similar directional transport was achieved compared to natural spider silk, where droplets were transported from joint regions to the spindle regions. The authors suggest that the mechanism and design principles used in this study would aid in fiber development for water and aerosol collection in manufacturing settings.