Abstract
The cuticles of most springtails (Collembola) are superhydrophobic, but the mechanism has not been described in detail. Previous studies have suggested that overhanging surface structures play an important role, but such structures are not a universal trait among springtails with superhydrophobic cuticles. A novel wetting experiment with a fluorescent dye revealed the extent of wetting on exposed surface structures. Using simple wetting models to describe the composite wetting of the cuticular surface structures results in underestimating the contact angles of water. Including the three-phase line tension allows for a prediction of contact angles in the observed range. The discrepancy between the contact angle predicted by simple models and those observed is especially large in the springtail Cryptopygus clavatus which changes, seasonally, from superhydrophobic to wetting without a large change in surface structure; C. clavatus does not have overhanging surface structures. This large change in observed contact angles can be explained with a modest change of the three-phase line tension.
Highlights
Collembola, a group of small, terrestrial hexapods, have been known to possess remarkable water-repellent properties [1,2,3,4,5,6,7]
We propose that by using the equation of Zheng et al [16] the high contact angles observed in Collembola can be predicted in general, and a possible mechanism for the seasonal change of Cryptopygus clavatus in specific can be provided
Since all orders of Collembola and a wide range of surface structures and habitat types are considered in these two studies, it seems likely that the granule area fraction of most Collembola will fall within the two extremes of 0.111 to 0.709
Summary
Collembola, a group of small, terrestrial hexapods, have been known to possess remarkable water-repellent properties [1,2,3,4,5,6,7]. The apparent contact angle of a composite wetting state is predicted by the Cassie–Baxter equation, which underestimates the contact angle of Collembola cuticles [5] and sub-micrometersized surface structures in general [16]. The Cassie–Baxter equation fails to predict changes in contact angle without an accompanying change in surface structure, such as the seasonal change in wetting characteristics for the Collembola species Cryptopygus clavatus [6].
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