Wetting Properties of Springtail Cuticles: A Quantitative Analysis
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With the prospect of producing superhydrophobic and mechanically stable biomimetic surfaces, the water repellent properties of springtail cuticles are investigated quantitatively. The analysis is based on apparent contact angle measurements on nine springtail species and SEM and AFM images of their cuticles. It is argued that water exists in a metastable Cassie-Baxter state on the cuticle and that three-phase line tension of the order of 10^-8 J/m contributes to high apparent contact angles. A model is developed to assess the significance of a recently reported re-entrant geometry of the cuticle granules and to study the resistance of the cuticle against wetting. The model estimates that springtails resist wetting under hydrostatic pressures up to 10^4 - 10^5 Pa, and that re-entrant granule profiles increase the wetting resistance by 50 - 400%. To explain the low contact angle hysteresis observed on the cuticles, new equations are proposed to include the effect of three-phase line tension. Again, a line tension magnitude of the order of 10^-8 J/m can account for the experimental findings. This work indicates that the sub-micron size scale of the springtail cuticle granules provides an elegant approach for achieving water repellent and mechanically stable surfaces. Two springtail species are singled out as especially promising for biomimetic applications.