Wetting characteristics of Colocasia esculenta (Taro) leaf and a bioinspired surface thereof

Manish Kumar,Rajneesh Bhardwaj

doi:10.1038/s41598-020-57410-2

Abstract

We investigate wetting and water repellency characteristics of Colocasia esculenta (taro) leaf and an engineered surface, bioinspired by the morphology of the surface of the leaf. Scanning electron microscopic images of the leaf surface reveal a two-tier honeycomb-like microstructures, as compared to previously-reported two-tier micropillars on a Nelumbo nucifera (lotus) leaf. We measured static, advancing, and receding angle on the taro leaf and these values are around 10% lesser than those for the lotus leaf. Using standard photolithography techniques, we manufactured bioinspired surfaces with hexagonal cavities of different sizes. The ratio of inner to the outer radius of the circumscribed circle to the hexagon (b/a) was varied. We found that the measured static contact angle on the bioinspired surface varies with b/a and this variation is consistent with a free-energy based model for a droplet in Cassie-Baxter state. The static contact angle on the bioinspired surface is closer to that for the leaf for b/a ≈ 1. However, the contact angle hysteresis is much larger on these surfaces as compared to that on the leaf and the droplet sticks to the surfaces. We explain this behavior using a first-order model based on force balance on the contact line. Finally, the droplet impact dynamics was recorded on the leaf and different bioinspired surfaces. The droplets bounce on the leaf beyond a critical Weber number (We ~ 1.1), exhibiting remarkable water-repellency characteristics. However, the droplet sticks to the bioinspired surfaces in all cases of We. At larger We, we recorded droplet breakup on the surface with larger b/a and droplet assumes full or partial Wenzel state. The breakup is found to be a function of We and b/a and the measured angles in full Wenzel state are closer to the predictions of the free-energy based model. The sticky bioinspired surfaces are potentially useful in applications such as water-harvesting.

Highlights

Weber number (We) investigate wetting and water repellency characteristics of Colocasia esculenta leaf and an engineered surface, bioinspired by the morphology of the surface of the leaf
While the static contact angle on the taro leaf was reported by Kim et al.[10], a detailed morphology of the surface of the leaf, contact angle hysteresis and droplet impact dynamics on the leaf have not been reported in the literature, to the best of our knowledge
We have carried out an investigation of wetting characteristic and droplet impact dynamics on Colocasia esculenta leaf and bioinspired surfaces based on the morphology of the leaf

Summary

Introduction

We investigate wetting and water repellency characteristics of Colocasia esculenta (taro) leaf and an engineered surface, bioinspired by the morphology of the surface of the leaf. Patil et al.[13] investigated wetting characteristics and droplet impact dynamics on surfaces with micropillars, manufactured by mimicking first-tier structure of the lotus leaf They fabricated different surfaces by varying pitch of the pillars and investigated the role of impact velocity and the pitch. To the best of our knowledge, there has not been any attempt reported to fabricate an engineered surface based on the taro leaf using bottom-up approach This approach is suited to understand the role of micro- and nanoscale structures in determining wettability and associated droplet impact dynamics on a bioinspired surface. The present study shows a key difference between the bioinspired surfaces based on the two morphologies, namely, micropillars-like and honeycomb-like structure The former exhibit a discontinuous contact line as compared to that for a droplet on a smooth surface[21] and it results in a low contact angle hysteresis. We explain it using the help of a first-order model why a honeycomb-like surface exhibits a large hysteresis despite being superhydrophobic

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