Elucidating the surface geometric design of hydrophobic Australian Eucalyptus leaves: experimental and modeling studies

Hua Guo,Zonghan Xie,Jeremy Shaw,Kingsley Dixon,Zhong-Tao Jiang,Chun-Yang Yin,Xuemei Liu

doi:10.1016/j.heliyon.2019.e01316

Abstract

Three Australian native Eucalyptus species, i.e., Eucalyptus woodwardii, Eucalyptus pachyphylla and Eucalyptus dolorosa, were investigated, for the first time, with respect to the hydrophobicity of their leaves. It is well established that these leaves exhibit exceptionally high water repellency, in addition to an extraordinary ability to retain water, albeit their specific wetting mechanisms are still poorly understood. To identify the critical factors underlying this phenomenon, the surface topography of these leaves was subjected to micro-examination (SEM). Micro- and nanometer scale surface roughness was revealed, resembling that of the quintessential “lotus effect”. Surface free energy analysis was performed on two models based on the surface topographies of the study Eucalyptus species and lotus, in order to study wetting transitions on these specific microscopic surface features. The influence of surface geometrical parameters, such as edge-to-edge distance, base radius and cylindrical height, on surface free energy with different liquid penetration depths was studied with these two models. Larger energy barriers and smaller liquid-solid contact areas were more influential in the calculations for the lotus than for Eucalyptus. The information obtained from these two models may be useful for guiding the design of novel artificial surfaces in the collection and transport of micro-volume liquids.

Highlights

Many biological surfaces, such as plant leaves, bird feathers and animal furs, exhibit strong water repellency in order to adapt to environmental conditions
Given that the lotus is an emergent aquatic plant and the fact that the species of Eucalyptus thrive in drier environments, it is surprising that these Eucalyptus spp. exhibit such high levels of leaf hydrophobicity
Considerable hydrophobicity and strong adhesion were found on the leaves of three Australian native Eucalyptus species

Summary

Introduction

Many biological surfaces, such as plant leaves, bird feathers and animal furs, exhibit strong water repellency in order to adapt to environmental conditions. The wetting characteristics of the lotus leaf include high contact angles and low contact angle hysteresis [1, 2, 3, 4], attributed largely to a hierarchical surface structure. These properties have formed the basis of many surface-critical applications; for example, self-cleaning [5, 6, 7], corrosion prevention [8], drag reduction [9] and fouling control [10]. Energy barrier and energy potential were quantitatively identified during the speculated wetting transition process

Sample preparation

Instrumentation

Leaf surface wettability

Analysis of surface free energy

Conclusions