Seasonal changes in morphology govern wettability of Katsura leaves

Abstract

Deciduous broad-leaf trees survive and prepare for winter by shedding their leaves in fall. During the fall season, a change in a leaf’s wettability and its impact on the leaf-fall are not well understood. In this study, we measure the surface morphology and wettability of Katsura leaves from the summer to winter, and reveal how leaf structural changes lead to wettability changes. The averaged contact angle of leaves decreases from 147° to 124° while the contact-angle hysteresis significantly increases by about 35°, which are attributed to dehydration and erosion of nano-wax. Due to such wettability changes, fall brown leaves support approximately 17 times greater water volume than summer leaves.

How a raindrop gets shattered on biological surfaces

“…Preparation of Biological Specimens. Bird feathers, insect wings, and katsura leaves were used because of their superhydrophobic property with hierarchical structures (17,20,21,37,38). For bird feathers, the carcass of northern gannet (Morus bassanus) was obtained from the Smithsonian Museum of Natural History.…”

“…1 B and C (Movies S2 and S3). All these specimens have hierarchical superhydrophobic structures, where an array of micrometer-sized bumps exists with nanoscale structures (20,21).…”

An ecological perspective on water shedding from leaves

“…Epicuticular wax crystals can be reduced or absent in plants growing under low light conditions ( Hallam, 1970 ) or under elevated air humidity ( Koch et al , 2006 ). Seasonal changes of epicuticular wax load and composition are widespread and have been attributed to leaf ontogeny ( Jetter and Schäffer, 2001 ), temperature and water availability ( Ziv et al , 1982 ; Jordan et al , 1983 ), and erosion of wax crystals over time ( Neinhuis and Barthlott, 1998 ; Kang et al , 2018 ). Multiple studies report an increase in leaf wettability for broad-leaved trees towards the later part of the growth season, namely with increasing leaf age ( Neinhuis and Barthlott, 1998 ; Tranquada and Erb, 2014 ; Kang et al , 2018 ; Xiong et al , 2018 ).…”

Vapor flux on bumpy surfaces: condensation and transpiration on leaves

“…Three cases are presented: a single semicylinder, a single prolate semi-ellipse, and a single hair. These structures are inspired by the microstructures found on real plant leaves (Circular epidermal bumps in Cercidiphyllum japonicum [18]; prolate bumps in Viola tricolor [19]; hair-like structures in Phaseolus vulgaris [19]). As shown in Fig.…”

“…where R is the universal gas constant, T is the temperature, p vapor is the actual vapor pressure, and p sat is the saturated vapor pressure This relation explains that the droplet nucleation easily occurs on a hydrophilic surface (cos θ Equil > 0) at any groove sizes (r) in saturated air (p vapor /p sat > 1). Hierarchical double-layer roughness (i.e., nanowax and microbumps) is typical for leaf surfaces in nature (e.g., Lotus leaf [16,17], Katsura tree leaf [18], and a recent review in [19]). Especially, the nanowax might provide the groove lengthscale to initiate the droplet nucleation.…”