Microscale Mechanism of Age Dependent Wetting Properties of Prickly Pear Cacti (Opuntia)

Abstract

Cacti thrive in xeric environments through specialized water storage and collection tactics such as a shallow, widespread root system that maximizes rainwater absorption and spines adapted for fog droplet collection. However, in many cacti, the epidermis, not the spines, dominates the exterior surface area. Yet, little attention has been dedicated to studying interactions of the cactus epidermis with water drops. Surprisingly, the epidermis of plants in the genus Opuntia, also known as prickly pear cacti, has water-repelling characteristics. In this work, we report that surface properties of cladodes of 25 taxa of Opuntia grown in an arid Sonoran climate switch from water-repelling to superwetting under water impact over the span of a single season. We show that the old cladode surfaces are not superhydrophilic, but have nearly vanishing receding contact angle. We study water drop interactions with, as well as nano/microscale topology and chemistry of, the new and old cladodes of two Opuntia species and use this information to uncover the microscopic mechanism underlying this phenomenon. We demonstrate that composition of extracted wax and its contact angle do not change significantly with time. Instead, we show that the reported age dependent wetting behavior primarily stems from pinning of the receding contact line along multilayer surface microcracks in the epicuticular wax that expose the underlying highly hydrophilic layers.

Assessment of Orchid Surfaces Using Top-Down Contact Angle Mapping

“…The results observed in Figures 13 and 14 corroborates the existence of wettability change with leaf age, as expected according to [26] and [27]. In Prickly Pear Cacti, the young leaves are hydrophobic, and the old leaves are hydrophilic.…”

“…For the older leaf with a lower contact angle, the surface is much more hydrophilic. This difference occurs because of the impurities deposited on the leaf with age and the alteration of foliar microstructures with exposure to physical-chemical agents over time [26].…”

“…The results of novel top-down CA mapping measurements obtained in this leaf and flower wettability analysis of the orchid Cattleya warneri are in agreement and consistent with the theory found in the literature. Young leaves are hydrophobic, old leaves become practically hydrophilic mostly as a result of continued exposure to changes in the environment over time [26], [27]. These changes can include differences in air temperature, air relative humidity, soil moisture, frequency of precipitation events, and deposition of impurities.…”

Poly(3,4-propylenedioxypyrrole) Nanofibers with Branched Alkyl Chains by Electropolymerization to Obtain Sticky Surfaces with High Contact Angles

“…By contrast, other natural surfaces have extremely high θ w but extremely strong water adhesion also called parahydrophobic properties . This is the case of rose petals, Echeveria pulvinata leaves, gecko skin, peach skin, cacti spines or webspinner silks . These properties are extremely important, for example, to capture water droplets even in hot and arid environments and could be used for water harvesting systems ,.…”

Hydraulic Strategy of Cactus Root–Stem Junction for Effective Water Transport

“…As the root hydraulic conductivity is inversely related to the thickness of cortical layer, thin cortex cell layers allow water to reach the xylem vessels quickly from the root surface (Rieger and Litvin, 1999 ). To collect water quickly, cacti develop a shallow, widespread root system to absorb rainwater percolated through the upper soil surface before the rainwater drains away (Rykaczewski et al, 2016 ). Accordingly, the thin cortex cell layers of cactus root below the R–S junction can help the root to transport water quickly from the root surface to xylem vessels.…”