title: 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.
authors.affiliation: School for Engineering of Matter, Transport and Energy, Arizona State University , Tempe, Arizona 85287, United States.
authors.forename: Konrad
authors.initials: K
authors.lastname: Rykaczewski
authors.affiliation: School of Molecular Sciences, Arizona State University , Tempe, Arizona 85287-1604, United States.
authors.forename: Jacob S
authors.initials: JS
authors.lastname: Jordan
authors.affiliation: School for Engineering of Matter, Transport and Energy, Arizona State University , Tempe, Arizona 85287, United States.
authors.forename: Rubin
authors.initials: R
authors.lastname: Linder
authors.affiliation: School for Engineering of Matter, Transport and Energy, Arizona State University , Tempe, Arizona 85287, United States.
authors.forename: Erik T
authors.initials: ET
authors.lastname: Woods
authors.affiliation: School for Engineering of Matter, Transport and Energy, Arizona State University , Tempe, Arizona 85287, United States.
authors.forename: Xiaoda
authors.initials: X
authors.lastname: Sun
authors.affiliation: School for Engineering of Matter, Transport and Energy, Arizona State University , Tempe, Arizona 85287, United States.
authors.forename: Nicholas
authors.initials: N
authors.lastname: Kemme
authors.affiliation: School for Engineering of Matter, Transport and Energy, Arizona State University , Tempe, Arizona 85287, United States.
authors.forename: Kenneth C
authors.initials: KC
authors.lastname: Manning
authors.affiliation: School of Molecular Sciences, Arizona State University , Tempe, Arizona 85287-1604, United States.
authors.forename: Brian R
authors.initials: BR
authors.lastname: Cherry
authors.affiliation: School of Molecular Sciences, Arizona State University , Tempe, Arizona 85287-1604, United States.
authors.forename: Jeffery L
authors.initials: JL
authors.lastname: Yarger
authors.affiliation: Department of Research, Conservation and Collections, Desert Botanical Garden , Phoenix, Arizona 85008, United States.|School of Life Sciences, Arizona State University , Tempe, Arizona 85287-4701, United States.
authors.forename: Lucas C
authors.initials: LC
authors.lastname: Majure
mesh_terms: D057927:Hydrophobic and Hydrophilic Interactions; D029747:Opuntia / Q000502:physiology*; D013499:Surface Properties; D017155:Wettability