Dew harvesting efficiency of four species of cacti

Abstract

Four species of cacti were chosen for this study: Copiapoa cinerea var. haseltoniana, Ferocactus wislizenii, Mammillaria columbiana subsp. yucatanensis and Parodia mammulosa. It has been reported that dew condenses on the spines of C. cinerea and that it does not on the spines of F. wislizenii, and our preliminary observations of M. columbiana and P. mammulosa revealed a potential for collecting dew water. This study found all four cacti to harvest dew on their stems and spines (albeit rarely on the spines of F. wislizenii). Dew harvesting experiments were carried out in the UK, recording an increase in cacti mass on dewy nights. By applying a ranking relative to a polymethyl methacrylate (Plexiglas) reference plate located nearby, it was found that C. cinerea collected the most airborne moisture followed by M. columbiana, P. mammulosa and F. wislizenii respectively, with mean efficiency ratio with respect to the Plexiglas reference of 3.48 ± 0.5, 2.44 ± 0.06, 1.81 ± 0.14 and 1.27 ± 0.49 on observed dewy nights. A maximum yield of normalized performance of 0.72 ± 0.006 l/m(-2) on one dewy night was recorded for C. cinerea. Removing the spines from M. columbiana was found to significantly decrease its dew harvesting efficiency. The spines of three of the species were found to be hydrophilic in nature, while F. wislizenii was hydrophobic; the stems of all four species were hydrophilic. The results of this study could be translated into designing a biomimetic water collecting device that utilizes cactus spines and their microstructures.

Condensate droplet size distribution on lubricant-infused surfaces

“…The temperature drop ΔT i at the droplet-vapor interface arises due to an imbalance of evaporation and condensation of liquid and vapor molecules at the interface, respectively. The interface resistance can then be written as (2) where h i is the liquid-vapor interfacial heat transfer coefficient, and θ A is the advancing contact angle. The interfacial heat transfer coefficient can be derived from kinetic theory and can be described as [45,47,48] (3)…”

“…Condensation is pervasive both in nature [1,2] and industrial applications, including power generation [3], closed-loop electronics cooling systems [4,5], air conditioning [6], desalination [6,7], and water harvesting [8]. On industrial condenser applications, which are typically covered with a high surface energy metal oxide (i.e.…”

Hierarchical structures of cactus spines that aid in the directional movement of dew droplets

“…Dew has been observed to form on these finned regions of C. cinerea and is clearly not a deterrent in the formation of dew, if they are not compactly arranged as is the case with the fins of F. wislizenii. As was discussed in Malik et al [7], these microfins, which have also been observed on the Turninicarpus cactus by Mosco [26], render the surface hydrophobic and could explain their unsuccessful ability to harvest dew on their surfaces. Furthermore, Malik et al [7] found that the microgrooved nature of C. cinerea (along with the other dew-harvesting spines of P. mammulosa and M. columbiana), which were narrower at the tips of the spines compared with the wider grooves at their bases, creates a roughness gradient that drives water from the tip to the base of the spines; something that was also discussed by Roth-Nebelsick et al [16] for the microgrooves of the Stipagrostis sabulicola Namib grass.…”

“…This shows strong evidence of a transport system from the spines of some cacti and into the body of those cacti species in which the surface water flows on leaving the spines. With regard to wettability, the contact angles obtained by Malik et al [7] show the spines of three of the cacti species to be hydrophilic, with approximate contact angles of 60 • , 35 • and 40 • for C. cinerea, M. columbiana and P. mammulosa, respectively. The spine of F. wislizenii on the other hand was found to be hydrophobic with an approximate contact angle of 120 • (this could be explained by these microstructures trapping air and creating a surface that is hydrophobic).…”

“…In fact, biomimetic-inspired microstructured surfaces have been found to enhance surface transportation of water [11]. Unlike such studies that have focused on fog water harvesting on cactus spines, the spines of cacti have also been found to be an important feature in increasing the amount of dew harvested by certain cactus species [7]. Thus, the spines were the main points of interest in this study with the aim of informing a future airborne harvesting device.…”