Hydration-state-modulated morphology, wetting and vapor permeation of the Opuntia surface

Self-regulating membranes that adjust mass transfer in response to environmental stimuli occur in nature and have a wide range of potential industrial applications. Inspired by the vapor transport regulation by stomata on cactus epidermal surfaces, a novel type of such membrane that regulates vapor transport based on the widening of nanofissures in a wax coating of a swelling material was recently introduced. Here it is shown that the network of epicuticular wax microfissures on the surface of Opuntia cactus cladodes exhibits equivalent seasonal hydration-induced morphology and vapor transport modulation. Using wettability measurements during controlled dehydration experiments and using microscale imaging of plants harvested during the wet and dry seasons, it is shown that the average external width of these microfissures decreases from approximately 15 μm down to 6 μm. Using simulations, it is estimated that dehydration decreases the vapor transport across the fracture network by 20–30%. Consequently, the plant-hydration-dependent surface morphological changes induce moderate vapor permeation modulation of the microfissure network but cannot be considered to have an on/off type of response. Nevertheless, these fissures act as secondary vapor pathways, providing an illustration of natural hierarchical design employed to mediate water vapor loss from the surface.