The superhydrophilic and superoleophilic leaf surface of Ruellia devosiana (Acanthaceae): a biological model for spreading of water and oil on surfaces

Most leaves of plants are hydrophobic or even superhydrophobic. Surprisingly the leaves of the tropical herb of Ruellia devosiana Makoy ex E. Morr. Hort. (Acanthaceae) are superamphiphilic. Water droplets (10 µL) spread to a film with a contact angle of zero degree within less than 0.3 s. Such surfaces with a high affinity to water are termed superhydrophilic. Droplets of oil applied on R. devosiana leaves and replicas showed a similar spreading behaviour as water. These surfaces are superoleophilic, and in combination with their superhydrophilicity they are called superamphiphilic. Independent of the growing conditions, a reversibility of the superhydrophilicity in R. devosiana leaves was found. Additionally, on 90° tilted leaves a pressure free capillary transport of water occurs against the force of gravity. By using a low pressure environmental scanning electron microscope (ESEM), the water condensation and evaporation process on the leaves has been observed. The leaf surfaces are composed of five different cell types: conical cells, glands, multicellular hairs, hair-papilla cells and longitudinal expanded, flat epidermis cells, which, in combination with the surrounding papilla cells, form channel like structures. Replication of the leaf surface structure and coating of the replicas with hydrophilic Tween 20 and a water soluble extract gained from the leaf surfaces resulted in artificial surfaces with the same fast water spreading properties as described for the leaves.