Fig 1.
(a) Images showing the seasonal changes of a Katsura tree from spring to fall. The leaf color changes to reddish green—green—yellow—yellowish brown—brown. (b) Images of a sliding droplet on green and brown leaves to show the contact angles. The green leaf exhibits a superhydrophobic surface () whereas the brown leaf becomes less hydrophobic (
).
Fig 2.
Optical and scanning electron microscope images of a green leaf (a-d) and a brown leaf (e-h). The green leaf (summer) is covered with oblate spheroidal epidermal cells and homogeneous epicuticular waxes. However, on the brown leaf (fall), the epidermal cells are shrunk and the epicuticular waxes are eroded. Magnifications of SEM images: (b, f) = 500x, (c, g) = 50,000x, (d, h) = 2,000x.
Fig 3.
(a,c,e,g) Schematics illustrating wetting states for various leaf conditions; (a) green, (c) brown, (e) heat-treated, and (g) vacuum-treated leaves. Here, θ* is the apparent contact angle, ϕs is the areal fraction of solid/liquid interface, and α is the intact wax areal fraction. The scanning electron microscope images (b,d,f,h 2,000x) provide examples of the four leaf conditions; (b) green, (d) brown, (f) heat-treated, and (h) vacuum-treated leaves.
Table 1.
Theoretical and experimental contact angles, roughness, and the areal fraction of intact wax regions in different leaves.
Fig 4.
(a) Contact angles of different leaves; green, brown, heat-treated, and vacuum-treated leaves. Receding (green) and advancing (yellow) contact angles are measured on a tilting method. Here, the boxes represent the interquartile range (IQR) between first and third quartiles and the red line inside represents the median. The whiskers denote the lowest and highest values within 1.5 x IQR from the first and third quartiles, respectively. The averaged contact angle (red) is an average of the receding and advancing contact angles. Theoretical apparent contact angles (purple) are calculated from Eq (3). (b) Contact-angle hysteresis of different leaves. The contact angle hysteresis is the difference between the advancing and receding contact angles, which presents the stickiness of water droplets on a leaf. Theoretically Eq (4) predicted contact-angle hysteresis are shown as purple bars.