Wetting Properties of Natural and Plasma-Processed Biomimetic Surfaces - Biomimetic Architectures by Plasma Processing - page 114

Figure 4.2 Representative images for hydrophobic and hydrophilic

leaves: (a)

floating leaf, CA >150° (Reprinted
with permission from Ref. [14]. Copyright 2009 American
Chemical Society), (b) lady's mantle CA >150° (Reprinted with
permission from Ref. [15]. Copyright 2009 American Chemical
Society), (c) Cherry laurel CA < 90° (Reprinted with permission
from Ref. [16]. Copyright 2013 American Chemical Society).

Salvinia biloba

4.2 Theoretical Approach

What determines a surface to be hydrophobic, hydrophilic or
super- hydrophobic? The answer is hidden in the surface tension
at the solid-liquid interface or individual surface free energies
of the solid surface and the liquid involved. Surface tension or
surface energy is a tensile or contractile force which has the unit of
Newton/meter

2 2

(energy per
unit area). Surface tension acts as a stretched elastic membrane,
where each and every part is pulled by each other, and is caused by
the attractive or cohesive force between the liquid molecules in the
solution. What determines the shape of a water droplet resting on a
surface? Whether it will roll up like a spherical bubble (high CA) or
spread up completely like a film (low CA) on the surface? For this, we
have to determine the surface tension at three available interfaces:
(i) the liquid-air, (ii) the solid-air and (iii) the solid-liquid.

(force per unit area) or joules/meter

Figure 4.3 Schematic of a liquid droplet in contact with (a) a smooth solid

surface (CA,

q
0

), (b) a rough solid surface (CA,

q
C

) and (c) a

rough solid surface with air pocket (CA, q ).