Page 421 - Organic Electronics in Sensors and Biotechnology

398 Chapter Eleven

texture the solid surface. This increases the effective area between the
liquid and the solid polymer surface, thus modifying Eq. (11.2). The
wetting characteristics along textured surfaces have been treated and
investigated in the past and can be predicted using the Cassie-Baxter
11
or the Wenzel relationships. If a droplet, applied onto a textured
surface, stays standing on the pillar structures according to a fakir
situation, and the surrounding cavities are filled with vapor or air, the
Cassie-Baxter situation applies [see Eq. (11.3)]. In contrast, if the drop-
let wets the entire surface, Young’s equation is simply modified by
the roughness factor r, which equals the ratio between the real sur-
face area and the projected area [Eq. (11.4)].
cos θ = f cosθ + f cos θ (11.3)
∗
s s v v
∗
cos θ = rcosθ (11.4)

∗
where θ = apparent water contact angle
f = area fractions of the solid
s
f = area fractions of vapor
v
Polypyrrole (PPy) has been electrochemically synthesized to form
a mesh around pillars made of an insulating polymer (SU8), all made
on top of a PEDOT:PSS electrode surface. As the PPy mesh is electro-
chemically switched from the oxidized to its neutral state, both the
surface tension and its volume change. The result is that the effective
SU8 pillar height decreases at the same time as the wetting character
along the PPy phase is altered (Fig. 11.3). In its pristine oxidized state,
the droplet is “hanging” on the SU8 pillars in accordance to the fakir
situation (Cassie-Baxter’s case). As the PPy mesh is reduced, the
aspect ratio of the structured surface decreases, causing the water
droplet to contact both the SU8 pillars and the PPy mesh (Wenzel’s
case). This has a major impact on the wettability characteristics. In the
pristinely oxidized state, the apparent water contact angle is 129° while
it decreases to 44° when PPy is reduced (Fig. 11.4).

The Cassie-Baxter case The Wenzel case
FIGURE 11.3 Illustrations of water droplets added to textured surfaces
according to the Cassie-Baxter (left) and Wenzel (right) cases.