Static wetting angle

Lateral movement of the microscope stage, and hence the capillary system, is recorded via a position transducer. By observation of an interface within the capillary, and by maintaining it in the view of the fixed viewing optics, the displacement velocity can be directly determined. The interface can be photographed through the microscope optics, and the dynamic (or static) wetting angle measured (12,18). 

As an example. Figure 2.9 illustrates experimental results of the static wetting angle when a droplet of water or oil is deposited on a clean steel surface [23]. 

To sum up, the effects on static contact angles of the departures from ideality of solid surfaces are qualitatively well understood and some of these effects are used in practice to improve or reduce wettability. Moreover, for simple geometries, a semi-quantitative agreement is obtained between experimental results and theoretical predictions. For surfaces with random roughness, predictions of wetting hysteresis present a great difficulty because the relevant size of defects is not yet well-established. 

Fig. 10.1.4. The liquid may spread freely over the surface, or it may remain as a drop with a specific angle of contact with the solid surface. Denote this static contact angle by 6. There must be a force component associated with the liquid-gas surface tension (t that acts parallel to the surface and whose magnitude is a cos 0. If the drop is to remain in static equilibrium without moving along the surface, it has to be balanced by other forces that act at the contact line, which is the line delimiting the portion of the surface wetted by the liquid, for example, a circle. It is assumed that the surface forces can be represented by surface tensions associated with the solid-gas and solid-liquid interfaces that act along the surface, and tr i, respectively. Setting the sum of the forces in the plane of the surface equal to zero, we have...

The line that delimits the wetted portion of a surface by a liquid is called the contact line. In two-dimensional flows the contact line reduces to a point (see Fig. 3). For a static droplet, the angle of the interface between the liquid and the solid surface is caUed the static contact angle 0. 

Developing the analogy in our impact problem with a hydrophobic surface, we identify the wetting phase as air, so that the static contact angle with respect to the wetting phase is defined as o = (rt — %) and the triple contact line becomes no longer stable above u a (rr — 0o) - Furthermore, v is fixed by a critical capillary number Ca = u 7l/tlv that evolves like Ca l/9 . 

SLA, the LLR functionalized with a pure CHj-terminated SAM shows extremely high dynamic and static contact angles, an indication of Cassie-type wetting. The papillae are able to support the drop, even in the absence of the tubular wax of the original leaf. As soon as there is a hydrophilic contribution present in the SAM, Wenzel-type wetting occurs. Note the high standard deviations, which arise from the fact that in some measurements, the Cassie state still persisted, whereas in others, the drop penetrated into the structures. Both types of behavior were sometimes even observed on the same sample, which is an indication of the presence of metastable states. An explanation for this metastable Cassie regime... 

---