**3. Practical methods for powder wettability determination**

The wettability of powders can be determined by various techniques described in detail by Alghunaim et al. [19]. These include the sessile drop, Wilhelmy plate, and liquid penetration methods. In this section, only two practical methods of contact angle measurements based on the sessile drop and capillary rise are briefly described for their possible application to probiotic powders (Section 4).

#### **3.1 Sessile drop method**

The sessile drop method can be applied to solid surface prepared after compacting powder in a disc or pellet under well-defined conditions of high pressure (70– 700 MPa). It is the most common technique for determining the wettability of powders. The static contact angle measurement is performed by depositing a small amount of liquid drop on the disc or pellet while recording its spreading. A software is always

**Figure 5.** *Drop of water fitted by a software.*

used for analyzing the contact angles (edge detection) with the programmed functions (i.e. straight line, circle or Laplace fitting). In this way the contact angle is extracted by fitting the baseline between the drop/the substrate (interface liquid/ solid) and the shape of the drop as represented in **Figure 5**. The method provides the right and left contact angles θ<sup>R</sup> and θL.

For hydrophobic and superhydrophobic surfaces, other methods like Johnson and Dettre [73] and the sliding method can be used, respectively [74–78]. A hysteresis representing the difference between the advancing and the receding contact angle can be deduced in dynamic measurements [79].
