**1. Introduction**

Wettability is a functional property that represents the wetting degree of a solid. Among the macroscopic consequences of nano- and microscopic phenomena occurring at fluid-solid interfaces, it is governed by the balance between adhesive and cohesive forces, representing the assembly strength of different and identical surfaces, respectively [1]. Such phenomena are naturally encountered in our daily life, or technologically shaped for our necessities [2]. The common examples of solid wettability are observed with drops of water standing out on lotus leaves or repelling on waterproof materials, as well as with agrochemical dispersion preparations spreading

on plant surfaces [3–7]. Adhesives, anti-icing, bio-mimicking, boiling, coatings, fibers, freeze-casting, inks, micro/nanoelectromechanical systems, paper, and petrochemicals also take part of a non-exhaustive list of applications involving wettability [8–15].

When an amount of liquid (e.g. drop of water) comes into contact with a porous or nonporous solid surface (e.g. flat or granular solid), its displacement until an equilibrium position depends on the force resulting from the interfacial tensions at the contact line of three immiscible phases (solid/liquid/gas) (**Figure 1**). The changes in the resulting interfacial tension or free energy define the contact angle (CA), which expresses the wetting degree of the solid surface by the liquid phase [16]. When the CA is lower than an arbitrary value (e.g. θ < 90°C), the surface is qualified as hydrophilic whereas it is known as hydrophobic for high CA (e.g. θ > 90°C). A value superior to 150°C is a characteristic of a superhydrophobic surface [17].

For powder materials, the wettability is often quantified by the contact angle (θ) of compressed disc- or packed bed-prepared particles, which depends on the surface and bulk properties, composition of particles, wetting liquid characteristics, and physicochemical conditions such as the temperature, pH, ionic strength, and so on [18]. The powder wettability plays a crucial role in coating, dispersion as a precursor step to dissolution, and powder processing such as granulation and other practical usages. Several techniques are available for powder wettability determination through the contact angle measurement of solid particles reacting with dispersing media by the static and dynamic sessile drop method or by the capillary rise technique [19].

Most food and non-food (e.g. pharmaceuticals, detergents, minerals) ingredients and products are in powder form, which is constituted by numerous particles and granules, in pure or mixture form of multiple components [20, 21].

There are many powder wettability case studies of either organic or inorganicbased compounds reported in the scientific literature over the last five years (**Table 1**). However, less investigations have been conducted on powders that contain microorganisms such as probiotics and derivative products.

Probiotics are live microorganisms, when administered in adequate doses and under appropriate conditions, and can be benefits for the host through different action mechanisms [33]. In fact, they can act as (1) competitors for nutrients and adhesion sites with intestinal or plant pathogens; (2) metabolite producers, including bacteriocins, organic acids, antioxidants, enzymes, and biosurfactants; and (3) immunomodulators [34]. Often used as functional ingredients, probiotics find their potential applications in foods, feeds, cosmetics, pharmaceutics, and agriculture sectors [35–39]. Belonging mainly to lactic acid bacteria (LAB) such as Lactobacilli and

#### **Figure 1.**

*Illustration of liquid drop formed between three immiscible phases with different contact angles θ: (a) hydrophilic θ <90°C, (b) hydrophobic θ = 90°C, and (c) superhydrophobic surfaces.*

*Wettability of Probiotic Powders: Fundamentals, Methodologies, and Applications DOI: http://dx.doi.org/10.5772/intechopen.106403*


#### **Table 1.**

*Examples of wettability studies on various organic and inorganic powders.*

Bifidobacteria, or yeasts, probiotics are among the most investigated research topics today, owing to their beneficial effects on our overall ecosystem, involving activities and interactions among human, animal, and plant species, soils and the environment [40].

Their external surfaces are surrounded of components with different molecular classes (e.g. proteins, polysaccharides, peptidoglycans, etc.) and specific structures (e.g. pili), which are responsible for their most surface properties and functionalities such as hydrophobicity, adhesion, and aggregation capacities. Microbial cell surfaces are vital to their survival for interacting with the environment. It is assumed that microbial cells adhere to surfaces through the interactions between extracellular compounds biosynthesized by cells or from external (e.g. coatings) and surfaces [41]. It is generally recognized that there exists a correlation between cell adhesion capacity and surface hydrophobicity, which can be indirectly measured by the water contact angle [42]. Microorganisms are considered hydrophilic for values less than 20° and hydrophobic for values superior to 50° [43, 44].

Moreover, probiotic-based products are industrially manufactured and commercially available in powder particles, in most cases under various solid states rather than dispersed in liquid forms, by successive fermentation, liquid–solid phase separation, and drying processes [45]. It is therefore important to focus scientifically and technologically on the probiotic powder wettability, which is a performance indicator, directly or indirectly in terms of (1) cell capacity in adhering to surfaces; (2) product dispersibility in a fluid for some preparations before use in formulations or uptake as diet supplementation; (3) cell viability and product stability for limiting the contact with humidity and air through the powder porosity or permeability; and (4) coating material compatibility and processing efficiency for cell protection.

The current chapter deals successively with (1) the theoretical aspects of powder wettability; (2) the practical methods and techniques of wettability determination; and (3) the applications to probiotic powder wettability for food and agricultural products.
