## 1 Introduction
From the physical point of view, water absorption on the surface of a material is regarded as a system consisting of the solid phase (particles of the material), liquid phase (water) and gaseous phase (air). This system tries to achieve the most convenient energy state; in other words, there is an effort to minimize the surface energy. Such effort is manifested by reduced surface tension on the solid material/water phase boundary and a lower degree of dispersion. For the purpose of identifying absorptivity, it is very important to know the wettability value expressed as the size of the contact angle, i.e., the angle between the drop surface and the surface of the solid material. The directions of forces acting on the boundaries between individual phases are shown in Figure 2.  
The surface tension contracts the water drop; tension \(\sigma=2.3\) has the opposite direction and induces drop expansion over the surface. Tension \(\sigma=1.3\) acts in the direction of the tangent line. The angle formed between the tangent line and the surface of the wetted material is referred to as the contact angle with the symbol \(\uppsi\). If the forces with magnitudes \(\sigma=1.3\) and \(\sigma=2.3\) act in approximately identical directions, wetting occurs. If these forces act in the opposite directions, wetting does not occur. The lower the value of surface tension is, the better the wettability of solid surfaces; this increases the rigidity of pellets. This means that materials may be wetted by water or any other medium that exhibits lower surface tension. Various tensides may also be used as they are capable of wetting materials that are otherwise water proof [11]. Grains of biomass, which may be used in the sintering process, are characterised by a rough surface and a porous structure. The shapes of grains are strongly affected by processing and crushing techniques. The shapes of raw pellets depend on the pelletisability of the material and its homogeneity. By adding minimum amounts of the same material with a different grain size, or by adding different additives, or by changing the liquid, it is possible to produce regularly shaped pellets even from a mixture that is difficult to pelletise. As the charge materials are pre-pelletised before they are sintered, a biomass intended for substituting coke dust must be wettable to the extent that it is able to form rigid micro-pellets. This has been the purpose of the research on the absorptivity of coke dust, which resulted in an unambiguous conclusion that its absorptivity was excellent. Absorptive media selected for this investigation included water of a laboratory temperature of 20 \({}^{\circ}\)C, water of a temperature of 50 \({}^{\circ}\)C, industrial tensile, and soapy water. Prior to testing, all analysed materials were dried at the temperature of 110 \({}^{\circ}\)C. The temperature of water has a significant effect on the absorptivity of materials, and this has also been confirmed in the tests with biomass. Table 1 presents the correlation between changes in surface tension (\(\sigma\)) and water density (\(\rho\)) at different temperatures [12]. Figure 2: Representation of the contact angle for solid surfaces: (**a**) Water on the hydrophilic surface; (**b**) Water in the form of a drop on the hydrophobic surface. Soapy water and industrial tensile are liquid media that were used in order to compare their ability to wet wood or plant biomass. They belong to surfactants reducing the surface tension of liquids through adsorption on the liquid/gas interface. They reduce interfacial tension between oil and water by adsorbing on the liquid/liquid interface. A functional group of a soap solution consists of COOH, SO\({}_{3}\)H, etc. Tensides are surface-active substances, which are absorbed on the phase boundary already at low concentrations and thus reduce the surface tension of liquids; this may eventually lead to wetting materials [13].