(59b) Dynamic Wetting of Multicomponent Particle Systems

The wetting kinetics of a liquid-powder system is an important factor with regard to many practical applications, such as coating, granulation, agglomeration or powder reconstitution. With increasing liquid-solid contact angles, dynamic wetting processes are slowed down significantly, which commonly affects the final product quality in a negative way. In the food industry, it is often aimed to homogeneously disperse a food or beverage powder in water. For powders containing both poorly wettable and readily wettable particle surfaces, the content of the hydrophobic compounds may be decisive for the whole reconstitution process. While the impact of hydrophobic components within a powder mixture on the wettability was already studied in terms of granulation processes [1, 2], a detailed study of this effect with respect to wetting as first step of powder reconstitution is still missing in the literature. Although in both granulation and powder reconstitution the capillary pressure is the driving force for penetration of liquid into the powder system, the available water quantity makes the difference.

We studied wettability in terms of the content and the contact angle of hydrophobic material within a single pore as well as in a powder mixture. In a first step, cleaned and silanized glass was used as hydrophilic and hydrophobic model material, respectively, in order to focus on the wetting step in avoidance of swelling or dissolution of components. Secondly, the hydrophilic and hydrophobic glass materials were replaced by food components in order to progress towards real systems leading to changing solid and liquid properties during the wetting process. A two-wall setup was developed to study the capillary rise in a single gap containing walls of two materials with different wetting properties. Therefore, two slides were fixed around a spacer, which is changeable and determines the gap width. The dynamic rise was captured using a high speed camera (NX-S2, Imaging Solutions GmbH, Germany). Additionally, a Washburn setup (Washburn tool of the K100 tensiometer from Krüss, Germany) was used to determine penetration rates of water into powder beds. The experiment was performed with two particle fractions of different sizes in order to investigate the impact of pore size on the dynamic wetting as well. There was a significant deceleration of penetration rate observable into the powder mixture if hydrophobic surfaces are involved. Already low quantities of hydrophobic material within the powder mixture cause a significant change in wetting behaviour. Furthermore, a model equation was used to calculate theoretical penetration rates into pore networks with hydrophilic and hydrophobic walls. The results of the experimentally determined and calculated penetration rates showed a good accordance.

References:

[1] Charles-Williams, H., Wengeler, R., Flore, K., Feise, H., Hounslow, M.J., Salman, A.D., 2013. Granulation behaviour of increasingly hydrophobic mixtures. Powder Technology 238, 64–76. 10.1016/j.powtec.2012.06.009.

[2] Nguyen, T., Shen, W., Hapgood, K., 2009. Drop penetration time in heterogeneous powder beds. Chemical Engineering Science 64 (24), 5210–5221. 10.1016/j.ces.2009.08.038.