During reconstitution of food powders the wetting of solid surfaces is an important property since it is often the time limiting step. Most food powders are multicomponent powders which include hydrophilic as well as hydrophobic surfaces. Chemically heterogeneous surfaces result in heterogeneous wetting. Two experimental setups are used to determine the wettability of systems containing hydrophilic and hydrophobic surfaces. In a micro system, the wetting front in a single pore can be observed, while in a macro system the liquid penetration into a powder bed is investigated. The aim of this work is the determination of the key limiting parameters during wetting of multicomponent food powders.
Understanding the interactions between solid particles and liquid is an important requirement for several industrial processes such as granulation or agglomeration as well as for the final applications of food or pharmaceutical products. As the first step the wetting has a major role within food powder reconstitution being often the time limiting factor . The quality of wetting can be expressed by the contact angle. The wettability of particles with a certain liquid is a highly dynamic process and is mainly driven by capillary forces since liquid enters the pore space within the particle bulk due to compensation of capillary pressure. While high contact angles, in general, lead to an insufficient or slow wetting, small contact angles support a fast wetting process. Additionally, the pore size which is mostly determined by the particle size distribution, has a major impact on the capillary rise . Heterogeneity makes wetting complex whereby we distinguish between geometric and chemical heterogeneity. In this study, the focus lies on the impact of hydrophobicity on wetting kinetics.
A detailed study on the impact of hydrophobic surface in terms of the quantity as well as the contact angle of the hydrophobic component on the wettability in heterogeneous powder mixtures regarding food powder reconstitution is still missing in literature. Therefore, we study the impact of hydrophobicity on the wetting process in single pores as well as in powder mixtures in terms of contact angle and content. The aim of this study is to understand the effect of hydrophobic surface on the wetting process as first step in food powder rehydration and to develop a model to predict the wetting time.
We studied the capillary rise of water into single pores and pore systems containing hydrophilic and hydrophobic walls. Cleaned and silanized glass material was used as model hydrophilic and hydrophobic material, respectively, in order to focus on the wetting step in avoidance of swelling or dissolution of components. The material characterization is performed by sessile drop method which is schematically shown in Figure 1. Both, the contact angle and the diameter of the droplet, are measured in order to obtain the dynamic contact angle of the wetted materials.
A two-wall setup was developed to realize the capillary rise in a single gap containing walls with two different contact angles (Figure 2, left). Single gap experiments were performed in a tailor-made setup where two glass slides were fixed around a spacer. The spacer was changeable and determines the gap width. A movable beaker providing the water was installed below the gap setup and was moved upwards for starting the experiment. The water rise into the gap was captured by a high speed camera (NX-S2, Imaging Solutions GmbH, Germany) equipped with a microscope objective from OPTEM ZOOM 125 and ImageJ was used to analyse the images.
The results of the dynamic wetting process into the model powders containing hydrophilic (ϴhydrophilic = 26°) and three different types of hydrophobic (ϴ1 = 72°, ϴ2 = 83°, ϴ3= 93°) glass beads are presented in the following. Therefore, the mass gain over time was determined using the Washburn tool and analysed in terms of wetting times which are normalized by division by the fastest wetting time (100 wt.% hydrophilic). The impact of the amount of hydrophobic component and the level of hydrophobicity is shown in Figure 3. An increasing amount of hydrophobic beads in the sample leads to an increasing wetting time. Furthermore, a significant jump in wetting time by adding just 1 wt.% of the hydrophobic component can be observed. On the other side, the degree of hydrophobicity (ϴad = 81.7°, 94.7 ° or 95.7 °) affected the wetting time only at higher proportions in the sample (30 wt.% of hydrophobic fraction) in both size fractions.
 H. Schubert, Instantisieren pulverförmiger Lebensmittel, Chemie Ingenieur Technik 62 (11) (1990) 892–906.
Project started November 2014
Funded by Nestle Research Center Lausanne