Wetting of binary powder mixtures

Abstract

The wetting process involved when a liquid droplet comes into contact with a mixture of particles is a complex phenomenon which is often understood by reference to Cassie-Baxter theory. However, various authors have applied the Cassie-Baxter theory for the prediction of contact angles on two-component mixtures without success. We hypothesise that the main difficulty in applying the Cassie-Baxter theory to mixtures is that if the particles differ in size, it is possible for the small particles to coat the large particles, so reducing the available surface area of the large particles. This leads to the view that bulk volume fractions are not good estimates of surface fractions of the components within the mixture. We argue that the Cassie-Baxter theory over represents the influence of large particles and that below a certain critical volume fraction they exert no influence. We present a simple geometrical model that relates the critical surface coverage volume fraction to the Sauter mean particle size of the binary mixture components. As a consequence, the wetting behaviour can be determined from the bulk volume fractions and the calculated critical surface coverage volume fraction, by means of a simple geometric model. We show that the simple model describes the five two-components systems reported here and a further four systems reported in the literature, irrespective of whether the larger or small particles are hydrophobic/hydrophilic. With this model, it is possible to predict the wetting behaviour of mixtures of particles that coat each other using very simple characterisation methods, so reducing the development time in the creation of formulations in the pharmaceutical industry.

Effect of Filler Types on Cellulose-Acetate-Based Composite Used as Coatings for Biodegradable Magnesium Implants for Trauma

“…The lowest contact angle values are obtained for samples containing magnesium particles (CA-HAp-Mg and CA-Mg), which proves that by adding them, hydrophilic surfaces favorable for biological integration are obtained ( Figure 10 ). It is well known that when a liquid droplet comes into contact with a mixture of different particles, such as in our case (HAp and Mg particles), a complex contact angle behaviour can be observed [ 67 ]. As an analytical tool, the Cassie–Baxter theory can be used.…”

“…As an analytical tool, the Cassie–Baxter theory can be used. Mundozah et al [ 67 ] adapted this model by considering the average particle diameter. They showed that in the case of a mixture formed from larger particles (i.e., Mg powder) and small particles (i.e., HAp powder), if the larger particle diameter is higher than a critical volume fraction, the surface wettability could change considerably due to the fact that partial coverage of larger particles by the small ones appears.…”

The impact of channel fill level on internal forces during continuous twin screw wet granulation

“…One of the common equipment items evaluated to perform continuous granulation are the twin screw granulators (TSGs) (Mundozah et al, 2018;Silva et al, 2018). The advantage of this equipment is the flexibility offered from the high number of possible working environments achieved by changing different sections of the screw assembly, different segment geometries or feed port locations (Dhenge et al, 2011;Djuric and Kleinebudde, 2008).…”