Water interaction with MCC powder exposed to wetting–drying cycles: Comparison of capillary imbibition techniques

We investigate how wetting-and-drying (WD) cycles change the properties of microcrystalline cellulose (MCC) powder (Avicel PH101), due to hornification, by performing the following experiments: (i) drop absorption in a slightly compressed powder bed, (ii) capillary imbibition in a packed powder column, and (iii) drop absorption into tablets obtained by direct compression. We use the MCC powder as received, as well as the powder obtained after one or more WD cycles. In each case, the powder is sieved and fractionated into two samples, with different particle size distribution. First, we perform drop absorption experiments using both water and silicone oil (Polydimethylsiloxane - PDMS) drops. We observe that WD cycles increase the absorption time for water, a particularly marked effect after the first cycle, for both particle size distributions. Comparing the absorption times of water and oil drops we obtain a parameter β that characterizes the water-MCC interaction in lieu of the contact angle, and accounts for both capillary and swelling phenomena. We observed that the main effect is produced after the first WD cycle exhibited by a large (more than 50%) reduction in the β value. Column imbibition experiments performed using water and PDMS show that, initially, water penetrates at a larger rate than PDMS. This initialing faster water uptake is more significant for larger particles and fewer WD cycles. A power law regime is obtained in each case with a ζ exponent. In particular, a Washburn-like imbibition regime with ζ=0.5 is recovered at long times for both liquids. In the last experimental technique, we study the absorption of water drops deposited on tablets obtained by direct compression of the full distribution of powder particle sizes. We observe that absorption time increases with increasing number of WD cycles of the MCC powder. This is quantified by the 30% reduction of the ζ exponent that models the penetration rate of the drop. In summary, we use three kinds of experiments that cover a wide range of characteristic time scales to quantify the effect of WD cycles on the interaction of water with a swelling powder like MCC. The importance of comparing responses under different timescales is that the swelling phenomenon modifies the properties of the material in time, while imbibition happens. It is shown by this study that in some cases, the phenomena can be decoupled (droplet absorption in porous media) while in other cases it cannot be (droplet on