Characterization of powder wettability using capillary rise in a closed column
Creators: name: Li, Minglu
Publisher: name: No Publisher Supplied
Year: 2020

description: The wettability of powders plays a critical role in various industrial manufacturing processes and products. In particular, in the pharmaceutical industry, examples include wet granulation and the dissolution performance of solid doses obtained by powder compression. This study aims to develop a characterization method to study the wetting properties of powders, and finely divided solids in general, using a closed column packed with the material of interest. Using a closed column in contact with a liquid allows us to i) study both the advancing and receding process as the liquid penetrates and is later displaced from the column, ii) to characterize the powder composing the porous media inside the column in both static and dynamic ways for a wide range of powder-liquid systems, and iii) to control (reduce) the characteristic time of the experiments by changing the experimental (column height). The advancing, receding and bubbling pressure are defined to characterize the system in a static way. Analytical solutions are provided to study the dynamics during the capillary rise process. The explicit solutions are obtained in terms of the pressure differential and the liquid mass, two independent variables in the system that can be measured directly in the experiments. The hydrostatic effects and the non-linear pressure dependence on the penetration height are considered in the solutions without any approximations. Therefore, the solutions are general and can be used to characterize a wide range of solid-liquid systems, especially for systems with large capillary pressure. Two non-dimensional parameters governing the system are identified: the capillary pressure and the initial pressure in the closed column, both normalized by the hydrostatic pressure corresponding to the effective column height. The non-dimensional description provides valuable information on how to optimize the experimental setup depending on the application. As an important example we discuss how to reduce the equilibrium time.Experiments were performed using two sets of glass beads, 10μm and 45μm with Polydimenthysiloxane (PDMS) as the model system. The experimental data are fitted with the analytical solutions to study different imbibition regimes and obtain the effective capillary pressure and permeability. Three imbibition regimes are determined: the early/Washburn imbibition, the intermediate and the late imbibition. It is shown that the intermediate imbibition stage is the preferred region to obtain the effective capillary pressure and permeability values from fitting. The importance of these different stages and their importance to characterize systems presenting a relatively large heterogeneity of pore sizes is discussed. After validating the characterization method with the model system, it is applied to other solid-liquid systems with larger heterogeneity, including the larger glass beads and Deionized water, as well as pharmaceutical powders such as lactose, Microcrystalline cellulose (MCC) and alumina. In cases of highly heterogeneous system such as lactose and MCC, a spontaneous bubbling process is observed and thus the advancing pressure cannot be measured. The contact angle is estimated using the effective pressure obtained from the fitting with the analytical solutions. Different column heights were used in the experiments with lactose and a reduced time to reach the same completion factor is achieved by a shorter column. Performing shorter experiments was shown crucial when working with a powder (lactose) that is soluble in the penetrating liquid (water).