Date of Graduation

2007

Document Type

Dissertation/Thesis

Abstract

The behavior of a single liquid droplet impacting a solid surface is a complex phenomenon and is a basic component of various industrial processes. One such process is the film coating process in the pharmaceutical industry, where coating uniformity is important especially if the coating is for functional purposes. Coating variability on a tablet could be affected by several factors, one of which is the impingement of droplets on its surface. The spreading behavior of a droplet on a solid surface was reported to be affected by the surface properties, particularly the surface roughness. To understand these phenomena completely, a series of experiments were conducted to investigate the impact behavior of a droplet on tablet surface with different roughnesses. These studies were considered in two stages; namely, a short-term phenomenon and long-term phenomenon. A 2-mm droplet was used in the short-term phenomenon study since for a micro-droplet this event occurs in a micro-second time scale, which is too fast for our high-speed camera that has a maximum framing rate of 2000 per second. However, a 60-μm droplet was used to study the long-term phenomenon, in which penetration and recoiling of droplet were observed. The results from the short-term phenomenon study showed that the initial spreading behavior of a droplet on tablets and stainless steel surfaces were similar, which suggests that penetration is negligible during this period. Droplets were found to spread less and bounce higher on a rougher surface. The results of the long-term phenomenon study showed that the bigger droplet (2-mm) took a longer time to penetrate compared to the smaller droplet (60-μm). The penetration rate was also affected by the tablet hardness; the harder the tablet the longer the penetration rate. The long-term phenomenon was also performed on tablet edge with different curvature; namely, ‘sharp’ and ‘round’. The results showed that on the round edge of a tablet, the spreading and penetration rates were found to be similar to the rate on the flat surface. However, on the sharp edge of a tablet, the penetration rate was found to be much faster than the spreading rate. This is due to the non-homogeneous packing of granules at the sharp edge giving rise to a porous region around the edge. On the other hand, the packing of granules is more homogeneous on the round edge and surface of a tablet. A multiphase volume-of-fluid (VOF) model was developed to predict the short term spreading phenomenon of a droplet, which also indicates the maximum spreading diameter resulting from the impact. The results from the 2-mm droplet were used to verify the model and a good agreement was obtained between simulations and experiments. The same variables and computational schemes were used in modeling a 60-μm droplet on different surface roughnesses. The 60-μm model developed in this work is useful to the spray coating processes to predict the maximum coating coverage following the droplet impact.

Download

Share

COinS