Droplet Impact

Droplet impact is a phenomena prevalent in numerous industrial applications spanning from spraying insecticides to internal combustion engines. To date, most studies have aimed their efforts towards Newtonian fluids. However, many applications such as inkjet printing and the spray coating of pharmaceutical drugs involve complex rheological solutions. The outcome following impact can take on various outcomes but three of the most common are shown below in Figure 1.




Figure 1: Outcomes upon droplet impact: Deposition, Rebound, Splashing. Note: Outcomes other than the above have been observed such as partial rebound and various splashes.

Our study is aimed at the pharmaceutical industry but includes a more general study of non-Newtonian droplet impact. One of our aims is to study non-Newtonian droplet impact and determine the dominate non-Newtonian feature of such coating solutions in the pharmaceutical industry.

Buckingham Pi theorem provides the main dimensionless groups relevant to droplet impact:




where ρ, µ, and σ are the liquid density, viscosity, and surface tension, respectively. D and U are the drop diameter and impact velocity respectively.

Tablet coating operations are used for various reasons including brand recognition, time release of an active agent, or simple to mask unpleasant taste. Until now, pharmaceutical coating operations otherwise known as spray drying have been conducted on a trial and error scale up process. A simple schematic of this operation is shown below in Figure 2



Figure 2: Schematic of typical spray drying operation. Center: Atomizer sprays droplets as larger cylinder rotates clockwise.

Thus characterization of droplet impact of coating solutions on various tablet surfaces is critical to optimization of these processes. It is our goal to delineate the parameters which control impact outcome and determine a predictive model to prevent undesired coatings. The following images in Figure 3 shows a typical impact outcome from a coating solution impacting on a tablet.

Figure 3: Sequence of images from acetone based coating solution on tablet falling from a height of 300mm. Each image from left to right top to bottom is incremented by 0.2 ms.