Dynamic Adhesion of Pharmaceutical Powders: Measurement Using a Drop Test Method and the Influence of Surface and Crystal Properties

Powder adhesion, defined as the attraction between two unlike solid entities sharing a contact surface due to intermolecular forces acting over a distance, presents a significant challenge in pharmaceutical manufacturing by influencing powder flowability and contributing to defects such as poor content uniformity and equipment surface sticking. While existing techniques, such as atomic force microscopy and centrifuge methods, enable adhesion measurements, they are often time and cost-intensive. This thesis addresses this limitation by developing a practical and systematic approach for quantifying the effective work of adhesion, using the drop test method.
A robust standard operating procedure was established, supported by improvements to the drop test rig previously developed at the University of Leeds, to ensure reliable and repeatable measurements of impact velocity and contact time. The methodology was applied to both irregularly shaped ibuprofen and spherical aluminium-alloy powders. A key finding was the identification of a minimum particle count required to accurately determine the critical diameter.
The critical diameter, was defined as the hypothetical particle diameter at which the detachment and adhesive forces are balanced, separating particles that remain attached to the substrate from those that detach upon impact. An automated image analysis algorithm was developed to improve the efficiency and consistency of critical diameter determination, showing good agreement with manual analysis.
Overall, this thesis demonstrates that powder adhesion is governed by the interplay of measurement conditions, substrate characteristics, and particle properties. The developed methodology provides a practical and accessible framework for quantifying adhesion and offers a foundation for improving the design, manufacture, and handling of pharmaceutical powders.