Effect of surface treatment on drag coefficient of free-falling solid sphere in water

The study aims to examine the influence of different surface treatments on the drag coefficient of free-falling spheres in water. The spheres used in the experiment are classified into four categories and labelled as: ref – no surface treatment, FDTS – coated with hydrophobic perfluorodecyltrichlorosilane (FDTS) coating, E8 – 8 minutes etched in ferric chloride (FeCl3) and E32 – 32 minutes etched in FeCl3. No significant difference is observed in drag coefficient of the etched spheres compare with the reference sphere. However, the FDTS sphere’s drag coefficient was increased by 13%.