5. Conclusions

The experimental data provided a comprehensive overview of pressure drop in small pipes with 0, 50% and 100% hydrophobic coating of inner pipe surface over the Reynolds range from 150 to 10000. The main findings can be summarized as:
  1. The hydrophobic coating reduce the drag for laminar flow in small pipes of 1 and 2 mm by around 10% but increase the drag by around 10% in transition and turbulent flow in the larger pipes of 4 and 5 mm. This behaviour may attributed to the change from the liquid-air wetting surface condition to the liquid wetting surface of the pipes used in this paper.
  2. The increase of drag in transition and turbulent flow may be attributed to the very smooth surfaces of the pipes used in present work. This minimized/ eliminated any slip velocity at the wall which depend on the surface topology.
  3. There is a correlation between the percentage of surface coating and the drag reduction/increase. This correlation is not linear due the additional effect from the possible generation of vortices in the flow due to unbalanced shear forces on the walls.
  4. The data from 3mm pipe indicating a transition from small pipes where the viscosity have a significant influence on the velocity profiles to large pipes when momentum have more influence on velocity profiles.
  5. The high definition imagery has presented some interesting and intriguing information, in particular on the half-coated bubble trajectories. The complex interactions between fully coated and non-coated pipe walls will be invaluable to industry. The data provided within this article will present microfluidics and biometrics with opportunity to maximize the efficiency and reduce maintenance of any system which operated under hydrophobic conditions in small pipes.