On Drag Reduction with Super-hydrophobic Coating Experiments in a Laminar Couette Flow

1. On Drag Reduction with Super-hydrophobic CoatingExperiments in a Laminar Couette Flow Robert S. Brodkey, Konrad Koeltzsch, Abdullahi Yusuf, and H. Yildirim Erbil Ohio, Germany, and Turkey

2. The Old and the New

3. Purpose and Experiments Confirm that super-hydrophobic coating reduces skin friction drag for macroscopic size scales (Gordon & McCarthy 2000). Focused first on one Reynolds number, Re = 100 (laminar flow range).

4. Rational Polymers and surfactants are know to be excellent drag reduces. Microbubbles have been shown to be effective drag reduces. We hypothesize that super-hydrophobic coating might stabilize surface microbubbles and thus reduces skin friction.

5. Experimental Design Laminar Couette flow between two circular cylinders (inner diameter 100 mm, gap 7.15 mm, cylinder height 75 mm). Outer cylinder rotates; inner cylinder is at rest with no Taylor-G�rtler vortices! Torque measurement at inner cylinder. Further details in Koeltzsch et al. (2003). 550 ml water: aerated (at least 24 hours) or degassed (boiled and then air-cooled).

6. The Physical System

7. The Physical System

9. Results and Discussion Our preliminary experiments indicated that drag reduction occurs. However, the experimental error was very large.� The experimental technique was improved and compared with the preliminary tests. The new, much more accurate measurements did not confirm the preliminary experiments. We then retested the coating for hydrophobic activity and found that the activity had disappeared.

10. Results and Discussion Was the problem the lack of activity or was it in the measurements? The cylinder was sent back to Turkey and recoated. Second trip for the cylinder, none for Brodkey. This is two up-cylinder-ship. As reported before, the temperature effect is unimportant.

11. Results with the Newly Coated Cylinder Expanded scale transducer reading vs. number. There is less than 2% reduction in torque.

12. Observations During each data point acquisition, a sequence of pictures was obtained at 60 Hz. The sequence was averaged. The average was subsequently subtracted from the original images to form a new sequence of the differences. We hypnotize that it will take considerable time for the microbubbles to form from the saturated water solution.

13. Observations Thus, the test may need to extend for several hours for the microbubbles to form. As a result of jitter in the camera process, the average is a bit hazier than the individual frames, but shows no change of the short period of time.

14. An example animation Run 10 showing every 4th image Total time 1.3 sec

15. Observation The same frames for the subtraction process were generated. The idea was that if microbubbles formed the subtraction process, this would enhance their visibility. What were, however, nicely enhanced were any markings on the moving outer cylinder.

16. Sequence of Averages The next step was to take the sequence of averages; one for each of the image acquisition sequences, and composite them into a new sequence that would represent the 276 minutes of run time. This resulted in a sequence of 52 images. No effort was made at first to align the images horizontally at this point so the images jump horizontally from left to right and back.

17. Sequence of Averages The images were aligned to within a few pixels. Our hypothesis was that microbubbles would cover the surface over an extended period of time; however, viewing the time sequence does not show that this occurs. The small light spots shift and change a bit, but generally stay in the same position and additional spots do not accumulate.

18. The Long Time Sequence 276 minutes of time in sequence

19. Observations There appears to be little or no drag reduction and no real changes in the surface during the extended run. It was then necessary to make sure that the character of the surface did not change and that a drop of the test water would not be surface wetting. The ~ 2 mm drop of test water does not wet the surface.

20. ~ 2mm Water Drop on Surface

21. ~ 2mm Water Drop on Surface

22. References Choi, C.-H., K. J. A. Westin & K. S. Breuer (2003) Apparent slip flows in hydrophilic and hydrophobic microchannels. Physics of Fluids 15 (10), 2897-2902. Erbil, A. Y., A. L. Demirel, Y. Avci & O. Mert (2003) Transformation of a simple plastic into a super-hydrophobic surface. Science 209, 1377-1380. Gordon, T. D. & T. J. McCarthy (2000) Drag-reduction and slip: An investigation of size scale and hydrophobicity effects. FED-Vol. 253, Proceedings of the ASME Fluids Engineering Division 2000, 367-374.

23. References Koeltzsch, K., Y. Qi, R. S. Brodkey & J. L. Zakin (2003) Drag reduction using surfactants in a rotating cylinder geometry. Experiments in Fluid 34 (4), 515-530. Watanabe, K., Yanuar & H. Udagawa (1999) Drag reduction of Newtonian fluid in circular pipe with a highly water-repellent wall. J. Fluid Mech. 381, 225-238.

24. How old are you? The precursor of today�s 4000 frog and toad species

25. Adhesive Pads(Chemical Engineering)

26. DoneOverEnd