Music: Yellow Taxi Under a Blue Sky by Mindseye

Team 2nd Photo

Sierra Castillo

Introduction

Flow visualization give insight to how flow phenomena operates in our surroundings. For this assignment, teams were tasked with photographing these phenomena. Team 2 consists of Sierra Castillo, David Leng, and Katie Gresh. The team worked with a high speed camera operated by Professor Tad Truscott to visualize the water droplets falling on a hydrophobic surface. These surfaces allow water droplets to have high contact angles, resulting in spherical or semi-spherical droplets. Additionally, the droplets are able to bounce off of the surface without wetting. Using a Goretex rain jacket made by Marmot, the team dropped several water droplets on the surface and photographed the results with a high speed camera.

Apparatus

All of the photography completed for this assignment was captured in Prof. Hertzberg’s lab. The team laid the jacket across a cart such that the surface in the FOV for the camera was relatively flat. Next, the team set up a tripod over the surface to regulate the drop height and ensure that the drops would splash in the same spot (using reference tape) on the jacket to preserve the focus of the camera. Lighting was achieved through 2 sources; one lamp to the left of the apparatus at 120 degrees behind the drop zone and one held by a team member on the right close to the drop zone from 120 degrees in the back. For my submission, a cardboard box was placed under the jacket and angled to 45 degrees to achieve rolling droplets. Figure 1 shows the setup of the apparatus. Figure 2 is a picture of the tripod with a dime for scale. Figure 3 shows the setup before the tripod is added.

Figure 1: Apparatus used to capture videos using a high speed camera.

Figure 2: Dime placed on surface for scale.

Figure 3: Apparatus before tripod was added.

Fluid Physics

The physics of a hydrophobic surface are related to the microstructure of the surface. A detailed explanation of hydrophobic surfaces is presented in the report for the first team assignment found here. The distinction between a hydrophobic and superhydrophobic surface resides in the contact angles of the drops. Hydrophobic surfaces exert contact angles of 90 to 150 degrees. In this case, the contact angle is difficult to estimate due to the downward motion of the droplets. The surface is assumed to be hydrophobic as little to no wetting occurs.

Another interesting aspect of this video is the upward bounce of the droplet. The droplet bounces perpendicular to the surface, even though it is tilted at an angle. This is interesting as intuition tells us that the drop should bounce perpendicular to the ground or gravity. Unfortunately, these physics are widely unexplored and I could not find a decent reference to explain why this might happen. My best guess is that the droplet is behaving as a solid object. To explain, if you bounce a ball on an angled surface, it will bounce perpendicular to where it hit (depending on the material properties of the surface). This is one possible explanation of why the water droplets behave similarly, even though it is a fluid.

Photographic Technique

The camera used to complete this assignment was the Phantom V2511 high speed camera, operated by Professor Tad Truscott. The specifications are tabulated below.

Frames per Second	4000
Field of View	~4”
Distance: Obj to Lens	~6”
Resolution	1280×800
Pixels	12-bit, 28 microns

Once the images were compiled, very few edits were made to the video. I adjusted the contrast slightly and sped up the video. Additionally, I added background music of a snippet of “Yellow Taxi Under a Blue Sky” by Mindseye, found here. This artist employs a Creative Commons license, the details of which can be found here.

Conclusion

The outcome of the footage using the high speed camera was exciting and gave insight to the intermediate steps of what the naked eye can perceive and what actually happens when a drop of water falls on a hydrophobic surface. Overall, I am very pleased with my submission. The drops of water take on an almost animalistic life that reminds me of the movie Flubber. It almost looks like they are marching down a mountain.

References

[1] Youtube.  https://www.youtube.com/watch?v=HCGiwSghrqQ.  [Online]. [Accessed: 10- Oct-   2016].
[2] Cheng, Y. T. Superhydrophobic Surfaces. et. al. Applied Physics Letters 2005, 87, 194112. [Online]. [Accessed: 20- Oct- 2016].