Adding a tracer to the surface of a water droplet I have a 2 mm water droplet generated by a syringe and falling down. I am using two perpendicular cameras to capture simultaneous frames from it. I need to track the droplet during the time and reconstruct it (through consecutive frames). However, only the border of the droplet (which is an ellipse) is distinguishable in each frame and I do not have any traceable point on the surface of the droplet to use it as a tracer (each point on the droplet surface will give me a point inside the corresponding ellipse on each frame). I want to add one or few very small particles/tracers in the order of my pixel dimension (e.g. 0.05 mm or even less which are still detectable through my cameras) such that while the droplet wobbling or revolving during falling down, always remain on the surface of the droplet and don't go inside it. Which material or technique do you think is more appropriate for this purpose?
Please let me know if you know any reference or paper used your suggestion.
for more information please see this Phys.SE question as well.

 A: I would consider using water with a dye like a deep blue to be nearly opaque. Illuminate with an array of LEDs. Each bright spot reflected from the droplet is an LED. You can add a few strategic red LEDs among all white as reference.
Working out the most convenient geometry will tell if you need the LEDs on some curved surface in space or if a flat panel will do. I think you can get pretty good data this way and quite high resolution as far as number of points on the drop.
A: There may be some problems with Teflon particles. Prior to droplet separation from the syringe, the Teflon particle can roll down along the surface of the droplet, so you'll only have your tracer at the bottom of the droplet. This movement of the particle along the surface will not take place when the droplet is in free fall. This movement will be less important for small particles, but it is more difficult to register a very small particle by a camera. So you may wish to consider using phosphorescent particles (http://www.cospheric.com/PHYGPMS_phosphorescent_yellowgreen_microspheres.htm ; see their note on the surfactant).
