When the drop re-bounces from the liquid it forms a column that shoots out towards in the opposite direction. With enough timing it is possible to have a second drop hit that column during its short life span to get other spectacular results.
Many parameters are now affecting the final result and some kind of programmable hardware is required to control the results. It is still possible to get nice results with “normal plumbing” things but, well, it may take a lot of time. And maybe quite frustrating.
In these pictures I used green food coloring in the water that was dropped.
The above picture shows the re-bounce of the the first drop and the arrival of the second drop. One rather amazing thing to note is that the initial drop is still quite intact. Notice the color difference between the first drop (the one behind) and the column? In this picture the alignment of the two drop were not perfect.
In this picture the alignment was better (with all windows closed!) and you can see how the two drops start to collide.
Mixing different liquids will make the collision stand out even more. In the following pictures I used milk as the liquid surface. I recommend whole milk as it has higher surface tension.
Similar setup but with colored water drop colliding with milk. Each time you prepare and start fine tune your setup it is important to decide what you want to achieve and with what materials. Since the behavior of for instance water and whole milk is so different all timing will have to be redone. The same is true if you, late in the work process, decide that you want smaller drops and add some rinse aid. After this the drop size will change and all timing. Still, it is great fun and I encourage you to try and experiment!
Once I had everything setup as I wanted and was experimenting with different drop sizes and intervals I realized that it was possible to sequence the initial stepping. If we consider the time from start to exposure of the frame as an equation of the form:
TimeToExposure = OpenTime0 + Delay0 + OpenTime1 + Delay1 where
- OpenTime0 is the amount of time the valve is open to release liquid for 1st drop (this determines the size of the drop).
- Delay0 is the amount we wait until the valve is opened for the second drop
- OpenTime1 is the time the valve is open for the second drop
- Delay1 is the time we wait until the picture is taken
Then altering Delay1 while keeping the other parameters unchanged will yield pictures where things will progress as though they were taken very fast during one single event. Of course there will be variations and many parameters will come into play. Make sure there is as little movement of air as possible (these drops are very light and will be moved by even small air flows. Other aspects are the drip device you use and whether there is randomness in the flow and release. Anyhow, once I had this working I realized it was possible to make a “fake high speed camera movie” of the collision of two water drops. And here is the result:
One tell-tale here of the fact that this is not a high-speed movie is the fact that I use food coloring for the drop. Since the movie consists of close to 300 picture – that means 600 drops – the color of the milk slowly turns green…Yes, I could have changed the milk every frame but I’m lazy . Still, it’s kind of nice to have reproducible results that your can alter to become the composition of your choice!