After a droplet session with my daughter my interest in water collisions has been restarted. It’s an interesting technique when photographing water droplets to try to get a 2nd droplet to hit the first droplet as it emerges from the water, this technique is called ‘water collisions’ or ‘double drops’. The technique is very dependant on accurate timing of the spacing between the two droplets and the moment the flash guns are fired.
This post is a record of my experiments with water collisions using electronics to remove some of the variables. At the moment I’m capturing the water collisions about 80% of the time once the initial set up is calibrated. Calibrating the set up is necessary because of the variables like:
- The head of the fluid above the nozzle, this affects the droplet velocity.
- The opening time of the direct acting solenoid valve, this affect the droplet size depending on the viscosity of the fluid being used and the head of the fluid.
- The delay between closing the valve and reopening the valve for the second droplet.
- The delay before the flashguns are fired after the 1st droplet triggers a IR beam positioned under the nozzle, this is affected by the height of the nozzle over the tray.
- The depth of the liquid in the tray, this affects the height of the rebounding drop can achieve and how fast it reaches the best height.
To see how a droplet forms a splash take a look at this droplet sequence
- and numerous other smaller factors like, wetting agents, temperature and mechanical delays in the components, that affect the end result.
The set up to create the water collisions is:
- A tank suspended from the ceiling containing the fluid being used.
- A tube from the tank goes to a direct acting solenoid clamped above the receiving tray.
- The solenoid is connected to an Arduino processor which is used to programme the timing required.
- The nozzle below the solenoid is positioned directly over a small IR trigger beam.
- The IR trigger beam is connected to an electronic timing delay which fires the camera after a programmed delay and the flash guns are fired wirelessly from the camera.
From these first trials I have found that a valve opening time of 25 milliseconds produces a good size droplet that makes a good splash and a rebounding column of liquid. The delay between droplets is variable but about 100 milliseconds is a good starting point. The third delay is the timing for the flashguns; the optimum time is dependant on the inherent camera shutter and flash delay.
I used a 335 milliseconds delay initially and triggered the camera directly rather than triggering the flash guns with the camera on its ‘bulb’ setting as the whole process isn’t particularly quick and I can use a shutter speed of 1/250 second and a small aperture of f16-f22 to remove any ambient lighting.
The image album shown below contains images that have not been cleaned up and have been automatically processed by the raw conversion programme.