The purpose of the drip photography system described here is to photograph drop-on-drop collisions.This requires two parts: i) the drip control and ii) the photo capture. The drip control creates in a reproducible way the desired collision. The photo capture uses a combination of drop detection, short-duration flash, and a camera to photograph the collision at the desired instant.
Drip control: In a typical collision scenario, one drop of liquid is released from a reservoir, falls into a pool, and rebounds by tossing up the familiar spout or column. A second drop, released soon after the first, collides with the top of the rebounding column. See Figures 1-3 for a series showing the development of the collision. The drop controller sets the time interval between release of the drops in order to produce the desired result. This interval will depend on how far the drops fall and how much time elapses as the first drop rebounds. While trial and error is involved in determining the time interval, once settings are determined, a good drip control system will provide reproducible results as well as the fine control needed to tweak the results to obtain different effects. This depends not only on the electronic timer, which is inherently consistent in generating time intervals, but also on maintaining a constant pressure head above the valve. The latter is typically done with a Mariotte bottle, something that is easy to set up and requires no specialized equipment.
A falling drop is on a collision course with a rebounding column produced by a previous drop.
As the column narrows, the collision with the falling drops ensues.
The collision proceeds to form the familiar splash coronet.
Photo capture: A brief burst of light at the right instant is required to photograph the collision. A photogate is typically used below the valve in order to detect the passage of the first drop. A timer then generates a time delay after which an electronic flash is discharged. The delay is adjusted so that the flash is discharged at the desired instant of the collision. In order to record the event with a camera, the process is carried out in a darkened room, and the camera shutter is opened when the first drop is released. The shutter is held open until after the flash discharges. While the camera shutter can be actuated manually, one can have it opened automatically by either the drop controller or the photogate timer.
Components of a Drip Photography System
There's room for much creativity and ingenuity in setting up a system for drip photography. However, there are certain components to include. We've divided them into two lists with notes on each. HiViz.com provides the components on the left with the exception of wireless components. The photographer/experimenter provides the components on the right.
1 to 3 valves may be controlled simultaneously with the AstroSplash. Adapters are used to connect the valve to the fluid supply. The fluid drip is released from a nozzle.
A minimum of one flash unit is required, but at least two would be used for creative lighting effects. Backlighting is typically used with the light being diffused by a frosted plastic sheet. Colored filters provide creative lighting.
Photogate and delay timer
Either the Multi-Trigger 3 (MT3i) or Multi-Trigger 2 (MT2) serve as delay timer. The interrupter-style photogate sensor is used to detect the passage of a drop, and the delay circuit generates the time interval before the flash is discharged.
Camera and lens
A digital SLR is recommended both for precise framing of the subject and immediate image review. A lens of at least 100mm focal length will allow the camera to be placed a good working distance from the splash site.
Cables for flash and camera
The flash units may be connected to the MT2/3 with cables. For 2 flash units, a cable splitter is used. Alternatively, the MT2/3 can trigger a wireless controller for the flash units. (HiViz.com doesn't provide wireless controllers.)
A framework is required for all the components. Supports are needed for flash, camera, and lighting accessories.
A number of events must occur in a predetermined, predictable sequence in order to photograph drop-on-drop collisions. The timeline below may help in visualizing the events in the sequence. (Click on the diagram for a larger version that will open in a separate tab.) In the timeline, time increases to the right. Note that the primary triggering event is the press of the trigger button on the drop controller. This is indicated by the dashed, vertical line labeled ADC Trigger on the timeline. This opens the camera shutter and actuates the valve(s) to release the drops, which, in turn, starts the delay timer when the first drop enters the photogate. If the timing is set correctly, the collision event occurs simultaneous with the flash discharge (2nd vertical dashed line).
Figure 4. Timeline
When the trigger of the AstroSplash (ADC Trigger) is pressed, a series of 1 to 3 pulses is produced. (Note that in many cases, only the first 2 pulses are used. This was the case for each of the photos in Figures 1-3 above.) Controls on the AstroSplash allow the duration (TIME ON) and spacing (TIME OFF) of the pulses to be preset. The valve is open during the TIME ON intervals. The TIME ON and TIME OFF intervals are adjustable from 8 to 120 ms (1 millisecond = 0.001 s).
When the camera is connected to the CAM output of the ADC, the shutter opens in response to the ADC trigger. The response is not immediate, as there is a lag time that depends on the particular camera being used. For a typical distance of fall of the drops, the shutter will be open before the collision is produced. The duration of the shutter is set on the camera to close after the flash discharge.
Although the delay timer is electrically separate from the ADC, the delay timer response is triggered by the ADC. When the trigger button on the latter is pressed, the valve releases the first drop. This drop passes through the photogate. This is the event that triggers the delay timer response. The delay time is preset on the timer to discharge the flash unit(s) during the collsion.
The Basic Drip System
The drop delivery system
Getting consistent results with your photography depends on more than the electronics and the valve response. The drop delivery system must operate reproducibly. An important requirement is that the pressure of the fluid at the valve inlet is constant. This can be achieved with a simple device known as a Mariotte bottle (named after Edme Mariotte). Refer to Figure 5 to the right for the assembly. (Click on the diagram to open a larger version in a new tab.) The reservoir used for the fluid can be as simple as a plastic soda bottle, although a Nalgene bottle would be sturdier. Another variation is a length of PVC pipe with end caps. Whatever you choose for the fluid reservoir, here are some things to keep in mind for your construction:
It has to be possible to open the bottle to fill and then reseal it. The cap must have an airtight seal.
You'll need to drill or punch a hole in the cap for the inlet tube. The tube can simply be a soda straw. Seal the tube around the cap with a waterproof sealant such as a silicone sealant.
You'll also need a hole in the bottom of the bottle for the outlet tube. Size the hole for a tube that will fit snugly over the fitting that connects to the valve. Seal around the tube where it enters the bottle.
Here's how the Mariotte bottle works. Suppose you fill the bottle to a level a little higher than H0 and put on the cap. Initially, fluid will fill the inlet tube to the same level as in the bottle. However, when you push the CLEAR button on the AstroSplash, fluid will drain through the valve, and the fluid level inside the inlet tube will fall to H1. The fluid level in the bottle will fall slightly to, say, level H0. The air pressure in the air space above H0 will drop slightly, while the air pressure in the inlet tube will be the same as the atmospheric pressure surrounding the bottle.
Now here's the part that explains how the Mariotte bottle achieves a constant pressure head above the valve. As long as the fluid level H0 remains above H1, the bottom of the inlet tube will remain constant at atmospheric pressure. Thus, the pressure at level, H2, where the fluid enters the valve is also constant and is equal to atmospheric pressure + the pressure of the fluid column of height H1 - H2. The pressure on the outlet side at the nozzle will just be atmospheric pressure. Therefore, the pressure difference from valve input to output will be due to the fixed height differential, H1 - H2.
In order for this to work, it's important that all your seals be airtight, including where the pipe fitting screws into the valve at the top. The fitting for the valve in the HiViz.com Valve Kit is a nylon 1/4-in NPT (also called MIP) to 1/4-in barb adapter. If you prefer a brass fitting, home improvement and plumbing stores carry them. Use pipe joint compound for a good seal. Don't use Teflon tape, as this will likely result in small leaks. When you screw in the adapter, don't screw in too tight, or you'll crack the casing of the valve. The barb side of the adapter is for the outlet tube. For the fittings that we provide, a tube of 1/4 inch inner diameter is needed. (This tube is also provided in the Valve Kit.)
The nozzle is another important component for getting consistent results. The pipe end of the fitting that screws into the valve is the same 1/4 in NPT as for the inlet end of the valve. The inner diameter of the outlet hole is an important consideration, as that will influence the drop diameter and possibly the formation of smaller satellite drops that can be produced when the main drop breaks free. A hole of 1/8-in inner diameter is a good starting point. A nylon nozzle of this size is provided in the Valve Kit.
The entire system
Figure 6 to the right shows a basic set up with one valve and one flash unit. See the photos below for pictures of one possible set up. (Click on the diagram for a larger version.) We discuss the components below.
The fluid path
The Mariotte bottle, valve, and interrupter photogate are mounted in a vertical arrangement directly above the target reservoir, which can be a bowl or tank of your choosing. Build sturdy supports that will hold the valve vertical so that the drops will break free cleanly at the same time around the perimeter of the outlet hole. For consistent timing, fix the interrupter firmly in place below the spout.
Distances: Position the interrupter about an inch below the spout and the fluid surface 1-2 feet below the interrupter. These distances should only be considered starting points for testing and experimentation.
Water drops and splashes are typically backlit through a frosted screen. If using more than one flash unit, you can try creative arrangements. Colored filters are frequently used to make the lighting more interesting. Food coloring may be added to the fluid.
Solenoid and camera control
These are the connections to the AstroSplash Drop Controller (ADC):
12V IN jack to a 12VDC power supply. Our wall transformer serves this purpose. If you use your own supply, it must provide 2A of current for 3-valve operation. A regulated supply will provide for consistent valve response.
VALVE1 jack to the valve. You can use any of the VALVE jacks. The same output goes to all the jacks.
Drop detection and flash control with the Multi-Trigger 2 or 3
Note: The description below assumes the use of the Multi-Trigger 2 or 3 (MT2/3). If you're using a different photogate-delay timer, make the appropriate modifications.
These are the connections to the MT2/3:
9VDC jack to the 9V wall transformer. Note that the MT2/3 can also be powered with a 9V battery. However, we recommend using the wall transformer for a constant voltage source.
Interrupter cable to the PG input of the MT2/3.
For the MT2, Delayed output to the flash unit. For the MT3, Delayed Flash output to the flash unit. If you're using a wireless transmitter to discharge your flash unit(s), connect the transmitter rather than the flash to the output. If you're using multiple flash units without a transmitter, you can connect the flash units with a cable splitter.
Photos of an assembled system are shown in Figures 7 - 9 below.
The photo above shows a complete set up. The AstroSplash and Multi-Trigger 2 are on the left. The flash unit (not visible) is behind the frosted screen supported on a mini-tripod. A glass bowl filled with water serves as the target reservoir. A glass plate catches the overflow.
The frosted screen was removed to show the position of the flash unit.
This close up shows the drop delivery and detection parts of the system. The Mariotte bottle is connected to the valve along a sealed pathway. Drops are released from the nozzle and detected by passage through the interrupter photogate. Lab hardware is used for holding the parts in place. The support framework is, of course, a matter of choice. For more information on the interrupter support shown in the photo, click here.
Configuring the electronics
We'll describe a procedure in the next section for initial testing designed to fine tune your timing. For now, here are some typical settings for the ADC and the MT2.
Push the CLEAR button initially to clear the fluid path and initialize the Mariotte bottle operation. Release the button to stop the fluid flow.
With both PULSE 2 and PULSE 3 turned off, a single drop will be released when the trigger button is depressed momentarily. (If you hold your finger down on the button too long, you may get multiple drops.) Switch on PULSE 2 for a second drop and PULSE 3 for a third drop.
The TIME ON setting adjusts the volume of fluid released in the drop. A greater TIME ON means that the valve is open for a longer time. Thus, the volume of fluid released in the drop is greater when TIME ON is greater. For initial testing, select a value toward the lower end of the range, say 24 ms.
The TIME OFF setting adjusts the time interval between the closing of the valve for the previous drop and opening of the valve for the current drop. Therefore, this setting will determine where and when the drop colllision occurs. A typical starting point is around 96 ms.
Set the INPUT switch to the PG position for photogate input.
For the MT2, set the DELAY/10 switch to OFF. For the MT3, set the Divide Delay switch to the ÷1 position.
Set all knobs to their halfway positions.
This applies only to the MT2: The Review/Focus switch for the Delayed output can be in either position. (This switch is only used when triggering a camera.)
With the unit turned on, the PG alignment indicator LED will be on to indicate that the photogate is operational. The LED goes out when the gate is blocked.
Capturing your first collision
Camera settings: You'll find that you can see the drops when the flash goes off. Therefore, it's possible to make adjustments without taking photos. You might try this until you get close to the result that you want. Then you'll need to take photos to examine details. Set your camera for completely manual operation. In order to focus critically, place a ruler at the position of the splash as shown to the right. Use a small aperture (large f/#) to increase depth-of-field, since the latter will be shallow for close up photography. An exposure time of 1 second is a good starting point. Depending on the ambient light level in the room, you may want to try decreasing the exposure time.
Dealing with shutter lag: If you find that when you try to take photos, the result is black even when the flash fires, the problem may be that the shutter lag of the camera is too long, and the shutter doesn't open until after the flash fires. In order to decrease shutter lag, make sure to wake your camera up before taking a photo by pressing the shutter button halfway and then releasing it. If you have the option in your custom camera settings to increase the amount of time before the camera goes to sleep, that's something to try. Locking the mirror up before taking a shot is another way to reduce lag time. Yet one other method is to raise the height of the valve so that the drop has to fall further, taking more time. If none of these things work, you can always open the camera shutter manually before you press the ADC triggger to release drops.
Flash duration: In order to minimize blur due to the motion of the drops, adjust your flash power to be as low as possible. This minimizes the duration of the flash burst. Later, you can try experimenting with higher flash power as you wish in order to find the greatest power for which blur isn't noticeable. This will enable you to get more light on the subject if you need it.
Here's a suggested procedure for initially adjusting the timing for a collision such as that shown in Figure 2 at the top of the page. Once you've achieved your first successful photo of a collision, then you can tweak the times to get different effects.
On the ADC, turn off PULSE 2 and PULSE 3 so that you're only dealing with one drop initially. Set the TIME ON for PULSE 1 to 24 ms. The TIME OFF doesn't matter at this point, since there's no second drop.
On the MT2/3, set the FINE and COARSE delays at about their halfway settings.
Press the ADC trigger to release a drop. The goal is to see the pillar rebounding from the fluid 's surface when the flash goes off or to take a photo of it if you have trouble seeing the event with the naked eye. If you're lucky, you may see the drop colliding with the fluid on the first try. Otherwise, adjust the coarse delay as needed to get close to what you want. Then use fine delay adjustments until you're able to see the pillar.
Now you're ready for the second drop. Flip the PULSE 2 switch on. Set the TIME ON for PULSE 2 to 24 ms and the TIME OFF for PULSE 1 to 96 ms. When you press the trigger button, you should see not only the pillar produced from the first drop but also the second drop on its way down. You may also see some smaller satellite drops. Don't worry about these for now. Now decrease the TIME OFF until you're able to capture the second drop colliding with the pillar.
Experimentation and results
Timing: Once you've achieved success with the above procedure, you're on your way. Here are things to try:
Tweak the FINE delay on the MT2/3 to see the collision at different times.
Tweak the TIME OFF to produce different collision effects.
Tweak the TIME ON to change the drop volume. This can also affect the production of satellite drops.
Lighting: There are countless ways you can change the lighting to produce different effects. Try more than one flash unit, different angles, different filter.
Perspective: Try different camera heights and angles relative to the fluid surface.
Physical variables: There are many things you can change here. These include nozzle diameter, drop height, size, shape, and depth of the target reservoir, pressure head, fluid temperature, fluid color, and so on.
Number of drops/valves: What could you do with a third drop? Try it and find out. With another valve or two, you could produce side-by-side collisions.
For more useful tips and information on fine-tuning your system as well as setting up a 3-valve system, see related photos and discussions at the HiViz.com flickr group.
Characteristics of the drop
Figure 10 (to the right) shows the apparatus in operation. A single Vivitar 283 flash unit with a blue filter provided backlighting. The flash duration was about 0.1 ms. The timing was adjusted so that the first drop was producing a coronet while the second drop was still several inches above the bowl. There's also a small, satellite drop following the second drop. These can result as a drop breaks free from the nozzle. In order to see how this happens, see the image sequence below.
Figure 11 (below) shows a series of 10 photos of a drop breaking free from a nozzle. Time increases from 1 to 10, but the time intervals between photos aren't equal. The red spot seen in the photos is the reflection of the laser beam that was used for triggering the flash unit. Click on the image for a full-size version.
The purpose of the sequence is to show how satellite drops are formed when a drop breaks free. Note in images 1 - 3 how the neck between the nozzle and drop narrows. Image 3 is very near the instant of separation. As the neck retreats upward in images 4 and 5, it segments into a series of nodules. After the neck itself breaks free, these nodules retract due to surface tension to produce a train of droplets. In order to eliminate these droplets, try adjusting the valve TIME ON. With experimentation, you may find other ways to influence the production of these drops.
The series of photos also make it clear that the main drop changes shape radically as it falls. Depending on what shape the drop has when it collides with the rebounding spout, you may obtain different effects. Changing the distance of fall is a way to adjust this variable.
Some photographic results
Photos taken with the apparatus of Figure 7 are shown below. Click on an image to see the full size photo. There was no image editing other than changing the resolution to screen resolution to reduce file sizes. (The full size images are about 2 MB.) A Nikon D810 camera was used with a Nikon 200mm micro lens at f/32 for Figures 12 - 16. For Figure 17, the lens was a 28-300mm zoom set for 300mm at f/22. For lighting, a single Vivitar 283 flash was used with a blue filter. The flash duration was approximately 100 μs ( 1 μs = 0.000001 s).
The spout is produced when the first drop rebounds after entering the water. One or more drops typically form at the top of the spout and break free. In this photo, a drop is on the verge of breaking free. If this happens before the second drop reaches it, the collision will occur in mid-air. Otherwise, the collision will occur at the top of the spout. Both scenarios produce interesting effects.
Note that for this photo, the collision isn't quite head on. Perhaps the second drop was deflected slightly as it was released from the nozzle.
The camera angle was higher for this photo. The TIME ON was adjusted to capture a collision before a drop separated from the top of the spout.
Adjustments in the TIME OFF setting of the AstroSplash alter the nature of the collisions, while adjustments in the delay time of the Multi-Trigger 2 alter what time during a particular collision that the flash discharge. In this photo, a collision is captured in an early stage in mid-air.
With the delay of the Multi-Trigger 2 turned higher, a collision similar to the one above was captured at a later stage.
This collision is similar to the previous one, but the plane of the collision is tilted more. If you enlarge the photo and then click on the center of the collision to produce a full-size enlargement, you'll see a bit of motion blur in the droplets that are thrown off. Motion blur can be reduced by lowering the flash duration. The lowest duration typically achievable with shoe-mount flash units is ~30 μs.
This photo taken from a greater distance away gives an idea of the size of the splash. The diameter of the bowl is 6 in (15 cm).