UChicago Instructional Physics Laboratories

We are currently interested in updating this demonstration on the second floor of KPTC. I will provide the documentation for the electronics, plumbing and structural designs.

This demonstration will serve to illustrate the periodic formation of water droplets at the opening of a pipe. To do so, it will implement the stroboscopic effect, whereby a strobe light of tunable frequency is used to capture the motion of the droplets that are falling. If the frequency of the light is equal to the frequency of droplet formation, the droplets, identical to the naked eye, will appear to be stationary, since the strobe light will capture the same displacement that each drop has traveled. If the frequency is slightly higher than the falling rate, then subsequent flashes will capture smaller displacements from subsequent drops, so that the drops appear to fall upward. Conversely, if the strobe frequency is smaller than the falling rate, larger displacements downwards will occur for each subsequent flash, so that the drops appear to move downward.

For this hallway demonstration, we want to use the stroboscopic effect to capture the periodic motion of the drops. Moreover, we want the spectator to interact with the demonstration by adjusting the strobe frequency in order to observe the three aforementioned regimes. Other considerations to keep in mind is that the demo should be minimalistic, so that the audience only focuses on the falling drops and controlling the strobe light. The mechanisms by which the drops are formed, how the apparatus is powered, the electronics, etc. must be hidden from view so as to not distract the person away from the phenomenon we want them to observe. Moreover, it should be a well crafted apparatus that meets the expectations of a person entering a research university and that invites them to interact with it. It should also provide immediate feedback to maintain interest in what is happening, require little maintenance, and be easily diagnosed and troubleshooted.

The above goals are in part a reaction to the shortcomings of the current iteration of the demonstration. The visibility is not great, it takes a while for the demo to start (which can cause lost of interest), there is a constant noise that is distracting, wires and electronics are exposed, enclosure seems unfinished and haphazardly assembled, materials are deteriorated, water needs to be added continuously due to evaporation.

To be able to showcase this behavior in a new, improved manner, we will need the following:

• a way to produce drops that fall at a periodic rate that also minimizes noise;
• a water reservoir that fills up automatically;
• a strobe light that is bright enough and provides uniform illumination of the drops;
• a one-touch start that immediately starts the demonstration without constant input from the user, provides feedback, and turns itself off;
• a way to hide electronics, valves, reservoir, lights, funnel, etc.

Figure 1 shows a rough schematic of a possible iteration of this demonstration.

Figure 1: Demonstration schematic

The apparatus would consist of a wooden structure with a closed box as its base and a closed box on top, visualized in the figure as the two red boxes. The only part that the observer would clearly see is the center, light blue, where drops would fall from a visible fine tube onto a funnel that is embedded into the lower box. The upper box would contain the mechanism for forming water drops, while the bottom box would contain the electronics controlling the demonstration, lighting, and water waste mechanism. Coupling between the upper and lower boxes would be made through the use of cables sent behind the wall. Inside the upper box, a water reservoir would store the water that would be supplied to the tube. This reservoir would be kept full by coupling it to the building water system using a solenoid valve (orange box) that could be controlled electronically. Another solenoid valve would be used to start the water flow electronically, passing the water through a needle valve that allows fine adjustment of the opening and thus of the drop formation rate.

Inside the upper box, a water reservoir would store the water that would be supplied to the tube. This reservoir would be kept full by coupling it to the building water system using a solenoid valve (orange box) that could be controlled electronically. Another solenoid valve would be used to start the water flow electronically, passing the water through a needle valve that allows fine adjustment of the opening and thus of the drop formation rate.

It is worthwhile to note the importance of an automatic mechanism that maintains the water level within the reservoir constant. Indeed, the demonstration depends on a constant, periodic rate of drop formation in order to use the stroboscopic effect to “capture” the drops mid fall. In our apparatus, the drops would be formed at the point of a fine tube. This rate will depend on the force that is pushing on the water above the entrance, which is to say, the water pressure, determined by the column of water pushing down. For small changes in volume caused by the dropping of water through the orifice, the pressure will remain relatively constant. However, evaporation and use of the demonstration will cause the water level to decrease over time, therefore changing the rate at which the drops fall. Thus, the water pressure and therefore the volume of water in the reservoir would need to be kept constant. To achieve this, we can use a water level sensor (dark red in the figure) and a solenoid valve that couples the reservoir to the building plumbing. Once the water level drops below an acceptable level at which the flow rate through the tube starts changing, the sensor would activate the solenoid valve to open for some time. The amount of time could be fixed, or determined by the water sensor itself once it detects that an acceptable level of water has been reached.

Sufficient, uniform illumination should be used to provide a clear view of the drops. For this we can use LEDs of adequate temperature (5000-6000K) inside a transparent enclosure that has a light-diffusing coating, which would uniformly disperse the light. The figure shows the LEDs (which could be flood-spots) positioned at the bottom pointing up towards the tube at an angle. Another, possibly cleaner solution would be to illuminate the drops from the direction of the viewer (ie, into the screen in our schematic), so that the light is better reflected into their eyes. Two rows of bright LEDs pointed toward the droplets' path could be used.

In addition to this, the surface of the wood in the observation area can also be painted matte black to provide high contrast and improve drop visibility.

One main change we should implement is to have the demonstration run on its own upon the touch of a switch, as opposed to the current use of a foot pedal that must be continuously held on to operate. Moreover, it should turn on and provide immediate feedback to the user, since now it takes a while for the pump and lights to turn on, which may cause the user to think it does not work and leave. To overcome this, we can run the demonstration on a timer, where on pressing a button, both the solenoid valve and the strobe lights are activated immediately. The user would only have to focus on the drops and tuning the strobe frequency with a variable resistor. The demo would run for a minute or so before turning off, at which point the person can leave or turn it on again if they're interested.

The electronics should also include a way to automatically refill the reservoir and easily troubleshoot the system.

The file below has a schematic of the circuit.

schematic_water_drops_demo_2023-08-11.pdf