Explore the Physics of Soap Films With the SoapFilmScope

The idea for this project came from reading an article by Gaulon et al. [1]. The authors describe in detail the use of soap film as an educational aid to explore interesting effects in the fluid dynamics of this system. In particular, they examine the impact of acoustic waves on the unique optical properties of the film. In this Instructable, we have designed a device called the SoapFilmScope to perform these experiments. This tutorial will guide you through the process of creating this device, showcasing the mesmerizing interaction between sound waves and liquid membranes. The SoapFilmScope offers an engaging way to explore the physics of acoustics, light interference, and fluid dynamics.

The Science Behind The SoapFilmScope

Imagine a vertical soap film, delicately suspended at the end of a tube, dancing to the rhythm of music. This is not just a vivid imagination but a reality where colours swirl, split, and merge in synchrony with the music, thanks to the iridescence in the soap film. This captivating display is not only beautiful but also rich in physics.

When a sound wave travels through the tube and vibrates the soap film, it creates dynamic patterns through several fascinating mechanisms:

• Acoustic Propagation: Sound waves travelling through the tube interact with the soap film, inducing capillary waves.
• Light Interference: Iridescence, caused by interference of light waves reflecting off the thin layers of the soap film, creates vibrant, shifting colours.
• Fluid Dynamics: The soap film responds to sound waves with both oscillatory and non-oscillatory flows, leading to vortex generation, complex diphasic patterns, and even film swelling under certain conditions.

These effects occur on various characteristic time scales, intricately linked to the timing of the music being played. By exploring these interactions, the SoapFilmScope not only serves as an artistic expression but also as an educational tool, making the science behind these phenomena accessible and engaging.

The article mentioned at the beginning, along with the references therein and classic books [2-4] on the subject, provide further information on the physics of soap films. Interested readers can consult these sources to gain a deeper understanding of the phenomena.

In this Instructable, we will guide you through the steps to build your own SoapFilmScope. Specifically, we will cover how to:

• Construct the Device: Assemble the necessary components to create a setup where a soap film can be vibrated by sound waves.
• Experiment with Sound and Film: Use different sound frequencies and types of music to observe various patterns and effects in the soap film.
• Explore the Physics: Gain insights into the underlying physics, including acoustic propagation, light interference, and fluid dynamics, that produce mesmerizing effects.

REFERENCES

1. Gaulon, C., Derec, C., Combriat, T., Marmottant, P. and Elias, F., 2017. Sound and vision: visualization of music with a soap film. European Journal of Physics, 38(4), p.045804.
2. Boys C V 1959 Soap Bubbles, their Colours and the Forces which Mould Them (New York: Dover)
3. Lovett DR1994DemonstratingScienceWithSoapFilms(Bristol:InstituteofPhysicsPublishing)
4. Isenberg C 1987 The Science of Soap Films and Soap Bubbles (Clevedon: Tieto) reprinted Isenberg C 1992 (New York: Dover)

• 1 PVC T-shaped pipe fitting.
• 3D printer and PLA filament to print the support mechanism of the T-shaped fitting.
• 2 M2 15 mm screws to fix the ring to the T-shaped fitting arm.
• 1 M5 80 mm screw to adjust the T-shape fitting in the horizontal position.
• An inexpensive speaker with a diameter fitting the larger connecting hole of the T-shaped fitting. In our case, we have used one with an internal diameter of 46 mm.
• Soap solution for making film. It can be used as the one sold in toy shops to make soap bubbles. Alternatively, a mixture consisting of 80% water, 20% glycerol and <1% dishwasher detergent, typically provides a stable and long-lasting film, as suggested by various sources in the literature.
• A shallow containing to hold the soap solution. We have used a Petri dish.
• A smartphone camera. We have used an iPhone 8.
• A uniform source of light. A window or a large LED light pad.

To construct The SoapFilmScope, we will use a PVC T-shaped pipe fitting and custom 3D-printed parts. These parts create a stable support that facilitates the procedure for the formation of the soap film.

Assembly Instructions

PVC T-Shaped Pipe Fitting: This forms the backbone of the device. We have used the one shown in the first picture with its sizes. However, the device can be easily adapted to other shapes and dimensions.

3D-Printed Support: The smaller right-end opening of the horizontal cylinder will be dipped into a soap solution. To facilitate this process, a 3D-printed support was designed. A custom-designed support makes it easy to immerse the cylindrical extremity of the T-shaped pipe into the soap solution, ensuring a smooth and consistent film formation. This support also allows the pipe to move in a manner reminiscent of a bird dipping its beak into water to drink, a movement we can call the “Dipping Bird Motion”. The attached STL files are designed for the T-shaped pipe fitting used in our prototype. However, OpenSCAD program (https://openscad.org/) files are also provided, allowing you to easily adapt the 3D-printed parts to fit your specific pipe fitting.

Steps to Assemble the Parts

By following these steps, you will have a fully assembled SoapFilmScope ready for your experiments.

1) Attach the Ring to the T-Shaped Fitting:

• Print the ring using your 3D printer.
• Using two M2 screws, attach the ring to the vertical cylinder of the T-shaped pipe fitting as shown in the figure.

2) Prepare the Base:

• Print the base and the base extension parts.
• Glue the base to the base extension part as shown in the central figure.
• Add the M6 80 mm screw in the hole of the extended base.

3) Insert the Ring Holders:

• Print the two ring holders.
• Insert them into the base.

4) Position the T-Shaped Fitting:

• Insert the T-shaped pipe fitting into the guide of the holders.
• Support the T-shaped fitting using the two-ring supports.

5) Adjust the Ring Supports:

• Adjust the length of the ring supports to ensure that when the T-shaped fitting is turned down, it dips into the soap solution contained in the petri dish in front of the device.

6) Speaker Attachment: See step 3.

Ensure all parts are securely attached and correctly aligned to facilitate optimal performance of the device.

The openSCAD script

To use it, copy and paste the following script into the openSCAD program and uncomment the module that you need to print. Customize the size of the parts to adapt to your T-shaped fitting.

$fn = 100;

// Module to create the Ring
module Ring() {
    difference() {
        union() {
            // Main cylinder
            cylinder(10, 25.5, 25.5, center=true);
            // Horizontal cube
            cube([70, 5, 10], center=true);
            // Vertical cylinder through the ring
            rotate([90, 0, 0]) cylinder(80, 4, 4, center=true);
        }
        // Subtract inner cylinder
        cylinder(12, 22.5, 22.5, center=true);
        // Subtract thin horizontal plane
        cube([100, 0.1, 15], center=true);
        // Subtract small vertical cylinders
        translate([30, 0, 0]) rotate([90, 0, 0]) cylinder(80, 1.5, 1.5, center=true);
        translate([-30, 0, 0]) rotate([90, 0, 0]) cylinder(80, 1.5, 1.5, center=true);
    }
}

// Module to create the Speaker Cup
module SpeakerCup() {
    difference() {
        // Main outer cylinder
        cylinder(30, 30, 30, center=true);
        // Subtract inner cylinder
        translate([0, 0, 7]) cylinder(40, 27.5, 27.5, center=true);
        // Subtract small vertical cylinder through the cup
        translate([0, 0, 8]) rotate([90, 0, 0]) cylinder(80, 1.95, 1.95, center=true);
        // Subtract cube on the side
        translate([30, 0, 5]) cube([20, 5, 30], center=true);
    }
}

// Module to create the Ring Holder
module RingHolder() {
    difference() {
        // Top holder
        translate([0, 35, 0]) cube([20, 5, 100], center=true);
        translate([0, 35, 7]) cube([8, 7, 60], center=true);
    }
    difference() {
        // Bottom holder
        translate([0, -35, 0]) cube([20, 5, 100], center=true);
        translate([0, -35, 7]) cube([8, 7, 60], center=true);
    }
}

// Module to create the Base
module Base() {
    difference() {
        // Main base cube
        translate([0, 0, -45]) cube([50, 80, 10], center=true);
        // Subtract holder slots
        translate([0, -35, 2]) cube([20.5, 5.5, 100], center=true);
        translate([0, 35, 2]) cube([20.5, 5.5, 100], center=true);
        // Subtract vertical cylinders
        translate([-10, 20, -30]) cylinder(80, 2.5, 2.5, center=true);
        translate([10, -20, -30]) cylinder(80, 2.5, 2.5, center=true);
        translate([-10, -20, -30]) cylinder(80, 2.5, 2.5, center=true);
        translate([10, 20, -30]) cylinder(80, 2.5, 2.5, center=true);
    }
}

// Module to create the Base Extension
module BaseExtension() {
    color("red", 1) {
        difference() {
            union() {
                // Main extension cube
                translate([50, 0, -45]) cube([45, 20, 10], center=true);
                // Side cube
                translate([62.5, 0, -35]) cube([20, 20, 20], center=true);
            }
            // Subtract vertical cylinder
            translate([62.5, 0, -45]) cylinder(100, 3, 3, center=true);
            // Subtract hexagonal cylinder
            translate([62.5, 0, -28]) cylinder($fn=6, 6, 5.5, 5.5, center=true);
        }
        // Additional support cube
        translate([26, 0, -45]) cube([3, 80, 10], center=true);
    }
}

// Module to create the Ring Support
module RingSupport() {
    difference() {
        // Top ring support
        translate([0, 35, -3.5]) cube([7, 7, 39], center=true);
        translate([0, 0, 16]) rotate([90, 0, 0]) cylinder(80, 4, 4, center=true);
    }
    difference() {
        // Bottom ring support
        translate([0, -35, -3.5]) cube([7, 7, 39], center=true);
        translate([0, 0, 16]) rotate([90, 0, 0]) cylinder(80, 4, 4, center=true);
    }
}

// Main assembly
color("orange", 1) Base();
color("green", 1) translate([0, 0, 15]) Ring();
RingHolder();
color("pink", 1) RingSupport();
BaseExtension();
// SpeakerCup();  // Uncomment to include the Speaker Cup

Adjust the M6 screw to ensure that the tube is horizontal and balanced, as shown in Figure 1A. To form the soap film, dip the open right end of the device into the soap solution located in the petri dish, as shown in Figure 1B.

To document the evolution of the soap film, we used a uniform light source generated by a large LED light pad placed in front of the tube end with the film. The camera used was an iPhone 6, filming the soap film at a slight angle relative to the incident light provided by the LED light pad. Refer to the arrangement shown in the figure for guidance (see Figure 2).

After dipping, gravity pulls the water downward in the film, causing it to thin out. This effect is visible through the formation of colored bands that traverse horizontally across the film as the liquid sinks between the two surfactant layers forming the surface of the film. The colors result from the refraction of light by the film. Over time, the top of the film becomes black as its thickness drops below 200 nm. The Figure 3 shows a sequence of the film’s evolution over time, illustrating the thinning of the soap film, the sinking of the colored bands, and the expansion of the black area at the top.

In their article, Gaulon et al. demonstrated how sound waves can be used to study the interaction between acoustic waves and a soap film. This interaction generates complex effects that influence the flow of liquid within the film, leading to several spectacular phenomena: vortex generation, diphasic dynamical patterns, and the swelling of the soap film under certain conditions.

To reproduce these effects with the SoapFilmScope, a speaker will be attached to the other end of the T-shaped connector. This speaker will generate sound waves that propagate through the tube and interact with the soap film, creating visual effects. We used an inexpensive speaker, the same one described in another Instructable, which fits perfectly into the left hole of the horizontal tube of the T-shaped fitting. To secure it in place, you will need to print the SpeakerCup, which holds the speaker securely, as shown in the figure.

The speaker was connected to a MacBook, and the tone was generated using the Audacity program (https://www.audacityteam.org/) tone generator (Figure 2). The snapshots in Figure 3, taken from the embedded video, were obtained using a frequency of 850 Hz with a volume of 0.8 and a triangular wave. Note that the video was recorded in slow motion, which explains the change in tone at the beginning.

Alternatively, you can connect the speaker to a Raspberry Pi and use the Python program presented in our Instructable project: The KaleidoPhoneScope: A Dance of Light, Sounds, and Mathematics.

More details on the physics of this process can be found in the Gaulon et al. article and the cited literature.

In this Instructable, we have proposed the construction of a handy device that can facilitate the observation of soap film physics by capturing the refraction fringes with a camera and examining the effects of sound waves on the film’s structure and dynamics. The SoapFilmScope offers a unique and engaging way to explore the fascinating interaction between sound waves and liquid membranes, revealing the intricate dance of colours and patterns that arise from this interplay.

Whether you are a science enthusiast, an educator looking for a compelling classroom demonstration, or someone who enjoys DIY projects with a touch of artistry, The SoapFilmScope provides an intriguing blend of science and art. By building and experimenting with this device, you can gain a deeper understanding of the principles of acoustics, light interference, and fluid dynamics, while also creating visually captivating displays.

We hope that this project inspires you to dive into the world of soap film physics and discover the beauty and complexity of this phenomenon. So, gather your materials, follow the steps, and let your soap film dance to the music!