Fluid Dynamics
From the gentle rise of colored water through a paper towel to the swirling chaos of turbulence in a draining sink, fluid dynamics reveals some of the most visually captivating and conceptually rich phenomena in physics. It describes the behavior of liquids and gases, both at rest and in motion, and touches nearly every aspect of the natural world — from weather systems and blood flow to ink diffusion and the flight of airplanes.
With the help of smartphones, we can bring this dynamic branch of physics to life through sensors, video analysis, thermal imaging, and even audio recordings. Many fluid phenomena are subtle, transient, or difficult to observe — but with the right tools and a bit of creativity, they become measurable and shareable. In this chapter, we explore surface tension, pressure, buoyancy, flow, and viscosity through hands-on, low-cost experiments that invite curiosity, playfulness, and discovery.
Air Pressure and Temperature
Pressure Changes When Climbing Stairs (FLUID-01)
Sensors Used: Barometric pressure sensor
What’s Measured: Pressure difference across floors, estimated change in elevation
Description
Smartphones equipped with barometric pressure sensors can be used to investigate how atmospheric pressure varies with elevation — even over small height changes such as climbing stairs or riding an elevator. In this experiment, students record barometric pressure readings at different floors within a building and apply the barometric formula to estimate the corresponding change in height. This hands-on approach provides a tangible demonstration of how pressure decreases with altitude and how even modest vertical movements can be detected using sensitive smartphone instrumentation. By correlating pressure differences with the physical distance traveled, students explore foundational concepts in fluid statics and atmospheric science — all while ascending or descending the steps of everyday environments.
Measuring Water Pressure and Temperature Profiles in a Lake (FLUID-02)
Sensors Used: Barometric pressure sensor, temperature sensor
What’s Measured: Pressure and temperature as a function of depth
Description
In this experiment, a smartphone sealed in a watertight zip-lock bag is submerged at various depths in a lake, pond, or large container. At each depth, students record the pressure and temperature readings using built-in sensors or appropriate apps. These measurements can then be used to construct a pressure-temperature profile of the water column. The experiment provides a clear and intuitive demonstration that water pressure increases with depth due to the weight of the overlying fluid, following the relation P=ρghP = \rho g hP=ρgh. It also opens the door to observing natural temperature gradients, which can reveal stratification or mixing in the water body. This activity combines environmental exploration with core physics principles, offering an engaging way to study fluid statics and thermodynamics in a real-world setting.
Detecting Low-Frequency Sound Using a Pressure Sensor (FLUID-03)
Sensors Used: Barometric pressure sensor (high-sampling-rate enabled)
What’s Measured: Low-frequency pressure oscillations caused by sound waves
Description
While smartphone barometers are typically used to detect slow atmospheric pressure changes, this experiment explores their unexpected potential as acoustic sensors. If sampled at a sufficiently high rate — ideally 50 Hz or more — these sensors may capture low-frequency oscillations produced by sources such as subwoofers or environmental infrasound. To investigate this, students can place a smartphone near a large speaker playing deep bass tones and observe any periodic fluctuations in pressure readings. By comparing the sensor output with the known frequencies of the audio input, they can determine whether the pressure sensor is responding to acoustic waves. This crossover experiment blends principles from fluid dynamics, wave physics, and atmospheric science, revealing how even subtle pressure changes can be detected — and measured — with tools already in your pocket.
Creating a DIY Potato Gun to Demonstrate Gas Pressure and Force (FLUID-04)
Sensors Used: None required (optional: high-speed video or slow-motion camera)
What’s Measured: Qualitative observation of pressure buildup and projectile motion
Description
This hands-on experiment offers a playful yet powerful way to explore the physics of gas pressure and force. Using nothing more than a hollow pen tube and small plugs of potato, students can construct a simple potato gun that demonstrates how compressed gases can store and release energy. By inserting one potato plug at each end of the tube and then pushing the rear plug forward, the trapped air between the two plugs becomes compressed. As pressure builds, it eventually overcomes the static friction holding the front plug in place — launching it forward with a satisfying burst. The rapid expulsion showcases how pressure can be converted into kinetic energy, much like in real pneumatic systems. Capturing the motion with a smartphone camera, especially in slow-motion, allows for analysis of acceleration, force, and even air compression dynamics. This simple device reveals the core mechanics behind everything from air rifles to internal combustion engines.
References:
[1] “Root vegetable cannon - DIY potato gun,” https://www.thenakedscientists.com/get-naked/experiments/root-vegetable-cannon-diy-potato-gun
Constructing and Understanding a Water Barometer (FLUID-05)
Sensors Used: Barometric pressure sensor (smartphone)
What’s Measured: Atmospheric pressure changes correlated with water level variations
Description
This elegant experiment allows students to visualize atmospheric pressure in action by building a simple water barometer, also known historically as a “Goethe barometer” or “weather glass.” Using a clear plastic tube partially filled with water and sealed at one end, the device features a narrow spout left open to the atmosphere. Changes in air pressure outside the container cause the water level in the spout to rise or fall: higher pressure pushes water down into the main reservoir, while lower pressure allows it to rise. By observing these fluctuations over time and comparing them with real-time barometric pressure readings from a smartphone, students gain a tactile understanding of how barometers work. This experiment not only illustrates fundamental fluid and atmospheric physics but also connects directly to weather forecasting and historical instrumentation — a beautiful blend of hands-on science and environmental awareness.
References:
[1] “Water barometer,” http://practicalphysics.org/water-barometer.html
Pressure and Fluid Statics
Investigating the Pressure of a Water Column (FLUID-06)
Sensors Used: Barometric pressure sensor, smartphone camera
What’s Measured: Pressure vs. depth, jet range vs. hole height
Description
This experiment vividly demonstrates the principle that the pressure at the bottom of a water column depends solely on the height (or depth) of the liquid above, not the overall volume. Students can use a smartphone with a barometric pressure sensor or a simple manometer to observe how pressure increases linearly with depth, affirming the hydrostatic relationship P=ρghP = \rho g hP=ρgh. For a compelling visual, a plastic bottle can be filled with water and pierced with small holes at varying heights. As water jets from these openings, the lower holes will project water farther than the higher ones, clearly showing that deeper water exerts more pressure.
The behavior of the water jets links directly to Torricelli’s law, which states that the speed of fluid exiting a hole under gravity is proportional to the square root of the height of the fluid above it. Additionally, when students measure the range of these water jets and relate it to their height, they can engage with Bernoulli’s equation, exploring how pressure energy is converted into kinetic energy in fluid flow. This experiment introduces fluid statics and dynamics in a way that is both intuitive and experimentally rich — perfect for building an understanding of real-world phenomena like dam design, plumbing systems, and blood pressure.
References:
[1] “Investigating the pressure of a water column,” http://practicalphysics.org/investigating-pressure-water-column.html
[2] “Water Pressure,” https://www.thenakedscientists.com/get-naked/experiments/water-pressure
Demonstrating the Relationship Between Pressure and Force (FLUID-07)
Sensors Used: None required, optional smartphone video for observation
What’s Measured: Relative force, area, and pressure behavior
Description
This hands-on experiment reveals the fundamental relationship between pressure, force, and area in fluids, encapsulated by the equation P=FAP = \frac{F}{A}P=AF. Using simple and easily available materials like syringes or plungers connected by tubing and filled with water or air, students can explore how a force applied to one piston is transmitted through a confined fluid to move another.
By pressing down on a small piston and observing the resulting movement of a larger one — or vice versa — students witness Pascal’s principle in action: any change in pressure applied to an enclosed fluid is transmitted undiminished throughout the fluid. For instance, a small force applied to a small-area piston can lift a much larger piston with proportionally greater area, demonstrating how hydraulic systems can multiply force.
This experiment not only reinforces key concepts of fluid statics but also provides a conceptual gateway into real-world applications such as car brakes, hydraulic lifts, and syringes. Using a smartphone to film the setup can assist with slow-motion review and quantifying piston movement, especially in more refined versions of the experiment involving weight or spring-loaded force indicators.
References:
[1] “Pressure and force,” http://practicalphysics.org/pressure-and-force.html
Capillarity and Surface Tension
Capillary Action: Paper Towel vs. Cardstock (FLUID-08)
Sensors Used: Thermal camera (e.g. CAT S60), standard smartphone camera
What’s Measured: Height of water rise, temperature differences due to evaporation
Description
This experiment explores capillary action in porous materials by comparing how water rises through a strip of paper towel versus a piece of cardstock. Begin by preparing a shallow tray filled with room-temperature water, optionally colored for better visibility. Vertically dip the lower end of each material into the water and observe how quickly the liquid travels upward. Using your smartphone’s standard camera, you can document the rise over time via time-lapse or sequential photos. At the same time, a thermal camera can reveal a subtle but fascinating secondary effect: evaporative cooling.
As water wicks through the paper towel, the surface area and porosity facilitate rapid absorption, while evaporation along the front draws heat, creating a visible cool zone in thermal imagery. In contrast, the denser, less porous cardstock absorbs water more slowly and exhibits little to no thermal variation, highlighting the impact of material properties on capillary action.
This experiment vividly demonstrates not only the forces driving liquid movement in porous media but also how thermal imaging can bring hidden processes — like heat loss from evaporation — into plain view. Students can extend the activity by comparing different liquids (e.g., saltwater or oil), altering ambient temperature or airflow, or adding markings to estimate the capillary rise rate over time — perhaps with help from ChatGPT.
References:
[1] Capillary Action Visualization with Thermal Camera, https://www.youtube.com/watch?time_continue=55&v=_mqISJZxTzQ
Exploring Capillary Action in Narrow Tubes (FLUID-09)
Sensors Used: Smartphone camera (optional for time-lapse or measurement)
What’s Measured: Height of liquid rise in tubes of different diameters or materials
Description
This classic experiment reveals the phenomenon of capillary action using thin tubes inserted vertically into a liquid. When placed in water or another polar liquid, the liquid spontaneously rises within the narrow tube, climbing higher as the diameter decreases. This rise occurs due to the balance of cohesive forces (which pull molecules together within the liquid) and adhesive forces (which draw the liquid up along the tube’s inner surface).
Depending on the interplay between the liquid and the tube material, the liquid may either rise or be depressed, as seen in the contrast between water in glass and mercury in glass. Using a smartphone camera, students can document the steady rise and potentially measure the final height reached for different tube diameters, making it possible to relate observations to Jurin’s Law.
Capillary action is not just a physics curiosity — it underlies critical biological processes like water transport in plants and has practical implications for inkjet printing, porous materials, and microfluidics. Observing this quiet defiance of gravity opens a window into the microscopic tug-of-war between molecular forces and fluid structure.
References:
[1] “Capillary action,” http://practicalphysics.org/capillary-action.html
Demonstrating Surface Tension by Floating a Needle (FLUID-10)
Sensors Used: Smartphone camera (optional for video or photo documentation)
What’s Measured: Qualitative observation of floating and sinking behavior
Description
In this visually compelling experiment, a simple sewing needle becomes a gateway to understanding surface tension. Although denser than water, a dry needle can be gently placed on the water’s surface, where it floats—seemingly defying gravity. This works because of the cohesive forces between water molecules that create a taut surface “film,” strong enough to support lightweight objects if placed carefully and without breaking the surface.
Using a smartphone camera, students can document the process, observing how the water slightly dips around the needle while still holding it afloat. The effect is delicate; even the smallest disturbance can cause the needle to sink. To further illustrate the role of surface tension, a drop of soap can be added to the water. As the surfactant disrupts the hydrogen bonds between water molecules, the surface tension collapses—and the needle promptly sinks.
This experiment demonstrates not only the strength and fragility of intermolecular forces, but also how surfactants modify the physical behavior of fluids. A similar demonstration can be done using a small coin, such as a fifty-heller piece, to further test the limits of water’s cohesive strength.
References:
[1] “Floating a Needle,” https://www.thenakedscientists.com/get-naked/experiments/floating-needle
[2] Fifty-heller-coin on water surface, physicsexperiments.eu/2082/fifty-heller-coin-on-water-surface
Exploring Surface Tension with Soap Film and Loops (FLUID-11)
Sensors Used: Smartphone camera (for video and photographic documentation)
What’s Measured: Film stability, lifetime, interference patterns
Description
In this elegant and hands-on experiment, students explore the powerful yet subtle forces of surface tension using nothing more than a simple looped wire and soap solution. When the wire loop is dipped into the soapy water, a thin film forms across the frame. This soap film behaves like a minimal surface — it naturally adjusts to the shape that minimizes its area, reflecting the balance of cohesive forces within the liquid.
The most visually striking part of the experiment comes when sections of the film are popped. The remaining film instantly rearranges itself into the most efficient form, typically pulling taut into a circular membrane. This dynamic reshaping demonstrates surface tension’s role in minimizing energy across the surface of a fluid.
Students can use a smartphone camera to film the soap film from various angles, capturing not only the changing shapes but also the vibrant color fringes caused by thin-film interference. These color patterns vary depending on the film’s thickness and can be analyzed to understand the interplay between light and fluid surfaces. The lifetime of the film can also be recorded under different conditions — smooth vs. jagged loops, pure vs. impure soap mixtures — revealing how physical and chemical factors influence surface stability.
This experiment brings together concepts from fluid mechanics, optics, and geometry, offering a beautiful and tactile demonstration of how nature prefers efficiency.
References:
[1] “Going loopy for bubbles,” https://www.thenakedscientists.com/get-naked/experiments/going-loopy-bubbles
[2] “Surface tension,” http://practicalphysics.org/surface-tension.html
Fluid Flow and Viscosity
Viscosity Race (FLUID-12)
Sensors Used: Smartphone camera (with slow-motion or video mode)
What’s Measured: Terminal velocity, fall time, qualitative comparison of fluid resistance
Description
In this visually engaging experiment, students explore the concept of viscosity — a fluid’s resistance to flow — through a friendly competition between common household liquids. Transparent bottles are filled with different fluids such as water, cooking oil, glycerine, syrup, or shampoo. A small, dense object like a marble or bead is dropped into each container, and the descent is recorded using the smartphone’s slow-motion camera feature.
As the objects fall, their speed through each liquid reveals the unique viscous properties of the fluid. In thinner liquids like water, the bead will fall quickly, while in thicker substances like honey or shampoo, the descent slows dramatically. The comparative fall times provide an intuitive sense of viscous drag, the resistive force a fluid exerts on a moving object. This setup forms a practical and accessible demonstration of Stokes’ Law, which relates the drag force experienced by a sphere moving through a fluid to its velocity, the fluid’s viscosity, and the sphere’s radius.
Though qualitative by design, this experiment can also be extended to estimate relative viscosities if students measure terminal velocities and use objects of known size and density. It’s a fun, hands-on way to visualize a concept that is often hidden in equations — and to appreciate why molasses flows more slowly than milk.
References:
[1] “Falling through a high viscosity liquid,” https://spark.iop.org/falling-through-high-viscosity-liquid
Drip Time: Measuring Viscosity with a Dropper (FLUID-13)
Sensors Used: Smartphone stopwatch, slow-motion camera (optional)
What’s Measured: Time per drop, flow rate
Description
This simple but elegant experiment explores the viscosity of liquids by measuring how quickly they drip from a standard dropper or pipette. Fill the dropper with different fluids — water, glycerin, oil, syrup, or dish soap — and release drops one at a time while timing how long it takes for a fixed number of drops (e.g., 10 or 20) to fall. Record your results using a smartphone stopwatch or film the process in slow motion to observe differences in droplet formation and detachment.
More viscous liquids flow out more slowly, with longer intervals between drops and larger drop sizes due to increased cohesion. Less viscous liquids drip more quickly and cleanly. By measuring drop times and optionally calculating drop volume (using a small graduated cylinder), students can estimate flow rates and compare fluid properties quantitatively.
This experiment illustrates the relationship between viscosity, surface tension, and gravity in a hands-on way — and invites students to explore real-world applications like eye drops, syrups, or even industrial fluid dispensing.
Investigating Flow Rate and Flow Regimes Through Nozzles (FLUID-14)
Sensors Used: Smartphone camera (preferably with slow motion)
What’s Measured: Flow behavior, nozzle diameter, qualitative laminar/turbulent regime
Description
Water flow can be smooth or chaotic — and the transition between these states depends on the diameter of the nozzle, the velocity of flow, and the viscosity of the fluid. In this experiment, students explore how water behaves as it exits a series of nozzles with different diameters. By filming the flow with a smartphone camera — especially in slow motion — they can observe whether the stream is smooth and coherent (laminar) or wavy and chaotic (turbulent).
To begin, fill a bottle or reservoir with water and attach interchangeable nozzles made from pipettes, straws, or plastic tubing of various sizes. Record the outflow from each nozzle. Narrower nozzles typically increase the velocity of the exiting fluid due to pressure buildup, and may transition into turbulent flow depending on the conditions. Wider nozzles promote more laminar flow under the same driving pressure. These visual observations introduce core concepts such as Reynolds number, flow rate, and the continuity equation, as well as the broader idea of how geometry influences fluid behavior.
This experiment can be extended by measuring the volume of water expelled over a set time to calculate flow rate, or by using dye to better visualize the internal motion of the fluid. It offers a highly accessible and visual introduction to the physics of flowing fluids, making complex ideas visible in everyday materials.
Waves & Interference Patterns
Visualizing Water Waves: Standing Patterns and Ripple Propagation (FLUID-15)
Sensors Used: Smartphone camera (slow-motion and overhead video), optional flashlight for contrast
What’s Measured: Wavelength, frequency, interference patterns, wave behavior at boundaries
Description
Using a shallow tray of water and a vibrating source such as a speaker, metal rod, or even a finger tapping at regular intervals, students can generate and observe a variety of wave behaviors. Filming from above with a smartphone camera — ideally in slow motion and with angled lighting — makes it possible to capture the formation of standing waves, identify nodes and antinodes, and explore how interference creates stable patterns.
Expanding the setup, students can introduce barriers or objects into the tray to observe wave reflection, refraction, and diffraction. By altering the water depth, they can also explore how wave speed changes, providing an intuitive demonstration of refraction. These ripple patterns serve as analogues to wave behaviors in light and sound, allowing for hands-on engagement with abstract wave phenomena.
This experiment beautifully bridges conceptual understanding with visual experience — a low-cost, high-impact way to explore the physics of waves through real-time observation and smartphone-enhanced analysis.
References:
[1] “Waves on water,” http://practicalphysics.org/Waves-on-water.html
Advanced Concepts
Visualizing Air Currents Using Candle Smoke (FLUID-16)
Sensors Used: Smartphone camera (video), thermal camera (optional)
What’s Measured: Direction and flow of air currents, convection patterns, relative heat sources
Description
This elegant experiment uses the subtle behavior of candle smoke to reveal invisible air currents and convection flows. By lighting a candle in a quiet room and observing the curling motion of the smoke, students can visualize how heated air rises and interacts with surrounding cooler air. The experiment becomes even more insightful when performed near open windows, heaters, or fans, as these external influences shape the smoke’s path in real time.
To extend the exploration, students can compare the heat signatures and rising air patterns produced by various heat sources — such as incandescent, halogen, or LED bulbs, as well as stoves, lighters, or even an IR remote control — using either direct observation of smoke movement or a thermal camera like the one on the CAT S60. This allows them to correlate visible and infrared cues with air movement, offering a vivid introduction to convection and fluid flow in gases.
The candle becomes a quiet but powerful probe of the atmosphere — showing how air, though invisible, is constantly moving in response to temperature differences.
References:
[1] “Candle in the Wind,” https://www.youtube.com/watch?v=O9w9RX7cJe8
DIY Schlieren Imaging with a Smartphone (FLUID-17)
Sensors Used: Smartphone camera (video), optional mirror or screen background
What’s Measured: Air density variations, heat and sound waves, convection currents
Description
Schlieren imaging is a stunning technique that makes the invisible visible — revealing air movement, heat plumes, sound waves, and even shock fronts by detecting small changes in air density. Traditionally reserved for advanced laboratories due to the cost of specialized optics, Schlieren setups can now be built affordably and effectively using smartphones, thanks to a wave of innovative DIY adaptations.
One approach, known as Smartphone Schlieren, uses a concave mirror, a sharp-edged cutoff (like a razor blade), and a phone camera to detect refracted light caused by air density gradients. With careful alignment, this classic setup allows users to visualize thermal plumes from a candle or convection from a radiator. An especially elegant extension of this method is shown in [5], where Schlieren optics are used to visualize ultrasound waves in air — capturing momentary pressure fronts that would otherwise be completely invisible. The result is a mesmerizing, real-time demonstration of acoustic physics and fluid dynamics interacting at the edge of visibility.
Another method, Pocket Schlieren, uses background-oriented Schlieren (BOS), comparing subtle distortions in a background image as refractive index changes alter the path of light. This can be run on a smartphone alone, with software handling the image comparison and distortion mapping. It’s well-suited for visualizing heat rising from hands or appliances, as well as airflow from small fans.
Whether using mirrors or backgrounds, razor blades or digital overlays, Schlieren imaging is one of the most visually powerful demonstrations of fluid dynamics and wave behavior — and with modern tools, it’s well within reach for student projects or classroom displays.
References:
[1] Smartphone Schlieren (original smartphone-based Schlieren imaging system), https://arxiv.org/abs/1609.04298
[2] Pocket Schlieren (real-time background-oriented Schlieren processing on smartphones), https://arxiv.org/abs/2404.16060
[3] DIY Schlieren iPhone Photography anyone can do 5 minute setup, https://www.youtube.com/watch?v=teceK9Vdwec
[4] How To See Air Currents, https://www.youtube.com/watch?time_continue=5&v=4tgOyU34D44
[5] Visualizing Ultrasound with Schlieren Optics Part I, https://www.youtube.com/watch?v=MBPh410Gnes
Exploring Airflow and Lift: Bernoulli Papers and Flying Tubes (FLUID-18)
Sensors Used: Smartphone camera (slow motion), optional pressure sensor
What’s Measured: Qualitative airflow behavior, trajectory deviation, lift and drag effects
Description
Airflow can behave in surprising and often counterintuitive ways — but with simple materials and a smartphone, students can visualize some of the core principles behind flight and fluid motion. This experiment combines two engaging demonstrations of aerodynamic lift and pressure differentials, grounded in the Bernoulli principle and the behavior of turbulent air.
Begin with the classic Bernoulli paper demonstration. Suspend two narrow strips of paper vertically and blow a stream of air between them. Rather than being pushed apart, the papers are drawn together. This happens because the fast-moving air between them creates a region of lower pressure compared to the still air outside, resulting in a net inward force. Recording this with a smartphone in slow motion enhances the observation and makes the invisible airflow more apparent. If pressure logging tools are available, students can even attempt to correlate airflow with measured pressure drops.
Then transition to a more dynamic application with the flying tube activity. Take a lightweight, open-ended plastic mailing tube or paper cylinder, seal one end with tape, and flick it underhand into the air with a spin. Rather than tracing a standard parabolic arc, the tube will often curve unpredictably, deflect, or hover in surprising ways. This is due to the complex interaction of lift, drag, spin-induced airflow asymmetry, and internal air pressure dynamics — all elements that illustrate real-world aerodynamics. Recording the flight from the side with a slow-motion camera allows students to analyze the trajectory frame by frame and observe how the shape, spin, and orientation of the object influence its motion.
Together, these two parts introduce foundational concepts in fluid dynamics, from Bernoulli’s principle to the subtle forces that make birds soar and frisbees glide. They also underscore the idea that air — though invisible — exerts powerful, measurable influence on the motion of objects around us.
References:
[1] “Flying Tubes,” The Naked Scientists, https://www.thenakedscientists.com/get-naked/experiments/flying-tubes