Fluids and Surface Tension Demonstrations

See also: Pressure and Fluids, Water and Solubility

Fluids are substances that flow, such as liquids and gases. Surface tension is a property of liquids that creates an elastic-like effect at their surface. This category looks at the science behind these behaviours and how they influence natural systems and everyday experiences. Understanding fluids and surface tension helps explain a wide range of physical phenomena.

Demonstration	Materials	Difficulty	Safety	Summary
See Convection Currents	★☆☆	★☆☆	★★☆	A small container of hot, colored water is placed in a larger container of cold water. The warm water rises and circulates through the cold water, creating visible convection currents that illustrate heat transfer and density differences.
Celery Capillary Action	★☆☆	★☆☆	★☆☆	Celery is placed in colored water and the colors move upwards to it's leaves.
Color Changing Walking Water	★☆☆	★☆☆	★☆☆	Cups filled with colored water are connected by folded paper towels. The water climbs up the paper towels and travels into empty cups through capillary action. As the colors mix in the empty cups, a rainbow effect is created.
Color Swirling Milk	★☆☆	★☆☆	★☆☆	Food coloring on milk stays mostly still until a drop of dish soap touches the surface; swirling patterns then race across the milk as soap disrupts surface tension.
Cornstarch and Water	★☆☆	★☆☆	★☆☆	This experiment uses cornstarch and water to create "oobleck," a non-Newtonian fluid that behaves like a liquid when poured but like a solid when struck or squeezed. It provides a hands-on way to explore suspensions and unusual fluid dynamics.
Diffusion in Hot and Cold Water	★☆☆	★☆☆	★☆☆	By adding food coloring to hot and cold water, students can observe that molecules in hot water move faster, causing the dye to spread more quickly.
Double Slit Experiment with Water Waves	★★★	★★☆	★☆☆	A tank of water with two slits in a barrier demonstrates how waves interfere. Circular water waves passing through the slits overlap to produce alternating regions of reinforcement and cancellation, creating an interference pattern that illustrates wave behavior.
Floating Coin on Water	★☆☆	★☆☆	★☆☆	Aluminum coins, though denser than water, can be floated on the water’s surface due to surface tension.
Floating Paper Clip	★☆☆	★☆☆	★☆☆	A paperclip, normally too dense to float, can rest on the surface of water by using tissue paper and surface tension.
Glowing Oobleck	★★☆	★☆☆	★☆☆	This Halloween-themed activity creates glowing oobleck that behaves like both a liquid and a solid. Under a blacklight, the quinine in tonic water makes the mixture glow eerily, and when placed on a speaker playing spooky music, the oobleck appears to writhe and dance like a haunted slime.
Pepper and Water Surface Tension	★☆☆	★☆☆	★☆☆	Black pepper sprinkled on water races to the edges when a tiny amount of detergent touches the surface. Soap lowers surface tension and the water pulls away from the soap, carrying the floating pepper.
Pouring Water Down a String	★☆☆	★☆☆	★☆☆	Water is poured from one container into another along a piece of string. The demonstration shows how the adhesive and cohesive properties of water allow it to cling to the string and flow downward instead of falling freely.
Pythagoras Cup	★☆☆	★★☆	★☆☆	The Pythagoras cup looks like a normal drinking cup, but if filled past a certain level, it uses a siphon to empty itself completely. This ancient invention shows the principles of siphoning and was originally designed to teach moderation.
Snowstorm in a Jar	★★☆	★☆☆	★☆☆	Layer a water–paint mixture beneath baby oil and start an acid–base reaction with pieces of effervescent tablet. Carbon dioxide bubbles lift painty water droplets up through the oil; when bubbles burst, the droplets fall like snow.
Surface Tension Boat	★☆☆	★☆☆	★☆☆	A tiny boat made from a foam tray (or cardboard) zips across water when a trace of dish soap is applied at its notched stern.
Upside Down Water Glass	★☆☆	★☆☆	★☆☆	By filling a glass completely with water, covering it with a piece of stiff paper, and inverting it, the paper stays in place and holds the water inside the glass.
Viscosity With Marbles	★☆☆	★☆☆	★☆☆	Students compare how marbles fall through different liquids such as water, oil, syrup, and honey. This simple race demonstrates viscosity, or the internal friction of liquids, and helps students understand why some liquids flow faster than others.
Water on a Coin	★☆☆	★☆☆	★☆☆	Using an eyedropper, drops of water are placed on a coin and it's observed how many drops can pile up before spilling over. The activity demonstrates cohesion, adhesion, and surface tension in water compared with other liquids like oil and syrup.

Materials
★☆☆ Easy to get from supermarket or hardware store
★★☆ Available in most school laboratories or specialist stores
★★★ Requires materials not commonly found in school laboratories

Difficulty
★☆☆ Can be easily done by most teenagers
★★☆ Available in most school laboratories or specialist stores
★★★ Requires a more experienced teacher

Safety
★☆☆ Minimal safety procedures required
★★☆ Some safety precautions required to perform safely
★★★ Only to be attempted with adequate safety procedures and trained staff