A penny can hold many drops of water due to surface tension. Surface tension is a property of liquids that allows them to resist an external force due to the cohesive nature of its molecules. Water molecules like to stick together, and surface tension is the effect of the molecules on the surface “sticking” together more tightly than the molecules beneath them. This surface tension allows water to form dome shapes on a penny rather than just spreading out flat.
What is surface tension?
Surface tension is a phenomenon where the surface of a liquid behaves like a stretched elastic membrane. The molecules at the surface do not have other like molecules on all sides of them and are pulled inwards by cohesive forces from the other surface molecules. This creates an internal pressure and forces liquid surfaces to contract to the minimum area possible. Surface tension allows liquids to rise up capillary tubes against gravity, float small objects on the surface, and hold drops of liquid together rather than just spreading out flat.
Key Points about Surface Tension
- Caused by cohesive forces between liquid molecules at the surface
- Molecules on surface get pulled in by their neighbors, creating tension
- Minimizes surface area for amount of liquid volume
- Resists external force
- Allows liquids to form drops and domes
Water has a particularly high surface tension due to the hydrogen bonds between its molecules. The polar nature of water makes the molecules stick together tightly, creating a resilient surface. Other factors like temperature affect surface tension as well – higher temperatures disrupt intermolecular forces and decrease surface tension.
How does surface tension allow water to dome on a penny?
When a small volume of water is placed on a penny, the surface tension contracts the liquid to minimize surface area. This results in the water “beading up” into a dome shape on top of the penny, rather than spreading out flat. The water molecules around the edge are pulled in by their neighbors, creating an inward force vector. As more water is added in drops, it gets pulled into the existing dome shape.
Let’s look at the forces at play when a water droplet sits on a penny:
- Surface tension – Acts inward to minimize surface area, forming droplet dome
- Gravity – Pulls droplet down onto penny surface
- Adhesion – Water attracts to hydrophilic penny surface via hydrogen bonds
The combined result of these competing forces is that the water forms a raised dome shape. The dome progressively flattens out as more water is added and gravity gains more influence. But surface tension continues holding the water together in a bubble-like shape rather than spreading out flat.
Why Doesn’t the Water Just Spread Out?
You might wonder why the water doesn’t just keep spreading until it forms a flat puddle on the penny. This comes back to the properties of surface tension again. The inward force vector holding the water molecules together can resist the outward gravitational force trying to flatten and spread the liquid out. This allows the water to maintain a raised, curved meniscus against the penny.
How many drops can fit on a penny?
The exact number of drops that can fit on a penny depends on drop size. With an average drop size of 0.05 mL, a United States penny can hold around 40-50 drops before they overflow the edges. The dome shape that the water assumes allows more drops to be added without spilling over the penny’s sides.
Let’s assume an average water drop size of 0.05 mL (50 microliters). With a volume of 5 milliliters per teaspoon, that’s:
- 1 teaspoon = 5000 microliters
- 1 drop (50 microliters) is 1/100th of a teaspoon
A US penny has a surface area of about 327 mm2. Taking the volume of the spherical cap forming the water dome into account, as well as water’s density, 50 drops should take up around 150-200 mm2 on the penny’s surface. This means a penny can likely accommodate 40-50 drops before they reach the edges and overflow.
The exact maximum number also depends on factors like:
- The drops falling from the exact same height each time
- Ambient temperature (surface tension decreases with higher temperature)
- Any dirt or oil on the penny affecting adhesion forces
- Still air with no airflow disturbances
But under normal everyday conditions, expect a United States penny to hold somewhere around 40-50 drops of water before overflowing.
Drops on a Penny Experiment
Want to try this yourself? Here is a simple experiment to count the number of drops a penny can hold:
- Fill a medicine dropper, pipette, or eyedropper with water
- Hold it vertically above a penny laying flat on a flat surface
- Slowly squeeze out single drops, allowing them to dome on the penny’s surface
- Count each drop until the water spills over the edge of the penny
- Repeat several times and average your drop counts
This hands-on experiment allows you to see the surface tension effect in action as the dome shape grows. Notice how the droplet maintains an outward curvature, rather than immediately flattening out on the penny. You can try this on pennies of different sizes or even different liquids like alcohol or oil to observe how the dome shapes compare.
How does a water strider walk on water?
Water striders are insects that can literally walk or skate on the surface of ponds and slow moving streams. They rely on the same principle of surface tension that allows water drops to form domes on pennies.
The non-wetting, waxy coating on a water strider’s legs allows them to rest on top of the water’s surface tension film. Though the surface tension supporting the bug upward is much weaker than forces like gravity, the minuscule weight of the water strider prevents it from breaking through the surface.
Let’s examine the physics involved:
- Surface tension – Upward force supporting strider, though relatively weak
- Strider’s weight – Downward force but very minimal due to tiny mass
- Non-wetting legs – Repel water, don’t penetrate surface tension film
The weight of the average water strider is only about 5-15 milligrams. This allows the surface tension of water, around 0.07 N/m, to easily support the insect’s mass. However, larger insects would break through the film due to having more weight. The non-wetting distributed weight allows them to effectively walk on water.
Water Strider Facts
- Over 1500 species worldwide
- Most common in freshwater habitats
- Adults 2-15 mm long
- Legs spaced widely for better weight distribution
- Tips of legs are hydrophobic, repel water
- Can jump, propel, and turn rapidly on water surface
- Hunt other insects that fall into water trap
Understanding the science behind how water striders walk on the water’s surface gives insight into surface tension and other intermolecular forces in action.
Applications of surface tension
Beyond water domes on pennies, surface tension effects are incredibly important across many scientific disciplines and industries. Here are some of the key ways surface tension influences other materials and applications:
Microfluidics
In microfluidic devices dealing with tiny liquid volumes and channels, surface tension dominates over forces like inertia and gravity. Engineers must account for surface tension to understand how liquids will move and interact in microfluidic chips. Surface tension effects determine interface shapes, capillary flow rates, pressure drops, and more in these systems.
Self-assembly and patterning
Surface tension can cause molecules and nanoparticles to self-organize into patterned structures and 2D or 3D architectures. This effect is useful in fields like nanotechnology and materials science to construct complex assemblies from simple building blocks.
Foams and emulsions
Foams and emulsions rely on surface tension to stabilize gas or liquid interfaces. The formation and breakdown of foam bubbles is dictated by surface tension based on surfactants used. In emulsions like milk, surface tension keeps dispersed phase droplets separated in continuous phase.
Heat transfer and evaporation
Surface tension impacts boiling, condensation, and evaporation processes important in heat transfer. The wicking action from capillary effects can also influence heat pipes and vapor chambers. Surface tension helps set the meniscus shape in such liquid-vapor systems.
Surfactants and detergents
Surfactants lower surface tension and are added to solutions like detergents to help solubilize dirt and oils. Lower surface tension allows surfactants to wet surfaces and break down stubborn particles. Surfactants reduce intermolecular attractive forces at interfaces.
Biological processes
Surface tension is key for biological processes like lung inflation, water transport in plants, and diffusion across membranes. Cells have distinct interiors separated by hydrophobic lipid bilayer cell membranes. Proteins utilize capillary forces for locomotion and water resistance.
As you can see, surface tension directly impacts natural processes and technologies at all scales – from the micron size all the way up to large industrial applications. Anywhere liquids interact with gases, solids, or other immiscible liquids, surface tension forces are present and must be accounted for.
Conclusion
In summary, the ability of a penny to hold many water drops relies on the surface tension of water. Surface tension is a liquid property arising from unbalanced molecular cohesive forces at the surface. It causes water to contract and form dome shapes on a penny rather than spreading out flat. More drops can be added to the “water bubble” before surface tension is overpowered and the liquid spills over the penny’s edge. Real-world examples like water striders walking on water illustrate surface tension in action. Beyond just the party trick of water drops on a penny, surface tension has broad importance across many scientific and industrial applications.
