Water Surface Tension: The water’s surface or any other type of liquid acts like something similar to an elastic membrane, this is because of the surface tension.
Multiple groups of forces act on the molecules that are on the surface of the water and these forces are perpendicular to the surface and directed to the interior of the liquid.
These forces are capable of holding water if we pour it drop by drop, but it’s broken easily if we’re not careful. This experiment for kids is perfect to show the little ones the behavior of water. In this article, we will show you some of the surface tension experiments which you can do at your homes easily with your family.
Water Surface Tension
Surface tension is a phenomenon in which the surface of a liquid, where the liquid is in contact with the gas, acts as a thin elastic sheet. This term is typically used only when the liquid surface is in contact with gas e.g air. If the surface is between two liquids e.g water and oil, it is called “interface tension.”
Water Surface Tension Causes
Various intermolecular forces such as Van der Waals forces, draw the liquid particles together. Along the surface, the particles are pulled toward the rest of the liquid, as shown in the picture to the right. It is measured in Newton per meter.
Water Surface Tension Examples
In the above video, there is a perfect example of the surface tension test at home which can be executed very easily.
Following are some more examples of the Surface tension experiment.
Drops of Water
When you use a water dropper you will observe that water does not flow in a continuous stream, but rather in a pattern of drops. The shape of the drops is caused by the surface tension of the water. There is only one reason the drop of water isn’t completely spherical is that of the force of gravity pulling down on it.
In the absence of gravity, the drop will minimize the surface area in order to minimize tension, which will result in perfect spherical shape.
Insects Walking On Water
Several insects are eligible to walk on water, such as the water strider or there are many more. Their legs are formed in such a way that it distributes their weight, causing the surface of the liquid to become depressed, minimizing the potential energy to create a balance of forces so that the strider can move across the surface of the water without breaking through the surface and the equilibrium is maintained. This is similar in concept to wearing snowshoes which are long to walk across deep snowdrifts without your feet sinking.
Needle Floating On Water
Even though the density of these objects is greater than water, the surface tension along the depression is enough to counteract the force of gravity pulling down on the metal object.
Anatomy Of A Soap Bubble
When you blow a soap bubble, you are creating a pressurized bubble of air which is contained within a thin, elastic surface of the liquid. Most liquids cannot maintain a stable surface tension to create a bubble, which is why soap is generally used in the process … it stabilizes the surface tension through something called the Marangoni effect.
When the bubble is blown, the surface film tends to contract.
This causes the pressure inside the bubble to increase. The size of the bubble stabilizes at a size where the gas inside the bubble won’t contract any further, at least without popping the bubble.
In fact, there are two liquid-gas interfaces on a soap bubble – the one on the inside of the bubble and the one on the outside of the bubble. In between the two surfaces is a thin film of liquid.
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The spherical shape of a soap bubble is caused by the minimization of the surface area – for a given volume, a sphere is always the form which has the least surface area.
The pressure inside A Soap Bubble
To consider the pressure inside the soap bubble, we consider the radius R of the bubble and also the surface tension, gamma, of the liquid (soap in this case – about 25 dyn/cm).
We begin by assuming no external pressure (which is, of course, not true, but we’ll take care of that in a bit). You then consider a cross-section through the center of the bubble.
Along with this cross section, ignoring the very slight difference in inner and outer radius, we know the circumference will be 2pi R. Each inner and outer surface will have a pressure of gamma along the entire length, so the total. The total force from the surface tension (from both the inner and outer film) is, therefore, 2gamma (2pi R).