# Force of surface tension quandry?

• jlyu002@ucr.e
In summary, Chet explained that the surface tension is the force per unit length within the surface of a spherical drop, and that it always acts in the direction tangent to the free surface. Additionally, he clarified that there is no force between neighboring outer water molecules, only between the drop and the needle. Lastly, he said that the surface tension is due to the hydrogen bonds between neighboring atom.
jlyu002@ucr.e
I have went through the thread and looked at the explanations for surface tension, and I have also went online to about 10 different websites, but I have not yet been able to figure out this concept. I think it is just my inability to understand and I need perhaps a different way of explaining.
I am trying to see how the force of surface tension is pointing
Here is what I know.

Fw is the weight of the object, and Fs is parallel at the point where it contacts the object which is the force due to surface tension.

Also, the water wants to stay at it's most energetically favorable state. And a sphere will allow minimum surface area for a given volume.

Given all this information, I think the answer to my question is, 1. due to the net inward force, the outer water molecules, e.g. in space, are brought inward and into a sphere in which the outerwater molecules are more tightly bound in an inward fashion. Therefore, due to the fact that water is incompressible, there is a pressure inside the water molecule sphere and the pressure is also radiating outward.(I'm not sure if this pressure idea is right)

2. The energy wants to keep it at this nice mode, therefore if say an object like a needle sits on the water, the water wants to maintain the energy that it was at.

Please, if anyone can assist, I would be extremely thankful!Jonathan

I like to think of surface tension as the same as that of a stretched membrane acting along the free surface. In the case of a spherical drop, it is like a balloon that is stretched around the drop. The surface tension is the force per unit length within the surface of the balloon. Unlike an ordinary balloon, however, no matter how much the surface area increases, the force per unit length doesn't change. In the case of the needle, the surface tension is the force per unit length along the line of contact between the fluid free surface and the needle. The surface tension always acts in the direction tangent to the free surface. Hope this helps.

Chet

shangriphysics
Hi Sir Chet,

From your explanation I feel very close to understanding. I just have two more questions: I was wondering if there was no needle, if the direction of force per unit length would be horizontal with the neighboring outer water molecules for the outer most water layer for ex. a ball of water in space. Lastly, is the force due to the hydrogen bonds of neighboring atom?

Thank you Chet!

jlyu002@ucr.e said:
Hi Sir Chet,

From your explanation I feel very close to understanding. I just have two more questions: I was wondering if there was no needle, if the direction of force per unit length would be horizontal with the neighboring outer water molecules for the outer most water layer for ex. a ball of water in space.
I don't understand this question, particularly the part about the ball.

On a flat horizontal free-surface, the surface tension acts horizontally within the surface.
Lastly, is the force due to the hydrogen bonds of neighboring atom?
I'm not knowledgeable about the molecular explanation. What I do know is how to model surface tension effects.

Chet

Ahh I think I get the satisfying gist of the concept! Thank you so much Chet for your benevolent help!

## 1. What is surface tension?

Surface tension is a physical property of liquids that causes the surface of a liquid to behave like a thin elastic sheet. It is caused by the cohesive forces between molecules at the surface of the liquid.

## 2. How is surface tension measured?

Surface tension is typically measured in units of force per unit length, such as newtons per meter or dynes per centimeter. It can be measured using various methods, such as the drop weight method or the capillary rise method.

## 3. What factors affect surface tension?

The strength of surface tension is affected by several factors, including the type of liquid, temperature, and the presence of impurities or additives. Generally, surface tension decreases with increasing temperature and with the addition of impurities.

## 4. How does surface tension relate to the shape of liquid droplets?

Surface tension is responsible for the spherical shape of liquid droplets. This is because the surface tension forces act equally in all directions, causing the liquid to minimize its surface area and form a shape with the least surface area - a sphere.

## 5. What are some real-life applications of surface tension?

Surface tension has many practical applications, such as in soap bubbles, water droplets on a surface, and the behavior of insects and other small creatures that can walk on water. It also plays a crucial role in industrial processes, such as coating and printing, and in the functioning of our respiratory system.

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