Surface tension and Young's contact angle

In summary, the conversation discusses the unclear nature of surface tension forces in diagrams depicting capillary action. It is mentioned that the tensions are acting on the point where different substances meet and reach equilibrium, but it is not clear which substances are being affected. The conversation also touches on the concept of surface tension in different substances and its role in maintaining equilibrium. A reference is provided for further clarification on the topic.
  • #1
etotheipi
I came across this diagram, the ##\gamma##'s are supposedly forces per unit length of the respective interfaces:
1592667384469.png


It's not clear what these forces are acting on. ##\gamma_{SL}## and ##\gamma_{LG}## look like they could be acting on a small bit of water right at the end, but I have no idea what ##\gamma_{SG}## is supposed to be acting on.

Likewise there's this diagram of capillary action:

1592667531755.png


It's not clear here either what bodies any of those forces are acting on.

I wondered if someone could clarify? Thanks
 
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  • #2
etotheipi said:
It's not clear what these forces are acting on. γSL and γLG look like they could be acting on a small bit of water right at the end, but I have no idea what γSG is supposed to be acting on.
Those are surface tension - Solid Liguid interface, Liquid Gas, and Solid Gas.
At the point where all substances S, L, or G meet, there is an equilibrium and the tensions have to balance statically.

You are right, it is not readily apparent on what the tensions are acting.
But usually we are interested in the liquid surface.

Here is what I think
Consider the Water - Air interface ie the tension LG.
"Both" surface tensions, ie either that of the water in air, or that of the air in water ( if we can call that a surface tension, as usually we are interested in the energies at the liquid surface - consider an interface of two liquids ie water and oil - the oil particles could be more attracted to each other than to water particles, and the same for the water particles being attracted more to each other than the oil particles - and vice-versa ), would have to be equal statically or one would have movement. The surface tension of an air bubble in water is manifested as a pressure - a smaller bubble has a higher pressure within than a larger bubble. The surface tension of the water counteracts this pressure with surface tension of the molecules at the liquid-air interface. If less than that to counteract the internal bubble pressure, the bubble grow in size. If more, the bubble shrinks.

Hopefully that stands up ( or gets picked apart ) to PF scrutiny, which I hope it gets, as your question does delve farther into the common general descriptions of contact angle and surface tension.
 
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  • #3
I also found a nice reference here

In their words:
In Young’s law, the system on which the forces act is a corner of liquid bounded by the contact line

1592835318287.png
 
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  • #4
The reference does go into more detail.
thanks for the link.
 

1. What is surface tension?

Surface tension is the force that causes the molecules on the surface of a liquid to be pulled together, creating a "skin" or surface layer. It is caused by the imbalance of intermolecular forces between the molecules on the surface and those in the bulk 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 (N/m). There are various techniques for measuring surface tension, including the drop weight method, Wilhelmy plate method, and Du Nouy ring method.

3. What is Young's contact angle?

Young's contact angle is the angle formed between the tangent of a liquid droplet at the point where it meets a solid surface and the solid surface itself. It is a measure of the wettability of the surface by the liquid, with a smaller contact angle indicating better wetting.

4. How is Young's contact angle related to surface tension?

The Young's contact angle is related to surface tension through the Young-Laplace equation, which states that the pressure difference across a curved liquid surface is equal to the product of the surface tension, the cosine of the contact angle, and the curvature of the surface.

5. What factors affect surface tension and Young's contact angle?

Surface tension and Young's contact angle can be affected by various factors such as temperature, type of liquid and solid, presence of surfactants, and roughness of the solid surface. Additionally, the contact angle can also be influenced by the chemical interactions between the liquid and solid surfaces.

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