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How do we define the direction in which surface tension will act? Surface tension is a kind of hypothetical tension in which liquid molecules undergo tension force at the surface. Thus it should be a pulling force due to asymmetry.But many times in my books i have seen it acting in opposite direction.Why?

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There are a couple of different things to think about here.

First - surface tension can be thought of as "energy needed to create surface area". In that regard, you can think of a liquid water surface like a 2D spring: the more you stretch it, the more energy it costs. But it is nothing like a spring in the sense that the energy per unit area is constant (where in a spring energy goes as the square of the displacement).

You can then imagine that the force will act to make the surface smaller (that is energetically more favorable) - in whatever direction it needs to push in order to achieve that.

But there's a second important factor in most situations: there is often another (solid) surface in the picture, and there will be some forces between the liquid and the surface. If the surface likes water, we call it hydrophilic, and it is energetically favorable to create a large surface area where they touch. Unfortunately, if there is a large contact area of liquid and solid, there is an equally large area of liquid not touching solid (in the case of a plane) which is energetically not favorable. And so you get an equilibrium, where the liquid will be at a certain contact angle. At this point, you have some forces pulling on the liquid to make it bigger, and others to make it smaller.

If you have a hydrophobic surface, then the liquid will try to minimize its contact area ("beading"). Again, as it does so the area exposed to air will increase.

Once you ask yourself "what is the shape the liquid wants to be to minimize the energy" you will figure out the direction of the force.

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Surface tension is a very interesting phenomenon. One idea as to its origin is to realise that the molecules near the surface of a liquid are not bonded to molecules above the surface. To counteract the attractive forces from the liquid below the separation of the molecules near the surface is greater than that for molecules in the bulk of the liquid.
This increase in separation means that the potential energy of surface molecules is larger than those in the bulk. To minimise the potential energy the liquid tries to minimise its surface area.
This minimization of area manifests itself in a number of ways.
Liquid drops try and become spherical as a sphere has the smallest surface area for a given volume. Capillary rise can also be explained using the idea of surface minimization given that a layer of liquid molecules are always present on the surfaces of solids near the liquid. In such a case the force of adhesion (glass molecule to liquid molecule) is greater than the force of cohesion.

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However if the force of cohesion is greater than the force of adhesion, as at a mercury –glass interface, you would get capillary depression.

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  • $\begingroup$ Topic of direction of surface tension-can you explain why surface tension acts towards the centre of the AIR bubble? In the case of a liquid drop, liquid is inside the drop.If we consider one hemisphere of the LIQUID drop, the other surface is also liquid and hence pulls the hemisphere considered along the periphery by a total net force S(surface tension of liquid)times circumference of the hemisphere. But in case of a AIR bubble,where will surface tension act?How is difference in pressure inside and outside an AIR bubble still 2S/R where S is surface tension of LIQUID? $\endgroup$ – Hola Apr 1 '18 at 10:56
  • $\begingroup$ @Ola The bubble is trying to reduce the surface area of the air/liquid interface. $\endgroup$ – Farcher Apr 1 '18 at 10:58
  • $\begingroup$ Yes but how? Where is the force and how is it directed to perform the task of reducing surface area? Surface tension arises due to asymmetric force distribution, correct? Where is this asymmetric force distribution in an air bubble inside a tank of water and what is its direction? $\endgroup$ – Hola Apr 1 '18 at 11:02
  • $\begingroup$ The water in contact with the air bubble must be pulled away from the centre due to surface tension right ? How is it towards the centre of bubble. Can you tell me how this force is not away from centre. Since there are water molecules surrounding the water in contact with air bubble, it must experience a force ourward right? (Im considering only one hemisphere) can you please tell me where i'm going wrong? $\endgroup$ – Hola Apr 1 '18 at 13:29
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Surface tension acts inwardly, towards the body of the object. It is perpendicular to the surface of the object. If the object is curved, it is perpendicular to the tangent of the object's surface.

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You can think of the surface as a rubber sheet which has been stretched. This means that it will try to retain and decrease its area. This can only happen if it applies a force and thus this force is defined as surface tension times length

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