Why is surface tension parallel to the interface?

A text says:

The surface tension of a liquid results from an imbalance of intermolecular attractive forces, the cohesive forces between molecules: A molecule in the bulk liquid experiences cohesive forces with other molecules in all directions. A molecule at the surface of a liquid experiences only net inward cohesive forces

If this is the case: Why is surface tension considered or experienced as a force parallel to the interface while it is so obvious that it must be perpendicular to it?

• I came here to ask the exact same question that net force experienced by the molecules at the surface is downward but then why is surface tension parallel/tangential to the surface? I read the answers given and I read answers of all the duplicate questions given. But none of the answers clearly address why surface tension is tangential even though net force is downward. Kindly explain just in terms of forces, as Feynman said let things be explained without using new words/concepts. Feb 26, 2017 at 12:29
• @claws I have the same question too. There is an interesting paper: doc.utwente.nl/79082/1/why_is_surface.pdf. See "Question 1". Mar 3, 2017 at 1:50
• @claws Their explanation is amazing and I am fully convinced. The paragraph III/B and III/C answer this question just in terms of forces. Mar 3, 2017 at 3:08
– udrv
Mar 4, 2017 at 5:44

Let's look at what is going on when there is a liquid-gas interface. We know that in the liquid, there will be some high concentration of liquid particles, and in the gas there will be a very small concetration of particles. This mean as you move from the liquid to the gas you will see a continuous density gradient between the liquid and gas. (In practice this density gradient is so sharp that we think of it as a discontinuous change in everyday life, but actually it is continuous.)

To see why this concentration gradient gives us a stress, and why this stress is in the plane of the surface, you can view the situation in two different pictures: the microscopic picture and the thermodynamic picture. The difference between the two pictures is that the microscopic picture views each particle as an object moving deterministically in the potential of each other object and basically uses newtonian mechanics to reason about what is going on. The thermodynamic picture doesn't care about the position of individual beads, but will only think about the total energy of the system. First let's consider the microscopic picture.

Microscopic picture

In the bulk of the fluid (i.e., away from the surface) each particle has bounces around with the other particles. Since the particles make a fluid, we know they attract each other when they are far apart. On the other hand, if they get very close, they must repel each other, because you can't have two atoms taking up the exact same space. This means that a particle in the bulk is sometimes pulling on other particles and sometimes pushing on other particles. If the particles were pushing on each other more than pulling, then the liquid would expand, because everything is pushing against everything else, as if the liquid were trying to explode. Conversely, if all the particles were pulling on each other, the liquid would contract. This means that when the liquid is at its equilibrium density, the particles are on average pushing on each other as much as they are pulling on each other.

Another way of thinking about this pushing and pulling is in terms of stress. If the particles are all pushing on each other, then that is called a positive or compressive stress, and if the particles are all pulling on each other that is called a negative or tensile stress. The conclusion of the previous paragraph is that particles in the bulk are under no stress.

We also know that since the particles in the bulk are being pulled in all directions there is no net force on any particle. Notice the difference between stress and force. If a particle is being pulled left and right by the same amounts at the same time, then there is no net force, but there is a tensile stress.

Now let's turn to the interface. We saw in the very first paragraph of this answer that the density at the interface is lower. This means that the particles are farther away from each other on average. Since they only repel when they are close, the particles on the surface must be pulling on each other more than pushing, so that there is a tensile stress in the surface. Also since there are more particles on the liquid side than on the gas side, a particle also feels a net force toward the liquid.

Now the net force toward the liquid on the particle at the surface will not cause the interface to move because as the first particle is sucked back into the bulk of the liquid, another one will randomly bounce out of the liquid to take its place.

However, the tension can be used to do work. Since all the particles at the surface are pulling on each other similar to stretched elastic sheet, that is exactly how the interface will behave. So if you have a soap film, for example, which is attached to a movable boundary, then the surface liquid particles adhered to the boundary will give a pull to the boundary and that pull will move the boundary inward, just as a stretched elastic sheet would. Notice here that all the relevant forces are in the plane of the sheet. This should hopefully explain the microscopic picture.

Thermodynamic picture

In the thermodynamic picture, I don't consider there being any net force on a bead at the surface. There is an energetic force in the direction of the liquid due to the interactions, but I consider this to be balanced by an entropic "diffusion" force generated by the concentration gradient. This diffusion force causes particles to want to move to high concentration to low concentration. Thus it points toward the gas. Thus in equilibrium, there is no force anywhere so nothing should change.

However, there is a tension in the surface. This can be seen because a particle at the surface only experiences half the interaction energy it is supposed to. Thus there is a certain energy you have to pay to create a unit of surface area. The surface tension is exactly the constant of proportionality between energy and surface area. Since the energy doesn't change if you move the interface in the direction normal to the interface, there is no component of stress normal to the interface, so the stress is entirely in the plane of the interface. And it is a tensile stress because it wants to reduce the area, which is achieved by the interface contracting inward.

• Is it that surface tension acts only when there is an object on the surface? Dec 8, 2014 at 3:45
• No, surface tension always acts. Dec 8, 2014 at 3:52
• @NowIGetToLearnWhatAHeadIs: If surface tension is due to the less density at the interface, then why does the inward force come in discussion of surface tension??
– user36790
Feb 5, 2015 at 7:09
• The imbalance in pushing and pulling does indeed result in a stress. This stress is commonly referred to as absolute pressure and is rarely exactly zero. The pushing and pulling are typically not balanced. There's usually more pushing than pulling, resulting in a positive pressure, but that's dependant on the material, temperature, and density.
– Rick
Sep 22, 2017 at 23:23
• This paper by Michael Berry extends it to solid-liquid interface too. Nov 21, 2017 at 14:37

This is indeed very confusing. I think of it like this. Note this approach is purely conceptual and not at all rigorous! For a general definition of Force as the change of potential

$$F_x= -dV/dx$$

As you can see in the image, when a water molecule reaches the surface there has been work done on that molecule via the Hydrogen Bonds, an electrostatic force.

The potential is created by the Electrostatic Force generated between watermolecules, also called 'Hydrogen bonds'. So the potential of a molecule at the surface is

$$V= F_y \cdot y = dm \cdot a_y \cdot y$$

Where F is the work done by the electrostatic force or 'hydrogen bonds' and y is the distance along where the Force has acted (e.g. in a small spherical droplet the midpoint.) dm is the mass of a watermolecule.

Now imagine the linear density, which is defined:

$$dm/dx = \lambda$$

Rearranging

$$dm = \lambda \cdot dx$$

Substituting:

$$V = \lambda \cdot dx \cdot a_y \cdot y$$

$$F_x= -dV/dx = - \frac {d}{dx} (\lambda \cdot dx \cdot a_y \cdot y)$$

$$F_x= \lambda \cdot a_y \cdot y$$

And as you can see the acceleration in the y-direction creates the force in the x-direction. Of note is that y is the depth to which the hydrogen bonds cannot be approximated to be equal on both sides, which is actually quite small. In other words surface tension isn't really strongly correlated with the depth of the body of water like the last equation suggests.

TL;DR: The force parallel to the surface is created by a potential difference per horizontal distance caused by the electrostatic force in the perpendicular direction.