# Surface tension in CFD

The surface tension force refers to the gravity force applied on the free surface between two phases. The value of this force is calculated using the coefficient of surface tension depending on the fluid form.

This force has two normal and tangent components, the normal component is the flat zones of large curvatures, which induces strength and increases the pressure inside the droplet. The tangent component causes the fluid to move on the interface of the phases to a medium with a tension coefficient. Often it is called Maragoni effect.

#### Interactions of particles on each other

In low volume fractions (dilute mixtures), the probability of the interaction between the phases distributed with each other is lower than the interaction between the particle and the continuous phase. In such situations, particle-particle interactions are ignored, and single-particle / droplet-bubble relations are used.

By increasing the volume fraction, the particle-particle interaction effects are higher and the estimates of dilute systems are no longer valid. For solid particles dispersed in a fluid, the maximum volumetric component of solid particles is an amount that is greater than the particles as close as possible. There will still be fluid in the spaces between the elements and the maximum available fraction of solid particles will be less than 1. If we want to apply this maximum value for a solid fraction, additional forces must be added to the solver to prevent excessive fraction of solid particles from being exceeded. Since the liquid particles (bubbles and droplets) are deformed, the maximum amount of compression of particles can be applied to the first one.