Van der Waals forces are a term used in chemistry to describe the sum of repulsive or attractive forces on a molecular level. Van der Waals forces do not include forces caused by covalent bonds or electrostatic interactions.
All particles are polarized one way or another, and these polarizations fluctuate due to quantum dynamics. These fluctuating polarizations sometimes correlate in a way that creates attraction or repulsion forces. These attraction and repulsion forces are van der Waals forces, named after Dutch scientist Johannes Diderik van der Waals.
Van der Waals forces depend on the orientation of molecules involved (Except in the case of interaction between noble gas atoms). The forces are always attractive forces, causing atoms to bond together, but the rotation of the molecules can change the sign of the electrostatic interaction. This could cause the attraction to become a repulsion, depending on the state of molecular orientation.
As the size of particles involved decreases, van der Waals forces decrease with it. However, proportionally, van der Waals forces decrease much slower than other forces, such as gravitational pull, drag, and lift. Because of this, van der Waals forces become much more prevalent in smaller particles. These particles formed into powders that are significantly more difficult to fluidize, due to their stronger cohesion.
The relative strength of van der Waals forces is also determined by the type of surface particles are interacting with. Porous surfaces that allow for more area of contact are affected more strongly by van der Waals forces than perfectly flat surfaces.
A good macroscopic demonstration of van der Waals forces is the climbing ability of the gecko. The gecko is able to climb glass due to the microscopic projections from hair on their footpads. The relatively porous surface of the glass allows the van der Waals forces to have more effect, supporting the gecko's body weight.