Adhesion Force: vander waal force

    Forces Acting on Powder

    When considering the behavior of powders, there are forces other than contact forces to take into account. Among them, the adhesion force, which occurs between particles or between particles and walls as they attempt to adhere to each other, is a crucial element when handling powders.

    van der Waals Force

    The van der Waals force is one of the adhesion forces and is a force that arises between atoms or molecules. It is based on the electrical imbalance (electric dipole) within atoms or molecules, and as a result, it is a force that constantly acts between particles.
    • Magnitude of the Van der Waals Force
    The magnitude of the van der Waals force is expressed as follows:
    In this context, the unit vector indicates direction and does not affect the magnitude. Therefore, the magnitude of the van der Waals force is determined by the particle diameter, particle-surface separation distance, and Hamaker constant.
    So, how significant is the force calculated in this manner? The graph (*) below compares the magnitudes of various forces.
    From this graph, it can be observed that for particles smaller than about 1 μm, the influence of van der Waals forces and capillary forces is substantial. On the other hand, for larger particles, the influence of electrostatic forces like Coulombic forces is more significant. This implies that van der Waals forces need to be considered when dealing with particles around 10^-6 m (micrometers) or 10^-9 m (nanometers) in size, while they have a smaller impact on larger particles.
    Furthermore, the derivation of the van der Waals force equation is not covered here, but in summary:
    Focus on an individual atom or molecule and calculate the force due to electrical imbalances.
    Sum up these contributions for all volume elements composing the particle.
    • Hamaker Constant
    The Hamaker constant is one of the three factors required to calculate the van der Waals force. Particle diameter and particle-surface separation distance might be more comprehensible. So, what is the Hamaker constant?
    The Hamaker constant is a characteristic value that defines the magnitude of the van der Waals force. It is determined by the composition, surface condition, and shape of an object. Thus, determining a specific value for the Hamaker constant is not straightforward. However, it generally falls within the range of 10^-21 to 10^-19. Here is a table of indicative values for different materials as a reference (source: Chemical Engineers’ Handbook).

    What Makes Lizard Stick to Walls?

    Lizards, commonly found around residential areas, might have caught your attention at some point. Have you seen them clinging to walls or ceilings? Well, the factor allowing lizards to do this is also the van der Waals force. The feet of lizards are covered in tiny hair-like structures, and it’s through the van der Waals force between these hairs and the surface that they manage to cling.
    You might wonder, “Sticking due to hair?” Many may think so, but let’s consider the nature of the van der Waals force. The hair-like structures on a lizard’s feet are on the order of 10^-6 m (micrometers) in size, and their tips are even finer, branching to around 10^-9 m (nanometers) in thickness. As I mentioned earlier, the van der Waals force has a more significant impact when acting on smaller structures. Hence, the fine hairs enhance the influence of the van der Waals force. Coupled with the fact that these forces act on numerous points, they enable lizards to stick to walls.

    Conclusion

    In conclusion, we’ve covered the topic of van der Waals force as part of adhesive forces. Van der Waals force is a significant factor to consider when dealing with particles on the order of 10^-6 m (micrometers) or 10^-9 m (nanometers). The force’s strength depends on the surrounding environment and particle surface conditions, which determine the Hamaker constant.
    [Reference materials]
    (*) Masuda, “Adhesion and cohesive force of powder particles”, Journal of the Electrophotography Society, Vol. 36, No. 3 (1997) https://www.jstage.jst.go.jp/article/isjepj/36/3/36_3_169/_pdf