Normal Forces Between Two Surfaces 8211 Van der Waals Forces

Van der Waals forces are relatively weak intermolecular interactions which exist between atoms, molecules, or particles. These interactions were first documented by Johannes Diderik van der Waals in 1873 for developing a theory to account for the properties of real gases. Unlike other kinds of forces, Van der Waals forces are always present. They may be small compared to other kinds of forces, but become significant when dealing with large bodies. Figure 8.1 presents the various types of Van der Waals forces between molecules as well as macroscopic solids.

Van der Waals force is believed to be responsible for observing reversible dry adhesion in natural surfaces like gecko toes. In order to investigate the principal mechanism of adhesion in gecko toes, Autumn et al. [1] measured the adhesion force between the toes of a live gecko and a highly hydrophobic substrate as well as a highly hydrophilic one. The toes of the gecko adhered equally well to both the surfaces. It is in agreement with the van der Waals hypothesis which predicts high adhesion force on polarizable surfaces, regardless of hydrophobicity.

Fig. 8.1 Different types of Van der Waals interactions

Van der Waals Forces

Between Molecules

Between Macroscopic Solids

Coulomb Internet ion Monopole-Dipole Internetâ„¢ Dipole-Dipole Interaction o Keesom Interaction o Debye Interaction o London Interaction

Hamaker Approach Lifshitz Theory

Coulomb Internet ion Monopole-Dipole Internetâ„¢ Dipole-Dipole Interaction o Keesom Interaction o Debye Interaction o London Interaction

Hamaker Approach Lifshitz Theory

Fig. 8.2 Schematic of double layer in water at contact with a solid

Fig. 8.2 Schematic of double layer in water at contact with a solid

Lin et al. [52] investigated the adhesion mechanism of pedal foot of a marine species, the abalone. The experimental pull-off force measured by Atomic force microscopy (AFM) was in agreement with the theoretical vand der Walls adhesion force obtained by using the Johnson-Kendall-Roberts (JKR) equation (Sect. 8.2.2). It was also shown that the pull-off force increased with increasing humidity for the hydrophilic substrate while, it remained unchanged for the hydrophobic substrate. The combination of van der Waals force and capillary force was believed to be responsible for the strong adhesion of abalone to various substrates.

Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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