Surface Forces on Nanoparticles Determined by Direct Measurement
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Interaction forces between two colloidal size bodies have been studied for several decades. They are known to originate from the interatomic forces acting between all of the atoms of those bodies, as well as the atoms of any intervening medium. Surface forces especially play a role in many particulate systems, where they control powder flow, viscosity of ink and paint pigments, adhesion, coating, and interactions of biological molecules. Surface properties of material can dominate bulk properties when the size is smaller than about 100 µm. This is especially important as modern technologies require smaller dimensions and higher performance in application.
Understanding of surface forces is a matter of great importance for colloid science, where the stability and rheology of the colloidal suspension depend on the interaction forces between particles suspended in a liquid medium. The development of both theoretical and experimental approaches that can give qualitative and quantitative results has widely been performed for particles of varying sizes, ranging from several millimeters to several micrometers, and they fit each other well. However, the scalability of those theories down to the nanometer size range is questionable because the premises that were required to develop the current theories directly exclude nano size. For instance, the Derjarguin approximation, which translates from the interaction energy between two flat surfaces into other shapes, such as sphere–sphere and sphere–plane, is only valid when the radius of the particle is much greater than the separation distance between two particles. Besides, only a few novel force measurement techniques are capable of directly measuring the surface forces of nanoparticles.
This entry will briefly describe the types of surface forces that are used to describe forces in colloidal suspensions, explaining the mechanisms and parameters attributing to those forces as well as the limitations to nanoparticles. Then, the colloidal surface force measurement techniques will be reviewed, followed by new development of the direct force measurement for nanoparticles.