Force Measurement for a Single Nanoparticle

Abstract

Micrometer- and Nanometer-sized particles suspended in liquids and in air experience various kinds of forces [1–5]. Gravity and viscous drags exerted on a single microparticle are as weak as femto-Newtons, though they, of course, cannot be neglected for determining the mechanical motion of the particles. Van der Waals interactions between microparticles and those between a particle and a solid surface play an important role in the adhesion mechanism. Surface double layer forces also act on microparticles dispersed in solution. Particles and solid surfaces are usually charged by ionic dissociation of surface groups or by adsorption of ions from solution onto particles, so that counter-ions are attracted to the charged surfaces against thermal diffusion, forming electric double layers. Electrostatic forces can also be induced on the charged particles by applying an electric field. Short-range forces due to hydrophobic and hydrophilic interactions, hydration and solvation energy, and hydrogen bonding networks sometimes govern the behavior of microparticles in solution. In addition to these mechanical and electromagnetic forces, particles always undergo Brownian motion with thermally induced random forces. Aggregation, adhesion, and sedimentation processes of organic, metallic, semiconductor colloidal particles, surfactant micelles, macromolecules, and so on, can be determined by the strength of those forces and their balances. Equilibrium states and the dynamics of particle motion have been theoretically investigated by Derjaguin, Landau, Verway and Overbeek, the so-called DLVO theory. It has been clarified that those forces are dependent on surface roughness, particle shapes, the physical and chemical structures of the surfaces, the hydrogen exponent and electrolyte concentrations of the medium, as well as temperature.