Measurement of Force Curve due to Electrostatic Charge on a Single Particle Using Atomic Force Microscope
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概要
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A single particle is brought into contact with a metal target, and the force acting on the particle is measured by using an atomic force microscope (AFM). Focusing on the measurement of force curve rather than the maximum adhesive force, the electrostatic interaction was successfully observed by separating it from other interations such as liquid bridge and intermolecular forces. In order to evaluate the force curve and the method, a force curve was numerically calculated with an approximation of disk-to-disk interaction with image force and a good analytical approximation was developed. The successful agreement between the observed force curve and the theory revealed that the force curve observed can be explained by the electrostatic interaction, and that the amount of charge on the particle and the radius of the charged (contact) area can be estimated from the analysis. The order of magnitude of the measured charge density was 10-2C/m2, which is much greater than that obtained with the impact charging experiment of 10-4C/m2. From this it was concluded that the force curve measurement with AFM can measure the net amount of the charge generated before the charge relaxation due to gas discharge or other charge relaxation took place. In the experiment using 8 kinds of metal (Al, Au, Cr, Ni, Pt, Ti, Zn, and Zr) and polystyrene particles, the net charge generated was shown to be compared fairly well with the conventional simple condenser model based on metal-to-metal contact model with the term of contact potential difference in its order of magnitude. Although clear correlation was not obtained between the measured charge density and the work functions of metal targets because of a large scatter in the data, a strong relationship between the charge density and contact area was found. The underlying mechanisms for the relationship is not known at this moment, however the finding gives a good hint for the next attempt.
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