Measurement of Force Curve due to Electrostatic Charge on a Single Particle using Atomic Force Microscope

A single particle is brought into contact with a metal target, and the force acting on the particle is measured by using atomic force microscope (AFM). By focusing on measuring the force 'curve', rather than looking only at the maximum adhesive force, electrostatic interaction was successfully observed by separating other interactions such as liquid bridge and intermolecular forces. To evaluate the force curve, the force curve with an approximation of disk-to-disk interaction based on image force was numerically calculated and a good analytical approximation was developed. The fact of successful agreement between the observed force curve and the theory revealed that the force curve observed can be surely attributed to 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(-2)C/m(2), which is much greater than that obtained with impact charging experiment as 10(-4)C/m(2). From this it was concluded that the force curve measurement with AFM can capture the net amount of the charge generated before charge relaxation due to gas discharge taking place. In the experiment using 8 kinds of metal (Al, Au, Cr, Ni, Pt, Ti, Zn, and Zr) and polystyrene particle, the net charge generated was shown to be compared fairly well with the conventional simple condenser model based on metal-to-metal contact model in terms of contact potential difference in its order of magnitude. Although a 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.