Sphere-Sphere Forces

Hello. Firstly, greetings to everyone, I am new here.

So, I am trying to build a model based on this article:
www.google.com/url?sa=t&rc...mp;sig2=HC5XPGRvo0qippVGnfngLg

I am also new to electrostatic modeling, but I have some basic Comsol knowledge.

My model contains two copper spheres with potentials Va and Vb and a large sphere representing surrounding medium (vacuum). I have selected Force Calculation domain condition and I tried Electric Potential and Terminal boundary conditions on the spheres.

In the article, it is described: '' When Va=Vb the force is repulsive at all distances, otherwise the force becomes attractive when the approach is close enough. Attraction occurs between spheres both at positive potential because of mutual polarization: for spheres with equal radii, the sphere at lower potential obtains a negatively charged region neighbouring the other sphere.''

I tried many different settings and I can't seem to get the described result, especially the reversal of the force direction. Am I missing something fundamental or just any detail? Is this a phenomenon that can be modeled using Comsol?

Regards,
Tomi