Direct measurement of field-induced polarization forces between particles in air

We have measured the effect of DC and AC electric fields (up to 15 kV/m) on the force between two 15 μm radius BaTiO3 glass spheres in air in the separation range 0-6 μm. These fields cause attractive forces that are much greater than van der Waals forces and therefore can be used to control and separate particles in many applications. The attractive force has a static and dynamic component. The static forces are about 9000 times greater than the prediction by multiple-moment methods but otherwise follow the expected trends for polarization forces. For example, the force scales with the square of the field, is constant over a range of field frequencies, and has the same force-separation profile predicted by multipole-moment methods. In contrast to the point dipole approximation, which depends inversely on the fourth power of the distance between the centers of the spheres, the measured static force approximately follows a power law, F ∝ -s(-0.75), that depends on the separation, s, between the nearest points of the spheres. This power law is very similar to the prediction by multipole-moment method for separations less than 1/10th of the radius. The dynamic response force occurs at twice the frequency of the drive and has a similar amplitude to the static force. The electrical field also causes a large increase in adhesion.