We use a computational method based on the discrete-dipole approximation (DDA) to calculate the gradient and nongradient contributions to optical forces on nanometer sized silver particles in water. We find that, due to a contribution that is usually neglected, nongradient forces are often non-negligible. This result is not a consequence of an approach to the dipole limit. We suggest that this method could provide useful input for a more detailed understanding of the physics relevant to optical trapping and binding phenomena.

• Received 9 September 2005

DOI:https://doi.org/10.1103/PhysRevB.73.075416