  The forces acting on an optically trapped particle are usually assumed to be
conservative. However, the presence of a non-conservative component has
recently been demonstrated. Here we propose a technique that permits one to
quantify the contribution of such non-conservative component. This is an
extension of a standard optical tweezers calibration technique and, therefore,
can easily become a standard test to verify the conservative optical force
assumption. Using this technique we have analyzed different-sized optically
trapped particles under different trapping conditions. We conclude that the
non-conservative effects are effectively negligible and do not affect the
standard calibration procedure, unless for extremely low-power trapping, far
away from the trapping regimes usually used in experiments.
