We report new observations that question the validity of using the escape method to deduce optical trapping forces in optical tweezers. The results were obtained by measuring the minimum beam power required to trap a particle subject to fluid flow, created by movement of the sample stage. The power was found to increase by approximately 50% over the range of amplitudes of movement used. This suggests that ;the measured trapping force is not solely dependent upon the velocity of the fluid past the trapped particle, as commonly assumed. These effects were observed at all fluid velocities and accelerations employed. We consider various causes for the observed effects but an explanation remains an outstanding challenge.
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