Influence of the axial displacement of a trapped bead on the calibration of the force parameters of optical tweezers

As a force probe, optical tweezers have two important parameters: trap stiffness and escape force. The drag-force method is often used to calibrate these parameters in the lateral direction. In that case the axial displacement of the trapped bead is usually neglected and its influence on the calibration precision has not been studied. In this work the information entropy method is used to measure the axial displacement in the calibration. Then its influence on the calibration is analyzed. The result shows that the trapped bead displaces axially as well as laterally when the surrounding medium flows. So the trap stiffness measured in this way is not really on the same horizontal plane, which is one of the error sources of the calibration. The experimental date also shows that there is a jump in the axial displacement of the bead when the fluid velocity reaches the critical value. This fact indicates that the bead escapes from the trap upward in stead of laterally and hence the measured escape force is not the maximal radial or lateral trapping force as commonly accepted.