作为力探针的光阱有两个重要参数,在垂直光束轴的横向上的刚度和逃逸力。对它们的标定精度直接影响到光阱测力的精度。利用信息熵方法,研究了基于流体力学法标定的光阱刚度和光阱逃逸力。结果表明,光阱中的小球在不同横向速度的水流作用下,不仅有横向位移,而且有不同的轴向位移,即所标定的光阱刚度不是同一水平面的光阱刚度,而是一定轴向范围内的平均横向刚度。这是流体力学法标定光阱横向刚度的主要误差来源之一。在逃逸速度下小球从光阱中沿轴向向上逃逸,所以这样测得的“代表光阱的捕获能力的参数——光阱逃逸力”只是在轴向逃逸的临界条件下的横向光阱力(等于此时的横向粘滞力),小于最大横向阱力。

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.