Trapping force calibration in biological applications of optical tweezers

In this work we analyzed the calibration of optical trapping forces. One calibration technique utilizes the controlled motion of a trapped object in a fluid with known viscosity where the trapping,force is calculated from the-Stokes' Law based on inertia-free assumptions (i.e., neglecting velocity and acceleration of the trapped object). In our study, we. calculated the displacement of-the trapped object from the trapping center using Fourier analysis of the equation of motion. Waveforms of different frequencies were used both in theoretical modeling and experiments to control the motion of the trapped object in an aqueous solution. Calibration data obtained experimentally were compared to theoretical results. The dynamic analysis of the trapped object showed that trapping force can significantly differ from theoretically predicted values under inertia-free assumptions. Various factors including type of the waveform used to control the motion of a trapped object during calibration, its frequency, viscosity of the calibration fluid, mass and dimensions of the trapped object, stiffness of the optical trap and frequency response of the equipment used to control the motion of the trapped object contribute to the differences.