Figure 9.11 — Mean square displacement analysis Figure 9.11 — Mean square displacement analysis. Mean squared displacement analysis. MSDs corresponding to a polystyrene sphere (a = 1.03 μm) optically trapped with 2.3, 6.0 and 10.8 mW laser power (λ0 = 1064 nm) at the sample. The symbols and error bars are the experimental MSDs [Eqs. (9.30)] and the solid lines are the theoretical MSDs [Eq. (9.29)] corresponding to the fitted parameters. The fitted values of the stiffness are κx(ex) = 14.3 ± 0.3, 36.2 ± 0.6 and 68.9 ± 0.6 fN/nm and of the unit conversion factor [Eq. (9.31)] are S(ex) = 478, 191 and 106 nm/V. At short time scales the MSD is diffusive and at long time scales, it reaches a plateau due to the trap confinement – the stronger the trap, the smaller the value of the plateau.