Determining errors in force measurements for colloids close to contact

Due to diffraction patterns in light microscopy, the overlapping of colloid images introduces minor errors in determining particle positions, which affecting the accuracy of observing interactions between colloids near contact. This study quantitatively investigates the errors arising from the overlapping effect in force measurements between two closely positioned colloids using digital video microscopy (DVM). For this aim, computer simulations were conducted to obtain the positions of two optically trapped colloids interacting based on a force model and particle images were generated as a time series using experimental particle intensity profiles for two different particle sizes. These generated images were analyzed using tracking algorithms from the literature and the interaction forces were extracted and compared with theoretical force values. The results demonstrated that diffraction patterns significantly influence the accuracy of particle position detection and interaction force calculations. Large diffraction patterns and intensity fluctuations in an image introduce significant errors. However, these errors can become significantly smaller in the presence of large forces and may therefore be considered negligible.