We present an experimental method based on video microscopy to perform nanometer scale position detection of a micrometer bead in the direction along the propagation of the detection light. Using the same bead for calibration and detection significantly improves the in depth resolution in comparison to video microscopy methods from literature. This method is used together with an optical trap to measure interaction potentials between a glass surface and colloids made of polystyrene or silica at different electrolyte concentrations. The results are confirmed by an independent method where the optical trap is used in connection with a quadrant photodiode. Also, we present a maximum likelihood analysis method which considerably improves the spatial resolution of interaction potentials by optimizing the underlying potential function to fit all observed position distributions. The measured interaction potentials agree well with DLVO theory for small electrolyte concentrations; however, for larger electrolyte concentrations the potentials differ qualitatively from both DLVO and Lifshitz theory.
Hansen, P. M., Dreyer, J. K., Ferkinghoff-Borg, J., & Oddershede, L. (2005). Novel optical and statistical methods reveal colloid-wall interactions inconsistend with DLVO and Lifshitz theories. Journal of Colloid and Interface Science, 287, 561. https://doi.org/10.1016/j.jcis.2005.01.098