Colloidal force measurements using atomic force microscopy and total internal reflection microscopy

Measurements of colloidal forces at deformable interfaces (compared to rigid systems) are complicated by deformation of the interfaces making both measurement and interpretation more difficult, although understanding the colloidal forces at a liquid-liquid and liquid-gas interface is relevant to many industrial processes. Atomic Force Microscopy (AFM) measurements between a rigid probe and a deformable interface have become commonplace, yet interpretation of the data is not straightforward due to the deformation. A semianalytic model was developed to predict the behavior of this type of AFM force measurement. The results of the theory demonstrate that analysis techniques for rigid systems are not valid for deformable cases.; A method for extracting quantitative data from an AFM measurement was developed using the above model, where planar interaction energy per unit area was deconvoluted from the AFM force measurement without any prior knowledge of the interaction energy. Interaction energies extracted from measurements with an anionic surfactant absorbed at the oil water interface compared favorably to theoretical predictions for van der Waals and electrostatic force models. The experimental results indicated that local surface tension changes or local charge rearrangement of the surfactant occur during the force measurement. Electrostatic forces and steric forces were observed in interaction energies extracted from measurements with a tri-block copolymer (Pluronic F-108) absorbed at both interfaces in the presence of an inorganic salt. The decay lengths of the electrostatic force indicated that the oil-water interface was not charged, consistent with a nonionic polymer absorbed at the oil-water interface.; An existing Total Internal Reflection Microscopy (TIRM) fluid cell was modified to measure the interaction energy of an oil droplet levitated below a flat silica plate. Methods to improve the experiment to minimize noise, which obscured most quantitative analyses on the data, were discussed.; A novel method to account for the effects of surface roughness on van der Waals force predictions using Lifshitz theory was developed. The model behavior was probed at contact by variation of the type of surface roughness and at larger separations by comparison to previously published TIRM measurements of retarded van der Waals forces between a sphere and plate.