The atomic force microscope (AFM) has provided unprecedented opportunities to study velocity-dependent interactions between deformable drops and bubbles under a range of solution conditions. The challenge is to design an experimental system that enables accurate force spectroscopy of the interaction between deformable drops and thus the extraction of accurate quantitative information about the physically important force-separation relation. This step requires very precise control and knowledge of the interfacial properties of the interacting drops, the drive conditions of the force-sensing cantilever, the disposition of the interacting drops on the substrate and on the cantilever, and transducer calibrations of the instrument in order to quantify the effects of approach velocities and interfacial deformation. This article examines and quantifies in detail all experimental conditions that are necessary to facilitate accurate processing of dynamic force spectroscopy data from the AFM using the well-defined system of tetradecane drops in aqueous solutions under surfactant and surfactant-free conditions over a range of force magnitudes that has not been attained before. The ability of drops to deform and increase the effective area of interaction instead of decreasing the distance of closest approach when disjoining pressure exceeds the Laplace pressure means that the DLVO paradigm of colloidal stability as being determined by a balance of kinetic energy against the height of the primary maximum is no longer valid. The range of interfacially active species present in alkane-aqueous systems investigated provides insight into the applicability of the tangentially immobile boundary condition in colloidal interactions.
Quantitative interpretation of the dynamic forces between micrometer-sized deformable droplets and bubbles has previously been limited by the lack of an independent measurement of their absolute separation. Here, we use in situ confocal fluorescence microscopy to directly image the position and separation of oil droplets in an atomic force microscopy experiment. Comparison with predicted force vs. separation behavior to describe the interplay of force and deformation showed excellent agreement with continuum hydrodynamic lubrication theory in aqueous films less than 30 nm thick. The combination of force measurement and 3D visualization of geometric separation and surface deformation is applicable to interactions between other deformable bodies.
A systematic study of collisions between surfactant-free organic drops in aqueous electrolyte solutions reveals the threshold at which continuum models provide a complete description of thin-film interactions. For collision velocities above ~1 μm/s, continuum models of hydrodynamics and surface forces provide a complete description of the interaction, despite the absence of surfactant. This includes accurate prediction of coalescence at high salt concentration (500 mM). In electrolyte solutions at intermediate salt concentration (50 mM), drop-drop collisions at lower velocity (<1 μm) or extended time of forced drop-drop interaction exhibit a strong pull-off force of systematically varying magnitude. The observations have implications on the effects of ion-specificity and time-dependence in drop-drop interactions where kinetic stability is marginal.
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