Combined AFM-Confocal Microscopy of Oil Droplet Interactions and Novel Characterization Methods of the Oil-Water Interface

Our research has focused on the development of innovative methods using Atomic Force Microscopy (AFM) to quantitatively study the dynamic forces between drops, bubbles, particles on the nano-scale. The dynamic interactions on the nano-scale between these objects mediate or control behaviour on the macroscopic scale in complex fluids (e.g. emulsions, foams and particle suspensions). This talk will focus on two aspects of the complexities involved in measuring the collision and coalescence of micro-drops or micro-bubbles using AFM.  First, we present a method to directly measure the interfacial or surface tension of the micro-drops or micro-bubbles used in the AFM force measurements.  Quantitative theories to analyze these force data require the interfacial tension, obtained from macroscopic measurements.  We present a novel method using a nano-needle probe to measure the interfacial tension of the drops in the AFM liquid cell directly¹.  Second, we present a method to directly measure the inter-droplet separation.  AFM methods lack an independent measure of the absolute separation distance.  For rigid surface this can be inferred from the data, but for deformable surface a model or fitting procedure is required.  For the first time, we use in situ confocal fluorescence microscopy to directly image the position and separation of oil droplets in an atomic force microscopy experiment². 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.

References

1. Uddin, M. H.; Tan, S. Y.; Dagastine, R. R., Novel Characterization of Microdrops and Microbubbles in Emulsions and Foams Using Atomic Force Microscopy. Langmuir 27, (6), 2011, 2536-2544.

2. Tabor, R. F.; Lockie, H.; Mair, D.; Manica, R.; Chan, D. Y. C.; Grieser, F.; Dagastine, R. R., Combined AFM-Confocal Microscopy of Oil Droplets: Absolute Separations and Forces in Nanofilms. The Journal of Physical Chemistry Letters 2011, 961-965.