Soft colloidal probes for AFM force measurements between water droplets in oil

Here we introduce an extension of the atomic force microscopy (AFM) colloidal probe technique, as a simple and reliable experimental approach to measure the interaction forces between small water droplets (∼80–160 μm) dispersed in oil. Small water droplets are formed by capillary breakup of a microscale water jet in air, which is forced out of a fine capillary nozzle, and deposited on a superhydrophobic substrate immersed in tetradecane oil medium. In these conditions the water droplets are very loosely attached to the superhydrophobic substrate and are easily picked up with a hydrophobic AFM cantilever to form a soft colloidal probe. Sample force measurements are conducted to demonstrate the capability of the technique.