(243a) Hydrophobic Forces Between Deformable Oil Drops

The hydrophobic force is a fundamental interaction in nature, and is central to many processes ranging widely from dispersing oil spills to forming cell membranes.  Yet, many studies by different groups over the past 30 years to directly measure the hydrophobic force have led to the observations of extra attractions that have varied in range from tens to hundreds of nanometers.  Perhaps most surprisingly, the hydrophobic effect is often invoked to explain results using solid (mainly mica) surfaces in contact with water, but rarely in the study of drops and bubbles, arguable some of the most hydrophobic interfaces in nature. For solid surfaces many secondary phenomena, surface chemical heterogeneity and roughness, and the presence of nano-bubbles, have been uncovered without a clear picture of how to describe the hydrophobic effect on a microscopic scale.

We employ molecular smooth and inert oil droplets in aqueous solutions to isolate the intrinsic hydrophobic attraction, showing it to be far a shorter range and stronger attraction than the conflicting results in the literature suggest.  This is also more consistent with an expected molecular origin. We have measured this force directly by suppressing any extraneous forces leaving only a “hydrophobic force” between two drops in aqueous solution.  We also introduce separately different mild repulsive forces of quantum electrodynamic, electrical and hydrodynamic origin between the drops, and in each case the same attraction is observed.