  We directly measured the interactions between a hydrophobic solid and a
hydrophobic liquid separated by water using force spectroscopy, where colloidal
probes were coated with graphene oxide (GO) to interact with immobilized
heptane droplets in water. We detected attractions with a long range of ~0.5
microns, which cannot be readily explained by standard
Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. When the GO was reduced to
become more hydrophobic, these forces increased in strength and ranged up to
1.2 microns, suggesting that the hydrophobic nature of the involved surfaces
critically influences the observed long-range forces. Previous studies have
addressed such hydrophobic attractions, but were limited to solid/water/solid
and solid/water/air scenarios. Here we expand this knowledge to include the
solid/water/liquid situation. Based on our results, we propose air bubbles
attached to the colloidal probe and molecular rearrangement at the water/oil
interface as possible origins of the observed interactions. The proposed
mechanism expands insights gained from previous to the solid/water/liquid
situation and is universally applicable to describe attractive interactions
between hydrophobic bodies of any kind separated by water. Our work will be
useful to understand and motivate the formation of many colloid and interface
phenomena, including emulsions using 2D materials and other
amphiphilic/hydrophobic particles.
