Surface Forces in the Thin Liquid Films (TLFs) of Water Confined between n-Alkane Drops and Hydrophobic Gold Surfaces

In flotation, air bubbles are commonly used to selectively collect the hydrophobic particles suspended in water, with the collection efficiencies strongly affected by contact angles and the properties of wetting films. Knowing that oil drops form substantially larger contact angles than air bubbles on a hydrophobic surface, many investigators explored the possibility of using oil drops rather than air bubbles for flotation. In the present work, the surface forces present in the thin liquid films (TLFs) of water confined between n-alkanes of different chain lengths and thiol-coated gold surfaces have been measured to better understand the drop-surface interactions, which are controlled by surface forces. The force measurements are conducted by monitoring the changes of local curvatures as an oil drop approaches a flat surface slowly, while monitoring film thicknesses by microinterferometry, constructing spatiotemporal film profiles, and analyzing the profiles to derive disjoining pressure and free energy isotherms using the Frumkin-Derjaguin isotherm. The results show that drop-hydrophobic surface interactions are driven by the hydrophobic force, which was first measured by Israelachvili and Pashley in 1982, and that both short- and long-range hydrophobic forces increase with contact angle and decrease with the chain lengths of n-alkane oils. It has been found also that the hydrophobic forces measured with oil drops have longer decay lengths than those measured with air bubbles, which may provide a kinetic advantage over the method of using air bubbles for flotation.