Understanding the coalescence and non-coalescence of underwater oil droplets

Oil collection requires effective methods to accelerate coalescence of oil in water. The coalescence behaviors of n-hexane, n-heptane, n-octane, dichloromethane and carbon tetrachloride droplets on oleophobic surfaces in water are investigated by experiments and simulations. Experimental results show that the n-hexane, n-heptane and n-octane droplets coalesce in water while the dichloromethane and carbon tetrachloride droplets do not coalesce. Theoretical calculations indicate that the critical thickness of water film between oil droplets to rupture in experiment is several nanometers, which is exactly in the size scale of molecular dynamics simulation. To investigate the mechanisms underlying the coalescence/non-coalescence, molecular dynamics simulations on the oil/water/oil interface are carried out. The simulation results indicate that the orientated oil molecules and increased number of hydrogen bonds in water are main reasons why oil/water/oil interface is stable. To undermine the stability, oleophilic surface in water is constructed, resulting in the coalescence of carbon tetrachloride droplets.