Crude oil derivatives on sea water: Signatures of spreading dynamics

The dynamics of different liquid hydrocarbons (including crude oil and their derivatives) spreading over a deep layer of an original sea water surface is studied by video-enhanced microscopy and dynamic tensiometry in laboratory conditions. The water subphase had well-defined viscoelastic properties (derived from supplementary stress-relaxation Langmuir trough measurements) resulting from the presence of natural surfactants at sea. The latter method was not suitable for measuring the oil spreading rate at interfaces where surface active material layer of particular elastic properties is already present, because changes in surface tension then were caused by compression of the interfacial film rather than by the spreading of oil. Classical tension-gradient-driven spreading theory, developed for pure, nonvolatile, and immiscible liquid spreading on a second liquid predicts lens expansion rates that are higher by a factor of 6–9 than those experimentally observed for natural sea water. The fitting constants K (0.11–0.94) and power-law exponents n (0.10–0.79) of the spreading oil lens radius–time equation rL ~ K tn, however widely distributed, are apparently dependent on the dilational viscoelasticity of the sea water surface (and remaining air/oil and water/oil interfaces as well). The spreading data revealed the two spreading regimes: short-time diffusive (n = 1/2) and long-time convective (n = 3/4), limited by the network phase. An important observation is that oil viscosity and density have a minor effect on the rate of spreading. In addition, the obtained parameters together with surface properties of the natural surfactant-containing water body stand for principal input data required in modeling of surface tension gradient-driven oil spill spreading at sea.