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Using cinematic digital holography, we demonstrate that turbulent breakup of crude oil mixed with dispersants into microdroplets starts with the formation of very long and quite stable, single or multiple microthreads that trail behind 300-1400 microm droplets. These threads extend from Reynolds number dependent regions with high surfactant concentration, which, along with associated viscosities and stretching by turbulence stabilizes the threads. The subsequent breakup, producing 2.8 microm droplets, is due to an increasing surface area and diffusion of dispersants into the continuous phase.

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High-speed inline digital holographic cinematography is used for studying turbulent diffusion of slightly buoyant 0.5-1.2 mm diameter diesel droplets and 50 m diameter neutral density particles. Experiments are performed in a 50ϫ 50ϫ 70 mm 3 sample volume in a controlled, nearly isotropic turbulence facility, which is characterized by two dimensional particle image velocimetry. An automated tracking program has been used for measuring velocity time history of more than 17 000 droplets and 15 000 particles. For most of the present conditions, rms values of horizontal droplet velocity exceed those of the fluid. The rms values of droplet vertical velocity are higher than those of the fluid only for the highest turbulence level. The turbulent diffusion coefficient is calculated by integration of the ensemble-averaged Lagrangian velocity autocovariance. Trends of the asymptotic droplet diffusion coefficient are examined by noting that it can be viewed as a product of a mean square velocity and a diffusion time scale. To compare the effects of turbulence and buoyancy, the turbulence intensity ͑u i Ј͒ is scaled by the droplet quiescent rise velocity ͑U q ͒. The droplet diffusion coefficients in horizontal and vertical directions are lower than those of the fluid at low normalized turbulence intensity, but exceed it with increasing normalized turbulence intensity. For most of the present conditions the droplet horizontal diffusion coefficient is higher than the vertical diffusion coefficient, consistent with trends of the droplet velocity fluctuations and in contrast to the trends of the diffusion timescales. The droplet diffusion coefficients scaled by the product of turbulence intensity and an integral length scale are a monotonically increasing function of u i Ј/ U q .