This study investigates the chaotic behavior of breaking waves by laboratory experiments and numerical modeling. Repeated laboratory runs with different initial velocity perturbations show that the wave profile before the wave breaks can be accurately reproduced, but the subsequent breaking process varies among runs, indicating the lack of repeatability of breaking waves in reality. Numerical simulations based on the Smoothed Particle Hydrodynamics method are further carried out to examine the repeatability of wave breaking process. Consistent with the laboratory observation, multiple numerical simulations with variations in initial conditions present highly repeatable velocity field and free surface profile in the potential flow region but considerable variation at the breaking and post-breaking processes. Comparison also shows that 3D vortex structures induced by breaking waves are different among cases. Analysis of particle trajectory reveals that there is a similar trajectory thus a minor trajectory divergence among particles that are initially located at the pre-breaking region and the flume bottom, which are not directly impacted by the breaking process. However, a much more significant particle trajectory divergence is observed among particles that are initially located at the wave-splash region and the bore propagation region. The rate of divergence of particle trajectory under breaking waves is further examined by computing the Lyapunov exponent, a widely used indicator of chaos. This study reveals that different initial velocity perturbations lead to variations of near-surface velocity at the onset of wave breaking, which eventually cause the development of drastically different breaking wave jets and splashes. Therefore, the process of wave breaking, like many other dynamic processes in nature, exhibits a chaotic behavior.
The displacements of ensembles of colloids at the interface between oil and suspensions of the bacterium Pseudomonas aeruginosa PA14DpelA indicate enhanced colloid mobilities and apparently diffusive motion driven by interactions with the bacteria. However, inspection of individual trajectories of B500 particles reveals prolonged, directed displacements inconsistent with purely hydrodynamic interactions between swimming bacteria and colloids. Analysis of the properties of colloid paths indicates trajectories can be sorted into four distinct categories, including diffusive, persistent, curly, and mixed trajectory types. Non-diffusive trajectories are the norm, comprising 2/3 of the observed trajectories. Imaging of colloids in the interface reveals anisotropic assemblies formed by colloids decorated with one or more adhered bacteria that drive the colloids along these paths. The trajectories and enhanced transport result from individual colloids being moved as cargo by these adhered bacteria. The implications of these structures and open questions for interfacial transport are discussed and related to the active colloid literature.
In response to the Deepwater Horizon oil spill, critical research has tracked the changes in petroleum hydrocarbons with environmental weathering. There are limitations, however, whereby single analytical techniques cannot always identify the wide breadth of petroleum and petroleum-derived compounds. We explore the analytical capabilities of ramped pyrolysis-gas chromatography–mass spectrometry (Py-GC–MS) to evaluate environmental samples of petroleum hydrocarbons from the Deepwater Horizon oil spill. We show that bulk flow Py-GC–MS can quantify the overall degree of petroleum hydrocarbon weathering. Furthermore, thermal slicing Py-GC–MS can quantify specific compounds in the “thermal desorption zone” (50–370 °C), as well as characterize pyrolyzed fragments from non-GC-amenable petroleum hydrocarbons (including oxygenated hydrocarbons) in the “cracking zone” (370–650 °C). Our data also suggest an increase in thermochemical stability, concentration of oxygenated products and complexity of high molecular weight and/or polar components with advanced weathering. This analysis not only elucidates weathering trends in Deepwater Horizon oil over several years, but also illustrates the analytical capacity of this method for future petroleum hydrocarbon investigations, filling a void in research connecting Py-GC–MS and environmentally weathered oil samples.
ABSTRACT: After oil spills and dispersant applications the formation of red tides or harmful algal blooms (HABs) has been observed, which can cause additional negative impacts in areas affected by oil spills. However, the link between oil spills and HABs is still unknown. Here, we present experimental evidence that demonstrates a connection between oil spills and HABs. We determined the effects of oil, dispersant-treated oil, and dispersant alone on the structure of natural plankton assemblages in the Northern Gulf of Mexico. In coastal waters, large tintinnids and oligotrich ciliates, major grazers of phytoplankton, were negatively affected by the exposure to oil and dispersant, whereas bloom-forming dinoflagellates (Prorocentrum texanum, P. triestinum, and Scrippsiella trochoidea) notably increased their concentration. The removal of key grazers due to oil and dispersant disrupts the predator−prey controls (“top-down controls”) that normally function in plankton food webs. This disruption of grazing pressure opens a “loophole” that allows certain dinoflagellates with higher tolerance to oil and dispersants than their grazers to grow and form blooms when there are no growth limiting factors (e.g., nutrients). Therefore, oil spills and dispersants can act as disrupters of predator−prey controls in plankton food webs and as indirect inducers of potentially harmful dinoflagellate blooms.
The instrument LISST (laser in situ scattering and transmissiometry) has been widely used for measuring the size of oil droplets in relation to oil spills and sediment particles. Major concerns associated with using the instrument include the impact of high concentrations and/or out-of-range particle (droplet) sizes on the LISST reading. These were evaluated experimentally in this study using monosized microsphere particles. The key findings include: (1) When high particle concentration reduced the optical transmission (OT) to below 30%, the measured peak value tended to underestimate the true peak value, and the accuracy of the LISST decreased by ~8% to ~28%. The maximum concentration to reach the 30% OT was about 50% of the theoretical values, suggesting a lower concentration level should be considered during the instrument deployment. (2) The out-of-range sizes of particles affected the LISST measurements when the sizes were close to the LISST measurement range. Fine below-range sizes primarily affected the data in the lowest two bins of the LISST with >75% of the volume at the smallest bin. Large out-of-range particles affected the sizes of the largest 8–10 bins only when very high concentration was present. The out-of-range particles slightly changed the size distribution of the in-range particles, but their concentration was conserved. An approach to interpret and quantify the effects of the out-of-range particles on the LISST measurement was proposed.