During the Deepwater Horizon oil spill, the unprecedented injection of millions of liters of chemical dispersant at the wellhead generated large quantities of submillimeter oil droplets that became entrained in a deep sea plume. The unexpected generation of these droplets has resulted in many studies in the last decade aiming to understand their transport and fate during and after the spill. Complicating matters, the plume coincided with a microbial bloom, and in addition to ocean dynamics these droplets were subjected to biological processes such as biodegradation and microbial aggregation. A lack of field observations and laboratory experiments using relevant conditions has left our understanding of these biotic processes and the role they played in the fate of the oil droplets poorly constrained. Furthermore, while biodegradation has been incorporated into drop transport models using available data, the effects of microbial aggregation involving extracellular polymeric substances (EPS) on their transport has seldom been incorporated into modeling efforts particularly due to our lack knowledge of these processes. We use a microfluidic platform to observe bacterial suspensions interacting with a single ~200 μm oil drop in conditions relevant to the drop rising through the microbial bloom. We observe the development of individual, invisible bacterial EPS threads extending from the drop surface which can capture additional passing bacteria and form bacteria-EPS aggregates. Using high speed imaging, we make high resolution flow measurements both with and without EPS threads present and analyze the momentum balance to elucidate the hydrodynamic impact of these filaments. Surprisingly, these thin individual EPS filaments alter significantly the pressure field around the drop and increase the drag, which would drastically reduce the drop's rising velocity in the water column. We demonstrate that this mechanism which plausibly occurred in the deep sea plume would have major impacts on both the drop and bacteria transport during and after the Deepwater Horizon oil spill.

The association between phytoplankton blooms and oil spills is still controversial despite numerous studies. Surprisingly, to date, there have been no studies on the effect of bacterial communities (BCs) exposed to crude oil on phytoplankton growth, even though crude oil changes BCs, which can then affect phytoplankton growth and species composition. Co-culture with crude oil-exposed BCs significantly stimulated the growth of Prorocentrum texanum in the laboratory. To gain more direct evidence, oil-degrading bacteria from oil-contaminated sediment collected after the Texas City “Y” oil spill were isolated, and changes in dinoflagellate growth when co-cultured with single bacterial isolates was investigated. The oil-degrading bacterial isolates significantly stimulated the growth of dinoflagellates (axenic and xenic cultures) through releasing growth-promoting substances. This study provides new evidence for the potential role of oil-degrading bacteria in the formation of phytoplankton blooms after an oil spill.

Bacteria are important examples of active or self-propelled colloids. Because of their directed motion, they accumulate near interfaces. There, they can become trapped and swim adjacent to the interface via hydrodynamic interactions, or they can adsorb directly and swim in an adhered state with complex trajectories that differ from those in bulk in both form and spatiotemporal implications. We have adopted the monotrichous bacterium Pseudomonas aeruginosa PA01 as a model species and have studied its motion at oil–aqueous interfaces. We have identified conditions in which bacteria swim persistently without restructuring the interface, allowing detailed and prolonged study of their motion. In addition to characterizing the ensemble behavior of the bacteria, we have observed a gallery of distinct trajectories of individual swimmers on and near fluid interfaces. We attribute these diverse swimming behaviors to differing trapped states for the bacteria in the fluid interface. These trajectory types include Brownian diffusive paths for passive adsorbed bacteria, curvilinear trajectories including curly paths with radii of curvature larger than the cell body length, and rapid pirouette motions with radii of curvature comparable to the cell body length. Finally, we see interfacial visitors that come and go from the interfacial plane. We characterize these individual swimmer motions. This work may impact nutrient cycles for bacteria on or near interfaces in nature. This work will also have implications in microrobotics, as active colloids in general and bacteria in particular are used to carry cargo in this burgeoning field. Finally, these results have implications in engineering of active surfaces that exploit interfacially trapped self-propelled colloids.

Inhalation of PM2.5, particles with an aerodynamic diameter <2.5 mm, from sea spray after crude oil spills could present serious health concerns. The addition of dispersants to effectively spread the crude oil throughout the water column has been practiced in recent years. Here, we investigated the possibility of an increase in the toxic content of fine PM after adding dispersant. A laboratory setup consisted of a vertical tank filled with seawater, 31.5 L airspace for aerosol sampling, and a bubble generating nozzle that aerosolized the oily droplets. Four different cases were studied: no slick, 0.5-mm-thick slick of pure crude oil (MC252 surrogate), dispersant (Corexit 9500A) mixed with crude oil at dispersant to oil ratio (DOR) 1:25, and DOR 1:100. The resulting airborne droplets were sampled for gravimetric and chemical analyses through development of a gas chromatography and mass spectrometry technique. Also, PM2.5 particles were size-fractioned into 13 size bins covering <60 nm to 12.1 mm using a low-pressure cascade impactor.
The highest PM2.5 concentration (20.83 ± 5.21 mg/m3) was released from a slick of DOR 1:25, 8.83x greater than the case with pure crude oil. The average ratio of crude oil content from the slick of DOR 1:25 to the case with pure crude oil was 2.37 (1.83 vs 0.77 mg/m3) that decreased to 1.17 (0.90 vs 0.77 mg/m3) at DOR 1:100. For particles <220 nm, the resultant crude oil concentrations were 0.64 and 0.29 mg/m3 at DOR 1:25 and 1:100, both higher than 0.11 mg/m3 from the slick of pure crude oil.

In this work, a large-eddy simulation of bubble plumes in linearly stratified environments is presented. The gas bubbles are treated as Lagrangian particles. The intrusion and peeling are clearly manifested in the computed flow fields. The results of about 50 simulations with different parameters reveal the importance of bubble source area for plumes on the laboratory scale. A new type of bubble plume with rapid and distinct peelings is observed which is favored by large source areas. With a proper normalization, the present data points collapse onto a single straight line after applying a virtual-source correction which reflects the source-area effect. These results provide a plausible explanation for the scatter of the previous experimental and computational data in literature. A simple relation between the trap height and the peel height is observed and its mechanism is discussed

Understanding of oil-water interfacial phenomena is essential for predicting mixing and/or phase-separation in environmental and industrial systems. Time-resolved digital holography and planar laser-induced fluorescence are used for examining processes occurring after ascending buoyant oil droplets of varying viscosity cross a stratified oil-water interface. Previous studies have focused on the crossing process, and the current belief is that once this droplet becomes immersed in the oil, its content can mix with the bulk fluid. In contrast, we show that the droplets remain encapsulated by a stable continuous submicron thin water film, which prevent them from mixing. This film forms even in pure oil and water with minimal surfactant concentration and persists for periods that are three to four orders of magnitude longer than those of the crossing process. Observations following the film evolution reveal that segments located close to the interface appear to be attracted to the bulk water, causing the entire droplet to flatten slowly. The resulting reduction in the peripheral radius of curvature
eventually breaks up the film into suspended submicron droplets. The morphology of this flattening process varies with oil viscosity, and its duration increases from seconds to nearly one hour as the oil viscosity increases from one to fifty cSt. In processes involving multiple oil droplets crossing the interface, they form a separate persistent long-lasting layer containing a complex thin-film structure that does not mix with the bulk oil. In contrast, thin oil films do not form around a descending water droplet after it crosses the interface.

Phytoplankton blooms have been occasionally observed to occur after oil spills, and changes in bacterial communities (BC) associated with phytoplankton are known to affect phytoplankton growth. In the present study, to examine the effects of BC exposed to crude oil on phytoplankton blooms, established free-living (FL) BC in Karenia brevis (Dinophyceae) culture were collected and then exposed to crude oil under light or dark conditions. These exposed FLBC were then added to K. brevis culture to investigate the effects on growth rate of this dinoflagellate. Enhanced growth of K. brevis was observed following addition of FL (24.7%) BC exposed to crude oil and light. Whereas BC grown with crude oil in the dark did not enhance growth, and BC without treatment showed a slight growth inhibition (13–15%) of K. brevis. In addition, the growth-promoting effect had a positive correlation with the inoculated bacterial density; the treatment with a higher (,1.5 times) density of FL (42.9%) BC that were exposed to crude oil and light showed an increase in the growth-promoting effect. Taken together, BC exposed to crude oil and light may play an important role in enhancement of K. brevis growth.

Competing time scales involved in rapid rising micro-droplets in comparison to substantially slower biodegradation processes at oil-water interfaces highlights a perplexing question: how do biotic processes occur and alter the fates of oil micro-droplets (<500 μm) in the 400 m thick Deepwater Horizon deep-sea plume? For instance, a 200 μm droplet traverses the plume in ~48 h, while known biodegradation processes require weeks to complete. Using a microfluidic platform allowing microcosm observations of a droplet passing through a bacterial suspension at ecologically relevant length and time scales, we discover that within minutes bacteria attach onto an oil droplet and extrude polymeric streamers that rapidly bundle into an elongated aggregate, drastically increasing drag that consequently slows droplet rising velocity. Results provide a key mechanism bridging competing scales and establish a potential pathway to biodegradation and sedimentations as well as substantially alter physical transport of droplets during a deep-sea oil spill with dispersant.

Functionalization of a surface with biomimetic nano-/micro-scale roughness (wires) has attracted significant interests in surface science and engineering as well as has inspired many real-world applications including anti-fouling and superhydrophobic surfaces. Although methods relying on lithography include soft-lithography greatly increase our abilities in structuring hard surfaces with engineered nano-/micro-topologies mimicking real-world counterparts, such as lotus leaves, rose petals, and gecko toe pads, scalable tools enabling us to pattern polymeric substrates with the same structures are largely absent in literature. Here we present a robust and simple technique combining anodic aluminum oxide (AAO) templating and vacuum-assisted molding to fabricate nanowires over polymeric substrates. We have demonstrated the ecacy and robustness of the technique by successfully fabricating nanowires with large aspect ratios (>25) using several common soft materials including both cross-linking polymers and thermal plastics. Furthermore, a model is also developed to determine the length and molding time based on nanowires material properties (e.g., viscosity and interfacial tension) and operational parameters (e.g., pressure, vacuum, and AAO template dimension). Applying the technique, we have further demonstrated the confinement e ects on polymeric crosslinking processes and shown substantial lengthening of the curing time.

This paper presents the results of a scaling study of bubble and drop plumes in a stratified ambient. Use is made of a standard integral model of the top-hat type, which can be reduced to one of the Gaussian type by a simple transformation. The focus of the work is on the effects of the dissolving material on the plume dynamics. It is pointed out that, for a drop plume, the loss of buoyancy due to dissolution can be compensated by a lightening of the ambient liquid associated with the dissolved drop material, or even aggravated if the density of the solution is greater than that of the undissolved drops. For bubbles, these effects are compounded by the volume expansion due to the falling hydrostatic pressure. This process is not important in deep water, where the peel height is smaller than the water depth, but can be significant in shallow water, where the two may be comparable. With a focus on the analysis of a point-source, three important parameters are identified. The first one compares the drop/bubble dissolution rate with the rise time to the neutral height (the level at which the plume density equals the ambient density), the second one accounts for the effect of the dissolved material on the liquid density, and the third one is the drop/bubble rise velocity compared with the characteristic plume velocity.

Oil spills are one of the most dangerous sources of pollution in aquatic ecosystems. Owing to their pivotal position in the food web, pelagic copepods can provide crucial intermediary transferring oil between trophic levels. In this study we show that the calanoid Paracartia granican actively modify the size-spectrum of oil droplets. Direct manipulation through the movement of the feeding appendages and egestion work in concert, splitting larger droplets (Ø = 16 µm) into smaller ones (Ø = 4–8 µm). The copepod-driven change in droplet size distribution can increase the availability of oil droplets to organisms feeding on smaller particles, sustaining the transfer of petrochemical compounds among different compartments. These results raise the curtain on complex small-scale interactions which can promote the understanding of oil spills fate in aquatic ecosystems.

Airborne toxic compounds emitted from polluted seawater polluted after an oil spill raise health concerns when inhaled by humans or other species. Inhalation of these toxic compounds as volatile organic compounds (VOCs) or airborne fine particulate matter (PM) may cause serious pulmonary diseases, including lung cancer. Spraying chemical dispersants to enhance distribution of the crude oil into the water was employed extensively during the Deepwater Horizon spill. There is some evidence that dispersion of the crude oil decreased the emission rate of the VOCs but increased the emission rates of fine PM that may carry toxic compounds. In this study, the cancer risks and non-cancer hazards of the detected VOCs and particulates for spill-response workers were estimated with and without use of dispersant under action of breaking waves. A subchronic exposure scenario was modeled to address the inhalation health threat during initial phases of an oil spill response. A dosimetry model was used to estimate regional deposition of PM. Use of dispersant reduced benzene cancer risks from 57 to 37 excess life- time cancer cases per million for 1 h of daily exposure that continues for 3 months. Adding dispersant resulted in emissions reductions of the lighter VOCs (up to 30% lower). However, hazard quotients (HQs) of the non- carcinogenic VOCs even after dispersant addition were above 1 meaning there are serious concerns about expo- sure to these VOCs. Inhalation of airborne particles emitted from the slick containing dispersant increased the total mass of deposited particles in upper respiratory regions compared to the slick of crude oil only. This study showed the application of dispersant onto the pollution slick increased the total mass burden to the human respiratory system about 10 times, an exploratory HQ analysis is presented to evaluate the potential health risk.

 

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.

Field data from the first several days after an oil spill is rare but crucial for our understanding of a spill’s impact on marine microbiota given their short generation times. Field data collected within days of the Texas City “Y” oil spill showed that exposure to crude oil can rapidly imbalance populations of marine microbiota, which leads to the proliferation of more resistant organisms. Vibrionales bacteria were up to 48 times higher than background concentrations at the most impacted sites and populations of the dinoflagellate Prorocentrum texanum increased significantly as well. Laboratory microcosm experiments with a natural plankton community showed that P. texanum grew significantly faster under oiled conditions but monocultures of P. texanum did not. Additional laboratory experiments with natural communities from Tampa Bay, Florida showed similar results although a different species dominated, P. minimum. In both cases, tolerance to the presence of crude oil was enhanced by higher sensitivity of grazers led to a release from grazing pressure and allows Prorocentrum species to dominate after an oil spill. The results suggest careful monitoring for Vibrionales and Prorocentrum during future spills would be beneficial given the potential implications to human health.

Crude oil spill incidents occur frequently causing a verity of occupational, ecological and environmental problems. Dispersants are applied to enhance the dispersion rate of crude oil slicks into the water column. In this study, the aerosol size distribution from 10 nm to 20 μm, total particle-bound aromatic hydrocarbons (pPAH) and volatile organic compounds (VOCs) are measured in a 6 x 0.3 x 0.6m tank as plunging breaking waves entrain oil slicks. The experiments are performed for seawater with slicks of crude oil, crude oil-dispersant mixture and dispersant only. The measurements investigate the effects of wave energy and slick properties on the temporal evolution of the emissions. The total number concentrations of particles originating from the oil dispersant
mixture are 1–2 orders of magnitude higher than those of crude oil across the entire nano-scale range, reaching 100x for 20 nm particles. Conversely, the differences in concentration are small in the micron range. The average concentrations of pPAH are variable but similar (150–270 ng/m3). The VOC concentrations for crude oil-dispersant mixtures are 2–3 times lower than those of crude oil, presumably due to the surfactant effect on mass diffusion. The drastic increase in ultrafine particle concentrations may raise concerns about effects of inhalation by cleanup workers and downstream communities though VOC emissions reduce. Findings through this study provide insight into how the spray of dispersant may change the ratio of airborne particulate matter and VOC emissions from seawater due to natural processes.