Behaviour of floaters
Substances classified as ‘floaters’ have the greatest tendency to remain on the water surface. In addition, the rates of evaporation of the products designated as floater/evaporators and floater/dissolver/ evaporators are such that the possibility of a vapour or gas plume developing cannot be ruled out. Substances in the subclasses floater/evaporated/dissolver and floater/ dissolver are likely to diffuse/disperse in the water column.
On being released, substances that are classified, as floaters will tend to spread out on the water surface, where they are influenced by external factors such as the wind, waves and tidal currents. In addition to the effect of dispersal phenomena, products of this type will also be subjected to various natural degradation processes.
Movement
The movement of a floater on the water surface is determined by:
- Wind velocity
- Water current velocity
- Waves
- The Coriolis effect
Because of the tides, the water mass in the sea and in the estuaries is in constant motion the strength and direction of the enduing currents being time and place dependent. Over a complete tidal cycle the currents will approximately cancel each other out.
The wind blowing over the water surface will ‘drag’ the slick in the same direction as it does the surface layer of water the slick moving at about 3% of the wind speed in the same direction.
Though the wave train moves in the direction of the wind it has no effect on the transportation of the slick the physical wave motion being vertical. Thus, while have waves are a result of the wind; it is the wind shear on the floating layer, which causes the lateral transfer of the slick.
When a slick remains on the water for several days the effect of the Coriolis force may become obvious as a deflection from the wind direction caused by the earth’s rotation.
Spreading
Spreading results in ever decreasing layer thicknesses.
It occurs in 3 phases:
- Spreading under the influence of gravity as modified by inertia
- Spreading under influence of gravity as modified by fluid viscosity
- Spreading as modified by viscosity and surface tension.
Phase 1 start immediately after release of the substance, when the slick is the thickest. Once a certain smaller thickness has been reached phase 2 takes over as inertia becomes a less important factor and viscosity becomes more of influential After reaching even smaller thickness gravity the surface tension of both fluids start to play its part with the onset of phase 3. Meanwhile, the enhancement of surface area promotes the evaporation, droplet dispersion and solution, which ultimately remove the slick from the sea surface to ever-decreasing concentrations in the air and water column.
The following table shows some spreading coefficients.
Table 17 examples of spreading coefficients
| Name | Surface tension | Interfacial tension | Spreading Coefficient |
| Cyclohexane | 0,0246 | 0,05101 | - 0,00233 |
| Hexane | 0,0184 | 0,0511 | + 0,0035 |
| Toluene | 0,029 | 0,0357 | + 0,00803 |
| Xylenes | 0,0312 | 0,0352 | + 0,0052 |
| Nonane | 0,0229 | 0,035 | + 0,01955 |
| Fish oil | 0,038 | 0,05 | - 0,015 |
| Crude | 0,0343 | 0,011 | + 0,0227 |
| Rape seed oil | 0,035 | 0,0172 | + 0,0208 |
| Vegetable oil | 0,025 | 0,01624 | + 0,03176 |
| Benzene | 0,0289 | 0,035 | + 0,0091 |
| Aniline | 0,0455 | 0,006 | + 0,0215 |
Meanwhile, the enhancement of surface area promotes the evaporation, droplet dispersion and solution, which ultimately remove the slick from the sea surface to ever-decreasing concentrations in the air and water column.
Meanwhile, the enhancement of surface area promotes the evaporation, droplet dispersion and solution, which ultimately remove the slick from the sea surface to ever-decreasing concentrations in the air and water column. shows some spreading coefficients.
Evaporation
Volatile floaters gradually evaporate with increasing surface having the major role while ambient temperature and wind speed have no appreciable effect.
Important influence factors in the evaporation of a substance are:
- Surface area
- Temperature
- Wind speed
Dissolving/Dispersing
Floating liquids that are soluble or disperse as droplets in water slowly move from the slick into the water phase. The large contact area and the small layer thickness due to the spreading of the slick, enhance these processes.
Factors that influence the solution process are
- Contact area
- Solubility
- Sea state which provides the energy for droplet formation
The solution process occurs only at the contact area between the water column and the slick, as does droplet formation. This means that a larger contact area will result in faster solution/dispersion processes.
The solubility of a substance determines at which concentration equilibrium between both phases is reached, though this only applies when the available water is limited. It does not apply to the cean.
Increasing sea state may influence the rate of solution for realtively insoluble substances by ‘refreshing shallow water, though it certainly increases the energy supply for droplet formation.
