Alternatively, heavier refined oil products may pose a lesser fire and toxic hazard and do not spread on water as readily. Heavier oils are more persistent, however, and may present a greater remediation challenge. The rate at which an oil spill spreads will determine its effect on the environment. Most oils tend to spread horizontally into a smooth and slippery surface, called a slick , on top of the water. Factors which affect the ability of an oil spill to spread include surface tension, specific gravity , and viscosity .
Surface tension is the measure of attraction between the surface molecules of a liquid.
The higher the oil's surface tension, the more likely a spill will remain in place. If the surface tension of the oil is low, the oil will spread even without help from wind and water currents. Because increased temperatures can reduce a liquid's surface tension, oil is more likely to spread in warmer waters than in very cold waters. Specific gravity is the density of a substance compared to the density of water. Since most oils are lighter than water, they lie flat on top of it.