Once booms have concentrated oil in sufficiently thick layers on top of the water, mechanical methods such as skimming, separating and vacuuming can be mobilized where conditions are favorable. But because these techniques take place at the surface, they are subject to the same disruptions that applied to booms, particularly those posed by wind, waves and currents.
Skimmers are slow yet very effective machines used for surface removal in calm or sheltered waters and along shorelines. They work by taking advantage of the adhesive nature of the oil, which will cling to any surface that it comes into contact with. Using rotation, suction, gravity or other forces to drive motion, these machines: 1. provide a never-ending surface for the spilled petroleum to cling to, 2. clean the surface, and 3. repeat that process continuously. 
The primary types of oil skimmers include belt, brush/disk/drum, mop and floating suction.  Stainless steel belt oil skimmers are lowered into the contaminated water and then passed through special wiper blades, which remove the oil from both sides as it passes through. Brush/disk/drum skimmers float on the surface and are fitted with a brush, disk or drum that collects oil as it rotates. The oil is then scraped off and discharged from the skimmer. Rope mop skimmers consist of a drive unit and a length of oil-attracting rope. Oil sticks to the rope and is carried back to the drive unit for removal.
For very thick layers of oil, floating weir oil skimmers (designed with pivoting flaps that trap the oil while letting water recycle outward through a filter) are most effective. However, these work best when sitting on highly concentrated oil or they tend to take on large amounts of water with the oil.  These devices feature multiple floats and a central intake weir, which is adjustable to help accommodate oil layer thickness. 
Water can contain oil in three major forms: 1. free oil (droplets larger than 20 microns in diameter), 2. emulsified oil (droplets that have been mechanically or chemically reduced in size to smaller than 20 microns in diameter), and 3. dissolved oil (molecular-scale particles capable of dissolving in water). 
Because they take advantage of the differences in the specific gravities of water and oil to achieve separation, traditional gravity-type separators will not separate emulsified or dissolved oil from water. A gravity separator has a chamber designed to provide controlled flow conditions that help globules of free oil rise to the surface of the water and form a separate oil mass that can be removed mechanically.
The "rise rate" of oil globules (their vertical velocity) is the speed at which oil particles move toward the separator surface because of the difference in densities of oil and water. The closer the density of the oil or the grease is to that of water, the longer it takes for separation to occur.
High-tech centrifuge separators also leverage the density difference
between oil and water, but by using centripetal force to quickly
spin and separate the two fluids, they can "slurp up as much as
200 gallons of oil every minute."
Although centrifuge separators currently allow small amounts of
oil and water to cling together during the spin (especially in
water containing dissolved and emulsified oil from the heavy use
of dispersants), another invention could soup-up separators by
blasting these stubborn hangers-on with microwaves. Idaho National
Laboratory chemist David Meikrantz, on whose patented technology
Kevin Costner's Ocean Therapy Solutions based its separators,
says microwaves "add a little turbo boost" that can give centrifuge
separators higher throughput and better efficiency.
The video below demonstrates the separation principle for one of these commercially available vortex-driven separators.
When the primary fluid enters the separator, an impeller creates a cyclonic flow that forces the heavier water outside the stream, while the lighter oil is drawn inward to form a central core. At the exit of the separation chamber, the separated streams are collected and the primary (clean) fluid is released for discharge.
Mechanical collection by skimmers and separators cannot recover all the petroleum dumped in a major spill, a deepwater leak or a spill in rough waters where oil is rapidly churned and emulsified. These situations require chemical, microbial and other more experimental types of intervention such as airplane-deployed robots that cordon off the oil and use centrifuge separators to collect oil for refining,  non-toxic superabsorbent polymer powder that forms a sponge-like material when sprinkled on an oil slick and is easily removed from the surface,  new high-speed skimming vessels that not only work fast, but can handle the rough seas that hamper current mechanical methods, and products like Aerogel, a reusable sponge-like material (composed of 2 percent clay, 2 percent plastic and 96 percent air) that's capable of absorbing oil and leaving water behind. 
Go To Cleanup:
Chemicals and Microbes