  Separating petroleum hydrocarbons from water is an important problem to
address in order to mitigate the disastrous effects of hydrocarbons on aquatic
ecosystems. A rational approach to address the problem of marine oil water
separation is to disperse the oil with the aid of surfactants in order to
minimize the formation of large slicks at the water surface and to maximize the
oil-water interfacial area. Here we investigate the fundamental wetting and
transport behavior of such surfactant-stabilized droplets and the flow
conditions necessary to perform sieving and separation of these stabilized
emulsions. We show that, for water soluble surfactants, such droplets are
completely repelled by a range of materials (intrinsically underwater
superoleophobic) due to the detergency effect; therefore, there is no need for
surface micro/nanotexturing or chemical treatment to repel the oil and prevent
fouling of the filter. We then simulate and experimentally investigate the
effect of emulsion flow rate on the transport and impact behavior of such
droplets on rigid meshes to identify the minimum pore opening (w) necessary to
filter a droplet with a given diameter (d) in order to minimize the pressure
drop across the mesh and therefore maximize the filtering efficiency, which is
strongly dependent on w. We define a range of flow conditions and droplet sizes
where minimum droplet deformation is to be expected and therefore find that the
condition of is sufficient for efficient separation. With this new
understanding, we demonstrate the use of a commercially available
filter--without any additional surface engineering or functionalization--to
separate oil droplets from a surfactant stabilized emulsion with a flux of
11,000 L m$^{-2}$ hr$^{-1}$ bar$^{-1}$. We believe these findings can inform
the design of future oil separation materials.
