Synergy of in-situ heating and wettability gradient enables highly efficient separation of oil-in-water emulsions.

Abstract

Surfactants form stable interfacial films on the surface of emulsified oil droplets, which makes the separation of oil-in-water emulsions extremely challenging. Herein, a superhydrophilic-hydrophobic stainless steel mesh (UWSOB-OL@SSM) was prepared by sequentially modifying a carbon nanotube electrothermal layer, a resin insulating layer, a superhydrophilic coating, and hydrophobic bumps onto mesh. The UWSOB-OL@SSM exhibits macroscopic superhydrophilicity/underwater superoleophobicity, with a microscale wettability gradient between hydrophobic bumps and the superhydrophilic coating. It also exhibits excellent in-situ Joule heating performance, with surface temperatures reaching 300 °C in 10 s at a voltage of 2.0 V. The synergistic effect of the wettability gradient and in-situ heating rapidly destabilizes the interfacial film of emulsified oil droplet, promoting their demulsification, coalescence, and flotation during oil/water separation. As a result, UWSOB-OL@SSM successfully separated various oil-in-water emulsions, including high-viscosity crude oil and motor oil. Its separation efficiency reached 99.0 % with in-situ heating, significantly higher than the 71.8 % efficiency of the traditional superhydrophilic/underwater superoleophobic mesh which lacks hydrophobic bumps and in-situ thermal effect. Moreover, the synergistic mechanisms of wettability gradient and in-situ heating to enhance demulsification and antifouling were revealed by observing the dynamic coalescence process of emulsified oil droplets. This synergistic mechanism provides new fundamental insights into separating highly emulsified oil-in-water emulsions.