Electric field driven on demand separation of oil-water mixtures

A. Tuteja, G. Kwon, A.K. Kota, J.M. Mabry
University of Michigan-Ann Arbor, US

Summary:

There is an acute need for the development of new solutions to separate oil-water mixtures as both the production of oil and oil-transport engender a severe environmental risk in sensitive ecosystems. In many ways 2010 was a banner year highlighting this risk, as evidenced by the oil-spill disaster off the coast of Louisiana, the Chinese tanker that ruptured on the Great Barrier Reef in the Indian Ocean and the Deep Horizon Gulf Rig that exploded and sank. Mixtures of oil and water are classified based on the size of oil droplet (doil) – free oil if doil > 150 μm, dispersed oil if 20 μm < doil < 150 μm and emulsified oil if doil < 20 μm. When a liquid contacts a textured substrate, it can assume either the Cassie-Baxter state or the Wenzel state. In the Cassie-Baxter state, air is trapped between the liquid and the solid forming a composite (liquid-air-solid) interface. In the Wenzel state, liquid fills all the cavities present on a textured surface leading to fully-wetted surface. Recent Electrowetting on a Dielectric (EWOD) experiments on textured substrates reveal that polar liquids can transition from the Cassie-Baxter state to the Wenzel state in response to an applied electric field. However, non-polar liquids on textured substrates do not undergo such a transition. In this work, we utilize this preferential wettability transition of polar liquids (e.g., water) on porous membranes to separate them from non-polar liquids (e.g., oils). For effective on demand separation of oil-water mixtures, the membranes must be designed in a manner that they support both water and oil in the Cassie-Baxter state before the electric field is applied. While it is easy to support water in the Cassie-Baxter state because of its relatively high surface tension, it is significantly more difficult to do so with oils because of their low surface tension. In our recent work, we have utilized surface chemistry in conjunction with re-entrant texture geometry to develop superoleophobic membranes that can support oils in the Cassie-Baxter state. Utilizing such membranes, in this work, we are able to achieve the on-demand separation of various oil-water mixtures such as free oil and water, oil-in-water emulsions and water- in-oil emulsions, with > 99.9% separation efficiency. In addition, we have also developed an apparatus for the continuous separation of oil-water emulsions that can be triggered on demand.