Reports: DNI551512-DNI5: Hygro-Responsive Surfaces: A New Approach for Oil-Water Emulsion Separation

Anish Tuteja, PhD, University of Michigan

We published one manuscript in 2012 based on our support from ACS-PRF, as listed below. A 2nd manuscript is currently in press.

The first manuscript published based on our work was:

On-demand separation of oil-water mixtures”, Gibum Kwon, Arun K. Kota, Yongxin Li, Ameya Sohani, Joseph M. Mabry, and Anish Tuteja#, Advanced Materials, 2012, 24 (27), 3666-3671. (Impact Factor: 13.877).

Overview of the work:

In this work, we developed the first-ever membrane-based unit operation that allows for the electric field driven, on-demand separation of various oil-water mixtures.

There is an acute need for the development of new solutions to separate oil-water mixtures, especially in the presence of surfactants, as both the production of oil and oil-transport engender a severe environmental risk in sensitive ecosystems. Several recent events, including the Deepwater Horizon oil-spill disaster off the coast of Louisiana and its aftermath, have highlighted this risk. In particular, the merits of the decision to spray 1.1 million gallons of chemical dispersants (or surfactants) after the oil-spill have been widely debated. Allowing the oil to naturally rise up to the ocean surface, rather than dispersing it within the water, would have facilitated easier oil-collection, as conventional gravity separators and skimming techniques are unable to separate surfactant-stabilized oil-water mixtures or emulsions.

In addition to improved oil-spill response, novel technologies for oil-water separation are expected to impact a wide range of other commercial technologies, including waste-water treatment, oil-drilling and extraction, fuel-purification, as well as, the separation of a range of commercially significant emulsions in a variety of industries including food and beverages, petroleum, pharmaceuticals and cosmetics.

Membranes are widely used for emulsion separation. However, traditional membrane-based separation technologies are energy-intensive and further limited by the inability of a single membrane to separate all types of oil-water mixtures such as free oil and water, oil-in-water emulsions, water-in-oil emulsions, or any combination of these phases.

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 a 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 utilized this preferential wettability transition on oleophobic (or superoleophobic) membranes, to develop a single unit operation that, for the first time, enables the on-demand separation of free oil and water, oil-in-water emulsions and water-in-oil emulsions, with ≥ 99.9% separation efficiency. The ease of scalability of the developed apparatus allows us to separate relatively large volumes of oil-water emulsions(> 1 Gallon). We also engineered the first-ever continuous oil-water emulsion separation apparatus that can be triggered on-demand. This separation apparatus has a separation efficiency ≥ 99.9% and removes at least 99% of the emulsified droplets.

Our separation methodology is expected to have a significant impact on a wide range of commercial applications including clean-up of oil-spills, waste-water treatment, separation of numerous commercially-relevant oil-water emulsions, fuel-purification, and microfluidic valves for lab-on-a-chip type of applications.

The second publication based on our work is currently in press, and is listed below:

“Superomniphobic Surfaces for Effective Chemical Shielding”, Shuaijun Pan, Arun K. Kota, Joseph M. Mabry, and Anish Tuteja#, Journal of the American Chemical Society, In Press. (Impact Factor: 9.907)

 Overview of the work:

Superhydrophobic surfaces display apparent contact angles greater than 150° and low contact angle hysteresis (the difference between the advancing and receding contact angles) with water. Superoleophobic surfaces display apparent contact angles greater than 150° and low contact angle hysteresis with low surface tension liquids. Superomniphobic surfaces display both superhydrophobicity and superoleophobicity. While surfaces that display superomniphobicity with certain Newtonian liquids have been previously engineered, there were no previous reports of surfaces that can repel a wide variety of Newtonian liquids including organic and inorganic concentrated acids, concentrated bases and solvents. Furthermore, there were no reports of surfaces that display superomniphobicity with non-Newtonian liquids.

In this work, we fabricated surfaces that, for the first time, display superomniphobicity not only with a wide variety of Newtonian liquids, but also with non-Newtonian liquids. Our surfaces were synthesized by electrospinning beads of a poly(dimethylsiloxane) and fluorodecyl polyhedral oligomeric silsesquioxane  blends onto wire meshes. The resulting hierarchical structure with re-entrant texture and low surface energy renders our surfaces superomniphobic to a wide variety of Newtonian and non-Newtonian liquids.

We demonstrated the superomniphobicity of our surfaces with 35 different Newtonian liquids and 25 different non-Newtonian liquids. Newtonian liquids including concentrated inorganic chemicals (e.g., sulfuric acid, nitric acid, phosphoric acid, hydrofluoric acid, hydrogen peroxide, potassium permanganate, potassium hydroxide and ammonium hydroxide) and organic chemicals (e.g., dimethylformamide, toluene, chloroform, acetic acid, tetrahydrofuran, acetone, methanol, ethanol and hexylamine) can easily bounce and roll-off from our surfaces at tilt angles of 2° or less. More significantly, non-Newtonian liquids including viscoelastic polymer solutions (e.g., PMMA, PS, PVP and PVDF in solvents such as acetic acid, dimethylformamide, methylpyrollidone and toluene) of different polymer molecular weights (35,000 Da to 5,000,000 Da) and concentrations (1 mg mL-1 to 100 mg mL-1) can also easily bounce and roll-off from our surfaces. Such effective chemical repellency against virtually all contacting liquids – organic or inorganic, acids or bases or solvents, polar or non-polar, Newtonian or non-Newtonian – has never been reported before. Consequently, our surfaces serve as effective chemical shields against virtually all liquids.

Overall, we envision that our surfaces will have numerous applications including stain-free clothing, spill-resistant breathable protective wear, bio-fouling resistant surfaces, self-cleaning, drag reduction, and light-weight corrosion-resistant coatings.