Polymers for Miscible Liquids Separation

Separating liquid mixtures, either miscible or immiscible, is a critical step in a wide range of practical processes that include wastewater remediation, biofuel production, water purification, and removal of contaminants from groundwater. Conventional separation methods such as thermal distillation and liquid-liquid extraction are often limited either by their inability to separate an azeotrope, their high energy consumption or by the need to add an extractant which must be removed after separation occurs. 

We have recently developed a polymer absorbent that can separate a miscible polar-non-polar azeotrope by selectively absorbing a polar liquid (ethanol) while repelling a non-polar one (n-heptane) at the ambient conditions. This is in striking contrast to the conventional wisdom regarding absorbents. First, traditionally, absorbents have been thought to be unable to separate miscible liquids and have only been used to separate immiscible liquids such as oil and water. Secondly, conventional absorbents typically absorb a low surface tension liquid (oil) while repelling a high surface tension liquid (water). Lastly, to our knowledge, an azeotrope has never been separated by an absorbent without altering its chemical composition or operating conditions. Therefore, we anticipate that our absorbent will have a potential to substitute energy-intensive and complicated thermal distillation or liquid-liquid extraction that are currently in use. [See Also]

Self-Healable Surfaces with Superomniphobic Wettability

A superomniphobic surface that can repel liquids with both high (e.g., water) and low (e.g., oil) surface tension has demonstrated potential for a wide range of practical applications including oil-water separation, self-cleaning, drag reduction, and corrosion resistance due to its extreme liquid repellency. Designing such a superomniphobic surface involves a low solid surface energy coating along with a re-entrant surface texture (i.e., convex or overhang topography) which enables a composite solid-liquid-air interface even with a low surface tension liquid such as an oil. The composite interface can be further reinforced by hierarchical surface topography. However, such a re-entrant surface texture with a hierarchical topography often results in poor mechanical durability.

In a new vein, superomniphobic surfaces that can repair physical damage and restore their inherent physicochemical properties are desirable. Recently, we have developed a water- responsive self-repairing superomniphobic surface by utilizing cross-linked hydroxypropyl cellulose (HPC) composited with SiO2 nanoparticles that are treated with a low-surface-energy perfluorosilane. The surface demonstrates that it can repair a deep scratch upon exposure to water vapor for ≈10 s and restore its inherent superomniphobic wettability and mechanical hardness. This can be attributed to the reversible hydrogen bonds between the free hydroxyl groups of the HPC-SiO2 which can be readily dissociated upon exposure to water vapor. Consequently, the HPC-SiO2 film acquires sufficient mobility and demonstrates a viscous flow into a scratch resulting in repairing the damage. The surface demonstrates that it can repair damage by a water droplet pinned on the damaged area or consecutive rolling water droplets at ambient conditions. We envision that our surface can provide a viable solution for a protective coating against hostile environments in the marine, automotive, and aviation applications. For more information, read this article. [See Also]

Membranes for Oil-Water Separation

Oil-water separation is a crucial step in a wide variety of industries. For example, 140,000 L of oil-contaminated water is produced during conventional mining operations on a daily basis. Additionally, oil leakage and spillage during marine transportation not only pose a threat to the marine environment and ecosystem but is a waste of valuable natural resources. Typically, an oil-water mixture can be classified into three categories based on the dispersed phase size (diameter, d)-as free oil-water, if d > 150 μm, as a dispersion if 20 μm < d < 150 μm, or as an emulsion if d < 20 μm. Oil-water emulsions are stable in the presence of the adsorbed interface-active chemicals (e.g., surfactant). Spontaneous separation of stable oil-water emulsions can be impractically time-consuming. Further, the separation process becomes more challenging with the decrease in the size of the dispersed phase.

We have committed to developing membrane-based technologies for the separation of oil-water mixtures with high flux and high separation efficiency at a lower cost and energy consumption level. This has led us to develop a hygro-responsive membrane that enables solely gravity-driven separation of oil-water emulsions. Also, we have fabricated a self-decontaminating membrane that can clean itself upon illumination of light which can recover its inherent flux after being fouled by organic matters. Furthermore, we have developed porous media that enable reversible adsorption-desorption of contaminants from water upon either temperature change or alternating electric field. These separation membranes and technologies are expected to have a wide range of commercial applications, including wastewater treatment, solvent recovery, and produced water separation.

Photoresponsive Surfaces

Photo-responsive titania (TiO2) surfaces have demonstrated wettability change upon exposure to ultraviolet light. Photo-driven manipulation of liquid motion on a TiO2 surface is highly attractive because it would eliminate any need for either direct electrical contact with liquids or complex electronic circuitry. However, their practical applications are often limited either by the inability to respond to the visible light spectrum of natural sunlight or by the slow kinetics and the need for special environments (that is, storage in dark or heat) to recover the original wetting state.

We have recently demonstrated that a dye-sensitized TiO2 surface can be engineered to have its wettability state optically modulated upon illumination by visible light. We showed that this wettability change arises due to the electric potential difference established between the surface and the liquid upon incident illumination. A systematic study of the relationship between the energy levels of the dye and the contacting liquid has revealed that the highest occupied molecular orbital (HOMO) energy level of the dye and the reduction potential of the liquid govern the ensuing wetting behaviors. Utilizing this photo-induced wetting of our dye-sensitized TiO2 surface, we demonstrated light-guided manipulation of liquid droplet motion along the surface. Furthermore, we showed demulsification of surfactant-stabilized brine-in-oil emulsion via interfacial coalescence of brine droplets under visible light illumination. Such surfaces thus offer a wide range of potential applications including optically driven, microfluidic devices with customizable wettability and continuous solar-driven self-cleaning and oil–water separation technologies. For more information, read this article. [See Also]