Liquid-infused porous membranes (LPMs) have emerged recently with rich interfacial properties in various fields including separation, gas capture, sensors, electronics, and drug release. The inherent property of porous membrane combined with the molecular smoothness and dynamic nature of the functional liquid, have afforded LPMs with a wealth of omniphobic, anti-fouling, energy-saving, anti-adhesive, gating, and adaptive functions. Specifically, LPMs have brought new chances in membrane processes. LPMs can achieve reconfigurable gating behavior for permeating specific substances (gas, liquid, suspension, emulsion, etc) at a tailorable threshold pressure, eliminating fouling issues and requiring lower driven energy. LPMs can also be adaptive to various external stimuli to achieve tunable wettability, responsive permeability, and logic fluid manipulation. However, developing LPMs with multifunctionalities is still a persistent quest, especially, how to construct a membrane with reverse functions such as on-demand emulsification and demulsification with both desirable performances remains challenging.

To address this issue, the colleagues in Suzhou Institute of Nano-Tech and Nano-Bionics (SINANO), Chinese Academy of Sciences (CAS) introduce a selectively in-situ liquid-infused aerogel membrane (SILAM) with reverse functions, enabling on-demand emulsification and demulsification capability, sustained energy-saving property, and outstanding anti-fouling behavior (Figure 1). 

Figure 1. Design strategies and application models of a SILAM within the hierarchical porous aerogel.

To selectively confine a functional liquid, the development of a hierarchical porous aerogel membrane (HPAM) is a prerequisite. HPAM is established by reprotonation of aramid nanofibers (ANF), and simultaneous liquid-liquid phase separation and sol-gel transition of aramid nanofiber/polyvinylpyrrolidone (ANF/PVP) followed freeze-drying (Figure 2). The multiple pore length scales in HPAM render the in-situ selective confinement of a functional liquid (aqueous phase of the contacting liquid) during the membrane process. The functional liquid can be continuously self-replenished as long as the feeding liquid is flowing. The thermodynamic and kinetic study of HPAM is conducted to rationalize the pore structure and surface area. Both the phase separation and sol-gel transition can be regulated by controlling the three components: ANF/PVP, solvent (DMSO), and non-solvent (water) to affect the thermodynamic and kinetic aspect of HPAM formation. A series of surfactants has been searched for designing the emulsions, and thus optimum premix emulsion is selected to create a stable SILAM.

Figure 2. Fabrication and the thermodynamic and kinetic study of HPAM.

Just like the split attributes of human mental status, a single SILAM can be switched between the emulsification and demulsification modes by playing with the driven pressures. On one hand, under a positive pressure, SILAM is used for emulsification with outstanding uniformity and superb stability, as well as energy-saving behavior (Figure 3). During the formation of SILAM and emulsification process, different confinement effects exist due to multiscale porous structures. Initially, both macropores and mesopores are infiltrated with water. After applying external pressure, water in the macropores is easier to flow due to lower capillary force, thus allowing for the gating function of oil droplets. On the contrary, water in mesopores is firmly confined due to higher capillarity, retaining the functional liquid during oil transport through macropores. When the premix emulsion passes through SILAM, the flow resistance is small since there is no liquid-liquid interface resistance between the water confined in the membrane and the water phase in the premix emulsion. The hierarchical pores also contribute to the multiple deformation and extrusion of the emulsion. Notably, compared with current emulsification membranes, SILAM can realize finer emulsion sizes with less energy input (48% lower transmembrane pressure than the oil pre-infused membrane) and long emulsion stability (up to ten weeks). Furthermore, SILAM is also able to discriminate among micro-emulsions and nano-emulsions under different driven pressures.

Figure 3. SILAMS for homogeneous emulsification.

On the other hand, under a negative pressure, SILAMs achieve a demulsification function with efficient oil/water separation from premix emulsions or oil-water mixtures. The separation factor can be up to 99.97% and the membranes can be cycled up to 30 times without obvious degradation. Moreover, the separation efficiency of different membrane structures fabricated with variant coagulation baths or polymer molecular weights was also determined, all greater than 99.8%. Importantly, the real ship cleaning oil wastewater can be purified with SILAM, where the oil content can be reduced from 450 ppm to 25 ppm. SILAM exhibits a fairly stable membrane flux of 400 L/m²h up to 30 cycles. Compared with other oil-water separation membranes, SILAM has a relatively low operating pressure. Moreover, SILAMs can eliminate fouling in the demulsification process. The scale-up production of HPAM in SILAMs has been showcased by a roll-to-roll methodology. We anticipate that this selectively in-situ liquid-infused aerogel membrane system might shed light on the development of smart membranes in various application scenarios, ranging from water treatment, materials fabrication, food industry, to the petrochemical industry, and beyond.

Figure 4. SILAMs for demulsification.

For further readings, you are invited to read our complete paper, “Liquid-infused aerogel membranes with reverse functions enable on-demand emulsification and demulsification”, published in Nature Water:

https://www.nature.com/articles/s44221-024-00290-x