Title: Synthesis of Stretchable and Durable Underwater Superoleophobic Membrane for Filtration-based Oil/Water Separation *
3.3. Results and Discussions
3.3.3. Oil/water separation under practically relevant severe conditions
hierarchical topography highly water-compatible and (b) to protect such interfaces from any unwanted deposition of chemicals (e.g. proteins, and surfactants). As a result, the underwater anti-oil-fouling property was preserved even after prolonged (10 days) exposure to BSA protein (5 wt%; Fig 3.7F and L) contaminated aqueous phase. The investigation of the effect of UV irradiation on the coated fibrous substrate was another important and relevant durability test with regard to its prospective outdoor applications. The ‘fish-scale’-mimicked interface was exposed to both short (254 nm) and long (364 nm)-UV radiation for 30 days, and the advancing OCA and OCA hysteresis were examined at regular intervals. The anti-oil-fouling property remained unaltered with the advancing OCA of above 160º, as
shown in Fig. 3.8. Furthermore, the stretchable underwater superoleophobic interfaces, which were pre-treated with various physical abrasions and chemical treatments, were exposed to large (150%
tensile strain) physical deformation. However, the embedded ‘fish-scale’-inspired wettability remained unperturbed, as noted in Table 3.2. The exemplary physical, chemical and mechanical durability of the as-synthesized interface with ‘fish-scale’-mimicked wettability is expected to greatly enhance its performance in practically relevant challenging settings.
from the aqueous phase, but most of them are energy-inefficient and/or other causing secondary pollutions in the environment. As an alternative, biomimicked interfaces have emerged as a prospective candidate for energy-efficient and eco-friendly separation of oil/water mixtures. Though the ‘lotus leaf’-inspired wettability has been useful in selective filtration and collection of oil from oil/water mixtures, the ‘fish-scale’-mimicked wettability has allowed to selectively filtrate aqueous phase from various forms (i.e., floating oil, sediment oil and emulsified oil) of oil/water mixture even at practically relevant extremely challenging conditions.
However, the demonstrations of oil/water separation in practically relevant different severe settings are limited in the literature. In this chapter, the highly durable and stretchable fibrous substrate was used as a membrane for energy-efficient and eco-friendly collection of oil-free aqueous phases in various physically and chemically harsh scenarios. In this demonstration of oil/water separation, a prototype was developed by customizing a falcon tube (50 mL capacity), where the opening of the falcon tube was wrapped with the ‘fish-scale’-inspired stretchable membrane for selective filtration of the aqueous phase from various oil/water mixtures, and a side hole was made near the closed end of the tube for both (1) pouring the oil/ water mixture and (2) collecting the residual oil phase, simply by tilting the falcon tube and rotating the side hole of the tube upside down (Fig. 3.9). To facilitate the
Figure 3.9: (A-H) Digital images depicting the selective filtration/collection (A-C) of aqueous phase (dyed with methylene blue) from oil (soybean oil, dyed with Nile red)/water mixture. (D-H) Digital images illustrating the collection of water-free residual oil phase in a separate container through the side hole of the prototype.
gravity-driven and eco-friendly oil/water separation, the prototype was tilted at 25º angle. Moreover, the end of the tube, which was wrapped with the underwater superoleophobic membrane, was kept at
a downslope, as shown in Fig. 3.10A. The oil-water mixture consisting of both light and heavy oils was poured from the side hole with the help of a funnel. This resulted in the selective passage of the
Figure 3.10:(A-B,D-E) Digital images illustrating the gravity-driven selective filtration/collection of aqueous phase (methylene blue dye aided visual inspection) from oil/water mixtures, where both light (A,D) and heavy (B,E) oils (Nile red aids visual inspection) were individually used for preparing respective oil/water mixture. (C,F) However, the uncoated membrane allowed to pass both oil and water phases. (G) Plot accounting the performance of oil/water separation through the biomimicked interface, irrespective of the viscosity of the used oil phases.
Before SeparationAfter Separation
Light Oil
(Motor oil) Heavy Oil (DCE)
Uncoated PU membrane Coated PU membrane
D F
C
E B A
0 20 40 60 80 100 120
DCE Kerocene Soybean Oil Motor Oil
Separation efficiency (wt%) 249 cP
1.64 cP 40.46 cP
0.836 cP
Heavy
Oil Light Oil
G
DCE Kerosene Soybean Oil Motor Oil
Figure 3.10
aqueous phase through the ‘fish-scale’-mimicked stretchable membrane (Fig. 3.10A and B) and the aqueous phase was collected in a separate beaker (placed under the prototype), as shown in Fig. 3.10D and E. The residual oil phase was extremely repelled by the super-oil-repellent membrane and was
collected in another separate beaker by a downward rotation of the side hole of the prototype, as shown in Fig. 3.9. However, the uncoated fibrous substrate which soaked oil under water (Fig. 3.2C-D), was incapable of separating the oil from the aqueous phase as both the oil and water phases easily passed through the membrane, as shown in Fig. 3.10C,F. The prototype, wrapped with the coated membrane having stretchable ‘fish-scale’-mimicked wettability, was found to be highly efficient in separating the oil/water mixture irrespective of the density and viscosity of the oils that were used in the preparation of the oil/water mixtures. The separation efficiency was measured to be above 99 wt% on an average, as shown in Fig. 3.10G. Then, the same prototype was used for the separation of oil/water mixtures under severe physical and chemical conditions to simulate the harshness might be faced during outdoor applications. A variation in the temperature is a practical issue and some of the bioinspired interfaces
Figure 3.11:(A-B,D-E) Digital images demonstrating the successful oil (dyed with nile red dye) / water (dyed with methylene blue) separation through biomimicked interfaces at high (100°C; A,D) and low (10°C; B,E) temperatures.
(C,F) Digital images accounting the oil/water separation performance of the ‘fish-scale’ mimicked interface―which was repetitively exposed to high (150 %) tensile deformation for 1000 times.
are prone to loss of anti-wettability under extreme conditions of temperature.51,52 However, the as- synthesized underwater superoleophobic coating on fibrous substrate remained highly capable of surviving the exposures of extremes of temperatures, without compromising the anti-oil-fouling property under water. Therefore, oil/water separation at both high and low temperatures was demonstrated with the current prototype, where both the hot (100ºC) and cold (10ºC) aqueous phases (blue colour aided visual inspection of water phase) were selectively and efficiently filtrated through the underwater superoleophobic membrane, as shown in Fig. 3.11A-B,D-E. Then, the underwater superoleophobic membrane, which was manually and repetitively deformed with 150% tensile strain for 1000 times, was used for demonstrating the oil/water separation performance. The membrane was noticed to be efficient in complete separation of the aqueous phase from the oil/water mixtures (red colour aids visual inspection of oil phase) even after incurring successive tensile deformations, as shown in Fig. 3.11C,F. Next, the current prototype was further used to investigate the oil/water separation performance, where the aqueous phases were contaminated with harsh chemicals. Highly acidic (pH 1)/alkaline (pH 12) aqueous phases, artificial seawater and river water were successfully
separated from the respective oil contaminated aqueous phases by using the superoleophobic membrane, where the separation efficiency was calculated to be 99%, as shown in 3.12A. Moreover, the current prototype was repetitively used for separating the aqueous phase from the contaminated oil phase for 50 times (Fig. 3.12B). The same stretchable membrane, decorated with ‘fish-scale’- mimicked wettability, was repetitively used in oil/water separation processes without washing or application of any additional treatment, and there was barely any change in the selective filtration performance. Thus, the current approach provided a highly prospective biomimicked interface for outdoor applications, and it remained efficient for performing at practically relevant
Figure 3.12: (A) The bar graph depicting the water separation efficiency (wt%) from bulk oil/water mixtures under various practically relevant severe conditions. (B) The plot estimating the water separation efficiency (wt%) after repetitive separation of oil/water mixtures using the same biomimicked membrane for consecutive 50 times.
physically/chemically challenging scenarios. Among other forms of oil contaminations, emulsified
form is the most difficult to separate. So, the current setup was extended to investigate the performance of this underwater superoleophobic membrane for oil-in-water emulsion separation. While, the ‘lotus leaf’-inspired extremely water-repellent interfaces are inherently incapable of separating oil droplets that are dispersed in the aqueous phase, but the ‘fish-scale’- inspired wettability has been successfully explored in the separation of oil-in-water emulsion droplets in the recent past.11,55 As expected, the stretchable underwater superoleophobic interface was observed to be highly efficient in removing oil droplets (with a diameter ranging from 600 nm to 3500 nm, coloured using Nile red for facilitating the visual inspection) that were suspended in the aqueous phase, as shown in Fig. 3.13. Further, the separated aqueous phase was examined using an optical fluorescence microscopy. The oil droplets that were present initially (refereeing before separation process) in the oil-in-water emulsion were completely removed from the separated aqueous phase, which was confirmed using the fluorescence
Figure 3.13: (A-F) Digital images demonstrating the ability of the underwater extreme oil-repellent membrane in the oil-in-water emulsion separation, where the emulsion solution consisted of 2% (v/v) of DCE dyed with water-insoluble Nile red.
images (Fig. 3.14A-B) and DLS (Fig. 3.14C) study, where the oil phase was labelled with the water immiscible Nile red dye. Thus, the synthesized underwater superoleophobic membrane has a high
potential for removing oil/oily contaminants from various forms of oil/water mixtures in various severe and challenging scenarios.