Frequent oil leakage accidents during offshore oil exploration and marine transportation have resulted in substantial losses of valuable oil resources and severe damage to marine ecosystems, [1][2][3][4] underscoring a dire need for advanced and effective oil spill recovery solutions. Physical absorption based on materials with selective wettability (oleophilic yet hydrophobic) offers an eco-friendly method to recycle oil sources from oil-contaminated wastewater. [4][5][6][7][8] This method differs from conventional methods such as in situ combustion, 9 bioremediation, 10 chemical interventions, 11 and oil containment booms. 12,13 Specically, two primary strategies characterize physical absorption-based oil recovery. The rst one is constructing ltration-type oil collectors using oleophilic/ hydrophobic membranes [14][15][16][17] or membranes featuring underwater superoleophobicity 18,19 as the lter. These membranes allow oil to pass through while rejecting water, thus achieving oil/water phase separation. However, as oil/water separation progresses, accumulated water droplets obstruct the membrane pores, which impedes continuous operation. The second strategy is in situ oil collection from water surfaces driven by external forces. [20][21][22][23][24] While this method ensures continuous oil recovery, it demands energy-consuming post-treatments, such as pumping 25 and squeezing, 26 thereby increasing operational complexities.
A new strategy in this domain involves oil skimmers based on the siphon effect. [27][28][29] Such skimmers employ capillary adsorption for self-starting oil recovery, transporting oil from polluted sites to collecting containers using just gravitational energy. Unlike the conventional physical absorption strategies, siphon-assisted oil skimmers eliminate the need for external power sources or manual interference, enabling direct oil recovery from oil-contaminated wastewater in a spontaneous, continuous, low-cost, and environmentally friendly manner. However, a challenge with siphon-assisted oil skimmers lies in their relatively low oil transport rate (generally ∼ 100 L m -2 h -1 ), [27][28][29] especially when compared with other pump-assisted devices (e.g., 500-600 L m -2 h -1 ). 21,25 The ow behavior of liquids driven by siphon effects is critically dependent on the channel structure. 27,30,31 Consequently, there is an urgent demand for innovative channel designs to improve the oil transport rates of siphon-assisted oil skimmers. Directional freeze casting offers a promising solution, which is known for its effectiveness in craing lowtortuosity and aligned channels to facilitate liquid transport. [32][33][34][35] This method involves directional solidication of a suspension along a controlled temperature gradient, followed by the sublimation of the solidied solvent (e.g., water) to yield a well-shaped three-dimensional (3D) monolith mirroring the negative imprint of the ice. [36][37][38] However, designing channels for siphon-assisted oil skimmers is complex due to their inherent U-shape structure, particularly at the highly curved juncture where two "legs" of the U-shape converge. Rapid oil transport in these U-shaped skimmers relies heavily on the strategic design of channel structures, especially at this critical juncture.
In this study, we introduce a new design for siphon-assisted oil skimmers with interconnected well-aligned graphene channel structures, denoted as well-aligned oil skimmers (WOS). These are craed through a specialized "legs-to-head" directional U-shape freeze casting process (LHFC), as illustrated in Fig. 1. The freezing process of graphene solution is carefully manipulated to start from both ends of the U-shaped "legs", converging ultimately in the "head" region. We nd that the channel structure within the "head" is critically contingent upon the connection angle (q) between the two "legs". At q = 0°, channels in the "head" remain isolated from each other and align perpendicularly to the direction of oil ow. As a result, oil skimmers developed via directional freeze casting even underperform in comparison to those craed through random freeze casting at q = 0°. In contrast, at q = 60°, channels formed in the "legs" coalesce into an interconnected network in the "head" region. This interconnection substantially lowers the transport barriers in the "head", accelerating oil transport across the skimmers. Enhanced by the signicant reduction in oil viscosity attributed to the exceptional solar-heating capability of graphene, the WOSs achieve a high oil recovery rate of 620.2 L m -2 h -1 under 1 sun irradiation (i.e., solar ux of 1 kW m -2 ). Furthermore, outdoor eld tests exhibit a peak oil recovery rate of 938.2 L m -2 h -1 at noon, with a cumulative daily yield of 14 001 L m -2 and an annual yield of 32 143 barrels per m 2 . Remarkably, the WOS outperforms even pump-assisted oil recovery systems, demonstrating its substantial promise for practical, electricity and labor-independent oil spill remediation applications.
U-Shape WOSs with various connection angles were prepared through an LHFC process with details described in ESI. † In brief, specic volumes of graphene oxide (GO) solutions were introduced into a 3D-printed U-shaped mold. During the LHFC process, the solidication of the GO solutions initiated from both ends of the U-shaped "legs", as illustrated in Fig. 2a. Following a vacuum drying process and subsequent thermal annealing, 3D graphene monoliths were obtained. To augment the solar absorption capability, vertically-standing graphene petals (GPs) were grown on the surface of graphene monoliths using plasma-enhanced chemical vapor deposition. 27,28,39 Each WOS is denoted based on its connection angle, q, between the two "legs" of U-shaped oil skimmers. For instance, WOS-60 implies a connection angle of 60°. For comparison, oil skimmers with a randomized pore conguration (ROS) were constructed using similar procedures but frozen directly in a chilled condition at approximately -150 °C. The concentration of GO solutions signicantly affects the microstructure, wettability, and oil recovery performance of the fabricated WOS-60s, as demonstrated in ESI Fig. S1. † For this study, the GO concentration used to fabricate the oil skimmers is xed as 10 mg mL -1 unless otherwise mentioned. Cross-sectional scanning electron microscope (SEM) images in Fig. 2b present the morphology of the WOSs' two "legs" (highlighted in the inset) along the solidication trajectory, revealing prominent well-aligned channels spanning several tens of microns. Conversely, ROSs display a chaotic porous structure (Fig. 2c) due to the uncontrolled solidication direction.
The oleophilic and hydrophobic characteristics of graphene 34 make WOSs naturally suited for selective oil extraction from contaminated wastewater. As demonstrated in Fig. 2d and ESI Fig. S2, † oil droplets permeate the WOSs quickly, while water droplets remain on the surface, exhibiting a contact angle of 135°. Furthermore, the oil transport efficiencies of WOSs and ROSs were assessed using a customized oil-wicking test setup (ESI Fig. S3 †). Corresponding mineral oil masses per area wicked by the "leg" section in different skimmers were measured and plotted as a function of time, as shown in Fig. 2e. The oil-wicking rates of WOSs substantially surpass those of ROSs, demonstrating the advantages of the low-tortuosity and well-aligned channels within the "legs" of WOSs. Moreover, the WOSs exhibit outstanding light-absorption properties across the entire solar spectrum (Fig. 2f). Upon decorating GPs on the surface (ESI Fig. S4 †), the overall solar absorption of WOSs jumps from 82.9% to 98.9%, which is attributed to the lighttrapping effect of the vertically standing GPs. [39][40][41] Moreover, the maximum stress of WOSs increases from 1.6 kPa to 53.8 kPa as the applied compressive strain rises from 10% to 90% (ESI Fig. S5 †), exhibiting good mechanical properties. At a compressive strain of 70%, the stress/density ratio of the WOSs reaches 3505 Pa mg -1 cm -3 , which is comparable to or even exceeds those of graphene aerogels reported in prior studies. 34,42,43
The oil recovery performance of WOSs is evaluated in a laboratory-scale experimental setup. As illustrated in Fig. 3a and ESI Fig. S6, † the apparatus comprises two separate chambers connected by an inversely U-shaped oil skim with WOSs. The le chamber contains a mineral oil/water mixture at a volume ratio of 1 : 2, mimicking the scenario of oating oil spills on water surfaces. During the tests, mineral oil is extracted from the le chamber and transported to the right chamber via the WOSs. The collected oil is then quantied using an electronic balance.
To maintain a steady rate of oil recovery, an oil feedstock is supplied to keep a consistent height difference between the oil layers in both chambers (DH). The entire setup is covered with a 10 mm-thick polyethylene lm to minimize convective heat losses. For all the tests conducted under solar illumination, the le chamber is covered by an aluminum foil to prevent extra solar heating.
Beneting from the excellent light-absorbing capability, WOSs can efficiently absorb and convert solar energy to thermal energy under solar irradiation. The temperature distribution across the surface of the WOS-60 under 1 sun solar illumination is visualized by an infrared (IR) camera (Fig. 3b). Fig. 3c presents the temperature variations over time recorded at specic points identied in Fig. 3b. The surface temperature on top of WOS-60 reaches equilibrium at 82 °C aer 30 minutes. Furthermore, even at the end of the "legs", a notable temperature of approximately 60 °C is observed. Such elevated temperatures play a crucial role in substantially reducing oil viscosity (Fig. 3d), leading to improved oil recovery performance.
Fig. 3e compares the oil masses recovered by WOS-60 and ROS-60 at a xed DH of 35 mm. In dark conditions, WOS-60 shows superior oil recovery performance, achieving a rate of 152.5 L m -2 h -1 , which is approximately 2.6 times as high as the recovery rate (i.e., 61.7 L m -2 h -1 ) of the ROS-60. Additionally, graphene-based membranes fabricated by another method, ambient drying, were tested and compared to the WOS-60 and ROS-60 samples. These ambient-dried oil skimmers feature hierarchical porous structures, with large pore sizes of several tens of microns and smaller pore sizes of several microns, as shown in ESI Fig. S7. † This structural variation inuences their oil recovery performance compared to the ROS samples, resulting in a lower oil recovery rate of 46.8 L m -2 h -1 under dark conditions (ESI Fig. S7 †). When subjected to 1 sun solar irradiation, the rate of WOS-60 is boosted to 425.7 L m -2 h -1 , attributed to the decreased oil viscosity under the elevated temperature. The impact of the oil-to-water volume ratio on oil recovery performance is also investigated. We have tested the oil recovery performance of oil skimmers in oil/water mixtures at volume ratios of 1 : 1 and 1 : 4. The oil recovery rate remains relatively unchanged as the oil-to-water volume ratio varies (ESI Fig. S8 †), which can be attributed to the consistent and xed DH. Interestingly, increasing DH further amplies the oil recovery rate, as the siphon mechanism is predominantly governed by shis in gravitational potential energy. 27,29 As Fig. 3f Fig. 1 Schematic representation of the LHFC design for solar-heating siphon-assisted oil recovery. Channels in the "head" of WOS with q = 0°are disconnected, impeding oil recovery, while the well-aligned channels in WOS with q = 60°are interconnected in the "head", promoting rapid oil recovery.
demonstrates, the oil recovery performance of WOS-60 augments in tandem with DH. Specically, high rates of 269.1 L m -2 h -1 in dark conditions and 620.2 L m -2 h -1 under 1 sun irradiation are achieved at a DH of 55 mm. This performance not only surpasses that of previously reported siphonbased oil skimmers but also rivals that of other externally powered oil recovery apparatuses (ESI Table S1 †). Moreover, the oil/water mixture separation efficiency is calculated to be 99.92% (ESI Fig. S9 †), illustrating the excellent capability of WOS-60 in effectively recovering oil from oil/water mixtures. Additional tests of oil transport under different solar irradiation intensities of 0.2, 0.5, 0.8, 1, and 1.5 suns are also conducted to systematically evaluate how decreased oil viscosity enhances the oil recovery performance of the WOS-60 skimmers. Meanwhile, the surface temperatures of the oil skimmers under different solar irradiation intensities are recorded. As demonstrated in Fig. 3g and ESI Fig. S10, † the oil skimmers exhibit oil recovery rates of 318.1, 383.2, 489.9, 620.2, and 700.3 L m -2 h -1 at solar intensities of 0.2, 0.5, 0.8, 1, and 1.5 suns, respectively. The increase of oil recovery rate with solar intensity can be attributed to the more elevated surface temperatures induced by higher solar intensities. In addition, oil recovery tests based on a different type of oil with much higher viscosity (denoted as oil-II) and WOS-60 are conducted, as shown in ESI Fig. S11. † An average oil recovery rate of 111.4 L m -2 h -1 is achieved under dark conditions, which is increased to 270.7 L m -2 h -1 at 1 sun, indicating that oil-II can also be spontaneously and continuously recovered by WOS-60. Therefore, our proposed skimmers are effective in recovering oil with a broad range of viscosities.
To test the long-term stability of WOS-60, a continuous 72 hours oil recovery evaluation, alternating between 12 hours of sunlight exposure and 12 hours of darkness, was carried out. Throughout the test, collected oil was manually redirected to the feedstock at 2 hours intervals under sunlight and 6 hours intervals in darkness, ensuring a steady DH of 55 mm. Interestingly, oil recovery rate under both 1 sun irradiation and dark conditions remained consistent (Fig. 3h), demonstrating the robust cyclic stability of WOS-60. We note that although residual oil remains within the skimmers aer each oil recovery test, the oil skimmers are ready for next uses without performance degradation. As an illustration, aer initial tests on fresh WOS-60s, the oil skimmers were stored under ambient conditions for two months. Upon re-evaluation of their oil recovery performance, as shown in ESI Fig. S12, † the oil recovery rates remained consistent with that of the fresh samples, and the separation efficiencies of the oil/water mixture were maintained at a high level of up to 99.85% (ESI Fig. S9 †). These results highlighted the excellent reusability and stability of the WOS-60s, demonstrating that the used membranes can remain free from secondary pollution under ambient environmental conditions.
Our results indicate that the well-aligned channels in the two "legs" of WOS-60 signicantly improve oil recovery performance compared to ROS-60. This raises two intriguing questions: rst, how do the well-aligned channels in the two "legs" merge in the "head" region? Second, does the connection angle (denoted as q) between these "legs" inuence the channel structures established during the freeze-casting procedures, and, consequently, the oil recovery efficiency? To investigate these aspects, we fabricated WOSs and ROSs with various q values ranging from 0°to 100°at a xed DH of 35 mm. To eliminate the potential confounding inuence of the solar-heating effect, oil recovery tests were conducted under dark conditions. Fig. 4a presents the comparative oil performance of ROSs with varying q values. Notably, the oil recovery performance deteriorates progressively as q increases. This decrement can be attributed to an increased channel resistance resulting from an extended ow pathway at larger q values. Based on Bernoulli's equation, 44 the velocity (v) of the outgoing oil from the skimmers can be represented as:
where g is the gravitational acceleration constant, and f L represents the channel resistance, which is described by: 44
where C is the pressure drop constant, L is the total ow distance of oil, and DP base is the base pressure drop. Based on the relationship, an elongated oil ow distance clearly leads to a diminished oil recovery rate. Fig. 4b illustrates the temporal evolution of recovered mineral oil by WOSs for diverse q values under dark conditions. Subsequently, the corresponding oil recovery rates for both ROSs and WOSs are calculated and depicted in Fig. 4c. Interestingly, at a q of 0°, the oil recovery rate of WOS is unexpectedly inferior to that of ROS. This suggests that the oriented channel structure within the two "legs" in WOS-0 fails to enhance its overall oil recovery efficiency. As q increases from 0°to 60°, the oil recovery rate of WOSs display a pronounced increment, reaching a peak value of 152.5 L m -2 h -1 at q = 60°. Beyond this point, as q continues to rise, a decline in the recovery rate is observed. However, the ratio of the oil recovery rate between WOSs and ROSs, denoted as v WOS /v ROS , consistently increases as q augments.
To reveal the correlation between the oil recovery performance and q of WOSs, the channel architecture at the "head" of WOS-60 (Fig. 4d) was examined and compared with that of WOS-0 (Fig. 4e), of which the connection angle (i.e., 0°) is typical in conventional siphon-assisted devices. [27][28][29] In WOS-60, the freezing directions within its two "legs" converge at an angle of 60°. This leads to a connected channel structure in the "head" region, with a connection angle of approximately 60°(Fig. 4d).
Consequently, this conguration enables a low resistance to oil ow within the "head" of WOS-60 and thus enhances the overall recovery rate. In contrast, the freezing directions in both "legs" Fig. 4 (a) Mass evolution of collected oil by ROSs with different q under dark conditions. (b) Mass evolution of collected oil by WOSs with different q under dark conditions. (c) Calculated oil recovery rates for ROSs and WOSs with different q under dark conditions. (d) and (e) SEM images showing the channel structure in the "head" of (d) WOS-60 and (e) WOS-0. Yellow arrows denote the freezing direction during the freeze casting process. of WOS-0 run parallel to the x-direction, causing channels within its "head" to be orthogonal to the oil's ow direction (ydirection), as shown in Fig. 4e. This structure signicantly impedes oil movement across the "head" of WOS-0, thereby undermining the oil recovery rate compared to ROS-0 at q = 0°. As q increases from 0°to 60°, the resistance for oil traversing the "head" region diminishes, leading to an enhanced oil recovery rate. However, for q values over 60°, the elongated ow distance results in higher channel resistance, compromising the overall oil recovery performance. Even though oil ow resistance in the "head" region continues to decrease with larger q (evidenced by the ascending WOS-to-ROS recovery rate ratio in Fig. 4c), the recovery rate of WOSs decreases as q progresses from 60°to 100°. Hence, WOS-60 with a q of 60°is demonstrated to be the optimal conguration for oil recovery.
The morphology and structure of channels formed by freeze casting are intrinsically linked to the growth patterns of ice crystals. 36,45 The growth behavior of these crystals is inuenced by the temperature gradient present in the liquid precursor during the solidication process. Importantly, ice crystals tend to grow following the direction of the imposed thermal gradient. To shed light on the formation of distinct channel structures in the "head" regions of WOS-60 and WOS-0, we monitored the real-time temperature distributions using a highresolution IR camera (FLIR A600) with a 25-micron precision lens. Fig. 5a shows the photograph of the "head" region in the freeze-casting mold for WOS-60, and Fig. 5b-d present the sequential temperature distributions during solidication. In the initial phase of the GO solution freezing within the "head" region (as shown in Fig. 5b), the temperature gradients align with the directions of the two "legs" in the mold. As the freezing process advances, a consistent angle of approximately 60°is retained in the temperature gradients (Fig. 5c), leading to the merging of freezing within the "head" region (Fig. 5d). This phenomenon results in the formation of an interconnected framework of ice crystals that, upon sublimation, yields interconnected channels in the "head".
Contrarily, the freezing process in the WOS-0 mold (Fig. 5eh) shows a temperature gradient aligned solely in the x-direction right from the commencement of freezing in the "head" region. This orientation remains unchanged throughout the process, prompting the ice crystals to grow in a unidirectional manner along the x-axis. Consequently, the channels formed aer ice sublimation are well-aligned in the x-direction, impeding oil transport along the y-direction.
To gauge the oil recovery efficiency of the WOS-60 under realistic environmental conditions, we conducted outdoor eld tests during a mostly sunny day in Richardson, Texas, U.S. (coordinates: 96.73°W, 32.95°N). Fig. 6a depicts our experimental setup for outdoor oil recovery tests. We employed a pyranometer to measure the real-time incident solar ux (Fig. 6b) and a weather station to record ambient temperatures (Fig. 6c) and additional weather conditions (ESI Fig. S13 †). During the tests, to stabilize DH at an approximate value of 55 mm, the collected oil was manually cycled back to the feedstock container every 1 hour during daylight and every 4 hours aer sunset. The oil recovery rate of WOS-60, determined on an hourly basis, is presented in Fig. 6d. During the noon hour (between 12:00 and 13:00), the WOS-60 exhibited an exceptionally high oil recovery rate, reaching a peak value of 938.2 L m -2 h -1 under an average solar ux of 923 W m -2 . This rate is signicantly higher than that achieved under 1 sun irradiation in indoor experiments (i.e., 620.2 L m -2 h -1 ). Additionally, the cumulative daily oil collection of WOS-60 reaches up to 14 001 L m -2 , corresponding to an annual yield of approximately 32 143 barrels per m 2 . Note that the oil recovery performance could be further improved with a more substantial DH.
To evaluate the oil recovery capability of WOS-60 in comparison to other types of oil skimmers, we determine the oil ow ux (q) through dividing the oil recovery rate (v) by the driving pressure (DP): 21,22 q = v/DP
The driving pressure in the siphon-assisted system is given by: 46
where r oil is the density of mineral oil, and g denotes the acceleration of gravity. As shown in Fig. 6e, the WOS-60 exhibits excellent oil ow uxes of 136 982 and 207 218 L m -2 h -1 bar -1 during the indoor and outdoor tests, respectively, signicantly outperforming pump-assisted and other ltration-type oil recovery systems (details are shown in ESI Table S1 †). Importantly, the WOS-60 operates autonomously and continuously without the requirement for external electricity or human intervention. The high operational efficiency of the oil skimmers assisted by siphon effects and renewable energy demonstrates great potential for practical oil spill recovery.
In summary, we have reported a novel LHFC technique to fabricate siphon-assisted oil skimmers and systematically assessed their self-pumping oil recovery performance in oilcontaminated wastewater. By tracking real-time temperature distributions, we determined that the connection angle between the two "legs" of WOSs could signicantly affect the temperature gradient, subsequently impacting the channel formation within the "head". The optimal oil recovery performance was observed at a connection angle of 60°, which is the result of balancing the reduced transport barrier within the "head" against the augmented resistance in the "legs" due to the elongated ow pathway. Consequently, the WOS-60, beneting from the well-aligned channel structure in the "legs" and interconnected pathways in the "head", exhibits remarkable oil recovery performance compared to ROSs with an outstanding oil/water separation efficiency of 99.92%. During the indoor test, the oil recovery rates were measured to be 269.1 L m -2 h -1 in dark conditions and 620.2 L m -2 h -1 under 1 sun irradiation. Moreover, the rate reached a maxima value of 938.2 L m -2 h -1 at noon during an outdoor test, accumulating a daily oil collection of 14 001 L m -2 and an annual yield of 32 143 barrels per m 2 . The oil ow ux is calculated to be 207 218 L m -2 h -1 bar -1 , signicantly higher than the reported external power-driven oil recovery systems. Our research paves the way for innovative designs of channel structures with intricate shapes, showing great potential for rapid liquid transportation across diverse applications.
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