Continuous in Situ Extraction toward Multiphase Complex Systems

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Continuous In-Situ Extraction towards Multiphase Complex Systems Based on Superwettable Membrane with Micro-/Nano-Structures Zhe Xu, Zhongpeng Zhu, Ning Li, Ye Tian, and Lei Jiang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b04328 • Publication Date (Web): 26 Sep 2018 Downloaded from http://pubs.acs.org on September 27, 2018

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Continuous In-Situ Extraction towards Multiphase Complex Systems Based on Superwettable Membrane with Micro-/Nano-Structures Zhe Xu,†,‡ Zhongpeng Zhu,†,‡ Ning Li,† Ye Tian,*,† and Lei Jiang*,†,§ † Key Laboratory of Bio-inspired Materials and Interfacial Science, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China ‡ University of Chinese Academy of Sciences, Beijing 100049, P. R. China § School of Chemistry, Beijing Advanced Innovation Center for Biomedical Engineering, Beihang University, Beijing 100191, P. R. China E-mail address for corresponding authors: [email protected] or [email protected] KEYWORDS: hydrophobicity, oleophilicity, nano-structure, microporous membrane, extraction chemistry, multiphase complex system

ABSTRACT: Liquid-phase-extraction is widely used in chemical industry. Traditional extracting routes always involve multiple procedures, need large floor space, and take long operating time. “Continuous in-situ extraction” that can conduct a real-time integration of solutes-extraction and solvents-separation simultaneously would be of great significance. Superwettable materials offer

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us a good choice to separate different immiscible solvents; herein, we achieve continuous in-situ extraction of multiphase complex systems by using a porous polytetrafluoroethylene membrane with nano-structure-induced superwettability. It realizes a rapid, selective, and efficient real-time removal of various extracting agents during a continuous process due to their wetting differences. Compared with traditional extraction, our route shows a distinct superiority on saving operating time, enhancing liquid-recovery, and simplifying procedures, while still remains high extracting performances. In addition, our membrane possesses excellent durability even after long-term use in harsh chemical environments or under strong mechanical impacts. Thus, we believe that it will provide a potential alternative of current industrial extractions.

Liquid-phase-extraction, as an important technology for partitioning various substances owing to their distinct solubility in immiscible solvents, has been widely applied in many industrial fields, such as chemical analysis,1 environmental treatment,2 pharmaceutics,3 food manufacturing,4 biorefinery,5 and hydrometallurgy.6 Traditional extraction involves multiple procedures, needs large floor space, and takes long operating time. “Continuous in-situ extraction”7–9 that can conduct a direct, real-time integration of solutes-extraction and solvents-separation simultaneously would be of great significance. In the past 14 years, advanced membranes with superwettability10–12 have been widely developed for separation of immiscible solvents, including oil-water mixtures,13–26 oil-water emulsions,27–36 organic liquids,37,38 ionic liquids,39 and several other multiphase systems.40 However, few works on continuous real-time removal of complex extracting systems with multi-solutes41–43 by taking advantage of superwettability have been reported until now. Three critical challenges need to be responded in this field: 1) a simple method to fabricate superwettable membrane and an effective


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way to integrate it with extractor; 2) excellent durability to protect membrane’s superwettability against long-term contact with multi-components in complex systems; 3) a potential alternative of current industrial extractions. Herein, we create a specific model of continuous in-situ extraction by equipping a vertical-placed superwettable membrane (SWM) with an extracting device, which exhibits great advantages of saving time, enhancing recovery, and simplifying procedures against traditional extracting routes. It conducts rapid, selective and efficient real-time removal of a variety of extracting agents (EAs) during a continuous process, driven by the wetting differences of multiphase solvents containing complex solutes. Our SWM, which is made of polytetrafluoroethylene (PTFE) with laser-formed micro-/nano-structures, shows an extremely stable superwettability after long-term work in harsh chemical environments or under strong mechanical impacts. We anticipate that superwettabilitybased continuous in-situ extraction possesses a potential alternative of current extracting routes in industry. RESULTS AND DISCUSSION To prevent the complex solutes and multiphase solvents of common extracting systems44–46 from altering membrane’s wettability, PTFE47,48 is selected because of its good chemical inertness and excellent solvent resistance. The SWM here is fabricated by using a nanosecond pulse laser49 to construct both micro- and nano- structures on an original smooth, transparent PTFE film (Figure 1 a). A combined strategy (Figure S1) that consists of modifying and drilling processes based on “coherent effect” and “ablation effect” of laser-spots is proposed to form sophisticated nanoscale roughness and regular microporous array on PTFE (Figure 1 b and Figure S2), respectively. Our modifying method of manipulating a large number of interrelated laser-spots under a low energy


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below the thermal ablation threshold of PTFE50 easily induces nano-structures (Figure S3), which are generally observed not only around the surface (Figure 1 c and Figure S4) but also inside the channel (Figure 1 d and Figure S5) of micro-pores. It is worth noting that the fabricating strategy does not damage -CF2- bond of PTFE (Figure 1 e, with its characteristic FT-IR vibration peaks at wavenumber of 1150 and 1260 cm-1) so that the membrane can maintain its intrinsic low surface energy13 (XPS results, in Figure S6). Governed by nano-structures and fluorinated compositions, superwettability is finally obtained. Our SWM possesses classical superoleophilicity (Movie S1) but superhydrophobicity in air (Movie S2). Phenolic compounds (PCs) are among most important industrial contaminants51 that always lead to serious water-pollutions and death of fishes. Aqueous solution (AS) of phenol (colorless) and o-nitrophenol (yellow) is chosen as a typical model to investigate the treating effects of phenolic wastewater by liquid-phase-extraction. N-Butyl acetate is considered as an EA for extracting the PCs from initial AS. Due to the nano-structure-induced superwettability, our SWM also exhibits wetting differences towards the chosen extracting system (Figure 1 f). Superoleophilicity endows the membrane a strong affinity with EA, showing a very small contact angle (CA) ~ 0° (Figure 1 g), while superhydrophobicity brings an extreme repellence against the aqueous solution of PCs (PCs-AS) with a large CA ~ 139° (Figure 1 h). We further create a model of superwettability-based continuous in-situ extraction by assembling our SWM at the side-face of an extractor (Figure 2 a). This vertical-placed SWM can provide a horizontal flow so that all liquid-phases contact the membrane’s surface no matter what densities they possess. The in-situ extracting route is realized by applying a real-time removal of PCs-rich extracting agent (PCs-EA) while repelling final AS simultaneously during an extracting process. In this work, we successfully achieve an integration of solutes-extraction and solvents-separation


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by taking advantage of the wetting difference of SWM between EA and AS, respectively (Movie S3). It is worth noting that fresh EA can be also kept constantly dropping into the device so that a continuous extracting process is available. In the end, all of the as-extracted liquids are collected selectively and automatically (Figure S7). On the contrary, traditional extraction always needs much more vessels and consists of multiple procedures, including a thorough mix of the extracting system, gravity settlement of immiscible liquid-phases, and the subsequent separation (Figure 2 b). It requires long operating time, such as mixing time, waiting time, and separating time (Figure S8 and Movie S4). Our superwettabilitybased in-situ extraction, however, exhibits a superior advantage of saving time, especially during continuous extracting processes for treating larger throughput (Figure 2 c). For example, in-situ extraction takes only about 4 min to deal with a certain volume (64 ml) of the extracting system (EA:AS = 16:48); As for larger throughput of 256 ml, the operating time (< 14 min) is half the time of traditional extraction. Furthermore, real-time PCs concentration in EA-phase arrives at a steady state in no more than 1 min by our in-situ strategy (Figure S9). However, mixing time to reach a PCs-rich state by traditional extraction needs to be up to about 4 min (Figure S10), much less waiting time (~ 1.5 min) and separating time (1.37 ± 0.15 min). Volume recovery (Figure 2 d) of liquids (EA and AS) by in-situ extraction (~ 99.0%) is also higher than that conducted by a traditional route (~ 97.0%). This behavior will be more distinct with the increasing of throughput (97.0%, 98.5%, 99.0%, 99.3%, and 99.4%, from 64 ml to 320 ml), because the continuous in-situ extraction greatly reduces multiple transferring procedures that mostly lead to unwanted losses of liquids. It is worth noting our in-situ strategy maintains high efficiencies (> 99.38%) at different EA:AS ratios (16:48, 32:48, and 48:48), which is quite similar to traditional extraction (Figure 2 e). Therefore, we anticipate that the superwettability-based continuous in-situ extraction exhibits


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a comprehensive superiority compared with traditional methods. In addition, the specific model of extracting route could be further improved by equipping dual-inlets for simultaneously and constantly feeding fresh EA and PCs-AS (Movie S5 and Figure S11). It is well known that such continuous extracting manner is of great significance in the real industrial applications.52–54 To meet the urgent demands of dealing with multi-components in a variety of industrial fields,55– 57

extracting systems nowadays become much more complex. Three-liquid-phase system (TLPS)

that contains an organic-phase, a polymer-induced phase, and an aqueous-phase, is considered as a typical model for selective extraction of different solutes by each phase. Our in-situ strategy is proved to be further available for these multiphase complex systems (Figure 3 a and Figure S12). Besides the as-mentioned EA, AS and PCs, polyethylene glycol (PEG) is chosen to form another new polymeric liquid-phase with the help of multiple other chemical adjuvants (Table S1). Our SWM also endows wetting differences (Figure 3 b) towards TLPS, showing a strong affinity with EA (CA ~ 0°), while an extreme repellence against both PEG-phase (CA ~ 137°) and PCsAS (CA ~ 134°). It enables us to conduct continuous real-time removal of EA from the complex system (Figure S10). Real-time concentrations of PCs (phenol and o-nitrophenol) in EA (Figure 3 c, with the standard curves of PCs in Figure S13) illustrate that EA-phase is able to selectively enrich phenol (PC-1, concentration 4.86 ~ 5.02 mg/ml), but keep away from o-nitrophenol (PC-2, 0.36 ~ 0.37 mg/ml). The two solutes can be concentrated in EA and PEG respectively at the end of in-situ extraction for different interactions between both solute- and solvent- molecules.58,59 It is also proved by extraction efficiencies of PC-1 and PC-2 (Figure 3 d) in various liquid-phases (PC-1 enriched in EA 88.56 ± 0.07% while PC-2 enriched in PEG-phase 65.35 ± 0.23%), along with color comparison of the final filtrates (see inset pictures, PC-1-EA is colorless while PC-2PEG is orange). Similar high performance is obtained by traditional extraction (Figure S14), and


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it also accords with the results in a previous literature,60 in which multiple procedures of mixing, transferring, centrifuging, and waiting for phase-equilibrium are required. Thus, we believe that superwettability-based continuous in-situ extraction offers us a facile, rapid, and efficient method to realize selective extractions towards multiphase complex systems. Besides the chosen n-butyl acetate, a variety of common EAs (Figure 3 e and Table S2), including esters, aromatics, halides, alkanes, and ethers, also exhibit large permeating fluxes (> 2.70 L cm-2 h-1) through our SWM. It is further proved that such in-situ strategy is widely suitable for different extracting systems. It is of great significance to promote a novel technology for meeting the requirements in practical applications. Our fabricating strategy of SWMs via a commercial laser device provides a simple, fast, controllable, and easy-scaling-up way to manipulate micro-/nano-structures onto PTFE. The full-steps, such as modifying with nano-structures, drilling micro-pores, and cutting into desired pieces (with a fixed area at 34 mm × 34 mm), are quickly accomplished within only 25 s (Movie S6). Membrane size and micro-pore size can be precisely controlled by altering laser parameters, such as laser power and pulse number. Thus, we easily obtain a series of SWMs with increasing pore-sizes from SWM-1 to SWM-5 at almost every 50 μm (Figure 4 a and Table S3), along with excellent repeatability (Figure S15), which are able to match different requirements of separation flux (0.70 ± 0.06, 2.04 ± 0.21, 4.50 ± 0.22, 8.22 ± 0.36, and 14.00 ± 1.06 L cm-2 h-1, respectively) with high efficiencies (> 99.50%) for the continuous removal of EAs (Figure 4 a). We insist that it could provide selective real-time removing rates according to the practical extracting demands. Besides, a prolonged mechanical force cycles test (Figure 4 b) against SWM is conducted. Nanostructures of our membrane can be well kept even after 180 cycles under an alternating pressure (~ 5 N, for 10 s) and adhesive (~ 3 N, 2 s) loads, remaining stable superhydrophobic state (CA > 150°).61,62 In addition, our SWM also maintains its superwettability even in harsh environments


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of multiphase complex extracting conditions (Figure 4 c). Dynamic contact angles (DCAs) are performed to facilitate the evaluation of either the wetting property or the adhesive force state of membrane. The almost changeless DCAs during long-term work (> 32 h) in TLPS bring a strong evidence to reveal its good chemical durability. CONCLUSIONS In summary, we present a model of superwettability-based continuous in-situ extraction towards multiphase complex systems. The extracting strategy is achieved by assembling a vertical-placed superwettable membrane (SWM) with an extractor. It conducts a rapid and continuous real-time removal (fluxes > 0.70 L cm-2 h-1) of various organic extracting agents from multiphase complex situations due to the wetting differences, especially for dealing with large throughput. Compared with traditional extraction, continuous in-situ strategy exhibits outstanding advantages of saving operating time (~ 4 min for 64 ml), enhancing recovery (> 99.4%), and simplifying procedures, but remains high extraction performance. Our SWM, which is made of PTFE with micro-/nanostructures induced by a simple, fast, controllable, and easy-scaling-up laser-fabrication, possesses durable superhydrophobicity and superoleophilicity even after long-term work in harsh chemical environments (> 32 h) or under strong mechanical impacts (> 180 cycles). We anticipate that our superwettability-based continuous in-situ extraction will have a great potential for the alternative of current industrial extractions in the future. EXPERIMENTAL SECTION Laser-Fabrication of Superwettable Membrane with Micro-/Nano-Structures. First of all, a piece of the PTFE film (from Daxiang Plastic, China) was well cleaned and firmly fixed onto an operating platform. Secondly, nanosecond pulse laser (LSC30, from HuaGong Laser, China) was


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devoted to generating nano-structures in a confined area (14 mm × 14 mm) onto the film surface by “coherent effect”. And then, regular microporous array (spot matrix 26 × 26) was manipulated by “ablation effect”. Finally, the membrane was washed in deionized water, ethanol, and acetone subsequently by an ultrasonic treatment (100 Hz, 30 min), and transferred in a vacuum dryer (0.1 MPa, 60 °C) for 12 h. Pore-size of the membrane was precisely controlled by tuning either laser power (0 ~ 30 W) or pulse number (0 ~ 300). Full-steps of fabricating process were applied in air atmosphere. A series of SWMs were obtained by regulating laser parameters (Table S3), hereinto, SWM-4 was commonly applied in all extracting experiments, and SWM-0 was used in chemical and mechanical durability tests. Superwettability-Based Continuous In-Situ Extraction. SWM in in-situ extracting device was tightly sealed between a pair of flanges, assembled by a vertical-placed way. Aqueous solution of phenolic compounds (PCs-AS) was prepared by adding the mixture of phenol and o-nitrophenol (from Beijing Chemical, China) in water with concentrations of 2000 mg L -1 and 1000 mg L -1, respectively. A certain volume of PCs-AS (48 ml) was firstly poured in the in-situ extractor once under a condition of stirring (~ 500 r/min); Fresh n-butyl acetate (from Beijing Chemical, China) as extracting agent (EA) was continuously dropped (~ 6 ml/min) in; Phenolic compounds (PCs) were then extracted by EA, and after a short while, the phenolic compounds-rich extracting agent (PCs-EA) started to flow out through the SWM; If keeping the feed of EA, continuous extracting process was underway; In the end, final filtrates were entirely collected. Different volume ratios (EA:AS = 16:48, 32:48, 48:48) were applied by a fixed volume of PCs-AS (48 ml) with different volumes of EA (16 ml, 32 ml, and 48 ml). Traditional method for a comparison was manipulated by controlling similar parameters with in-situ extraction, involving volume ratio (EA:AS = 16:48, 32:48, and 48:48) and total throughput (64 ml, 128 ml, 192 ml, 256 ml, and 320 ml, respectively).


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Selective In-Situ Extraction towards Three-Liquid-Phase System. Polyethylene glycol (PEG, from Xilong Chemicals, China), (NH4)2SO4 and NaOH (from Beijing Chemical, China) were the main additives of three-liquid-phase system (TLPS). Polymer solution (PEG) with concentration of 50 w.t.% and (NH4)2SO4 aqueous solution (AS) with concentration of 50 w.t.% were prepared in advance. To form the TLPS, a mixture of the stock solutions (EA/PEG/AS) was prepared with the concentration ratios of 20 w.t.% n-butyl acetate, 10 w.t.% polyethylene glycol, and 10 w.t.% (NH4)2SO4, and pH value (~ 9.5) was adjusted by a saturated NaOH aqueous solution. Phenolic compounds (PCs) that involve both phenol (PC-1) and σ-nitrophenol (PC-2) were prepared with concentrations of 2000 and 1000 mg L -1, respectively. Before extraction, they were added into a certain volume of the stock solution with 10 w.t.% polyethylene glycol and 10 w.t.% (NH4)2SO4 (PCs-PEG/AS, 48 ml). After a continuous feed of EA (16 ml), the TLPS (PCs-EA/PEG/AS) was obtained, and in-situ extraction was completed for selective concentration of PC-1 in EA-phase (PC-1-EA) and PC-2 in PEG-phase (PC-2-PEG). Other extracting agents for the permeating tests (including ethyl acetate, methylbenzene, trichloromethane, carbon tetrachloride, petroleum ether, cyclohexane, hexane, and ethyl ether) were purchased from Sino-Pharmaceutical Group, China. All these reagents were of analytical grade. Chemical and Mechanical Durability Tests. All the tests were applied onto SWM-0. Chemical durability was evaluated by measuring water-in-air dynamic contact angles (DCAs) during longterm work. The membrane was equipped in our in-situ extractor and kept stirring in TLPS. It was taken out after every 4 hours, and then washed and entirely dried before each DCA measurement. Mechanical durability was tested by manipulating a large number of cycles under both pressure (~ 5 N, for 10 s) and adhesive (~ 3 N, 2 s) loads against the laser-fabricated region that involves


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micro-/nano-structures. After 180 cycles, a SEM image of the membrane’s surface morphology was captured and the water-in-air CA was also measured. Instruments and Characterization. Scanning electron microscopy (SEM) photos were taken on a field-emission scanning electron microscope (JEOL 7500-F, Japan). Digital pictures and videos were obtained by a SLR-camera (Canon EDS-60D, Japan) with an alternative macro lens (Canon EF 100mm f/2.8L, Japan). Optical microscopy (OM) photographs were grasped using an optical microscopy (Olympus BX51, Japan), and then recorded by an in-situ camera system (Nikon DSU3, Japan). Chemical analysis data were obtained from a Fourier transform infrared spectrometer apparatus (JASCO FT/IR-6600, Japan) or an X-ray photoelectron spectroscopy instrument (XPS, Thermo Fisher Scientific, ESCALAB 250Xi, USA). Contact angles were measured on a contact angle system (Data-physics OCA20, Germany), calculating an average value from at least eight measurements at different positions by using a 2 μL liquid (H2O, CHCl3, AS, or EAs) droplet per time for every static contact angle. Dynamic contact angles were measured by a 10 μL droplet per time to avoid the misleading values of receding contact angle.63 Concentration measurements were determined by high performance liquid chromatography equipment (HPLC, Shimadzu LC20AT, Japan), performing at 25 °C under a 1.0 mL/min flow with an injection volume (10 μL) of water/methanol (25v/75v) as a mobile phase each time; C18 column (5 μm, Thermo Scientific, USA) was used at 30 °C and column eluant was monitored at 270 nm. Real-time concentrations were given by HPLC from the filtrates picked up by a constant volume (1 ml) for every 1 min. Separation or permeating fluxes were recorded by measuring the time and volume simultaneously, focusing on the removal of extracting agents (EAs) from water; Separation efficiency was measured by detecting moisture from a Karl Fischer moisture titrator (Mettler Toledo V20, Switzerland) with three parallel tests to get one average value. Equivalent diameters


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of micro-pore were evaluated by professional analysis software (Nikon NIS-Elements, Japan), and counted beyond 50 pores for each data. Mechanical force curve was accomplished by a laptop-controlled motorized test stand (MARK-10 ESM301, USA) with a pressure sensor. ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website. The PDF file includes additional tables (Table S1−S3), figures (Figure S1−S15), and movies (Movie S1−S6). The authors declare no competing financial interest. AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected] Author Contributions Z.X. performed the experiments, collected and analyzed the data, wrote the manuscript. Z.Z. and N.L. analyzed the data and improved the manuscript. Y.T. analyzed the data and helped to revise the manuscript. L.J. conceived the project, designed the experiments, and analyzed the data. ACKNOWLEDGMENT This research is supported by the National Research Fund for Fundamental Key Projects (2017YFA0204504), National Natural Science Foundation (21722309, 21671194), and the 111 Project (B14009).


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Figure 1. A PTFE microporous membrane with nano-structure-induced superwettability towards extracting systems. (a) Digital picture of the laser-fabricated PTFE membrane with micro-/nanostructures, scale bar 5 mm; (b-d) SEM photos of the membrane’s surface morphology, including regular microporous array (b), nano-roughness around the surface (c) and inside the channel (d) of micro-pores, scale bar 100 μm; (e) FT-IR spectrum of membrane reflecting its -CF2- chemical bond of PTFE, which results in an intrinsic low surface energy; (f) This membrane exhibiting the wetting differences towards a typical extracting system (model to deal with phenolic wastewater), using n-butyl acetate as extracting agent (EA) and mixture of phenol/o-nitrophenol as phenolic compounds (PCs); (g) Superoleophilicity bringing the membrane a strong affinity with EA-phase, showing a small contact angle (CA) ~ 0°; (h) Superhydrophobicity giving an extreme repellence against aqueous solution of PCs (PCs-AS), showing a large CA ~ 139°.


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Figure 2. Advantages of continuous in-situ extraction based on superwettable membrane (SWM) with micro-/nano-structures. (a) Diagrammatic sketches of superwettability-based continuous insitu extraction by equipping SWM at the side-face of an extractor; (b) Diagrammatic sketches of traditional extraction that contains thoroughly mixing, gravity settling, and separating procedures; (c) Comparison of operating time between in-situ extraction and traditional extraction illustrating that in-situ extraction exhibits larger throughput per time due to its continuous performance; (d) Comparison of the volume recovery demonstrating that in-situ extraction shows a higher liquidrecovery than traditional route; (e) In-situ extraction also maintaining the similar high efficiency to traditional extraction.


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Figure 3. Superwettability-based continuous and selective in-situ extraction towards multiphase complex systems. (a) Schematic illustration of continuous in-situ extraction focusing on a typical three-liquid-phase system (TLPS), consisting of n-butyl acetate (EA-phase), polyethylene glycol (PEG-phase), and (NH4)2SO4 aqueous solution (AS-phase); (b) Wetting differences of our SWM towards the chosen TLPS; (c) Real-time concentration of PCs in EA-phase displaying a selective extraction by TLPS, with phenol (PC-1) enriched into the EA and o-nitrophenol (PC-2) into the PEG; (d) Final extraction efficiency of PC-1 (colorless) and PC-2 (yellow) conducted by the insitu strategy further proving its selective capacity of PCs, with inset digital images exhibiting the final filtrates of EA and PEG respectively; (e) Our in-situ strategy showing high permeating flux towards various kinds of common extracting agents, including esters, aromatics, halides, alkanes, and ethers.


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Figure 4. Potential of our superwettable membrane (SWM) for real applications. (a) Capacity for laser-fabrication of SWM with controllable micro-pore size that is suitable for different demands of separation flux, inset scale bar: 100 μm; (b) The SWM maintaining its surface nano-structures and superhydrophobicity (CA ~ 158°) even during 180 cycles of mechanical impacts, inset scale bar: 100 μm; (c) Dynamic water-in-air CAs of our SWM indicating excellent durability in harsh chemical environments, detected after long-term work in the as-mentioned TLPS (pH ~ 9.5).


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For Table of Contents Only: Herein, continuous in-situ extraction towards multiphase complex systems using a superwettable membrane with micro-/nano-structures is successfully achieved. It realizes a rapid, selective, and efficient real-time removal of various extracting agents due to the wetting differences. Compared with traditional extracting routes, our strategy shows great advantages of saving time, enhancing liquid-recovery, and simplifying procedures, while remains high extraction efficiency. TOC Graphic:


Continuous in Situ Extraction toward Multiphase Complex Systems

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