•A continuous flow reactor based on a gallium liquid metal droplet is demonstrated•Liquid metal droplet promotes continuous solution pumping and chemical reaction•Electrical signal parameters affect the pumping performance and products qualities Gallium-based liquid metals combine distinct features of high electrical conductivity and fluidity. When immersed in solutions, they establish chemically active and electro-responsive dynamic interfaces. Here, we harness these features and formulate a proof-of-concept liquid metal enabled flow reactor, that can operate in different environments, to produce high-value materials. Upon applying an electrical stimulus to a liquid metal droplet, placed in a fluidic channel filled with a precursor-electrolyte solution, a flow is generated, and these materials are produced as a result of the interfacial interactions of precursor ions with the liquid metal droplet. We show that the electrical signal parameters affect the system performance and control the quality of products. This liquid metal enabled continuous flow reactor is presented as a straightforward and efficient device for synthesizing different materials potentially for development in the pharmaceutical and chemical applications. Continuous flow reactors are used in a wide variety of chemical and biological processes. However, problems with mechanical pumps and lack of reactive components are the current challenges in the setup of contemporary flow reactors. Here we introduce a proof-of-concept liquid metal enabled continuous flow reactor that can promote both mass transport and reactions. We show that a continuously operating system based on a gallium metal droplet can pump a solution containing reagents, while spontaneously allowing a reaction to take place at the metal droplet interface. Model examples are demonstrated for generating Mn3O4, MoS2, and reduced graphene oxide. The system is characterized at different voltages and frequencies to show the gamut of its performance, while characterization of the produced materials showcases the controlled qualities of the synthesized materials and properties of the flow reactor. The proposed continuous flow reactor can find many applications within the food, chemical, and pharmaceutical industries. Continuous flow reactors are used in a wide variety of chemical and biological processes. However, problems with mechanical pumps and lack of reactive components are the current challenges in the setup of contemporary flow reactors. Here we introduce a proof-of-concept liquid metal enabled continuous flow reactor that can promote both mass transport and reactions. We show that a continuously operating system based on a gallium metal droplet can pump a solution containing reagents, while spontaneously allowing a reaction to take place at the metal droplet interface. Model examples are demonstrated for generating Mn3O4, MoS2, and reduced graphene oxide. The system is characterized at different voltages and frequencies to show the gamut of its performance, while characterization of the produced materials showcases the controlled qualities of the synthesized materials and properties of the flow reactor. The proposed continuous flow reactor can find many applications within the food, chemical, and pharmaceutical industries. IntroductionDespite the new fascination with the field of liquid metals, and a plethora of recent applications, they still offer many unprecedented possibilities.1Sun X. Yuan B. Sheng L. Rao W. Liu J. Liquid metal enabled injectable biomedical technologies and applications.Appl. Mater. Today. 2020; 20: 100722https://doi.org/10.1016/j.apmt.2020.100722Google Scholar, 2Tang J. Lambie S. Meftahi N. Christofferson A.J. Yang J. Ghasemian M.B. Han J. Allioux F.-M. 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The electrical double layer and the theory of electrocapillarity.Chem. Rev. 1947; 41: 441-501https://doi.org/10.1021/cr60130a002Google Scholar,10Han J. Tang J. Idrus-Saidi S.A. Christoe M.J. O’Mullane A.P. Kalantar-Zadeh K. Exploring electrochemical extrusion of wires from liquid metals.ACS Appl. Mater. Interfaces. 2020; 12: 31010-31020https://doi.org/10.1021/acsami.0c07697Google Scholar With such properties, liquid metals can, in principle, be used to devise continuous flow reactors.In continuous reactors, one or more fluid reagents are pumped through a fluidic system where a chemical reaction takes place as the reagents travel through the system. In comparison with more traditional batch and stirred-tank reactors, continuous flow reactors offer high-quality control and durability, which both are important for cost-efficient production.11Sun A.C. Steyer D.J. Allen A.R. Payne E.M. Kennedy R.T. Stephenson C.R. A droplet microfluidic platform for high-throughput photochemical reaction discovery.Nat. Commun. 2020; 11: 1-6https://doi.org/10.1038/s41467-020-19926-zGoogle Scholar,12Neyt N.C. Riley D.L. Application of reactor engineering concepts in continuous flow chemistry: a review.React. Chem. Eng. 2021; 6: 1295-1326https://doi.org/10.1039/D1RE00004GGoogle Scholar Conventional continuous flow reactors rely on pumping hardware that generally uses a piston or peristaltic pump.13Murray P.R. Browne D.L. Pastre J.C. Butters C. Guthrie D. Ley S.V. Continuous flow-processing of organometallic reagents using an advanced peristaltic pumping system and the telescoped flow synthesis of (E/Z)-tamoxifen.Org. Process. Res. Dev. 2013; 17: 1192-1208https://doi.org/10.1021/op4001548Google Scholar These mechanical pumps, however, have several drawbacks, including moving parts that can be fouled and blocked via precipitation of materials, energy loss due to the heat generated by friction, and high failure rates.14Zhang C. Xing D. Li Y. Micropumps, microvalves, and micromixers within PCR microfluidic chips: advances and trends.Biotechnol. Adv. 2007; 25: 483-514https://doi.org/10.1016/j.biotechadv.2007.05.003Google ScholarLiquid metals can play two vital roles in changing the fundamental aspects of continuous flow reactors. They can be used as efficient pumps, based on their deformation or Marangoni flow, without any moving parts.15Zavabeti A. Daeneke T. Chrimes A.F. O’Mullane A.P. Zhen Ou J. Mitchell A. Khoshmanesh K. Kalantar-zadeh K. Ionic imbalance induced self-propulsion of liquid metals.Nat. Commun. 2016; 7: 12402https://doi.org/10.1038/ncomms12402Google Scholar,16Khoshmanesh K. Tang S.-Y. Zhu J.Y. Schaefer S. Mitchell A. Kalantar-zadeh K. Dickey M.D. Liquid metal enabled microfluidics.Lab. Chip. 2017; 17: 974-993https://doi.org/10.1039/C7LC00046DGoogle Scholar Liquid metals can also offer a reactive interfacial media that promotes the reaction of reagents.17Ghasemian M.B. Mayyas M. Idrus-Saidi S.A. Jamal M.A. Yang J. Mofarah S.S. Adabifiroozjaei E. Tang J. Syed N. O'Mullane A.P. et al.Self-limiting galvanic growth of MnO2 monolayers on a liquid metal-applied to photocatalysis.Adv. Funct. Mater. 2019; 29: 1901649https://doi.org/10.1002/adfm.201901649Google Scholar, 18Wang Y. Mayyas M. Yang J. Tang J. Ghasemian M.B. Han J. Elbourne A. Daeneke T. Kaner R.B. Kalantar-Zadeh K. Self-deposition of 2D molybdenum sulfides on liquid metals.Adv. Funct. Mater. 2021; 31: 2005866https://doi.org/10.1002/adfm.202005866Google Scholar, 19Lertanantawong B. Lertsathitphong P. O'Mullane A.P. Chemical reactivity of Ga-based liquid metals with redox active species and its influence on electrochemical processes.Electrochem. Commun. 2018; 93: 15-19https://doi.org/10.1016/j.elecom.2018.05.026Google Scholar, 20Wang Y. Wang S. Chang H. Rao W. Galvanic replacement of liquid metal/reduced graphene oxide frameworks.Adv. Mater. Interfaces. 2020; 7: 2000626https://doi.org/10.1002/admi.202000626Google Scholar The surface of liquid metals can be deformed on demand, which helps in releasing the interfacial products, freeing the active surface, and allowing a continuous reaction to occur. In addition, they are conductive and deformable, which can allow for dynamic EDLs with an unlimited availability of source electrons. Altogether, the electron-rich interfaces and atomically smooth surface of liquid metals can naturally provide “super reaction media” for synthesizing materials.17Ghasemian M.B. Mayyas M. Idrus-Saidi S.A. Jamal M.A. Yang J. Mofarah S.S. Adabifiroozjaei E. Tang J. Syed N. O'Mullane A.P. et al.Self-limiting galvanic growth of MnO2 monolayers on a liquid metal-applied to photocatalysis.Adv. Funct. Mater. 2019; 29: 1901649https://doi.org/10.1002/adfm.201901649Google Scholar,21Carey B.J. Ou J.Z. Clark R.M. Berean K.J. Zavabeti A. Chesman A.S.R. Russo S.P. Lau D.W.M. Xu Z.-Q. Bao Q. et al.Wafer-scale two-dimensional semiconductors from printed oxide skin of liquid metals.Nat. Commun. 2017; 8: 14482https://doi.org/10.1038/ncomms14482Google Scholar,22Kalantar-Zadeh K. Tang J. Daeneke T. O’Mullane A.P. Stewart L.A. Liu J. Majidi C. Ruoff R.S. Weiss P.S. Dickey M.D. Emergence of liquid metals in nanotechnology.ACS Nano. 2019; 13: 7388-7395https://doi.org/10.1021/acsnano.9b04843Google ScholarControlling the surface tension of liquid metals is the key to realizing the aforementioned traits for developing continuous flow reactors. The surface tension can be controlled using an external stimulant such as an electric field.10Han J. Tang J. Idrus-Saidi S.A. Christoe M.J. O’Mullane A.P. Kalantar-Zadeh K. Exploring electrochemical extrusion of wires from liquid metals.ACS Appl. Mater. Interfaces. 2020; 12: 31010-31020https://doi.org/10.1021/acsami.0c07697Google Scholar,23Khan M.R. Eaker C.B. Bowden E.F. Dickey M.D. Giant and switchable surface activity of liquid metal via surface oxidation.Proc. Natl. Acad. Sci. U S A. 2014; 111: 14047https://doi.org/10.1073/pnas.1412227111Google Scholar Such possibilities allow for the incorporation of liquid metals as an active element, which does not fatigue with time, in the body of a continuous flow reactor. This work is an attempt to develop a prototype of liquid metal-based continuous flow reaction system, which seems plausible, but has not been shown previously.Here a gallium (Ga) droplet soft component is used as the core of a continuous flow reactor that converts external electrical energy into mechanical motion. Its surface provides a chemically reactive zone for reagents to interact. To show the capability of the system, we introduce three different precursor solutions into our reactor to produce three popular functional nanomaterials; Mn3O4 that is widely utilized for the synthesis of soft ferrites,24Imboon T. Khumphon J. Yotkuna K. Tang I.M. Thongmee S. Enhancement of photocatalytic by Mn3O4 spinel ferrite decorated graphene oxide nanocomposites.SN Appl. Sci. 2021; 3: 653https://doi.org/10.1007/s42452-021-04644-yGoogle Scholar reduced graphene oxide (rGO) that is used in many applications such as those in water purification and energy storage/conversion,25Flouda P. Shah S.A. Lagoudas D.C. Green M.J. Lutkenhaus J.L. Highly multifunctional dopamine-functionalized reduced graphene oxide supercapacitors.Matter. 2019; 1: 1532-1546https://doi.org/10.1016/j.matt.2019.09.017Google Scholar and MoS2 that is known for its new applications of microelectronics.26Ou J.Z. Chrimes A.F. Wang Y. Tang S.-y. Strano M.S. Kalantar-zadeh K. Ion-driven photoluminescence modulation of quasi-two-dimensional MoS2 nanoflakes for applications in biological systems.Nano Lett. 2014; 14: 857-863https://doi.org/10.1021/nl4042356Google Scholar The solutions contain potassium permanganate (KMnO4), graphene oxide (GO), or ammonium tetrathiomolybdate ((NH4)2MoS4). Upon contact between the Ga droplet and precursor solutions, the ions or solid flake-like material (GO) are reduced on the surface of Ga. Using the stimulation generated by a polarizing voltage signal, the Ga droplet performs two functions for the continuous flow reactor: repulsing and inducing mass transport. The repulsion of the product occurs as a result of the pulsing of the liquid metal droplet under the application of a polarizing signal.27Mayyas M. Mousavi M. Ghasemian M.B. Abbasi R. Li H. Christoe M.J. Han J. Wang Y. Zhang C. Rahim M.A. et al.Pulsing liquid alloys for nanomaterials synthesis.ACS Nano. 2020; 14: 14070-14079https://doi.org/10.1021/acsnano.0c06724Google Scholar As the liquid metal droplet is pulsed, the product material at the interface is repelled away from the liquid metal surface. The large liquid metal deformation induces corresponding pulses that assist in efficiently exfoliating the materials formed on the surface of the liquid metal to sustain the continuous flow and reaction. The mass transport effect is enabled by Marangoni flow at the surface, which leads to the circulating flow in the reactor. As a reactor, we validate the viability of continuous production and system performance using different parameters. The products from the continuous reactor, Mn3O4, reduced GO (rGO), and MoS2, are carefully characterized, which demonstrates the capability of the unit to produce high-quality materials.Results and discussionThe liquid metal droplet, which is stimulated by an external voltage signal, plays a key role in the continuous flow reactor. The liquid metal droplet functions as a dynamic component for driving the chemical reaction, repelling the products formed on its surface, and circulating the solution through the channel.Figure 1A is a schematic illustration of the system that includes a fluidic channel filled with a precursor-electrolyte solution and a liquid metal droplet located in a recess that is stimulated by a square wave voltage signal. The details regarding how the fluidic system was printed and the design parameters are found in the experimental procedures (Figure S1).Before applying the external voltage, the liquid metal forms a near-spherical droplet with its surface either negatively or positively charged, depending on the type of electrolyte. This charged liquid metal droplet surface attracts counter ions to its surface, resulting in an accumulation of ions in a diffusion layer to form an EDL. According to Lippmann's equation, the surface tension between the droplet and electrolyte is a function of the potential across it:γ=−12C(φ−φ0)2+γ0(Equation 1) where γ is the surface tension, C is the EDL capacitance per unit area, φ is the EDL potential, φ0 is the potential of zero charge, and γ0 is the maximum surface tension, when the potential difference is φ=φ0.The surface polarity of the liquid metal droplet alternates when an external polarizing voltage is applied as schematically shown in Figure 1B. The Ga droplet plays a vital role as an electron donor because Ga metal itself is a strong reducing species.18Wang Y. Mayyas M. Yang J. Tang J. Ghasemian M.B. Han J. Elbourne A. Daeneke T. Kaner R.B. Kalantar-Zadeh K. Self-deposition of 2D molybdenum sulfides on liquid metals.Adv. Funct. Mater. 2021; 31: 2005866https://doi.org/10.1002/adfm.202005866Google Scholar,19Lertanantawong B. Lertsathitphong P. O'Mullane A.P. Chemical reactivity of Ga-based liquid metals with redox active species and its influence on electrochemical processes.Electrochem. Commun. 2018; 93: 15-19https://doi.org/10.1016/j.elecom.2018.05.026Google Scholar This promotes the formation of products from ionic precursors that are brought to the surface. The products formed on its surface can be detached and released when the polarity is reversed, due to the dramatic change in the liquid metal surface tension. The cycle can be repeated for continuous production. Although some Ga is released at the beginning, the reversible cycle allows it to return to the droplet, resulting in a sustainable system. This consumption of Ga, due to its oxidation into gallate ions,28Hurlen T. Anodic dissolution of liquid gallium in alkaline solutions.Electrochim. Acta. 1964; 9: 1449-1452https://doi.org/10.1016/0013-4686(64)85026-XGoogle Scholar,29Zhang J. Sheng L. Liu J. Synthetically chemical-electrical mechanism for controlling large scale reversible deformation of liquid metal objects.Sci. Rep. 2014; 4: 7116https://doi.org/10.1038/srep07116Google Scholar occurs during the anodic event, while the reverse reaction occurs during the cathodic event. To show this, cyclic voltammograms (CV) using a liquid metal droplet electrode in 1 mol L−1 NaOH electrolyte were run for 25 cycles; where the first, fifth, 10th, 15th, and 20th cycles are presented in Figure 1C. The reaction was carried out over a potential range from −2.0 to 2.0 V (versus a saturated calomel electrode [SCE]) at a scan rate of 4 V s−1. The CV shows two anodic peaks (A1 and A2) corresponding to the oxidation of Ga to its monovalent form and the ionization of adsorbed hydrogen on Ga liquid, respectively.30Chung Y. Lee C.-W. Electrochemistry of gallium.J. Electrochem. Soc. 2013; 4: 1-18https://doi.org/10.5229/JECST.2013.4.1.1Google Scholar,31Song M. Daniels K.E. Kiani A. Rashid-Nadimi S. Dickey M.D. Interfacial tension modulation of liquid metal via electrochemical oxidation.Adv. Intell. Syst. 2021; 3: 2100024https://doi.org/10.1002/aisy.202100024Google Scholar At more positive potentials, the monovalent form of Ga undergoes a transition to its higher oxidation state of Ga(OH)3, which in strong alkaline media, transforms into gallate ions, as described in Equation 2. The cathodic peak (C1) at −1.4 V versus SCE is attributed to the reduction of the oxidized Ga species at the interface, as described in Equation 3.32Perkins R.S. Anodic oxidation of gallium in alkaline solution.J. Electroanal. Chem. 1979; 101: 47-57https://doi.org/10.1016/S0022-0728(79)80078-9Google ScholarAnodic event: Ga(OH)3 + OH− → Ga[(OH)4]−(Equation 2) Cathodic event: Ga[(OH)4]− + 3 e− → Ga + 4 OH−(Equation 3) The CV curves show no significant change in current after 15 cycles. This observation suggests that the Ga dissolved into the electrolyte reaches an equilibrium state. The concentration of Ga in the electrolyte was monitored during the operation of the liquid metal enabled reactor. Figure 1D presents the Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) measurements of the Ga concentration, which shows that it reaches a steady-state after 300 min of operation. In addition, we performed the continuous operation of the reactor for 3 weeks and could not see any visible change in the mass of the droplet. The combined mass of the droplet and electrode did not show any alteration after the 3-week period when measured using a precision scale with the resolution of 10 μg.With the application of an external voltage on the liquid metal droplet, the surface charge becomes unequally distributed due to the nonuniform electric field, as shown in Figure 1E. This nonuniform electric field is a result of the distance variation across the droplet with reference to the counter electrode. On the electrically polarized liquid metal droplet, this unequal charge distribution induces a gradient in surface tension. The voltage difference across the EDL is less downstream qf (the furthest point of the droplet to counter electrode) compared with upstream qi (the closest point of the droplet to counter electrode), as shown in Figure 1E. As such, the change in the surface tension of the liquid metal droplet downstream would be relatively lower, as described by Lippmann's equation (Equation 1). As a result of this surface tension gradient across the liquid metal droplet, a pressure difference P is generated as given by the Young-Laplace equation:P=γ(2r)(Equation 4) where r is the radius of the liquid metal gradient droplet. The P across the two sides of the liquid metal droplet induces a flow of the surrounding solution from the low surface tension region of the droplet toward the high surface tension region through the channel. This type of flow is usually described as Marangoni flow.15Zavabeti A. Daeneke T. Chrimes A.F. O’Mullane A.P. Zhen Ou J. Mitchell A. Khoshmanesh K. Kalantar-zadeh K. Ionic imbalance induced self-propulsion of liquid metals.Nat. Commun. 2016; 7: 12402https://doi.org/10.1038/ncomms12402Google Scholar,33Tang S.-Y. Khoshmanesh K. Sivan V. Petersen P. O’Mullane A.P. Abbott D. Mitchell A. Kalantar-zadeh K. Liquid metal enabled pump.Proc. Natl. Acad. Sci. U S A. 2014; 111: 3304https://doi.org/10.1073/pnas.1319878111Google Scholar It is noteworthy to mention that because of the parabolic correlation of γ against φ, any applied voltage, either positive or negative, results in a decrease of the surface tension. Therefore, Marangoni flow occurs at both polarities. This effect helps to circulate the product away from the surroundings of the liquid metal droplet.Altogether, the liquid metal enabled reactor operates as a result of several synergistic mechanisms, as presented in Figure 1F. In this figure, (i) the static state of the liquid metal droplet without applying an external voltage is the initial state and the product is spontaneously formed on the liquid metal surface; (ii) the liquid metal droplet is then deformed by an anodic voltage, while the product is detached; and (iii) a cathodic event occurs where Marangoni flow mainly takes place and where the liquid metal recovers its original shape. The elastic behavior of the liquid metal in the cathodic and anodic events produces pulses that agitate and release the product from the surface of the droplet.We took digital snapshots of the motion of the liquid metal droplet at one period (Figures 2A and 2B ) and performed numerical simulations to explore the complex vortical (reversed) flows generated around the liquid metal droplet (Figures 2C–2F). These studies were conducted to exemplify the dynamic components involved in our reactor, as explained previously. Figure 2A shows the top and side views of a liquid metal droplet at the cathodic event. The purple color tracer indicates the flow direction from the upstream side. Figure 2C presents the front and rear views of the electric field simulation, which demonstrates that the droplet's front side (i.e., upstream) has the highest electric potential. This observation agrees with our flow simulations in Figure 2E, which show the formation of a vortex around the liquid metal droplet surface. The direction of the flow corresponds to the observation in Figure 2A.Figure 2Mass transport and relevant numerical simulations for the liquid metal enabled reactorShow full caption(A–D) (A and B) Top and side views of the droplet placed in the fluidic system (with a purple tracer) at the (A) cathodic event and (B) at the anodic event, which shows shape deformation. (C and D) Electric field (V m−1) distribution across the liquid metal droplet when placed in the electrolyte experiencing: (C) a cathodic event and (D) anodic event ([i] view from upstream and [ii] view from downstream).(E and F) Formation of vortices along the droplet surface (colored by velocity magnitude of the flow) during the cathodic and anodic events, respectively. It shows the significant impact of the Marangoni flow near the surface.See also Figure S2.View Large Image Figure ViewerDownload Hi-res image Download (PPT)When the voltage alternates to the anodic event, deformation of the liquid metal droplet occurs (Figure 2B).34Tokuda Y. Moya J.L.B. Memoli G. Neate T. Sahoo D.R. Robinson S. Pearson J. Jones M. Subramanian S. Programmable liquid matter: 2D shape deformation of highly conductive liquid metals in a dynamic electric field.Proc. 2017 ACM Intl. Conf. Interactive Surfaces and Spaces. 2017; : 142-150https://doi.org/10.1145/3132272.3134132Google Scholar,35Li M. Mohamed Cassim Mohamed Anver H. Zhang Y. Tang S.-Y. Li W. Automatic morphology control of liquid metal using a combined electrochemical and feedback control approach.Micromachines. 2019; 10: 209https://doi.org/10.3390/mi10030209Google Scholar The flow direction is the same as during the cathodic event, as the side of the liquid metal facing upstream always has the lowest surface tension. This is verified by the electric field simulation presented in Figure 2D. However, in this case, although the surface shear stress on the surface is higher, the overall flow is lower as the droplet symmetry is lost, which interferes with the flow. Consequently, the formation of the vortex demonstrated in Figure 2F drives the flow in the same direction as during the cathodic event. Altogether, the shear stress difference between the two sides of the droplets causes the solution in the vicinity of the liquid metal surface to move at a higher velocity around it, whereas the solution near the free surface of the electrolyte moves at a lower velocity. In addition, a control experiment was conducted using an anodic dc signal. Figure S2 indicates the application of the anodic dc signal that leads to the detachment of liquid metal droplet from the electrode.To showcase the capability of the liquid metal enabled reactor and analyze its characteristics during operation, we tested the system in three different scenarios: one for the production of Mn3O4 from KMnO4 and another for the reduction of GO flakes, which illustrates the reduction processes of ionic precursors and solid flake-like matter on the surface of Ga. The third example is the synthesis of MoS2 using (NH4)2MoS4, which demonstrates the capability of the system when using an ionic precursor in different electrolytes.To test the reduction of MnO4− on the liquid metal droplet surface, an electrochemical assessment was carried out. The electrochemical behavior of the KMnO4 solution (that forms MnO4−) was explored using a three-electrode system with a gold working electrode (Figure 3A). In the potential range from 0.6 to −1.45 V, a cathodic region is observed corresponding to MnO4− reduction. The onset voltage (Vonset) for the reduction of MnO4− was determined to be −0.30 V. For assessing the open-circuit potential (Voc) present on the liquid metal droplet, the liquid metal droplet was placed as a working electrode in an electrolyte solution with the pH adjusted as the precursor solution without MnO4− ions. As shown in Figure 3B, the liquid metal droplet showed a value of −1.57 V versus SCE, which is more negative than the estimated reduction potential of MnO4− (−0.30 V versus SCE), suggesting that the interfacial potential between the interface of the liquid metal droplet and the solution is sufficient to reduce the MnO4− ions into Mn3O4.36Kumar S.S. Basu S. Bishnoi N.R. Effect of cathode environment on bioelectricity generation using a novel consortium in anode side of a microbial fuel cell.Biochem. Eng. J. 2017; 121: 17-24https://doi.org/10.1016/j.bej.2017.01.014Google Scholar As such, we always have the target material of Mn3O4 produced on the surface of the liquid metal droplet and any applied voltage can contribute to repel the products and increase the mass flow.Figure 3The first example: Mn3O4 production in a basic mediumShow full caption(A) CV profiles of a KMnO4-NaOH solution acquired at a scan rate of 100 mV s−1, scanned in the potential window from 0.6 to −1.45 V (negative scan), initial and final voltages were both set to 0.0 V versus SCE.(B) The open-circuit voltage measurement of the liquid metal droplet in a KMnO4-NaOH solution recorded over 1,000 s.(C) CV profiles with and without KMnO4 acquired at a scan rate of 10 V s−1, scanned in the potential window from −5 to 5 V (negative scan), initial and final voltages were both set to 0.0 V versus SCE.(D) A schematic illustration of the mechanism for MnO2 and Mn3O4 formation on the liquid metal droplet and repelled from the liquid metal droplet (LGD).(E) Flow rates versus time at 1.0 Hz frequency and different voltage amplitudes from 2 to 10 Vp-p.(F) Flow rates versus time at 10 Vp-p and different frequencies from 0.5 Hz to 100 Hz. Data are represented as mean ± SD.(G) Sequential snapshots showing the reduction reaction in the reactor at the applied ac signal of 10 Vp-p and 1.0 Hz from 0 to 30 min.(H) Dynamic Raman spectra for the products taken out from the reactor from 10 to 30 min.(I) SEM image and EDS of the fin