The authors have no acknowledgments.

1. EHD Flow Induced by Rectified Ion Transport
<ol>
	<li>Development of a flow channel device to rectify ion transport pathways
	<ol>
		<li>Make a PTFE mold of the reservoir:
		<ol>
			<li>Cut a 13 x 30 x 10 mm3 mold from a polytetrafluoroethylene (PTFE) block using a milling machine (see Figure 2). Alternatively, purchase a custom-made product.</li>
			<li>Adhere acrylic plates of 15 x 18 x 1 mm3 at both ends of the PTFE mold with a plastic adhesive, which will make slits in the reservoir to settle the bias electrodes. These parts can be cut out from a large plate or purchased.</li>
			<li>Adhere the acrylic plates of 13 x 30 x 1 mm3 at the top and bottom surfaces of the PTFE mold with a plastic adhesive to make planar surfaces for clear observation.</li>
		</ol>
		</li>
		<li>Mix a silicone elastomer base and curing agent in the ratio of 10:1 in a 50 mL tube and shake the tube by hand.</li>
		<li>Settle the liquid PDMS in a vacuum vessel and degas it by using a rotary pump.</li>
		<li>Remove the tube from the vessel. Pour the PDMS into a 40 x 50 x 24 mm3 plastic vessel to mold the outer shape of the reservoir and place the reservoir mold (see step 1.1.1) in it.</li>
		<li>Bake the whole body of the liquid PDMS on a hotplate at 80 &#176;C for about 4 h.</li>
		<li>After the bake, isolate the PDMS reservoir from the PTFE mold and the outer vessel by hand. Make a slit across the center of the reservoir by using a surgical knife. This will be used to put the edges of an anion-exchange membrane (prepared in step 1.1.16) into it using tweezers. 
		NOTE: The PDMS reservoir is filled with electrolyte solutions later, as shown in Figure 2.</li>
		<li>Obtain glass plates (made by special order) with a circular shape of 18 mm in diameter or in a square with 18 mm edges.</li>
		<li>Wash the glass plates by soaking them in acetone, ethanol, and pure water (in that order) in an ultrasonication bath for 15 min each.</li>
		<li>Blow any residual liquids away with an air gun or heat the glass plates with a hotplate for 5 min at about 473 K.</li>
		<li>Using radio frequency sputtering, coat the glass surface with Cr or Ti exposed to Ar plasma for 1 min at 75 W and successively, deposit an Au thin film for 5 min at 75 W, setting the thickness at approximately 100 nm. 
		NOTE: Before coating the glass surface with the target metals, the samples were set in a vacuum chamber that was evacuated with a rotary pump and a molecular diffusion pump until the pressure decreased to 1 x 10&#8722;2 Pa.</li>
		<li>Solder a lead on the Au electrode surface using a soldering iron. 
		NOTE: The shape of the Au electrode may possibly be replaced by squares and helical wires, maintaining surface areas large enough to generate ionic currents.</li>
		<li>With tweezers, set the glass plates coated with an Au thin film at both ends of the reservoir. These are the bias electrodes.</li>
		<li>Cut an anion-exchange membrane into a rectangular shape of 20 x 18 mm2 by using scissors. A surface area of 13 mm in width and 10 mm in height is exposed to a liquid. Here, a box cutter or surgical knife may also be used to cut the membrane.</li>
		<li>Cut out a rectangular piece of 3 x 5.5 mm2 from one edge of the membrane with scissors. 
		NOTE: The thickness of the anion-exchange membrane is 220 &#181;m. The membrane is easily cut with scissors or a box cutter. The edges of the membrane are partly fixed with the slits in the chamber.</li>
		<li>Solidify a PDMS block with a stainless rod of a 1 x 1 mm2 cross-section in the same way as in steps 1.1.4 - 1.1.5, to create a flow channel that penetrates the membrane. Leave the construction overnight and then pull the stainless rod out of the PDMS block.</li>
		<li>Cut the PDMS block with a square flow channel into a 3 x 6 x 4.5 mm piece (see Figure 2) using a surgical knife. Make slits along the outer edges, then attach it to the membrane within the rectangular cutout. 
		NOTE: The top face of the channel has to be set horizontally for a clear observation of the particles in the flow channel via the transparent wall.</li>
	</ol>
	</li>
	<li>Preparation of solutions and pretreatments for experiments
	<ol>
		<li>Prepare NaOH aqueous solutions at concentrations of 1 x 10&#8722;1, 1 x 10&#8722;2, and 1 x 10&#8722;3 mol/L by diluting the stock solution.</li>
		<li>Make a dispersion of polystyrene particles of 2.93 &#181;m on average in diameter in each of the NaOH solutions prepared in step 1.2.1 by setting the concentration to 4.2 x 10&#8722;3 vol%. 
		NOTE: The size of the tracer particles may be changed appropriately to improve observability.</li>
		<li>Ultrasonicate the formatted anion-exchange membrane of 20 x 18 mm2 with a slit of 3 x 5.5 mm2 2x for 10 min in pure water at a power of 100 W.</li>
		<li>With tweezers, set the anion-exchange membrane with the PDMS flow channel into the PDMS reservoir. Fill the reservoir with 4 mL of NaOH solution using a micropipette. 
		NOTE: The membrane surface and flow channel are immersed in the solution, where the membrane surface exposed to the solution is at least 100x larger than the cross-section of the flow channel.</li>
		<li>Apply an electric potential of 2.2 V by using a DC power source in forward and backward directions for 2 h each in series, to improve the conductivity of the membrane before observation.</li>
		<li>Pull the Au electrodes out with tweezers. Remove the solution from the reservoirs using a micropipette.</li>
		<li>Set new Au electrodes in the reservoirs with tweezers. Fill the reservoirs with 4 mL of NaOH solution using a micropipette. Start observations when the solution is equilibrated. 
		NOTE: It may take a few minutes of waiting time until the natural convection settles down, which can be judged by observing the behavior of tracer particles.</li>
	</ol>
	</li>
	<li>Experimental setup and measurement systems
	<ol>
		<li>Set the frame rate and the exposure time of a high-speed complementary metal-oxide-semiconductor (CMOS) camera to 500 fps and 1 ms, respectively. 
		NOTE: As shown in Figure 1, the experimental device is set on the stage of a microscope connected to a high-speed CMOS camera to record the particle motions. The view is magnified in a 15 in monitor with a 100X lens.</li>
		<li>Remove any bubbles from the channel by inserting the tip of a micropipette into the channel end to push or pull out them, before applying an electric potential.</li>
		<li>Externally apply an electric potential of 2.2 V to the Au bias electrodes. Simultaneously monitor the electrical responses by using a potentiostat or a DC power source with a digital multimeter. 
		NOTE: The voltage value is determined to be the upper limit, avoiding the water electrolysis that generates O2 and H2 bubbles in the solution.</li>
		<li>Record the behavior of the tracer particles on the computer.</li>
		<li>Measure an electric potential difference between both ends of the flow channel by using Au probe electrodes and a digital multimeter to confirm that the concentration gradient of ions triggers an EHD flow38,39.</li>
		<li>Determine the origin of the Cartesian coordinate system at the center of the channel. 
		NOTE: The x-axis is along the longitudinal direction of the flow channel, and the y- and z-axes are in the horizontal and vertical directions in the cross-section of the channel, respectively, as shown in Figure 2. The transparent PDMS channel allows liquid flows to be visualized along the x-axis. The view is focused on the xy plane at z = 0 by controlling the depth of focus. The flow data are independent of x in the test section except just near the inlet and outlet of the channel, and the observation point is set at approximately 0.75 mm downstream from the origin, such that x = 0.75, y = 0, and z = 0 mm.</li>
		<li>After a single measurement (of 15 s), short-circuit the electrodes by connecting them to each other with a lead for 20 min until the solution is equilibrated.</li>
		<li>Next, move the entirety of the solution to another vessel (e.g., a 10 mL sample bottle) and stir it with a micropipette.</li>
		<li>Pour the stirred solution into the chamber again using a micropipette when iteratively performing the experiment. 
		NOTE: After observation, the EHD flow velocity is evaluated by using the particle image velocimetry (PIV) method39, which can be done by using the appropriate software to trace the displacement of particles and numerically evaluate the velocity. A detailed explanation of PIV methods and how to use them is omitted here because PIV analyses have been widely used and the procedures of computations depend on the software and operating system that is being used.</li>
	</ol>
	</li>
</ol>
2. Observation of Cation-induced EHD Flows
<ol>
	<li>Development of experimental device
	<ol>
		<li>Form Au bias electrodes with a 26 x 10 mm2 surface on the bottom glass plate according to procedures similar to those previously described in steps 1.1.5 - 1.1.7.</li>
		<li>Using radio frequency sputtering, coat a glass surface with Cr or Ti exposed to Ar plasma for 2 min at 75 W and deposit an Au thin film for 5 min at 75 W. 
		NOTE: This shape of electrode is determined in order to highly concentrate electric fields in the narrowest channel region. The ratio of the electrode surface, whose area of 10 x 10 mm2 is exposed to a liquid, to the cross-section of the channel is ideally 100:1; this ratio is predicted to be sufficient to drop the electric potential at the channel by a large amount16.</li>
		<li>Solder a lead line at an edge of the electrodes by using a soldering iron.</li>
		<li>From a large silicone rubber sheet, cut out 2 chambers, each made of a 1 x 1 x 1 mm3 flow channel placed between two 10 x 10 x 1 mm3 reservoirs, using a surgical knife (see Figure 3). These parts may be replaced by PDMS.</li>
		<li>Cut out a cation-exchange membrane with an average thickness of 127 &#181;m to 20 x 30 mm using a box cutter or surgical knife, as shown in Figure 3.</li>
		<li>Ultrasonicate each part in pure water for 15 min by applying 100 W.</li>
		<li>Insert a cation-exchange membrane between the chambers using tweezers, as shown in Figure 3. This will separate 2 electrolyte solutions of different concentrations.</li>
		<li>Press and seal the stack of the chambers and cation-exchange membrane with glass plates whose dimensions are 26 mm in width and 38 mm long.</li>
	</ol>
	</li>
	<li>Preparation of solutions
	<ol>
		<li>Prepare a dispersion of polystyrene particles of an average diameter of 1.01 &#181;m in a 1 x 10&#8722;2 mol/L tris(hydroxymethyl)aminomethane ethylenediaminetetraacetic acid (Tris-EDTA) buffer solution, where the volume ratio is adjusted to 1 x 10&#8722;2 vol%.</li>
		<li>Prepare a mixture of 1 mol/L of KCl and 1 x 10&#8722;2 mol/L of Tris-EDTA.</li>
		<li>Inject the Tris-EDTA/polystyrene particle and the Tris-EDTA/KCl solutions into the lower and upper chambers, respectively, via syringe needles inserted from the side walls of the chambers. 
		NOTE: The quantity of the solutions injected into each chamber is about 210 &#181;L.</li>
		<li>Wait for about 18 h until the solution is equilibrated as a result of a diffusion of the ions to relax the ion concentration difference between the upper and lower layers. 
		NOTE: In the diffusion process, K+ in the upper solution and H+ in the membrane are expected to penetrate the membrane first, and Cl&#8722; is expected to follow them.</li>
	</ol>
	</li>
	<li>Experimental setup and measurement systems
	<ol>
		<li>Set the experimental device developed in step 2.1 on the stage of the inverted microscope by hand, as shown in Figure 3. Connect the microscope to a high-speed CMOS camera to monitor the trajectories of the particle motions and record the observation data on a computer.</li>
		<li>Apply an electric potential difference of 2 V for 6 s between the two electrodes by using a function generator as a power source.</li>
		<li>To confirm that EHD flows are induced by ion transport, measure the ionic currents simultaneously by using an ammeter40.</li>
		<li>Analyze the recorded trajectories of the particles by the particle tracking velocimetry (PTV) method39. 
		NOTE: After the observations, the EHD flow velocity is evaluated by the PTV method, which is possible by using appropriate software, to trace the displacement of particles and numerically evaluate the velocity. A detailed explanation of PTV methods and how to use them is omitted here because PTV analyses have been widely used and the procedures of the computations depend on the software and operating system that is being used.</li>
	</ol>
	</li>
</ol>

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K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Effect+of+insulating+walls+on+the+structure+of+electrodynamic+flows+in+a+channel.">Effect of insulating walls on the structure of electrodynamic flows in a channel.</a> Technical Physics. 57, 1181-1187 (2012).</li><li>Yano, A., Doi, K., Kawano, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Observation+of+electrohydrodynamic+flow+through+a+pore+in+ion-exchange+membrane.">Observation of electrohydrodynamic flow through a pore in ion-exchange membrane.</a> International Journal of Chemical Engineering and Applications. 6, 254-257 (2015).</li><li>Doi, K., Yano, A., Kawano, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Electrohydrodynamic+flow+through+a+1+mm2+cross-section+pore+placed+in+an+ion-exchange+membrane.">Electrohydrodynamic flow through a 1 mm2 cross-section pore placed in an ion-exchange membrane.</a> The Journal of Physical Chemistry B. 119, 228-237 (2015).</li><li>Yano, A., Shirai, H., Imoto, M., Doi, K., Kawano, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Concentration+dependence+of+cation-induced+electrohydrodynamic+flow+passing+through+an+anion+exchange+membrane.">Concentration dependence of cation-induced electrohydrodynamic flow passing through an anion exchange membrane.</a> Japanese Journal of Applied Physics. 56, 097201 (2017).</li><li>Nagura, R., Doi, K., Kawano, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Characterisation+of+microparticle+transport+driven+by+ionic+current+conditions+in+electrically+polarized+aqueous+solutions.">Characterisation of microparticle transport driven by ionic current conditions in electrically polarized aqueous solutions.</a> Micro &amp; Nano Letters. 12, 526-531 (2017).</li><li>Doi, K., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Nonequilibrium+ionic+response+of+biased+mechanically+controllable+break+junction+(MCBJ)+electrodes.">Nonequilibrium ionic response of biased mechanically controllable break junction (MCBJ) electrodes.</a> The Journal of Physical Chemistry C. 118, 3758-3765 (2014).</li></ol>

The authors have nothing to disclose.

The purpose of this study was to separate cations and anions in aqueous solutions in terms of spatial distributions and transport numbers. Using an anion-exchange membrane, the transport of anions and cations could be rectified in the membrane and in a flow channel that penetrates the membrane, respectively. Alternatively, a cation-exchange membrane that separated high and low concentration solutions worked to generate electrically polarized solutions after a considerable waiting time. As a result, rectified ionic currents succeeded in reducing the applied voltages to induce ion-dragged EHD flows. 
The methods presented here are available for aqueous solutions with low application voltages in comparison to conventional methods that require extremely high voltages of tens of kV to inject electrical charges into non-polar solutions. It was clarified that EHD flows are effective in aqueous solutions as well as non-polar solutions. 
However, the present methods depend on water electrolysis to maintain constant ionic currents in which the ideal potential of water electrolysis is known to be 1.23 V. Thus, there is a limitation on the applied voltage to avoid generating O2 and H2 bubbles that change the properties of a liquid. To overcome this limitation, materials of the electrodes and electrolyte solutions have to be appropriately determined to set electrochemical reactions at the electrode surfaces to generate ionic currents in the solutions. At every trial, the electrode surfaces should be polished and bared to make a strong electric field in the solution, enhancing the electrochemical reactions.
In this study, the use of ion-exchange membranes was proposed to rectify the transport pathways of ion species. On the other hand, the efficiency of EHD flow generation seemed to depend on the capability of the membranes. As discussed in the protocol, the diffusion of ions takes a considerable waiting time until it becomes stable. Therefore, the preprocess to increase the conductivity of the membranes is crucial to improving the efficiency of the EHD flow generation. When maintaining ionic current conditions in externally applied electric fields, transport properties of ions are improved, and electrically polarized conditions are effectively achieved. 
In the future, EHD flows of aqueous solutions are expected to be applicable for liquid flow control systems in micro- and nanofluidic devices combined with EOFs and the like. Furthermore, applications for medical devices, in which ion transport has an important role to stimulate biological cells and signal transduction, are also challenging.

Recently, liquid flow control techniques have attracted much attention because of interest in the applications of micro- and nanofluidic devices1,2,3,4,5,6,7,8,9,10,11,12,13,14,15. In polar solutions, such as aqueous solutions and ionic liquids, ions and electrically charged particles usually bring about electrical charges in liquid flows. The transport of such polarized particles provides an expansion of various applications, such as single-molecule manipulation6,10,11,13,14,15,16,17, ion diode devices12,18, and liquid flow control19,20,21,22. EHD flow has been an applicable phenomenon for liquid flow control systems since Stuetzer1,2 invented the ion drag pump. Melcher and Taylor3 published an important article in which the theoretical framework of EHD flow was well reviewed and some outstanding experiments were also demonstrated. Saville4 and his coworkers23,24 contributed to the following expansion of EHD technologies in liquids. However, there were some limitations to inducing liquid flows driven by electric forces, because tens of kV have to be applied in liquids to inject electrical charges in non-polar solutions, such as oils, to polarize them1,2,3. This is a disadvantage for aqueous solutions because the water electrolysis that is induced by an electric potential higher than 1.23 V changes the characteristics of solutions and makes the solutions unstable.
In micro- and nanofluidic channels, surface charges of channel walls cause the concentration of counterions that effectively induce electroosmotic flows (EOFs) under externally applied electric fields25,26,27,28,29. Using EOFs, some liquid pumping techniques have been applied in aqueous solutions, reducing the electric voltages30,31,32. On the other hand, EOFs are limited to being generated in micro- and nanospaces in which surface areas become more dominant than liquid volumes. Furthermore, depending on the transport of highly concentrated ions very near the wall surfaces, such as in electric double layers, the slip boundary only causes the liquid flow, which may not be sufficient to make pressure gradients7,8,22,26,27. Fine tuning, such as to channel dimensions and salt concentrations, is required for the applications of EOF. In contrast, EHD flows driven by body forces seem to be available to transport masses and energies if the application voltages can be reduced to avoid degrading solvents. Recently, some researchers have suggested applications of EHD flows with low voltages33,34,35,36. Although these technologies have not yet been implemented, the frontiers are expected to expand.
In previous studies, we also conducted experimental and theoretical work on EHD flows in aqueous solutions37,38,39,40. It was supposed that the rectification of ion transport pathways was effective to generate electrically charged solutions that cause electric body forces under electric fields. By using an ion-exchange membrane and a flow channel crossing the membrane, we were able to rectify ionic currents. When applying an anion-exchange membrane, cations concentrated in the flow channel dragged the solvents and developed an EHD flow37,38,39. A difference in the mobility of ion species was an important factor when separating the cationic and anionic currents. Ion-exchange membranes effectively worked to modulate the mobility due to the ion selectivity. Ion transport phenomena were also investigated from the viewpoint of ionic current density influenced by applied electric fields41. These studies have been fruitful for developing manipulation techniques for single molecules, namely, micro- and nanoparticles, whose motions are strongly affected by thermal fluctuations11,16,17. EOFs and EHD flows are expected to expand the variety of precise flow control methods as well as pressure gradients.
In this study, we demonstrate two methods to drive EHD flows in aqueous solutions. First, an NaOH solution is used for a working fluid to drive an EHD flow37,38,39. An anion-exchange membrane separates the liquid into two parts. A polydimethylsiloxane (PDMS) flow channel with a cross-section of 1 x 1 mm and a length of 3 mm penetrates the membrane. By applying an electric potential of 2.2 V, the electrophoretic transport of Na+, H+, and OH&#8722; ions is induced along the electric fields. An anion-exchange membrane and a flow channel effectively work to separate the ion transport pathways, where anions dominantly pass through the membrane and cations concentrate in the flow channel, although both species usually move in opposite directions, maintaining the electroneutrality. Thus, such a condition does not cause a driving force for liquid flows. This structure is crucial to generating an EHD flow whose flow speed reaches on the order of 1 mm/s in the channel because highly concentrated cations accelerated by external electric fields drag solvent molecules. EHD flows are observed and recorded by using a microscope and a high-speed camera as shown in Figure 1. Second, a concentration difference between two liquid phases separated by an ion-exchange membrane causes an electrically polarized condition to be generated crossing an ion-exchange membrane40. In this study, we find the importance of a considerable waiting time to equilibrate ion distributions and a corresponding electric potential, which cause preferable conditions to apply to a body force in a liquid. Crossing the ion-exchange membrane, a weakly polarized condition is achieved. In such a condition, an externally applied electric field induces directional ion transport that generates a body force in a liquid, and as a result, the momentum transfer from the ions to the solvent develops an EHD flow.
As mentioned above, the present devices succeed in drastically decreasing the applied voltage difference to a few volts, and thus this method is can be used for aqueous solutions, although the conventional electrical charge injection methods required tens of kV and are limited to an application to non-aqueous solutions.

<table border="0" cellpadding="0" cellspacing="0" style="width:915px;" width="914">
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</table>

generation and control of electrohydrodynamic flows in aqueous electrolyte solutions

To drive electrohydrodynamic (EHD) flows in aqueous solutions, the separation of cation and anion transport pathways is essential because a directed electric body force has to be induced by ionic motions in liquid. On the other hand, positive and negative charges attract each other, and electroneutrality is maintained everywhere in equilibrium conditions. Furthermore, an increase in an applied voltage has to be suppressed to avoid water electrolysis, which causes the solutions to become unstable. Usually, EHD flows can be induced in non-aqueous solutions by applying extremely high voltages, such as tens of kV, to inject electrical charges. In this study, two methods are introduced to generate EHD flows induced by electrical charge separations in aqueous solutions, where two liquid phases are separated by an ion-exchange membrane. Due to a difference in the ionic mobility in the membrane, ion concentration polarization is induced between both sides of the membrane. In this study, we demonstrate two methods. (i) The relaxation of ion concentration gradients occurs via a flow channel that penetrates an ion-exchange membrane, where the transport of the slower species in the membrane selectively becomes dominant in the flow channel. This is a driving force to generate an EHD flow in the liquid. (ii) A long waiting time for the diffusion of ions passing through the ion-exchange membrane enables the generation of an ion-dragged flow by externally applying an electric field. Ions concentrated in a flow channel of a 1 x 1 mm2 cross-section determine the direction of the liquid flow, corresponding to the electrophoretic transport pathways. In both methods, the electric voltage difference required for an EHD flow generation is drastically reduced to near 2 V by rectifying the ion transport pathways.

Figure 4 (video figure) presents a representative result of an EHD flow generation, resulting from the rectification of ion transport pathways and highly concentrated cations that induced a liquid flow in the channel, according to step 1 of the protocol. Figure 5 shows a result of the PIV analysis, where 20 data points near the center of the channel (y = z = 0 mm) were averaged. In the case of the 1 x 10&#8722;1 mol/L NaOH solution, when an electric potential of 2.2 V was applied at t = 5 s, the velocity of the tracer particles quickly increased to a peak value. After that, the velocity decreased and converged to 0. The peak velocity reached a near 2 mm/s. This is a typical result of an EHD flow generated by using an anion-exchange membrane and a 1 x 10&#8722;1 mol/L NaOH solution.
It was also confirmed that the electrophoretic transport velocity of tracer particles was much lower than the peak velocity of the liquid flow in the 1 x 10&#8722;1 mol/L NaOH solution38,39. As discussed in the literature39, this kind of EHD flow is considered to consist of reversed flows dragged by OH&#8722; passing through the membrane and Na+ and H+ concentrated in the flow channel to compensate for the anion transport in the membrane. As the concentration decreased, the transport behavior tended to become slower. This means that the duration-until the velocity reached a peak-and the decay time seemed to be longer, decreasing the peak value of the velocity. This result indicated that the number of ions whose motion was driven by electric forces decreased, and consequently, the electric body force in the liquid was also reduced.
One important observation is that continuous ionic currents rectified by the ion selective interfaces caused solvent molecules to be dragged in one direction and this caused a liquid flow to develop. In this case, there is a possibility that the liquid flow was enhanced by ion concentration polarization crossing the anion-exchange membrane that triggered the reverse flow in the channel. This point was already mentioned in a previous study39. It was supposed that AC fields were also effective to control liquid flows periodically changing directions. The present EHD flow was limited to transient responses because of the finite number of Na+ ions; this situation was not conducive to maintaining a steady cationic current, even though the applied voltage of 2.2 V was enough to induce the electrolysis of water. To generate constant EHD flows, we propose dragging solvent molecules with ion species that are the dominant carrier of the ionic current. Further details will be verified in our future work. Here, we introduced a representative result of an EHD flow that could be induced in NaOH solutions by rectifying ion transport pathways. Details about the concentration dependency and electric potential differences are also discussed by Yano, Doi, and Kawano37,38 and Yano, Shirai, Imoto, Doi, and Kawano39.
Figure 6 (video figure) shows a representative result of the EHD flow generated in an electrically polarized solution under ionic current conditions. The velocity response of the EHD flow was also analyzed by tracking the tracer particles, as shown in Figure 7, which was a typical result obtained by tracking a single particle near the center of the flow channel. When an electric potential of 2 V was applied from t = 2 to 8 s, polystyrene particles responded to the applied electric field. At t = 2 s, the particle quickly translocated in the backward direction, corresponding to the electrophoretic transport of negative charges. After a short-time response, the flow changed to the forward direction and the velocity became steady at 30 &#181;m/s until the electric potential was turned off.
In this period, the negatively charged polystyrene particles moved in the direction of the transport of the positive charges. In general, the direction could not be reversed spontaneously under the one-directional electric field, even if the surface charge of the particles was fully shielded by the counter cations. Thus, this result indicated that cations dispersed in the solution were also electrophoretically transported along the electric fields, dragging solvent molecules that gradually developed a liquid flow. Negative charges highly concentrated on the particle surface caused an electric force stronger than that by cations distributed in the solution, and, thus, firstly drove the transport in the negative direction. After that, a liquid flow dragged by the cationic current increased a drag force on the particle. In this regime, velocity gradients were actually observed along the y-axis perpendicular to the flow direction, and, thus, a liquid flow generation was actually confirmed.
The behavior of polystyrene particles affected by EHD flows was also evaluated in a previous study, and it was found that the velocity of the EHD flow proportionally increased with an increasing ionic current. A waiting time of over 18 h before applying an external electric field is the most important factor for inducing a constant EHD flow, because it takes such a long time for the ion distributions to be equilibrated since they are almost uniform in the channel. As a result, Poiseuille-like flow patterns are steadily observed. On the other hand, we could not confirm a steady flow when the waiting time was not sufficient to achieve uniform ion distributions.
After observing a constant velocity, the electric potential was turned off at t = 8 s. Here, to quickly change the electric potential difference from 2 to 0 V, the shielded electrodes may require an excessive application of the electric potential to make both electrode surfaces equivalent. In that process, ions highly concentrated near the electrode surfaces receive repulsive electric forces, which result in reversed ionic currents. Especially, the cationic current that was dominant in the lower layer caused a liquid flow to be generated, and a transient response in the backward direction was actually observed in the experimental result, which immediately appeared when the electric potential was turned off and converged to 0 &#181;m/s. Such processes in the EHD flow generation were typical in this experiment. Next to the steady EHD flow, the reverse flows observed when switching the electric potential on and off are also interesting. In the transient responses, electrochemical reactions at the electrode surfaces possibly cause drastic ion concentration gradients that induce the diffusion potential as well as externally applied electric potentials. Such complicated ion transport phenomena have not yet been clarified enough and, therefore, are subjects to be solved in future work.
The mechanisms of the EHD flow generation are schematically presented in Figure 8. An EHD flow induced in NaOH solutions is shown in Figure 8a, corresponding to the case of Figure 4. The EHD flow dragged by Na+ in the channel is triggered by the transport of OH&#8722; in an anion-exchange membrane. The unsteady flow is caused by the dissipation due to mass flux damping, momentum flux damping, surface mobility, and electrowetting of the electrode surfaces. Another mechanism of EHD flow induced under cationic current conditions, which are more dominant than anionic ones, is represented in Figure 8b. K+ ions first penetrate a cation-exchange membrane, causing cation-dominant conditions, and, as a result, EHD flow is induced along the cationic current.
As described above, maintaining electrically polarized conditions under ionic current conditions by reducing the application of the electric potentials is key to generating steady EHD flows. By using the present methods, a few volts may be sufficient to induce EHD flows in aqueous solutions, although water electrolysis is necessary to maintain constant ionic currents to enhance the momentum transfer from electrolyte ions to solvent molecules.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/57820/57820fig1.jpg" /> 
Figure 1: Photograph of experimental setup for EHD flow observation. Motions of tracer particles are traced by a microscope connected to a high-speed camera, recording the trajectories in the controller. Electric potentials are applied by using a potentiostat or a DC electric power source. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/57820/57820fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/57820/57820fig2.jpg" /> 
Figure 2: Schematic illustration of an experimental device. A flow channel made of PDMS is fixed in an anion-exchange membrane and filled with an NaOH aqueous solution. Au electrodes are placed at both ends of the solution. The origin of the coordinate is set at the center of the square flow channel and an observation area is in an xy plane near x = 0.75 and z = 0 mm. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/57820/57820fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/57820/57820fig3.jpg" /> 
Figure 3: Photograph of experimental setup and schematic diagram of a device to induce a cation-dragged EHD flow in an electrically polarized solution. A 1 mol/L KCl and 1 x 10&#8722;2 mol/L Tris-EDTA buffer solution and a 1 x 10&#8722;2 vol% polystyrene (PSt) particle dispersion in a 1 x 10&#8722;2 mol/L Tris-EDTA buffer solution are separated with a cation-exchange membrane, where the average diameter of the PSt particles is 1.01 &#181;m. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/57820/57820fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/57820/57820fig4.jpg" /> 
Figure 4 (video figure): A movie of EHD flow driven by the transport of Na+ ions concentrated in the flow channel. The tracer particles are transported along the direction of the electric field when an electric potential of 2.2 V is applied at t = 5 s. Negatively charged polystyrene particles are brought to the cathode side in an EHD flow driven by the cationic current in the channel. In the case of a 1 x 10&#8722;1 mol/L NaOH solution, a peak velocity near 2 mm/s is reached quickly after applying an electric potential, and the velocity successively decays to zero. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/57820/fig4.mp4" target="_blank">Please click here to view this video. (Right-click to download.)</a>
<img alt="Figure 5" class="xfigimg" src="/files/ftp_upload/57820/57820fig5.jpg" /> 
Figure 5: The response of EHD flow observed in the flow channel, resulting from the PIV analysis for the recorded movie of Figure 4. The velocity response (blue solid line) was obtained by the average of 20 points near the center of the channel (y = z = 0 mm). The velocity is quickly increased after applying an electric voltage of 2.2 V at 5 s and gradually converges to 0 mm/s. The sequence of the applied voltage is also shown with a red dashed line. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/57820/57820fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 6" class="xfigimg" src="/files/ftp_upload/57820/57820fig6.jpg" /> 
Figure 6 (video figure): A movie of EHD flow observed in an electrically polarized solution, separating a 1 mol/L KCl solution and polystyrene dispersion by using a cation-exchange membrane. Applying an electric potential of 2 V from t = 2 to 8 s, the transport of tracer particles reflects an EHD flow driven by a cationic current. A constant flow velocity reaches 30 &#181;m/s during the application of the potential. Additionally, the particles also briefly respond in the negative direction when the electric potential is turned on and off because the electrical charge of a particle firstly affects the motion. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/57820/fig6.mp4" target="_blank">Please click here to view this video. (Right-click to download.)</a>
<img alt="Figure 7" class="xfigimg" src="/files/ftp_upload/57820/57820fig7.jpg" /> 
Figure 7:&#160;The response of EHD flow observed in the channel, resulting from the PTV analysis for the recorded movie of Figure 6. The velocity response (blue solid line) was obtained by tracking a single particle near the center of the channel. The sequence of the applied voltage is also shown with a red dashed line. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/57820/57820fig7large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 8" class="xfigimg" src="/files/ftp_upload/57820/57820fig8.jpg" /> 
Figure 8:&#160;Schematics of EHD flow generation mechanisms corresponding to Figures 4 and 5 (panel a) and 6 and 7 (panel b). (a) Liquid flows are induced in an NaOH aqueous solution that is separated with an anion-exchange membrane, where EOF induced by an OH&#8722; transport in the membrane triggers a flow dragged by an Na+ transport in the channel and is partly dissipated with mass flux damping, momentum flux damping, surface mobility, and electrowetting of the electrode surfaces. (b) The cationic current is more dominant than the anionic current because K+ firstly penetrates a cation-exchange membrane, which contributes to a liquid flow dragged by cations under constant current conditions involving water electrolysis. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/57820/57820fig8large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Generation and Control of Electrohydrodynamic Flows in Aqueous Electrolyte Solutions at JoVE.com

Generation and Control of Electrohydrodynamic Flows in Aqueous Electrolyte Solutions

The rectification of ion transport pathways is an effective method to generate one-directional ion-dragged electrohydrodynamic flows. By setting an ion-exchange membrane in a flow channel, an electrically polarized condition is generated and causes a liquid flow to be driven when an electric field is externally applied.

To drive electrohydrodynamic (EHD) flows in aqueous solutions, the separation of cation and anion transport pathways is essential because a directed ...

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8.8K Views.  Osaka University. The rectification of ion transport pathways is an effective method to generate one-directional ion-dragged electrohydrodynamic flows. By setting an ion-exchange membrane in a flow channel, an electrically polarized condition is generated and causes a liquid flow to be driven when an electric field is externally applied.

Video: Generation and Control of Electrohydrodynamic Flows in Aqueous Electrolyte Solutions

The Preparation of Electrohydrodynamic Bridges from Polar Dielectric Liquids

Horizontal and vertical liquid bridges are simple and powerful tools for exploring the interaction of high intensity electric fields (8-20 kV/cm) and polar dielectric liquids. These bridges are unique from capillary bridges in that they exhibit extensibility beyond a few millimeters, have complex bi-directional mass transfer patterns, and emit non-Planck infrared radiation. A number of common solvents can form such bridges as well as low conductivity solutions and colloidal suspensions. The macroscopic behavior is governed by electrohydrodynamics and provides a means of studying fluid flow phenomena without the presence of rigid walls. Prior to the onset of a liquid bridge several important phenomena can be observed including advancing meniscus height (electrowetting), bulk fluid circulation (the Sumoto effect), and the ejection of charged droplets (electrospray). The interaction between surface, polarization, and displacement forces can be directly examined by varying applied voltage and bridge length. The electric field, assisted by gravity, stabilizes the liquid bridge against Rayleigh-Plateau instabilities. Construction of basic apparatus for both vertical and horizontal orientation along with operational examples, including thermographic images, for three liquids (e.g., water, DMSO, and glycerol) is presented.

Horizontal and vertical electrohydrodynamic liquid bridges are simple and powerful tools for exploring the interaction of high intensity electric fields and polar dielectric liquids. The construction of basic apparatus and operational examples, including thermographic images, for three liquids (e.g., water, DMSO, and glycerol) is presented.

Horizontal and vertical liquid bridges are simple and powerful tools for exploring the interaction of high intensity electric fields (8-20 kV/cm) and ...

Non-aqueous Electrode Processing and Construction of Lithium-ion Coin Cells

Research into new and improved materials to be utilized in lithium-ion batteries (LIB) necessitates an experimental counterpart to any computational analysis. Testing of lithium-ion batteries in an academic setting has taken on several forms, but at the most basic level lies the coin cell construction. In traditional LIB electrode preparation, a multi-phase slurry composed of active material, binder, and conductive additive is cast out onto a substrate. An electrode disc can then be punched from the dried sheet and used in the construction of a coin cell for electrochemical evaluation. Utilization of the potential of the active material in a battery is critically dependent on the microstructure of the electrode, as an appropriate distribution of the primary components are crucial to ensuring optimal electrical conductivity, porosity, and tortuosity, such that electrochemical and transport interaction is optimized. Processing steps ranging from the combination of dry powder, wet mixing, and drying can all critically affect multi-phase interactions that influence the microstructure formation. Electrochemical probing necessitates the construction of electrodes and coin cells with the utmost care and precision. This paper aims at providing a step-by-step guide of non-aqueous electrode processing and coin cell construction for lithium-ion batteries within an academic setting and with emphasis on deciphering the influence of drying and calendaring.

Non-aqueous electrode processing is central to the construction of coin cells and the evaluation of new electrode chemistries for lithium-ion batteries. A step-by-step guide to the basic practices needed as an electrochemical engineer working with batteries in an academic experimental setting is furnished.

Research into new and improved materials to be utilized in lithium-ion batteries (LIB) necessitates an experimental counterpart to any computational ...

Free-form Light Actuators &#8212; Fabrication and Control of Actuation in Microscopic Scale

Liquid crystalline elastomers (LCEs) are smart materials capable of reversible shape-change in response to external stimuli, and have attracted researchers&#39; attention in many fields. Most of the studies focused on macroscopic LCE structures (films, fibers) and their miniaturization is still in its infancy. Recently developed lithography techniques, e.g., mask exposure and replica molding, only allow for creating 2D structures on LCE thin films. Direct laser writing (DLW) opens access to truly 3D fabrication in the microscopic scale. However, controlling the actuation topology and dynamics at the same length scale remains a challenge.
In this paper we report on a method to control the liquid crystal (LC) molecular alignment in the LCE microstructures of arbitrary three-dimensional shape. This was made possible by a combination of direct laser writing for both the LCE structures as well as for micrograting patterns inducing local LC alignment. Several types of grating patterns were used to introduce different LC alignments, which can be subsequently patterned into the LCE structures. This protocol allows one to obtain LCE microstructures with engineered alignments able to perform multiple opto-mechanical actuation, thus being capable of multiple functionalities. Applications can be foreseen in the fields of tunable photonics, micro-robotics, lab-on-chip technology and others.

Here, we fabricate 3D polymeric micro/nano structures in which both the shape and the molecular alignment can be engineered with nanometer scale accuracy by the use of direct laser writing. Light induced deformation of several types of liquid crystalline elastomer microstructures can be controlled in the microscopic scale.

Liquid crystalline elastomers (LCEs) are smart materials capable of reversible shape-change in response to external stimuli, and have attracted ...

Measurement of Quantum Interference in a Silicon Ring Resonator Photon Source

Silicon photonic chips have the potential to realize complex integrated quantum information processing circuits, including photon sources, qubit manipulation, and integrated single-photon detectors. Here, we present the key aspects of preparing and testing a silicon photonic quantum chip with an integrated photon source and two-photon interferometer. The most important aspect of an integrated quantum circuit is minimizing loss so that all of the generated photons are detected with the highest possible fidelity. Here, we describe how to perform low-loss edge coupling by using an ultra-high numerical aperture fiber to closely match the mode of the silicon waveguides. By using an optimized fusion splicing recipe, the UHNA fiber is seamlessly interfaced with a standard single-mode fiber. This low-loss coupling allows the measurement of high-fidelity photon production in an integrated silicon ring resonator and the subsequent two-photon interference of the produced photons in a closely integrated Mach-Zehnder interferometer. This paper describes the essential procedures for the preparation and characterization of high-performance and scalable silicon quantum photonic circuits.

Silicon photonic chips have the potential to realize complex integrated quantum systems. Presented here is a method for preparing and testing a silicon photonic chip for quantum measurements.

Silicon photonic chips have the potential to realize complex integrated quantum information processing circuits, including photon sources, qubit ...

Gain-compensation Methodology for a Sinusoidal Scan of a Galvanometer Mirror in Proportional-Integral-Differential Control Using Pre-emphasis Techniques

Galvanometer mirrors are used for optical applications such as target tracking, drawing, and scanning control because of their high speed and accuracy. However, the responsiveness of a galvanometer mirror is limited by its inertia; hence, the gain of a galvanometer mirror is reduced when the control path is steep. In this research, we propose a method to extend the corresponding frequency using a pre-emphasis technique to compensate for the gain reduction of galvanometer mirrors in sine-wave path tracking using proportional-integral-differential (PID) control. The pre-emphasis technique obtains an input value for a desired output value in advance. Applying this method to control the galvanometer mirror, the raw gain of a galvanometer mirror in each frequency and amplitude for sine-wave path tracking using a PID controller was calculated. Where PID control is not effective, maintaining a gain of 0 dB to improve the trajectory tracking accuracy, it is possible to expand the speed range in which a gain of 0 dB can be obtained without tuning the PID control parameters. However, if there is only one frequency, amplification is possible with a single pre-emphasis coefficient. Therefore, a sine wave is suitable for this technique, unlike triangular and sawtooth waves. Hence, we can adopt a pre-emphasis technique to configure the parameters in advance, and we need not prepare additional active control models and hardware. The parameters are updated immediately within the next cycle because of the open loop after the pre-emphasis coefficients are set. In other words, to regard the controller as a black box, we need to know only the input-to-output ratio, and detailed modeling is not required. This simplicity allows our system to be easily embedded in applications. Our method using the pre-emphasis technique for a motion-blur compensation system and the experiment conducted to evaluate the method are explained.

We propose a method to extend the corresponding frequency by using a pre-emphasis technique. This method compensates for the gain reduction of a galvanometer mirror in sine-wave path tracking using proportional-integral-differential control.

Galvanometer mirrors are used for optical applications such as target tracking, drawing, and scanning control because of their high speed and accuracy. ...

In Situ Monitoring of the Accelerated Performance Degradation of Solar Cells and Modules: A Case Study for Cu(In,Ga)Se2 Solar Cells

The levelized cost of electricity (LCOE) of photovoltaic (PV) systems is determined by, among other factors, the PV module reliability. Better prediction of degradation mechanisms and prevention of module field failure can consequently decrease investment risks as well as increase the electricity yield. An improved knowledge level can for these reasons significantly decrease the total costs of PV electricity. 
In order to better understand and minimize the degradation of PV modules, the occurring degradation mechanisms and conditions should be identified. This should preferably happen under combined stresses, since modules in the field are also simultaneously exposed to multiple stress factors. Therefore, two 'Combined Stress test with in situ measurement' setups have been designed and constructed. These setups allow the simultaneous use of humidity, temperature, illumination, and electrical biases as independently controlled stress factors on solar cells and minimodules. The setups also allow real-time monitoring of the electrical properties of these samples. This protocol presents these setups and describes the experimental possibilities. Moreover, results obtained with these setups are also presented: various examples about the influence of both deposition and degradation conditions on the stability of thin film Cu(In,Ga)Se2 (CIGS) as well as Cu2ZnSnSe4 (CZTS) solar cells are described. Results on the temperature dependency of CIGS solar cells are also presented.

In Situ Monitoring of the Accelerated Performance Degradation of Solar Cells and Modules: A Case Study for CuIn,GaSe2 Solar Cells

Two &#39;Combined Stress test with in situ measurement&#39; setups, which allow real-time monitoring of accelerated degradation of solar cells and modules, were designed and constructed. These setups allow the simultaneous use of humidity, temperature, electrical biases, and illumination as independently controlled stress factors. The setups and various experiments executed are presented.

The levelized cost of electricity (LCOE) of photovoltaic (PV) systems is determined by, among other factors, the PV module reliability. Better prediction ...

Study of Siphon Breaker Experiment and Simulation for a Research Reactor

Under the design conditions of a research reactor, the siphon phenomenon induced by pipe rupture can cause continuous outward flow of water. To prevent this outflow, a control device is required. A siphon breaker is a type of safety device that can be utilized to control the loss of coolant water effectively.
To analyze the characteristics of siphon breaking, a real-scale experiment was conducted. From the results of the experiment, it was found that there are several design factors that affect the siphon breaking phenomenon. Therefore, there is a need to develop a theoretical model capable of predicting and analyzing the siphon breaking phenomenon under various design conditions. Using the experimental data, it was possible to formulate a theoretical model that accurately predicts the progress and the result of the siphon breaking phenomenon. The established theoretical model is based on fluid mechanics and incorporates the Chisholm model to analyze two-phase flow. From Bernoulli&#39;s equation, the velocity, quantity, undershooting height, water level, pressure, friction coefficient, and factors related to the two-phase flow could be obtained or calculated. Moreover, to utilize the model established in this study, a siphon breaker analysis and design program was developed. The simulation program operates on the theoretical model basis and returns the result as a graph. The user can confirm the possibility of the siphon breaking by checking the shape of the graph. Furthermore, saving the entire simulation result is possible and it can be used as a resource for analyzing the real siphon breaking system.
In conclusion, the user can confirm the status of the siphon breaking and design the siphon breaker system using the program developed in this study.

The siphon breaking phenomenon was investigated experimentally and a theoretical model was proposed. A simulation program based on the theoretical model was developed and the results of the simulation program were compared with experimental results. It was concluded that the results of the simulation program matched the experimental results well.

Under the design conditions of a research reactor, the siphon phenomenon induced by pipe rupture can cause continuous outward flow of water. To prevent ...

Simultaneous Multi-surface Anodizations and Stair-like Reverse Biases Detachment of Anodic Aluminum Oxides in Sulfuric and Oxalic Acid Electrolyte

After reporting on the two-step anodization, nanoporous anodic aluminum oxides (AAOs) have been widely utilized in the versatile fields of fundamental sciences and industrial applications owing to their periodic arrangement of nanopores with relatively high aspect ratio. However, the techniques reported so far, which could be only valid for mono-surface anodization, show critical disadvantages, i.e., time-consuming as well as complicated procedures, requiring toxic chemicals, and wasting valuable natural resources. In this paper, we demonstrate a facile, efficient, and environmentally clean method to fabricate nanoporous AAOs in sulfuric and oxalic acid electrolytes, which can overcome the limitations that result from conventional AAO fabricating methods. First, plural AAOs are produced at one time through simultaneous multi-surfaces anodization (SMSA), indicating mass-producibility of the AAOs with comparable qualities. Second, those AAOs can be separated from the aluminum (Al) substrate by applying stair-like reverse biases (SRBs) in the same electrolyte used for the SMSAs, implying simplicity and green technological characteristics. Finally, a unit sequence consisting of the SMSAs sequentially combined with SRBs-based detachment can be applied repeatedly to the same Al substrate, which reinforces the advantages of this strategy and also guarantees the efficient usage of natural resources.

A protocol for fabricating nanoporous anodic aluminum oxides via simultaneous multi-surfaces anodization followed by stair-like reverse biases detachments is presented. It can be applied repeatedly to the same aluminum substrate, exhibiting a facile, high-yield, and environmentally clean strategy.

After reporting on the two-step anodization, nanoporous anodic aluminum oxides (AAOs) have been widely utilized in the versatile fields of fundamental ...

Electric-field Control of Electronic States in WS2 Nanodevices by Electrolyte Gating

A method of carrier number control by electrolyte gating is demonstrated. We have obtained WS2 thin flakes with atomically flat surface via scotch tape method or individual WS2 nanotubes by dispersing the suspension of WS2 nanotubes. The selected samples have been fabricated into devices by the use of the electron beam lithography and electrolyte is put on the devices. We have characterized the electronic properties of the devices under applying the gate voltage. In the small gate voltage region, ions in the electrolyte are accumulated on the surface of the samples which leads to the large electric potential drop and resultant electrostatic carrier doping at the interface. Ambipolar transfer curve has been observed in this electrostatic doping region. When the gate voltage is further increased, we met another drastic increase of source-drain current which implies that ions are intercalated into layers of WS2 and electrochemical carrier doping is realized. In such electrochemical doping region, superconductivity has been observed. The focused technique provides a powerful strategy for achieving the electric-filed-induced quantum phase transition.

Electric-field Control of Electronic States in WS2 Nanodevices by Electrolyte Gating

Here, we present a protocol to control the carrier number in solids by using the electrolyte.

A method of carrier number control by electrolyte gating is demonstrated. We have obtained WS2 thin flakes with atomically flat surface via scotch tape ...

Generation and Coherent Control of Pulsed Quantum Frequency Combs

We present a method for the generation and coherent manipulation of pulsed quantum frequency combs. Until now, methods of preparing high-dimensional states on-chip in a practical way have remained elusive due to the increasing complexity of the quantum circuitry needed to prepare and process such states. Here, we outline how high-dimensional, frequency-bin entangled, two-photon states can be generated at a stable, high generation rate by using a nested-cavity, actively mode-locked excitation of a nonlinear micro-cavity. This technique is used to produce pulsed quantum frequency combs. Moreover, we present how the quantum states can be coherently manipulated using standard telecommunications components such as programmable filters and electro-optic modulators. In particular, we show in detail how to accomplish state characterization measurements such as density matrix reconstruction, coincidence detection, and single photon spectrum determination. The presented methods form an accessible, reconfigurable, and scalable foundation for complex high-dimensional state preparation and manipulation protocols in the frequency domain.

A protocol is presented for the practical generation and coherent manipulation of high-dimensional frequency-bin entangled photon states using integrated micro-cavities and standard telecommunications components, respectively.

We present a method for the generation and coherent manipulation of pulsed quantum frequency combs. Until now, methods of preparing high-dimensional ...