Name: generation and control of electrohydrodynamic flows in aqueous electrolyte solutions
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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,...

<ol>
	<li>Stuetzer, O. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Ion+drag+pressure+generation.">Ion drag pressure generation.</a> Journal of Applied Physics. 30, 984-994 (1959).</li><li>Stuetzer, O. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Ion+drag+pumps.">Ion drag pumps.</a> Journal of Applied Physics. 31, 136-146 (1960).</li><li>Melcher, J. R., Taylor, G. I. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Electrohydrodynamics:+A+review+of+the+role+of+interfacial+shear+stresses.">Electrohydrodynamics: A review of the role of interfacial shear stresses.</a> Annual Review of Fluid Mechanics. 1, 111-146 (1969).</li><li>Saville, D. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Electrohydrodynamics:+The+Taylor-Melcher+leaky+dielectric+model.">Electrohydrodynamics: The Taylor-Melcher leaky dielectric model.</a> Annual Review of Fluid Mechanics. 29, 27-64 (1997).</li><li>Stein, D., Kruithof, M., Dekker, C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Surface-charge-governed+ion+transport+in+nanofluidic+channels.">Surface-charge-governed ion transport in nanofluidic channels.</a> Physical Review Letters. 93, 035901 (2004).</li><li>Dekker, C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Solid-state+nanopores.">Solid-state nanopores.</a> Nature Nanotechnology. 2, 209-215 (2007).</li><li>Schoch, R. 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A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Electric+field+modulation+of+the+membrane+potential+in+solid-state+ion+channels.">Electric field modulation of the membrane potential in solid-state ion channels.</a> Nano Letters. 12, 6441-6447 (2012).</li><li>Yasui, T., 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=DNA+manipulation+and+separation+in+sublithographic-scale+nanowire+array.">DNA manipulation and separation in sublithographic-scale nanowire array.</a> ACS Nano. 7, 3029-3035 (2013).</li><li>Ren, Y., 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=Particle+rotational+trapping+on+a+floating+electrode+by+rotating+induced-charge+electroosmosis.">Particle rotational trapping on a floating electrode by rotating induced-charge electroosmosis.</a> Biomicrofluidics. 10, 054103 (2016).</li><li>Ren, Y., 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=Flexible+particle+&#64258;ow-focusing+in+microchannel+driven+by+droplet-directed+induced-charge+electroosmosis.">Flexible particle &#64258;ow-focusing in microchannel driven by droplet-directed induced-charge electroosmosis.</a> ELECTROPHORESIS. 39, 597-607 (2018).</li><li>Qian, W., Doi, K., Uehara, S., Morita, 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=Theoretical+study+of+the+transpore+velocity+control+of+single-stranded+DNA.">Theoretical study of the transpore velocity control of single-stranded DNA.</a> International Journal of Molecular Sciences. 15, 13817-13832 (2014).</li><li>Qian, W., 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=Effect+of+polymer+length+and+salt+concentration+on+the+transport+of+ssDNA+in+nanofluidic+channels.">Effect of polymer length and salt concentration on the transport of ssDNA in nanofluidic channels.</a> Biophysical Journal. 112, 838-849 (2017).</li><li>Liu, W., 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=A+universal+design+of+field-effect-tunable+microfluidic+ion+diode+based+on+a+gating+cation-exchange+nanoporous+membrane.">A universal design of field-effect-tunable microfluidic ion diode based on a gating cation-exchange nanoporous membrane.</a> Physics of Fluids. 29, 112001 (2017).</li><li>Liu, W., 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=Control+of+two-phase+flow+in+microfluidics+using+out-of-phase+electroconvective+streaming.">Control of two-phase flow in microfluidics using out-of-phase electroconvective streaming.</a> Physics of Fluids. 29, 112002 (2017).</li><li>Osman, O. O., Shintaku, H., 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=Development+of+micro-vibrating+flow+pumps+using+MEMS+technologies.">Development of micro-vibrating flow pumps using MEMS technologies.</a> Microfluidics and Nanofluidics. 13, 703-713 (2012).</li><li>Osman, O. O., Shirai, 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=A+numerical+study+on+the+performance+of+micro-vibrating+flow+pumps+using+the+immersed+boundary+method.">A numerical study on the performance of micro-vibrating flow pumps using the immersed boundary method.</a> Microfluidics and Nanofluidics. 19, 595-608 (2015).</li><li>Daiguji, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Ion+transport+in+nanofluidic+channels.">Ion transport in nanofluidic channels.</a> Chemical Society Reviews Home. 39, 901-911 (2010).</li><li>Ristenpart, W. D., Aksay, I. A., Saville, D. 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B., Hann, J., Renaud, P. <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+the+surface+charge+on+ion+transport+through+nanoslits.">Effect of the surface charge on ion transport through nanoslits.</a> Physics of Fluids. 17, 100604 (2005).</li><li>Ross, D., Johnson, T. J., Locascio, L. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Imaging+of+electroosmotic+flow+in+plastic+microchannels.">Imaging of electroosmotic flow in plastic microchannels.</a> Analytical Chemistry. 73, 2509-2515 (2001).</li><li>Hsieh, S. -. S., Lin, H. -. C., Lin, C. -. 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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 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 curren...

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.

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		<tr height="42">
			<td height="42" style="height:42px;width:283px;">Sylgard 184</td>
			<td style="width:279px;">Dow Corning Corp.</td>
			<td style="width:187px;">3097366-0516, 3097358-1004</td>
			<td style="width:167px;">PDMS</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:283px;">Acetone</td>
			<td style="width:279px;">Wako Pure Chemical Industries, Ltd.</td>
			<td style="width:187px;">012-00343</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:283px;">Ethanol</td>
			<td style="width:279px;">Wako Pure Chemical Industries, Ltd.</td>
			<td style="width:187px;">054-00461</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:283px;">0.1 mol/L Sodium Hydroxide Solution</td>
			<td style="width:279px;">Wako Pure Chemical Industries, Ltd.</td>
			<td style="width:187px;">196-02195</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:283px;">Pottasium Chloride</td>
			<td style="width:279px;">Wako Pure Chemical Industries, Ltd.</td>
			<td style="width:187px;">163-03545</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:283px;">Tris-EDTA buffer 100x concentrate</td>
			<td style="width:279px;">Sigma-Aldrich Co. LLC.</td>
			<td style="width:187px;">T9285-10014L</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:283px;">2.93 &#956;m polystyrene particle</td>
			<td style="width:279px;">Merck KGaA</td>
			<td style="width:187px;">L300 Rouge</td>
			<td>Tracer particle</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:283px;">1.01 &#956;m polystyrene particle</td>
			<td style="width:279px;">Merck KGaA</td>
			<td style="width:187px;">K100(23716)</td>
			<td>Tracer particle</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Anion exchange membrane</td>
			<td style="width:279px;">ASTOM Corp.</td>
			<td style="width:187px;">Neosepta AHA</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:283px;">Gold (Au)</td>
			<td style="width:279px;">Furuuchi Chemical Corp.</td>
			<td style="width:187px;">AUT-13301X</td>
			<td>Sputtering target metal</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:283px;">Titanium</td>
			<td style="width:279px;">Furuuchi Chemical Corp.</td>
			<td style="width:187px;">TIT-72301X</td>
			<td>Sputtering target metal</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:283px;">Chromium</td>
			<td style="width:279px;">Furuuchi Chemical Corp.</td>
			<td style="width:187px;">CRT-24301X</td>
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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&#8...

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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