Generation and Control of Electrohydrodynamic Flows in Aqueous Electrolyte Solutions

Summary

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.

Abstract

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.

Introduction

Recently, liquid flow control techniques have attracted much attention because of interest in the applications of micro- and nanofluidic devices^{1,2,3,4,5,6,7,8,9,10,11,12,13,14,}

Protocol

1. EHD Flow Induced by Rectified Ion Transport

1. Development of a flow channel device to rectify ion transport pathways
   1. Make a PTFE mold of the reservoir:
      1. 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.
      2. 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.
      3. Adhere ....

Results

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^−1<.......

Discussion

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.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Sylgard 184	Dow Corning Corp.	3097366-0516, 3097358-1004	PDMS
Acetone	Wako Pure Chemical Industries, Ltd.	012-00343
Ethanol	Wako Pure Chemical Industries, Ltd.	054-00461
0.1 mol/L Sodium Hydroxide Solution	Wako Pure Chemical Industries, Ltd.	196-02195
Pottasium Chloride	Wako Pure Chemical Industries, Ltd.	163-03545
Tris-EDTA buffer 100x concentrate	Sigma-Aldrich Co. LLC.	T9285-10014L
2.93 μm polystyrene particle	Merck KGaA	L300 Rouge	Tracer particle
1.01 μm polystyrene particle	Merck KGaA	K100(23716)	Tracer particle
Anion exchange membrane	ASTOM Corp.	Neosepta AHA
Gold (Au)	Furuuchi Chemical Corp.	AUT-13301X	Sputtering target metal
Titanium	Furuuchi Chemical Corp.	TIT-72301X	Sputtering target metal
Chromium	Furuuchi Chemical Corp.	CRT-24301X	Sputtering target metal
Hight-speed CMOS camera	Keyence Corp.	VW-600M
Microscope	Keyence Corp.	VW-9000
Data logger	Keyence Corp.	NR-500, NR-HA08
Laser displacement meter	Keyence Corp.	LK-G5000, LK-H008W
PIV and PTV software	DITECT Co. Ltd.	Flownizer 2D
Potentiostat	AMTEK Inc.	VersaSTAT4
Inverted microscope	Olympus Corp.	IX73
High-speed CMOS camera	Andor Technology Ltd.	Zyla 5.5 sCMOS
Function generator	NF Corp.	WF1945B
Function generator	NF Corp.	WF1973
Ultrasonic cleaner	AS ONE Corp.	AS22GTU
Rotary pump	ULVAC, Inc.	G-100S	Degas liquid PDMS
Rotary pump	ULVAC, Inc.	GLD-201A	Sputtering
Molecular diffusion pump	ULVAC, Inc.	VPC-400	Sputtering