Concentration polarization is a di usion-limited phenomenon for ion transport in 1 electrodialysis based desalination processes. Once a so-called limiting current is 2 reached, the resistance of the system rises notably manifested as a plateau region 3 in the current-voltage curves. For long it is hypothesized that altering the surface 4 properties of the membrane can overcome the di usional transport limitation by the 5 induction of electroconvective vortices mixing up the laminar boundary layer. To 6 systematically investigate the in uence of geometrical and chemical membrane sur- 7 face topology on the evolution of electroconvection, circular patterns of polystyrene, 8 poly(2-vinylpyridine) and P2VP-microgels were inkjet printed on cation-exchange 9 membranes. All types of patterns cause an insigni cant increase in membrane resis- 10 tance but they reduce the plateau lengths indicating the desired accelerated onset 11 of electroconvection. In case of PS patterns, the drop in plateau length results in 12 a small reduction in transport resistance for overlimiting currents. However, mem- 13 branes modi ed with linear P2VP and P2VP microgel patterns do exhibit a signif- 14 icantly decreased resistance in this region at a simultaneous increase of the limiting 15 current density. Direct numerical simulations support our interpretation that the 16 surface charge of the printed patterns in uences the direction of the vortices being 17 advantageous during ion transport towards the membrane. 18 1 Introduction 19 Biological as well as synthetic membrane processes are limited by di usional resis- 20 tances at the membrane uid interface resulting in polarized concentration pro les. 21 In the eld of synthetic membranes for technical applications such as desalination 22 2 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 and bioprocessing, the phenomenon of concentration polarization frequently ham- 23 pers development and implementation. In pressure-driven membrane processes, such 24 as reverse osmosis, nano ltration and ultra ltration, the convective ux towards the 25 membrane at the feed side is higher than the di usive back transport of rejected 26 species. The ux through the membrane stagnates at the so-called critical ux even if 27 the transmembrane pressure is increased. Concentration polarization also severely 28 a ects ion transport through ion-exchange membranes in electrodialysis systems. 29 Here, an electrical potential applied over the membranes acts as the driving force. 30 At low applied potentials, the resulting current scales linearly with the potential as 31 seen in Figure 1 (a). When the potential is further increased, the di usion boundary 32 layer gets depleted due to higher transport numbers of counter ions in the ion ex- 33 change membrane as compared to the ones in the solution. This leads to a plateau 34 region analogous to that appearing in pressure driven processes, where the current 35 (the ux of ions) through the membrane cannot be increased. 36 As opposed to pressure-driven processes, in electrodialysis the current density 37 suddenly increases once a threshold voltage is surpassed. Di erent phenomena are 38 responsible for this so-called overlimiting current transfer [1, 2]. In most cases the 39 evolution of electroconvection is the governing mechanism [1]. Here, hydrodynam- 40 ical instabilities induce vortices at the membrane surface which mix the di usion 41 boundary layer. Therefore, new ions are transported to the membrane and higher 42 ionic uxes can be sustained. Many theoretical works have been conducted to un- 43 derstand the origin of the electroconvective uxes and their in uence on overlimiting 44 mass transfer [3, 4, 5]. For the rst time, the existence of these vortices was exper- 45 imentally proven by Rubinstein et al. [6] and also visualized in an electrodialysis 46 ow channel by Kwak et al.[7]. 47 3 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 Figure 1: Scheme of a) a typical i-v curve and b) the electric eld line distribution of patterned and unpatterned membranes. De Valen ca et al. [8] further investigated the speed of vortices evolving at di erent 48 stages of the ion depletion process. Druzgalski et al. [9] quanti ed the correlations 49 between current density in the membrane and the normal velocity in the adjacent 50 bulk uid by examining 3D direct numerical simulation data. Their analysis also 51 revealed that the enhancement in ion transport is dominantly contributed by large 52 vortices with sizes in the order of the thickness of the boundary layer. 53 54 Electrical and geometrical inhomogeneities of the membrane surface were math- 55 ematically predicted to lead to an intensi ed onset of electroconvection and there- 56 with a shortened plateau length [4, 10]. Figure 1 b) visualizes the reasoning be- 57 hind the earlier onset of electroconvection: the de ection of the electrical eld lines 58 due to a heterogeneous membrane surface causes a tangential component which, 59 in turn, induces an electrolyte slip, initiating and maintainig electroconvective vor- 60 tices. Balster et al. [2] experimentally proved that chemical heterogeneity as well 61 as micrometer-sized geometrical embossed undulations, indeed, may impact on the 62 evolution of electroconvection resulting in a decreased plateau length. Nanometer- 63 thick micrometer-sized polyelectrolyte patterns transferred through micro-contact 64 printing on top of a membrane have the same e ect on the emergence of electro- 65 osmotic induced uid instabilities [11]. Other works have shown that also hydropho- 66 4 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 bic surfaces lead to an intensi ed evolution of electro-convective vortices mixing 67 the di usion boundary layer[12, 13, 14]. Through direct numerical simulations, we 68 have recently discovered a complex chaotic mixing behavior induced by imperme- 69 able patches [15]. Yet, little is known whether and how the surface charge and the 70 structure of the patterns in uence the onset of electro-convection. 71 Here, we report a new fabrication process for patterned ion exchange membranes 72 based on ink-jet printing. Ink-jet printing is a versatile method achieving an accu- 73 rate deposition of polymers, particles or cells [16, 17, 18, 19]. Through the choice of 74 the nature of the ink, very di erent surface functionalities can be applied. In mem- 75 brane science, ink-jet printing has recently been used to modify composite mem- 76 branes for desalination and water treatment [20]. Yet, the challenge in using ink-jet 77 printing lies in nding appropriate properties of the ink to reproducibly print the 78 structures. For instance, the use of a mixture of solvents tailors their evaporation 79 rate to produce the desired structures [17, 21]. Another parameter in uencing the 80 printed structures is the hydrophobicity of the substrate[22]. For instance, printing 81 of hydrophobic patches on hydrophilic polymer lms that swell in a solvent is an 82 experimental challenge, addressed below. 83 With respect to printed patterns on electrodialysis membranes, we investigate three 84 very di erent materials: dots printed from (a) hydrophobic linear polystyrene (PS) 85 bearing a negative zeta potential (b) linear poly(2-vinylpyridine) (P2VP) polymers 86 having a positive zeta potential and (c) colloidal P2VP microgels. Polystyrene and 87 P2VP are relevant as they represent the situation in which the in uence of the 88 surface charge is depicted. Colloidal microgels are relevant as they can form col- 89 loidally assembled microstructures. They are crosslinked, water-swollen polymer 90 entities that swell or deswell due to di erent stimuli such as changes in pH, temper- 91 5 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 ature or ionic strength [23, 24, 25, 26]. A recently reported application in the eld 92 of membrane science is the generation of thermoresponsive hollow ber ltration 93 membranes by coating them with microgels [27, 28]. The potential of microgels to 94 in uence ion transport in electrochemical deionization processes is relevant as well. 95 Their e ect on monovalent ion selectivity was investigated in our recent paper [29]. 96 Moreover, their ability to exhibit distinct properties under the e ect of external 97 stimuli opens a variety of possibilities to reversibly tune ion transport according to 98 the needs of a speci c electro-membrane process. One possibility to tailor the ion 99 transport in these processes by means of microgels is shown below. In this paper, 100 we functionalize the surface of Na on 117 cation exchange membranes with ink-jet 101 printed dotted PS and P2VP patterns as well as colloidal P2VP microgels patterns. 102 This represents the rst systematic experimental and simulation study investigating 103 the in uence of dot nature and surface charge, dot spacing and dot density on the 104 accelerated onset of electroconvection. 105 2 Experimental details 106 2.1 Materials 107 The membranes used in the experiments are commercially available Na on 117 108 cation-exchange membranes (DuPont, USA) either as received or modi ed with ink- 109 jet printed patterns of linear polystyrene (PS), linear poly(2-vinylpyridine) (P2VP) 110 polymers or crosslinked P2VP microgels. For the printing procedure, 1 wt% of ei- 111 ther linear PS (average molecular weight 280 kDa) or linear P2VP polymers (average 112 molecular weight: 37.5 kDa) were dissolved in a mixture of (60/40) wt% ethyl ac- 113 etate and acetophenone. All chemicals were purchased from Sigma-Aldrich. P2VP 114 6 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 microgels were synthesized via surfactant free emulsion polymerization [29] with a 115 crosslinking degree of 1:100. Absorbed on a surface, the microgels have a thickness 116 of 300 nm in swollen state. For the patterning, the microgels were diluted to 0.05 117 wt% in ultra pure water. The di erently modi ed membranes were characterized 118 and compared with pristine membranes of the same batch.The ink-jet printing was 119 performed with an Autodrop Compact 2.21 ink-jet printer with a MD-K-140 print 120 head (microdrop Technologies, Germany). 121 2.2 Electrochemical Characterization of the Membranes 122 2.2.1 Current voltage measurements 123 The in uence of di erent polymer patterns on the onset and intensity of electrocon- 124 vection was characterized by current-voltage polarization curves and chronopoten- 125 tiometric measurements. The experiments were done in a six compartment electro- 126 dialysis cell [30]. The compartments surrounding the investigated membranes were 127 lled with 0.1 M NaCl solution. Prior to the experiments, the membranes were 128 equilibrated in this solution for at least 12 hours. All measurements were executed 129 using the four-electrode method. Two working electrodes were used to apply the 130 voltage inducing electric current, whereas the voltage drop over the Na on refer- 131 ence or ink-jet printed membranes was obtained by two calomel reference electrodes 132 (ProSense B.V., QM712X). To bring the reference electrodes as close as possible to 133 the membrane surface, the electrodes were extended using Haber-Luggin capillaries. 134 The electrolyte ow rate was kept constant at 0.5 L/min. During all experiments 135 the temperature was maintained at 25C. 136 In the current-voltage measurements, the voltage at the working electrodes was in- 137 creased stepwise with an Agilent 6634B power supply, every 30 s in the underlimiting 138 7 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 regime and every 60 s in the plateau and overlimiting regime. Conductivity and pH 139 values were recorded in every compartment. These parameters as well as the cur- 140 rent and voltage values were logged every second. No signi cant water splitting was 141 registered throughout all experiments. 142 2.2.2 Data analysis of current voltage measurements 143 The resistance of the membrane/solution system in the ohmic regime was determined 144 as the reciprocal of the slope of the current-voltage (i-V) curve in this regime. Here, 145 the limiting current density ( ilim) was calculated by subtracting the ohmic part from 146 the i-V curve. As a result of the subtraction, the ohmic part of the curve lies on 147 the y-axis. With the beginning of the plateau regime, the reduced voltage drop 148 starts to deviate from zero. Thus, this point is referred to as ilim. To obtain the 149 plateau length, a sigmoidal function was tted to the reduced current-voltage curve 150 to nd its in ection point [31]. Knowing the membrane voltage drop at which the 151 limiting and the overlimiting current densities are located, the plateau length is 152 easily calculated by the di erence between both values. 153 2.2.3 Chronopotentiometric measurements 154 Chronopotentiometric measurements enable the characterization of the time-evolution 155 of the concentration polarization at the membrane surface. For this purpose a con- 156 stant current is applied to the system and the voltage response is tracked [32, 33]. 157 These measurements were executed with an IviumStat XR potentiostat (Ivium Tech- 158 nologies, the Netherlands). The current and voltage values were logged every 0.1 159 seconds. Chronopotentiometric curves can be divided into four parts [32]. First, 160 after setting the current, the voltage rises instantaneously, reaching a value which 161 re ects the ohmic membrane and solution resistance. In the second part, the voltage 162 8 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 increases only slightly with time. After a certain time, the voltage rises steeply as 163 the polarization at the membrane surface intensi es. The time, corresponding to the 164 in ection point during the steep voltage increase, is known as the transition time 165 and it represents the time at which the salt concentration at the membrane surface 166 approaches zero. From this point on, the ion ux is maintained by electroconvective 167 uxes in case electroconvection is the governing mass transfer mechanism of the 168 overlimiting mass transfer. Finally, the response stabilizes to an average value with 169 uctuations representing the electro-convective mixing of the boundary layer. The 170 transition time can be correlated to the current by means of the Sand equation[32]: 171 =D 4 cbzF tmts!2 1 i2(1) Where D is the Di usion coecient of the permeating ion, cbis the bulk con- 172 centration, z the valency and F the Faraday constant. tmandtsstand for the ion 173 transport numbers in the membrane and in the solution respectively. 174 When inhomogenoeus membranes are investigated, the current density has to be 175 corrected by the fraction of conducting regions resulting in an increased current 176 density in the conductive areas of the membrane[34]: 177 ilocal=i (2) With this the Sand equation for inhomogeneous membranes can be written as: 178 =D 4 cbzF tmts!2 1 i2 local(3) A drawback of the Sand equation is that it only takes the di usive ion transport 179 into account. It was already mathematically and experimentally shown that exper- 180 imentally determined transition times are always larger than the ones obtained by 181 9 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 the Sand equation [13, 14, 35]. Thus, electroconvective ows may already promote 182 the ion transfer even before the membrane surface is fully depleted. In simulations of 183 electroconvection at wavily structured membranes, Pham et al. [36] found that rst 184 vortices even occur in the underlimiting regime. Rubinstein et al. [37] showed that 185 also any instability in the EDL leads to an onset of electroconvection already in the 186 underlimiting region. Since the transition time is proportional to the inverse square 187 of the current density, the product of the other parameters in the Sand equation 188 can be seen as a constant which is proportional to the mass transfer coecient de- 189 scribing the electro-di usive as well as the convective contributions to ion transport. 190 Merging these parameters into a single one leads to 191 =K1 i2(4) and 192 =Klocal1 i2 local(5) respectively. Further, a mass transfer enhancement factor P can be de ned as 193 P=Kpat Kref(6) whereKpatandKrefrepresent the mass transfer constants at the patterned and 194 the reference membranes respectively. 195 To determine the transition time of chronopotentiometric measurements, the in- 196 ection point of voltage response over time, the maximum of the rst derivative, 197 has to be found. Therefore, the derivative is approximated by forward di erential 198 quotients: 199 @Um @tUm(t+ t)Um(t) t(7) 10 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 t, here 0.1 s, represents the time between two measuring points. 200 2.3 Simulations 201 To investigate the impact of patterned ion exchange membrane surfaces on the 202 ion transport, two di erent 2D simulations of the membrane/solution system were 203 performed in a nite element based commercial multiphysics package, COMSOL 204 Multiphysics 5.3. Further details and schematic representations of the investigated 205 domains of the two modells can be found in the supporting information of this paper. 206 2.3.1 Steady-State Modelling 207 The steady-state model investigates the in uence of patterns on the ohmic behaviour 208 and the limiting current. It neglects the in uence of induced ow and the physics 209 describing the onset of electroconvection. The system modelled consists of a mem- 210 brane and two di usion boundary layers. Both sides of the membrane is adjacent 211 to NaCl solutions which are assumed to have uniform concentration of the bulk 212 soltuion as an initial conditions. 213 The area covered by impermeable and uncharged patches was altered to mimic 214 the experimental conditions. The set of equations to describe transport of N species 215 through the di usion boundary layers and the membrane is the following [38, 39]. 216 For a better comparison with the experiments, all the equations are considered in 217 the dimensional form. 218 0 =rji; 8i= 1:::N (8) Equation 8 is the conservation of ionic species, where jiis the ux of species i 219 described by the Nernst-Planck equation: 220 11 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 ji=Di(rci+ziciF RTr'); (9) whereciandziare the concentration and valence of species irespectively, Diis 221 the di usion coecient, Fis the Faraday constant, Ris the ideal gas constant, Tis 222 the absolute temperature and 'is the electric potential. 223 r2'=F ""0e: (10) Equation 10 is Poisson's equation, where "is the solution relative permittivity, "0 224 is the vacuum permittivity and eis the free electric charge density [38, 39]: 225 e=NX i=1zici: (11) To model the membrane domain, the di usion coecient Diis altered by a co- 226 ecient ( = 0:1) to account for the membrane porosity and tortuosity of the 227 di usion path. In the Poisson equation, a term is added to the free electric charge 228 density representing the membrane xed charge groups [38, 39]. Hence, for the 229 membrane domain, equations 9 and 10 have the form: 230 ji= Di(rci+ziciF RTr'); (12) r2'=F ""0(e+): (13) To obtain current-voltage curves, the electrical potential was step wise increased 231 over the three domains until the limiting current density was approached. The to- 232 tal current density was calculated after each step. Then, the distribution of the 233 counter-ions over the membrane thickness and in the di usion boundary layers was 234 analyzed in order to better understand the in uence of the membrane coverage on 235 12 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 the resistance of the system. 236 237 2.3.2 Dynamic Modelling 238 For the description of electrokinetic ow at zero Reynolds number, a dynamic model 239 was implemented in COMSOL, as described by Karatay et al.[40]. Here, the 2D 240 computational domain consists of an electrolyte above a cation exchange membrane 241 surface. The membrane is not included in the model. However, the boundary 242 conditions are set to describe the behavior of the electrolyte right at the membrane 243 surface. 244 The above mentioned equations 8, 9 and 10 are scaled in dimensionless form and 245 were modi ed to depict the time dependency and convection[41]. The set of equa- 246 tions, outlined above, is extended by the Navier-Stokes-equation and the continuity 247 equation for the electrolyte phases. 248 The modi ed Nernst-Planck and conservation equation result in: 249 ci t=r (ciuDirciDiziciV1 Tr ) (14) Here, uis the velocity vector in x and y direction and VTis the thermal voltage 250 (VT0:025V) and the dimensionless representation of the applied voltage. 251 Poisson's equation is in nondimensional form: 252 r2 =ccca+sig 22(15) Here,ccandcarefer to the cation and anion concentration, respectively. Sig is 253 the concentration of xed charge in the patches. It is set to 0 for all regions outside 254 the patches. Furthermore, =D Lis the dimensionless form of the Debye length D 255 13 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 divided by the height of the domain L. 256 Druzgalski et al.[41] showed that the inertial term of the Navier-Stokes-equation 257 can be neglected in this formulation. An incompressible uid is described by: 258 0 =rp+r2uer (16) and 259 ru= 0 (17) wherepandare the pressure and the viscosity. 260 Four patches are spread equidistantly above the membrane surface bearing a static 261 positive or negative charge. To mimic the di erent surface charge of the polymers 262 used in the experiments, sig was set to be 0.01 c bulkwithin the patches. 263 The applied potential was set to = 30VTas a suitable high potential for the 264 formation of vortices. The remaining variables are selected as suggested by Karatay 265 et al.[40]. 266 3 Results and discussion 267 3.1 Ink-jet printed patterns 268 The ink-jet printing of the dissolved linear PS and linear P2VP lead to homogenous 269 polymer patterns after evaporation of the solvents. Figure 2 b) shows a membrane 270 patterned with PS arrays with a coverage of 50%. The printing of the colloidal P2VP 271 microgels results in dots with a non-con uent P2VP microgel monolayer showing a 272 non-homogeneous distribution with a denser monolayer packing on the outer ring 273 and lower packing in the center of the drop Figure 2 c). To be able to investigate the 274 14 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 in uence of the membrane coverage on the onset of electroconvection, the distance 275 between the polymer dots, was varied to obtain coverages within 25% and 50% 276 (Figure 2 a). The diameter of the arrays was kept constant at 400 m throughout 277 all experiments. For the microgel patterns, it was not possible to coat the surface 278 with more than 31% because of a signi cant swelling of the Na on membranes when 279 coming in contact with water. 280 For a better comparison of the colloidal P2VP microgels and the linear P2VP 281 polymer in the further investigations, membranes were also printed with 25% and 282 31% of linear P2VP. The dots of the PS and P2VP had a thickness of about 2 283 microns. The dots of the microgels had a typical morphology shown in (Figure 2 c) 284 with a monolayer having less lateral microgel density in the center and a ring of 285 higher density on the outside. 286 3.2 Polystyrene polymer patterns on Na on 287 Current-voltage (i-V) curves of the patterned membranes were measured to charac- 288 terize their performance over a wide range of currents. Figure 3 a displays the i-V 289 curves of the PS-patterned Na on membranes in comparison to an uncoated refer- 290 ence. The membrane resistances in the ohmic region and the limiting current density 291 vary only slightly, although the coverages are altered by up to 50 % (Table 1). This 292 behavior was con rmed by stationary COMSOL simulations which serve to explain 293 the experimental data (Figure 4). Since the electric eld lines de ect in the solution 294 from the impermeable patch towards the membrane, they also de ect inside the 295 membrane, spreading into the region right behind the patterns (Figure 4 b). Thus, 296 the ions also di use into the region behind the patterns, so that the actual unused 297 membrane volume is much lower than the area covered by the non-conductive pat- 298 15 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 Figure 2: a) Scheme of the printed arrays. Printed patterns of b) polystyrene (PS) and c) of P2VP microgels of 400 m diameter on a Na on ion-exchange membrane. 16 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 terns implies. 299 Both the experiments and the simulations show the evolution of the plateau region 300 of modi ed membranes already at lower current densities. In case of the patterned 301 membranes, the earlier onset is caused by higher local current densities in the un- 302 covered regions, which leads to an earlier depletion of the boundary layer. 303 All investigated PS coatings improve the mixing of the boundary layer, as indi- 304 cated by decending plateau lengths in Figure 3 b. The degree of coverage or, more 305 precisely, the distance between the patterns has a decisive impact on the evolu- 306 tion and eciency of the electroconvective vortices. Davidson et al.[15] showed by 307 2D simulations, which investigated the evolution of electroconvection at patterned 308 membranes, that the threshold potential for the onset of stable vortices is lower. 309 These vortices arise in counter-rotating pairs of the same period as the pattern it- 310 self, thereby changing in size and eciency of mixing the boundary layer. Thus, in 311 this experimental case the inter-pattern distance at 35 % coverage leads to the most 312 ecient mixing re ected in the minimum plateau length and the biggest reduction 313 of the overall resistance in the overlimiting region. However, this resistance was 314 only slightly decreased by 5 %. The above mentioned 2D simulations revealed an 315 optimum in overlimiting mass transfer at 60 % coverage. The di erence between 316 the experiments and the simulations might be caused by the di erent ratios of pat- 317 tern sizes to ow channel height in each system. Another essential di erence is the 318 neglected third dimension in the simulations. In a more recent paper Duzgalski et 319 al.[9] did a comparison of 2D and 3D simulations. 2D simulations underpredict the 320 charge density in the mixed zone and the net current through the system. 321 322 Chronopotentiometric measurements depict the dynamics of ion depletion and 323 17 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 /s48 /s49/s48 /s50/s48 /s51/s48 /s52/s48 /s53/s48/s48/s46/s56/s48/s48/s46/s56/s53/s48/s46/s57/s48/s48/s46/s57/s53/s49/s46/s48/s48 /s48/s46/s48 /s48/s46/s53 /s49/s46/s48 /s49/s46/s53 /s50/s46/s48/s48/s49/s48/s50/s48/s51/s48/s52/s48 /s48/s46/s48/s48/s48/s52 /s48/s46/s48/s48/s48/s54 /s48/s46/s48/s48/s48/s56 /s48/s46/s48/s48/s49/s48 /s48/s46/s48/s48/s49/s50/s49/s50/s51/s52/s53 /s48 /s50 /s52 /s54 /s56 /s49/s48/s49/s46/s48/s49/s46/s53/s50/s46/s48/s50/s46/s53 /s80/s108/s97/s116/s101/s97/s117/s32/s108/s101/s110/s103/s116/s104/s32/s91/s86/s93 /s67/s111/s118/s101/s114/s97/s103/s101/s32/s91/s37/s93/s98/s41 /s32/s32 /s114/s101/s102/s101/s114/s101/s110/s99/s101 /s32/s32/s50/s53/s37 /s32/s32/s51/s53/s37 /s32/s32/s53/s48/s37/s67/s117/s114/s114/s101/s110/s116/s32/s100/s101/s110/s115/s105/s116/s121/s32/s91/s109/s65/s47/s99/s109/s178/s93 /s77/s101/s109/s98/s114/s97/s110/s101/s32/s118/s111/s108/s116/s97/s103/s101/s32/s100/s114/s111/s112/s32/s91/s86/s93/s97/s41 /s75/s84/s114/s97/s110/s115/s105/s116/s105/s111/s110/s32/s116/s105/s109/s101/s32/s91/s115/s93 /s49/s47/s105/s178/s32/s91/s99/s109/s52 /s47/s109/s65/s178/s93/s114/s101/s102/s101/s114/s101/s110/s99/s101 /s53/s48/s37/s99/s41 /s100/s41 /s50/s53/s37/s53/s48/s37 /s51/s53/s37/s86/s111/s108/s116/s97/s103/s101/s32/s68/s114/s111/s112/s32/s91/s86/s93 /s84/s105/s109/s101/s32/s91/s115/s93/s114/s101/s102/s101/s114/s101/s110/s99/s101/s50/s53/s37 /s51/s53/s37Figure 3: (a) Selected i-V curves of uncoated (reference) and of the polystyrene coated membranes with varying coverage and (b) mean values of the plateau length of all membranes investigated with the same cover- age.(c)Chronopotentiometry measurements of selected membranes ink-jet printed with PS at 31mA cm2and (d) representation of the mean values of the transition times of all investigated membranes over the reciprocal square of the current density1 i2. Table 1: Average values of characteristic parameters of the measured current-voltage curves of all investigated polystyrene patterned membranes. Ohmic Limiting current Plateau Mass transfer resistance density length constant ( cm2) (mA cm2) (V) (smA2 cm4) Reference 39.4 8.3 1.00 4405 25% 40.5 8.3 0.88 4077 35% 40.5 8.6 0.84 3789 50% 40.6 8.5 0.89 3841 18 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 Figure 4: a) i-V curves of membranes with di erent coverages of impermeable pat- terns simulated with COMSOL Multiphysics. b) Zoom into the feed-side di usion boundary layer and the membrane. The electric eld lines are de ected by the impermeable patches (white rectangles) and spread into the regions behind the patch inside the membrane again. the evolution of vortices in the di usion boundary layer. Figure 3 c shows the 324 voltage response over time of chronopotentiometric measurements of PS-modi ed 325 membranes and of a reference membrane at 31mA cm2. For increased membrane cover- 326 age, the transition time diminishes because of the increased local current density in 327 the permeable areas[33]. 328 The Sand equation (Eqn. 1) shows that the transition time is proportional to the 329 reciprocal square of the current density. Substituting all parameters in Equation 1 330 by a single proportional constant K (Eqn. 4 and 5), this parameter describes the 331 mass transfer towards the membrane up to the in ection point (transition time) of 332 the chronopotentiometric curve. 333 In Figure 3 d, the transition times of the investigated membranes are, therefore, 334 plotted against the reciprocal square of applied current density, where the slopes of 335 the curves depict the mass transfer parameter K (Table 1). 336 A comparison of K of the dot-patterned membranes and the reference membranes 337 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 shows that the ratio P=Kspot Krefdecreases from 0.92 for membranes coated with 25% 338 PS to 0.85 for membranes with 35% and 50% of coverage. In case of impermeable 339 patterns, this apparent decrease in mass transfer is explained by the enlarged current 340 density in the uncovered membrane areas because of the funneling of electric eld 341 lines. 342 In summary, the PS-modi ed membranes enhance the onset of electroconvection as 343 indicated by the the plateau length reduction up to 15%. Nevertheless, an operation 344 of electrodialyis in the overlimiting region is not favorable with these membranes, 345 since the overlimiting resistance compared to a reference membrane was only slightly 346 reduced. 347 3.3 P2VP patterns on Na on 348 The i-V curves of the membranes modi ed with linear P2VP polymers also reveal 349 only small increases in the ohmic resistance like the PS patterned ones. However, 350 the P2VP coated membranes show increased limiting current densities (Figure 5 a 351 and Table 3) and a much stronger reduction of the plateau lengths of up to 40% 352 compared to the reference membranes (Figure 5 b). Mishchuk et al. [5] state that 353 the maximum eciency in an enhanced onset of electroconvection is achieved for sig- 354 ni cantly di erent conductivities of patterns and membrane. Since P2VP is partly 355 permeable for ion and the printed geometries are similar to the PS coatings, other 356 properties of the polymers have to cause these drastic changes. The dynamic COM- 357 SOL simulations, also accounting for convective uxes, reveal that the surface charge 358 a ects the rotational direction of vortices (Figure 6 b and d). In case of P2VP, the 359 positive surface charge leads to an electrical double layer (EDL) consisting of anions 360 with a positive zeta potential. Here, the electro-osmotic slip is directed such that 361 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 the vortices transport the solution from above the patterns towards the pristine 362 membrane (Figure 6 a and b). This reduces the extent of concentration polarization 363 occurring at the patterns since higher concentrated solution is directly transported 364 to the ion transporting domains of the membrane. In addition to this, the velocity 365 of the vortices and the electrical eld are directed in the same direction facilitating 366 the ion transport even further. 367 On the contrary, PS bears a negative surface charge which results in an EDL consist- 368 ing of cations with a negative zeta potential. Based on our dynamical simulations 369 (Figure 6), the opposite sign of the zeta potential compared to P2VP microgels 370 leads to opposite rotational directions of vortices. As a consequence, at PS pat- 371 terned membranes, the vortices transport the solution from the pristine membrane 372 surface back towards the non-transporting patterns (Figure 6 c and d). Another 373 drawback of the PS patterns is that the direction of rotation velocity above the 374 pristine membrane is opposite to the direction of the electrical eld. 375 A detailed analysis of the ion transport at the patterned membranes with di erent 376 surface charges is beyond the scope of this work. Future work will focus on the 377 re nement of the simulations to provide a basis for such an analysis. 378 379 The increased limiting currents for the P2VP coated membranes might be ex- 380 plained by the positive surface charge of that polymer. Since the membrane and 381 P2VP bear di erently charged EDLs, the charge distribution next to the patterned 382 membranes is highly inhomogenious. These laterally induced concentration polar- 383 isations at the membrane-pattern transition are assumed to generate instabilities 384 in the uid already when the EDL is still in equilibrium so that electroconvection 385 evolves already at underlimiting currents [37]. 386 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 Table 2: Average values of characteristic parameters of the current-voltage curves of P2VP patterned membranes. Ohmic Limiting current Plateau Mass transfer resistance density length constant ( cm2) (mA cm2) (V) (smA2 cm4) Reference 42.4 8.3 0.94 3356 25% 46.9 11.9 0.58 3210 31% 46.1 11.5 0.6 3351 An earlier onset of electroconvective vortices at P2VP modi ed membranes are 387 also re ected in the results of the chronopotentiometric measurements (Figure 5 c and d). 388 Even though funneling of electric eld lines with its associated increased local cur- 389 rent density occurs in the pristine membrane areas, the transition times are almost 390 similar to the ones of the reference membranes. The comparison of the mass transfer 391 constants K (slopes of the curves in Figure 5 d, values in Table 2) by P=Kspot Kref392 reveals an increase in mass transfer for enlarged coverages. P rises from 0.95 for the 393 membranes covered with 25% to 0.99 for coverages of 31%. The latter value re ects 394 almost a similar mass transfer as the reference. In case of higher coverage, the in- 395 equality in the EDL grows and thus a more pronounced onset of electroconvective 396 vortices might be induced. 397 To conclude, the membranes patterned with the linear P2VP polymer increase 398 the limiting current density and reduce the plateau lengths by up to 40% resulting 399 in an enhanced ion transfer in the overlimiting region. 400 - 401 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 /s48 /s49/s48 /s50/s48 /s51/s48/s48/s46/s53/s48/s46/s54/s48/s46/s55/s48/s46/s56/s48/s46/s57/s49/s46/s48/s49/s46/s49 /s48/s46/s48 /s48/s46/s53 /s49/s46/s48 /s49/s46/s53 /s50/s46/s48 /s50/s46/s53 /s51/s46/s48 /s51/s46/s53 /s52/s46/s48/s48/s49/s48/s50/s48/s51/s48/s52/s48/s53/s48/s54/s48/s55/s48 /s48/s46/s48/s48/s48/s52 /s48/s46/s48/s48/s48/s54 /s48/s46/s48/s48/s48/s56 /s48/s46/s48/s48/s49/s48 /s48/s46/s48/s48/s49/s50 /s48/s46/s48/s48/s49/s52/s49/s46/s53/s50/s46/s48/s50/s46/s53/s51/s46/s48/s51/s46/s53/s52/s46/s48/s52/s46/s53/s80/s108/s97/s116/s101/s97/s117/s32/s108/s101/s110/s103/s116/s104/s32/s91/s86/s93 /s67/s111/s118/s101/s114/s97/s103/s101/s32/s91/s37/s93/s50/s53/s37 /s114/s101/s102/s101/s114/s101/s110/s99/s101/s67/s117/s114/s114/s101/s110/s116/s32/s100/s101/s110/s115/s105/s116/s121/s32/s91/s109/s65/s47/s99/s109/s178/s93 /s77/s101/s109/s98/s114/s97/s110/s101/s32/s118/s111/s108/s116/s97/s103/s101/s32/s100/s114/s111/s112/s32/s91/s86/s93/s51/s49/s37 /s51/s49/s37 /s50/s53/s37/s84/s114/s97/s110/s115/s105/s116/s105/s111/s110/s32/s116/s105/s109/s101/s32/s91/s115/s93 /s49/s47/s105/s178/s32/s91/s99/s109/s52 /s47/s109/s65/s178/s93/s114/s101/s102/s101/s114/s101/s110/s99/s101 /s48 /s50 /s52 /s54 /s56 /s49/s48/s49/s46/s48/s49/s46/s53/s50/s46/s48/s50/s46/s53 /s51/s49/s37 /s50/s53/s37/s86/s111/s108/s116/s97/s103/s101/s32/s68/s114/s111/s112/s32/s91/s86/s93 /s84/s105/s109/s101/s32/s91/s115/s93/s114/s101/s102/s101/s114/s101/s110/s99/s101/s97/s41/s98/s41 /s100/s41/s99/s41Figure 5: (a) Selected i-V curves of uncoated (reference) and of the P2VP coated membranes with varying coverage and (b) mean values of the plateau length of all membranes investigated with the same cover- age.(c)Chronopotentiometry measurements of selected membranes ink-jet printed with P2VP at 31mA cm2and (d) representation of the mean values of the transition times of all investigated membranes over the reciprocal square of the current density1 i2. 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 Figure 6: a) Schematic hypothesis of the formation of electroconvective vortices un- der the in uence of an electrical eld at P2VP patternend membranes and b) the ow direction of a vortex simulated with COMSOL. c) and d) show the scheme of vortex formation and the simulated results of ow direction of the uid for membranes coated with PS. In the simulated results, the patches are indicated with the white bars. In the scheme the (+) signs indicate a positive zeta potentials of the surface, the (-) signs a negative zeta potential. 3.4 P2VP microgel patterns on Na on 402 As opposed to the surface dots printig from the linear P2VP polymer, the P2VP 403 microgels used in this section consist of crosslinked polymer chains. The P2VP mi- 404 crogels have a spherical shape and are highly water-swollen in free solution. Little is 405 known about the swelling in printed con ned layers, certainly not under an electric 406 gradient. In contrast to the linear P2VP polymers forming an homogenous lms, 407 the P2VP microgels assemble into monolayer inside a spot with di erent microgel 408 density distribution shown in (Figure 2 c). 409 410 The P2VP microgel coated membranes have ohmic resistances similar to the ref- 411 erence membranes visible by overlapping i-V curves in the underlimiting region in 412 Figure 7 a and in Table 3. The reduction of the plateau length is comparable with 413 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 Table 3: Average values of characteristic parameters of the current-voltage curves of P2VP microgel patterned membranes. Ohmic Limiting current Plateau Mass transfer resistance density length constant ( cm2) (mA cm2) (V) (smA2 cm4) Reference 39.4 8.3 1.00 4405 25% 39.3 15.7 0.6 4594 31% 39.4 17.7 0.55 6079 that achieved for the spots with the linear P2VP polymers (Figure 7 b). 414 Also in case of P2VP microgels, such an improvement of ion transport was not 415 predicted since these microgels are neither fully impermeable nor do they completely 416 cover the whole patch. 417 In the recent past, we have presented con uent monolayers of P2VP microgels coated 418 on Na on membranes via inclined plane self assembly [29] to investigate the in u- 419 ence of the microgels on the ohmic resistance. In case of these homogeneously 420 covered membranes, the ohmic resistance only increases about 6% compared to a 421 bare membrane. Thus, for the two opposing e ects of (a) increasing resistance due 422 to dot coverage and (b) accelarated onset of electroconvection, the latter - induced 423 by di erent surface charges of PS and P2VP and the inverted rotational direction of 424 vortices for P2VP - dominates the overall mass transport through the concentration 425 polarisation boundary layer and membrane. 426 The P2VP microgel patterned membranes further reveal even higher limiting cur- 427 rent densities than the membranes modi ed with the linear P2VP, namely of about 428 factor two compared to the reference membranes. In case of the micropatterned 429 areas inside the P2VP microgel spots, the EDL is inhomogeneous on a microscopic 430 level, whereas in case of linear polymers the inhomogeneity is on a macroscopic level 431 only between the 400 m sized patches and the pristine membrane area. Conse- 432 quently, the extremely uneven concentration pro les in the micropatterned patches 433 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 /s48/s46/s48 /s48/s46/s53 /s49/s46/s48 /s49/s46/s53 /s50/s46/s48 /s50/s46/s53 /s51/s46/s48 /s51/s46/s53/s48/s49/s48/s50/s48/s51/s48/s52/s48/s53/s48/s54/s48 /s48 /s49/s48 /s50/s48 /s51/s48/s48/s46/s52/s48/s46/s53/s48/s46/s54/s48/s46/s55/s48/s46/s56/s48/s46/s57/s49/s46/s48 /s48/s46/s48/s48/s48/s52 /s48/s46/s48/s48/s48/s54 /s48/s46/s48/s48/s48/s56 /s48/s46/s48/s48/s49/s48 /s48/s46/s48/s48/s49/s50/s50/s52/s54/s56 /s48 /s50 /s52 /s54 /s56 /s49/s48/s49/s46/s48/s49/s46/s53/s50/s46/s48/s50/s46/s53/s98/s41/s97/s41 /s50/s53/s37 /s51/s49/s37 /s114/s101/s102/s101/s114/s101/s110/s99/s101/s67/s117/s114/s114/s101/s110/s116/s32/s100/s101/s110/s115/s105/s116/s121/s32/s91/s109/s65/s47/s99/s109/s178/s93 /s77/s101/s109/s98/s114/s97/s110/s101/s32/s118/s111/s108/s116/s97/s103/s101/s32/s100/s114/s111/s112/s32/s91/s86/s93 /s80/s108/s97/s116/s101/s97/s117/s32/s108/s101/s110/s103/s116/s104/s32/s91/s86/s93 /s67/s111/s118/s101/s114/s97/s103/s101/s32/s91/s37/s93 /s114/s101/s102/s101/s114/s101/s110/s99/s101/s51/s49/s37/s84/s114/s97/s110/s115/s105/s116/s105/s111/s110/s32/s116/s105/s109/s101/s32/s91/s115/s93 /s49/s47/s105/s50 /s32/s91/s99/s109/s52 /s47/s109/s65/s50 /s93/s50/s53/s37 /s51/s49/s37/s50/s53/s37/s86/s111/s108/s116/s97/s103/s101/s32/s68/s114/s111/s112/s32/s91/s86/s93 /s84/s105/s109/s101/s32/s91/s115/s93/s114/s101/s102/s101/s114/s101/s110/s99/s101/s99/s41/s100/s41Figure 7: (a)i-V curves of the reference and of the P2VP microgel coated mem- branes and (b) mean values of the plateau length of all membranes inves- tigated with the same coverage. (c)Chronopotentiometry measurements of selected membranes coated with P2VP microgels at 31mA cm2and (d) rep- resentation of the mean values of the transition times of all investigated membranes over the reciprocal square of the current density1 i2. 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 might result in an earlier onset of electroconvection even in the underlimiting regime. 434 Such interpretations require extensive experimental and simulative studies beyond 435 the scope of this manuscript. 436 The associated mass transfer enhancement can also be traced back in the chronopo- 437 tentiometric measurements (Figure 7 c and d). The transition times are larger com- 438 pared to the reference indicating that, in this case, electroconvection emerges before 439 the boundary layer is fully depleted and the extended space charge has been fully 440 developed. Analyzing the mass transfer up to the in ection point of the chronopo- 441 tentiometric curves by P=Kspot Kref(Table 3), we can con rm that the mass transfer in 442 that state of the depletion is enhanced for the membranes coated with P2VP mi- 443 crogels compared to the reference membranes. For the membranes coated with 25% 444 microgels, P has a value of 1.04 and reaches a value of 1.38 for membranes covered 445 with 31% of microgels. 446 In summary, the P2VP-microgel patterned membranes exhibit an even further 447 enhanced ion transfer in comparison to the linear P2VP dots. Firstly, they show 448 the highest reduction in overall resistance in the overlimiting region of all tested 449 membranes. This renders the suggested method of surface functionalization viable 450 to operate the electrodialysis desalination process in the overlimiting region. Sec- 451 ondly, their limiting current density is twice as high as the one of the reference 452 membranes, so that these microgel-patterned membranes also facilitate an enlarged 453 ion transfer already in the ohmic region. Thus, it is demonstrated that the synergy 454 of the di erent surface charge of pattern and membrane and the micro structure 455 of the patterns show the biggest mass transfer improvements. Simultaneous pH 456 measurements indicate no water splitting at the membrane surface. Such faradaic 457 side reaction would have reduced current eency and could have induced scaling 458 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 phenomena in real water system. Yet, the absence of pH changes supports tech- 459 nical viability. The results may also encourage the electro-catalysis community to 460 investigate lateral heterogeneity to overcome Nernst polarisation at electrodes. 461 4 Conclusions 462 In this paper, the impact of impermeable polystyrene (PS) and poly(2-vinylpyridine) 463 (P2VP) patterns as well as the impact of permeable P2VP microgel patterns on the 464 under- and overlimiting mass transfer was experimentally studied and simulated 465 with COMSOL Multiphysics. Further, the evolution of the electroconvection was 466 investigated. Impermeable PS and P2VP patterns printed on Na on membranes 467 enlarge the membrane resistance only slightly even though their coverage reached 468 up to 50%. Since the polymer patterns are only coated on top of the membrane, 469 the eld lines also spread into the regions behind the patterns once they entered the 470 membrane. Hence almost the whole membrane volume contributes to ion perme- 471 ation. The PS patterns enhance the mixing of the boundary layer, shown by the up 472 to 15% decreased plateau lengths. Despite this reduction of plateau length, only a 473 slight decrease in overall resistance in the overlimiting regime was achieved. 474 In contrast to that, P2VP polymer and P2VP microgel patterns of similar di- 475 mensions printed on Na on membranes exhibit enlarged limiting current densities. 476 P2VP microgels patterns doubled the limiting current density because of the for- 477 mation of lateral microgel density distribution inside of the printed areas. The 478 enhanced mass transfer might be enabled by an evolution of electroconvection al- 479 ready at underliminting currents. Further, a drastic drop in plateau length was 480 achieved. Both types of P2VP patterns reduce the plateau lengths by around 40%. 481 We suggest that these drastic reductions compared to the PS coatings evolve due 482 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 to opposite surface charges of the polymers. Time-resolved CFD simulations reveal 483 that the opposite surface charges result in reversed rotational directions of electro- 484 convective vortices contributing to a more ecient mixing of the boundary layer in 485 case of P2VP. We are con dent that the rst experimental evidence of an improved 486 electroconvection achieved by the synergistic e ects of micro patterns and surface 487 charge via microgel coatings will help to (a) further clarify the relevant factors to 488 enhance electroconvection and (b) render electrodialytic desalination processes more 489 ecient. 490 Supporting Information 491 Supporting Information is available from the Wiley Online Library or from the 492 author. 493 Acknowledgements 494 This work has been supported by the DFG through the project SFB 985 Func- 495 tional Microgels and Microgel Systems in project C4.This project has also received 496 funding from the European Research Council (ERC) under the European Union's 497 Horizon 2020 research and innovation program (grant agreement no. 694946). The 498 authors acknowledge Elif Karatay, Manuel C esar Mart  Calatayud and Tao Luo for 499 their input and the fruitful discussions. 500