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Electroosmosis drives a large slip velocity at the interface by altering the electrokinetic double layer effect at the fluid-solid interface, thereby generating high shear rates within the channel. In this paper, molecular dynamics simulations are used to construct an electroosmotic flow nanochannel model, and the fluid flow characteristics and wall slip reduction properties within graphene charged-wall nanochannels are investigated. The results show that the electroosmotic flow changes the structure of the bilayer to increase the mobility of its diffusion layer, and at the same time, the ions in the diffusion layer under the action of the applied electric field undergo directional migration and drive the overall fluid flow through the viscous effect, which enhances the mobility performance. After the introduction of ions, Na+ is adsorbed at the wall surface, which weakens the adsorption force between the fluid and the wall surface and enhances the driving force of the fluid in the confined domain space, thus increasing the slip length and flow rate. Finally, by modulating the charge size on the upper and lower wall surfaces, asymmetric channel wall charges are formed. The electric field gradient superimposed on the applied electric field further enhances the driving force of ions, changes the distribution of the of Na+ adsorption layer and the migration behavior of Cl–, thereby increasing the transport of the solution in the channel. Therefore, in this paper, a method is proposed to realize the ultrafast transport of solution in the channel by modulating the asymmetric wall charge of graphene, successfully achieving the slip reduction effect of the electroosmotic flow of solution in the graphene channel. A theoretical basis is laid for the fast and energy-saving transportation of microfluidics in the nano-limited space.
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Keywords:
- electroosmotic flow /
- wall charge /
- slip rent reduction /
- bilayer
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图 1 纳米通道流动模型示意图 (a) 石墨烯纳米通道内水流动模型; (b) 石墨烯纳米通道内离子溶液的流动模型(上下灰色层为三层石墨烯壁面, 中间蓝色区域为水溶液)
Figure 1. Schematic diagram of the nanochannel flow model: (a) The water flow model inside the graphene nano-channel; (b) the flow model of the ionic solution inside the graphene nano-channel. The upper and lower gray layers are the three-layer graphene wall, and the middle blue region is the water solution.
图 2 石墨烯纳米通道内壁面电荷为0.10 e下不同外加电场强度通道内水分子密度分布图
Figure 2. Density distribution of water molecules in the channel for different applied electric field strengths at a wall charge of 0.10 e inside the graphene nano-channel.
图 3 不同电压下密度曲线中上壁面(a)和下壁面(b)峰值处的局部放大图
Figure 3. Localized magnification of the density profiles at the peaks of the upper (a) and lower (b) walls at different voltages.
图 4 不同电场强度下石墨烯通道内水的流动特性分析 (a) 不同电场强度下通道内水分子归一化速度分布曲线; (b) 不同电场强度下通道内水溶液流动过程中受到的黏性阻力、范德瓦耳斯力和静电力的柱状分布; (c) 水流动过程中滑移长度的变化曲线与驱动力的柱状分布
Figure 4. Characterization of water flow in graphene channel under different electric field strengths: (a) Normalized velocity distribution curves of water molecules in the channel under different electric field strengths; (b) Columnar distributions of viscous resistance, van der Waals force and electrostatic force during the flow of aqueous solution in the channel under different electric field strengths; (c) variation curves of the slip length in the flow of water with the columnar distributions of the driving force.
图 5 电场强度为0.10 V/nm、壁面电荷为0.05 e条件下通道内水和离子溶液速度平均值归一化分布曲线
Figure 5. Normalized distribution curves of the mean water and ion solution velocities in the channel at an electric field strength of 0.10 V/nm and a wall charge of 0.05 e.
图 6 石墨烯通道内离子溶液流动过程中的水分子的密度以及离子排布 (a) 不同壁面电荷下离子溶液的密度分布; (b) 水分子在下壁面7.5 Å处密度分布峰值的局部放大图; (c) 壁面加电荷后离子溶液在流动过程中通道内离子的排布; (d) 壁面电荷为0.08 e下石墨烯通道内Na+与Cl–的电荷密度曲线. (其中不同颜色的曲线代表不同电荷下的密度曲线)
Figure 6. Density of water molecules during the flow of ionic solution in the graphene channel and ionic arrangement: (a) Density distribution of ionic solution at different wall charges of graphene; (b) local magnification of the peak density distribution of water molecules at 7.5 Å on the lower wall; (c) ionic arrangement of ions in the channel during the flow of ionic solution after the wall charge; (d) charge density profile of Na+ in the graphene channel at a wall charge of 0.08 e and Cl– charge density curves (curves of different colors represent density profiles at different charges).
图 7 石墨烯通道内不同壁面电荷下离子溶液流动过程中的速度、滑移以及受到的力的分析 (a) 不同石墨烯壁面电荷下离子溶液的归一化速度分布(其中不同颜色的电线代表不同电荷下离子溶液流动的速度曲线); (b) 不同壁面电荷下离子溶液流动过程中受到的范德瓦耳斯力、黏性阻力和静电力的柱状分布; (c) 不同壁面电荷下石墨烯通道内溶液的滑移量的变化曲线与溶液受到的驱动力柱状分布
Figure 7. Analysis of the velocity, slip, and the force applied during the flow of ionic solutions under different wall charges in the graphene channel: (a) Normalized velocity distribution of ionic solutions under different graphene wall charges (different colored wires represent the velocity profiles of ionic solutions flowing under different charges); (b) Columnar distributions of van der Waals force, viscous resistance, and electrostatic force applied to the flow of ionic solutions under different wall charges distributions; (c) the variation curves of the slip of the solution in the graphene channel under different wall charges with the Columnar distribution of the driving force on the solution.
图 8 不同壁面电荷差值下溶液中Na+和Cl–的电荷密度分布曲线 (a), (b), (c)分别是壁面电荷差值为0.01 e, 0.05 e和0.09 e下溶液中的离子密度分布曲线(其中蓝色点线为Na+的密度曲线, 红色点线为Cl–的密度曲线); (d) 不同壁面电荷差值下石墨烯上壁面吸附Na+的数量变化曲线
Figure 8. Charge density distribution curves of Na+ and Cl– in solution for different values of wall charge difference: (a), (b), (c) Ion density distribution curves in solution at wall charge difference values of 0.01 e, 0.05 e, and 0.09 e, respectively (the blue dotted line is the density curve of Na+ and the red dotted line is the density curve of Cl–); (d) variation curves of the amount of adsorbed Na+ on the graphene on the walls at different values of wall charge difference.
图 9 石墨烯通道内不同壁面电荷差值下离子溶液流动过程中的速度对比 (a) 石墨烯通道不同壁面电荷差值下离子溶液在纳米通道内的归一化速度变化曲线; (b) X方向0.10 V/nm的电场作用下, 壁面电荷差值为0.05 e、壁面电荷0.05 e、壁面电荷0.09 e的速度分布曲线对比
Figure 9. Comparison of velocities during the flow of ionic solutions in graphene channels with different wall charge differences: (a) Normalized velocity change curves of ionic solutions in nano-channels with different wall charge differences in graphene channels; (b) comparison of velocity distribution curves with 0.05 e, 0.05 e, and 0.09 e under the action of the electric field of 0.10 V/nm in the X direction.
图 10 石墨烯通道内不同壁面电荷差值下离子溶液流动过程中的受力以及滑移分析 (a) 不同壁面电荷差值下溶液流动过程中受到的范德瓦耳斯力、黏性阻力和静电力的柱状分布; (b) 不同壁面电荷差值下溶液的滑移曲线和溶液受到驱动力的柱状分布
Figure 10. Force as well as slip analysis during the flow of ionic solutions in graphene channels with different wall charge differences: (a) Columnar distributions of van der Waals force, viscous drag force and electrostatic force applied to the flow of solutions with different wall charge differences; (b) slip profiles of solutions with different wall charge differences and columnar distributions of driving force applied to the solutions.
表 1 原子对之间的相互作用参数
Table 1. Interaction parameters between atom pairs.
原子类型(i) 原子类型(j) σ/Å ε/meV 电荷q/e O (H2O) O (H2O) 3.154 6.72 –1.04 H (H2O) H (H2O) 0.00 0.00 0.52 Na+ (NaCl) Na+ (NaCl) 2.575 0.648 1.00 Cl– (NaCl) Cl– (NaCl) 4.448 4.607 –1.00 C (石墨烯) C (石墨烯) 3.40 3.73 –0.01 — –0.10 
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