A decade ago, two-dimensional microscopic flow visualization proved the theoretically predicted existence of electroconvection roles as well as their decisive role in destabilizing the concentration polarization layer at ion-selective fluid/membrane interfaces. Electroconvection induces chaotic flow vortices injecting volume having bulk concentration into the ion-depleted diffusion layer at the interface. Experimental quantification of these important flow patterns have so far only been carried out in 2D. Numerical direct simulations suggest 3D features, yet experimental proof is lacking. 3D simulations are also limited in covering extended spacial and temporal scales.

This study presents a new comprehensive experimental method for the time-resolved recording of the 3D electroconvective velocity field near a cation-exchange membrane. For the first time, the spatio-temporal velocity field can be visualized in 3D at multiples of the overlimiting current density. In contrast to today’s simulations, these experiments cover length and time scales typical for actual electrodialytic membrane processes.

We visualize coherent vortex structures and reveal the changes in the velocity field and its statistics during the transition from vortex rolls to vortex rings with increasing current density. The transition is characterized by changes in the rotational direction, mean square velocity, and temporal energy spectrum with only little influence on the spatial spectrum. These findings indicate a more significant impact of EC’s structural change on the mean square velocities and temporal spectra than on the spatial spectra. This knowledge is a prerequisite for engineering ion-selective surfaces that will enable the operation of electrically driven processes beyond the diffusion-limited Nernst regime.

十年前，二维微观流动可视化证明了理论上预测的电对流作用的存在以及它们在使离子选择性流体/膜界面处的浓差极化层不稳定方面的决定性作用。电对流引起混沌流涡流，将具有整体浓度的体积注入界面处的离子耗尽扩散层。迄今为止，这些重要流动模式的实验量化仅在 2D 中进行。数值直接模拟表明 3D 特征，但缺乏实验证明。3D 模拟在涵盖扩展的空间和时间尺度方面也受到限制。

本研究提出了一种新的综合实验方法，用于对阳离子交换膜附近的 3D 电对流速度场进行时间分辨记录。第一次，时空速度场可以在 3D 中以超限电流密度的倍数进行可视化。与今天的模拟相比，这些实验涵盖了实际电渗析膜过程的典型长度和时间尺度。

我们将相干涡旋结构可视化，并揭示了在从涡旋滚动到涡旋环的过渡过程中，随着电流密度的增加，速度场及其统计数据的变化。这种转变的特点是旋转方向、均方速度和时间能谱的变化对空间谱的影响很小。这些发现表明，EC 的结构变化对均方速度和时间光谱的影响比对空间光谱的影响更显着。这些知识是设计离子选择性表面的先决条件，这将使电驱动过程的运行超出扩散限制的能斯特机制。