pH-Mediated Aggregation-to-Separation Transition for Colloids Near Electrodes in Oscillatory Electric Fields
pH-Mediated Aggregation-to-Separation Transition for Colloids Near Electrodes in Oscillatory Electric Fields
复制标题
振荡电场中电极附近胶体的 pH 介导的聚集到分离转变
DOI：
10.1021/acs.langmuir.1c00671
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发表时间：
2021
期刊：
影响因子：
3.9
通讯作者：
Woehl, Taylor
中科院分区：
文献类型：
--
作者：
Rath, Medha;Weaver, Jacqueline;Wang, Mei;Woehl, Taylor
胶体在低频（<1 kHz）振荡电场附近的平面电极聚集在中性pH电解质由于电流体动力学（EHD）流，但在碱性pH电解质分离。胶体势能和电解质离子迁移率被认为在这种现象的潜在机制中发挥作用，但是为什么颗粒在某些电解质中聚集而在其他电解质中分离的统一理论仍有待建立。在这里，我们表明，增加局部pH值附近的电极与电化学反应引起的胶体聚集到分离过渡的振荡电场，诱导强烈的吸引力EHD流。一种电活性分子，对苯醌，在电极上电化学还原，局部增加附近的胶体溶液的pH值。叠加一个足够大的稳定的电化学电位上的振荡电位引起的可逆的聚集到分离的转变。与直觉相反的是，降低频率，增加吸引力EHD阻力，导致类似的聚集分离过渡。更有趣的是，当改变振荡电位时，观察到多个跃迁。这些结果表明，振荡势引起的排斥性流体动力拖曳力。最近发现的不对称整流电场（AREF）的缩放参数表明，排斥AREF诱导的电渗（EO）流竞争与吸引EHD流。一对胶体力平衡，包括这些竞争流表现出流动反转定性与实验观察到的聚集到分离的过渡。从广义上讲，这些结果强调的重要性，AREF诱导的EO流胶体聚集和分离在低频振荡电场。
Colloids in low-frequency (<1 kHz) oscillatory electric fields near planar electrodes aggregate in neutral pH electrolytes due to electrohydrodynamic (EHD) flow but separate in alkaline pH electrolytes. Colloid ζ-potential and electrolyte ion mobilities are thought to play roles in the underlying mechanism for this phenomenon, but a unifying theory for why particles aggregate in some electrolytes and separate in others remains to be established. Here, we show that increasing local pH near the electrode with an electrochemical reaction causes a colloidal aggregation-to-separation transition in oscillatory electric fields that induce strong attractive EHD flows. An electroactive molecule,para-benzoquinone, was electrochemically reduced at the electrode to locally increase the solution pH near the colloids. Superimposing a sufficiently large steady electrochemical potential onto an oscillatory potential caused a reversible aggregation-to-separation transition. Counterintuitively, decreasing frequency, which increases attractive EHD drag forces, caused a similar aggregation-to-separation transition. Even more interesting, multiple transitions were observed while varying the oscillatory potential. Taken together, these results suggested that the oscillatory potential induced a repulsive hydrodynamic drag force. Scaling arguments for the recently discovered asymmetric rectified electric field (AREF) showed that a repulsive AREF-induced electroosmotic (EO) flow competed with attractive EHD flow. A pairwise colloidal force balance including these competing flows exhibited flow inversions qualitatively consistent with experimentally observed aggregation-to-separation transitions. Broadly, these results emphasize the importance of AREF-induced EO flows in colloid aggregation and separation in low-frequency oscillatory electric fields.