Department of Mechanical Engineering, Indian Institute of Technology, Kharagpur 721302, India.
Anal Chim Acta. 2010 Feb 5;659(1-2):1-8. doi: 10.1016/j.aca.2009.11.029. Epub 2009 Dec 7.
In the present work, we outline the design and analysis of a micro-capillary filling mechanism through the aid of interfacial pH gradients (and hence interfacial tension gradients) generated by employing arrays of transverse electrodes inducing step changes in voltages, in a natural buffer system that requires low power and no synthetic ampholytes. The capillary transport is modulated by a dynamic and non-trivial coupling between the interfacial tension and viscous resistances, as a consequence of the underlying intermolecular interactions. The competing effects of the driving and the retarding forces effectively determine the displacement, velocity and acceleration characteristics of the capillary front, in a dynamically evolving manner. A comprehensive theoretical model of capillary dynamics is developed here to address these issues in details, thereby revealing the combined influence of the interfacial electrochemistry and the applied transverse voltages, as guided by the pertinent fundamental thermodynamic principles governed by free energy considerations and the physico-chemical phenomena over interfacial scales. Non-trivial implications of the pH-gradient driven micro-capillary transport are aptly emphasized, so as to offer significant physical insights on the adopted strategy as a guiding principle for facilitating capillary filling processes by inducing a modulation in the effective interfacial energy. Particular implications on the capillary filling time are also pinpointed, revealing the effectiveness of the adopted design strategy. Finally, a universal scaling relationship of the capillary filling time as a function of the pertinent operating parameters is derived, so as to provide a generalized guideline for implementing the design scheme. A non-dimensional parameter, depending simultaneously on the inter-electrode pitch and the transverse voltage, is identified, which may be kept to a minimal limit within the other operating constraints of the chosen system, towards minimizing the capillary filling time.
在本工作中，我们通过在自然缓冲体系中采用横向电极阵列诱导电压阶跃变化来产生界面 pH 梯度（从而产生界面张力梯度），从而概述了微毛细管填充机制的设计和分析。该自然缓冲体系需要低功率且无需合成两性电解质。在分子间相互作用的基础上，界面张力和粘性阻力之间存在动态且非平凡的耦合，从而调制了毛细管传输。驱动力和阻力的竞争效应有效地确定了毛细管前缘的位移、速度和加速度特性，其方式是动态演变的。这里提出了一个综合的毛细管动力学理论模型来详细解决这些问题，从而揭示了界面电化学和外加横向电压的综合影响，这些影响是由自由能考虑和界面尺度上的物理化学现象指导的相关基本热力学原理所引导的。强调了 pH 梯度驱动微毛细管传输的重要意义，从而为所采用的策略提供了重要的物理见解，以通过调节有效界面能来促进毛细管填充过程。还特别指出了对毛细管填充时间的影响，揭示了所采用的设计策略的有效性。最后，推导出了毛细管填充时间作为相关操作参数函数的通用比例关系，从而为实施设计方案提供了通用指南。该比例关系的一个无量纲参数同时取决于电极之间的间距和横向电压，可以在所选系统的其他操作约束范围内保持最小限制，以最小化毛细管填充时间。
