Abstract The development of long-time stable nanofluids for practical use in heat transfer processes is a tremendous scientific challenge because nanoparticles tend to precipitate and agglomerate when in a solution, affecting both their thermophysical properties and their stability. This work experimentally investigates the role of the electro-repulse force by electric charges around the nanoparticle, as a way of improving the stability of an electrolyte-based nanofluid. Nanofluid samples were prepared in a two-step method, with 1 wt% and 3 wt% concentrations (mass fraction) of titanium oxide (TiO2) nanoparticles added to a base fluid consisting of an electrolyte solution with a different concentration of potassium chloride (KCl) and deionized water. The pH of the base fluid was maintained constant, adding HEPES as a buffering agent. The stable condition of the nanofluid was established when the temporal variation of the thermal conductivity was negligible. When stability was established, the dynamic viscosity, zeta potential and the enhancement of the thermal conductivity were measured under controlled temperatures. Experimental results showed that the stable behavior of the nanofluid was directly influenced by the electric charge around the nanoparticles and the electro-repulse force between the nanoparticles (represented by the zeta potential), producing a consistent and homogenous stable condition for an extended 30-day period. Due to the greater number of nanoparticles in the 3 wt% solution, the dynamic viscosity of the nanofluid at 3 wt% was higher than at 1 wt%. It was noted that the addition of the nanoparticles did not affect the Newtonian nature of the fluid (except that it was slightly for higher KCl concentrations) and it produced an increase of a 41.75 ± 2.4% for 1 wt% and 59.32 ± 2.1% for 3 wt% of the nanofluid dynamic viscosity, with respect to that of the pure water. Significant enhancement of thermal conductivity enhancement was also obtained, ranging from 0.46 ± 0.11% to 1.47 ± 0.12% for the 1 wt%; and, 2.15 ± 0.11% to 4.7 ± 0.13% for the 3 wt% of nanoparticles added. This noteworthy improvement was attributed to the higher level of homogeneity of the nanofluid, caused by the high electro-repulse force between nanoparticles. Stable electrolyte-based nanofluids, such as KCl, which increase the electro-repulse forces between nanoparticles, can bolster the application of this type of nanofluid in energy conversion and electronic cooling. Enhanced stability properties (particularly in microchannel heat sinks) give these nanofluids the ability to use electric fields for the fluid motion, rather than traditional pumping devices.

中文翻译：

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基于 TiO2 电解质的纳米流体的粘度、增强的热导率和 zeta 电位的实验研究

摘要 开发用于传热过程的长期稳定纳米流体是一项巨大的科学挑战，因为纳米粒子在溶液中容易沉淀和团聚，影响它们的热物理性质和稳定性。这项工作通过实验研究了纳米颗粒周围电荷的电排斥力的作用，作为提高基于电解质的纳米流体稳定性的一种方式。纳米流体样品采用两步法制备，将 1 wt% 和 3 wt% 浓度（质量分数）的氧化钛 (TiO2) 纳米颗粒添加到由不同浓度氯化钾 (KCl) 的电解质溶液组成的基液中) 和去离子水。基液的 pH 值保持恒定，添加 HEPES 作为缓冲剂。当热导率的时间变化可以忽略不计时，纳米流体的稳定条件成立。建立稳定性后，在受控温度下测量动态粘度、zeta 电位和热导率的提高。实验结果表明，纳米流体的稳定行为直接受到纳米粒子周围电荷和纳米粒子之间的电排斥力（以zeta电位表示）的影响，产生了持续30天的一致且均匀的稳定条件时期。由于 3 wt% 溶液中纳米粒子的数量较多，因此 3 wt% 时纳米流体的动态粘度高于 1 wt% 时。值得注意的是，纳米颗粒的添加不会影响流体的牛顿性质（除了它对较高的 KCl 浓度略有影响）并且它产生了 41.75 ± 2.4% 的 1 wt% 和 59.32 ± 2.1% 的增加。纳米流体动态粘度的 3 wt%，相对于纯水的动态粘度。还获得了显着的导热性增强，对于 1 wt%，范围从 0.46 ± 0.11% 到 1.47 ± 0.12%；并且，对于添加的 3 wt% 的纳米颗粒，2.15 ± 0.11% 至 4.7 ± 0.13%。这一值得注意的改进归因于纳米流体的更高水平的均匀性，这是由纳米粒子之间的高电斥力引起的。稳定的基于电解质的纳米流体，例如 KCl，可增加纳米粒子之间的电排斥力，可以支持这种类型的纳米流体在能量转换和电子冷却中的应用。增强的稳定性特性（特别是在微通道散热器中）使这些纳米流体能够使用电场进行流体运动，而不是传统的泵送装置。