以狹縫黏度計研究磁性奈米流體之黏度

Study of the viscosity of magnetic nanofluids via slit viscometer

劉宇恩(Yu-En Liu)

指導教授 ： 李雨

國立臺灣大學/工學院/應用力學研究所/碩士(2019年)

https://doi.org/10.6342/NTU201903665

摘要

磁性奈米流體的黏度將透過真空幫浦油奈米流體(Fe3O4-VPO)於狹縫黏度計中以實驗的方式進行研究。透過X光繞射(XRD)分析化學沉積法所生成的Fe3O4磁性奈米晶體，其大小約為8nm。在配製奈米流體時，將使生成的Fe3O4奈米粒子吸附油酸，並分散於基底流體中，其目的在於提供空間排斥力以抗衡粒子間凡德瓦力，確保奈米流體之穩定性。煤油(kerosene)與真空幫浦油等兩種基底流體，將用於配製奈米流體。透過動態光散射(DLS)檢測得知，稀薄Fe3O4-kerosene奈米流體中的粒徑分布為8-300nm，且平均粒徑為27nm。對比DLS與XRD之量測結果，可以推斷奈米粒子應普遍以團聚之型態懸浮於應用層面的奈米流體。
本研究中的狹縫黏度計是以聚二甲基矽氧烷(PDMS)流道接合玻璃基底的方式製造，並藉由矩形流道中內置的壓力計量測流體壓力降，以獲取流體黏度。黏度計將放置於兩片永久磁鐵中間，透過不同的擺放，施加垂直或平行於流場渦度之磁場於流體，以研究磁場方向的影響。調整磁鐵片間距可改變磁場強度，而設定驅動流體的定流量幫浦則可改變流體的體積流率。本裝置與文獻的不同主要在於黏度計流道大小及磁場的施加方式，較小的流道可減少試驗流體的需求量，以永久磁鐵代替線圈則不須冷卻設備。實驗中的量測值皆透過改良的布氏黏度計加以驗證，而布氏黏度計的外加磁場則是透過纏繞於試驗區域的線圈所產生。
本文採用不同體積分率(1.2%及3%)之Fe3O4-VPO奈米流體，討論在不同剪應變率(1.25 – 2.5 s-1)、不同磁場強度(0 –160 Gauss)及不同磁場方向下的黏度變化值。而得到以下結論：(1) 透過施加中等強度之磁場即可增加奈米流體之黏度。以體積分率3%之Fe3O4-VPO奈米流體於160 Gauss、且剪應變率 1.25 s-1之情況為例，H_⊥及H_∥分別可使奈米流體黏度上升至300%及50%。(2) 奈米流體中的剪切稀化效果相當顯著，當剪應變率由1.25 s-1 增加至 2.5 s-1時，可使(1)中H_⊥的黏度增益值由300%下降至130%。(3) 當磁場低於100 Gauss時，奈米流體的老化效應並不顯著；當磁場增強後則有明顯改變。以體積分率1.2%之Fe3O4-VPO奈米流體於160 Gauss 且剪應變率 1.25 s-1之情況為例，黏度值因老化效應而提升了兩倍。

關鍵字

磁性奈米流體 ； 磁黏效應 ； 狹縫黏度計 ； 粒子粒徑

並列摘要

The viscosity of magnetic nanofluid was studied experimentally using Fe3O4-VPO (vacuum pump oil) nanofluid in a slit viscometer. Magnetic nanocrystals of Fe3O4 were prepared by the co-precipitation method, whose size was measured as about 8 nm using the X-ray diffraction (XRD). The nano Fe3O4 particles were then coated with oleic acid, and dispersed in base fluid, for forming the nanofluid. The coated oleic acid was em-ployed for providing the steric force to counterbalance the van der Waals force between particles, ensuring the stability of nanofluid. Two base fluids, kerosene and vacuum pump oil, were used for synthesizing nanofluids. The sizes of particles in diluted Fe3O4-kerosene nanofluids were measured as 8300 nm, with an average at 27 nm, using the dynamic light scattering (DLS) technique. By comparing the DLS measurement with the XRD measurement, it is suggested that the particles in nanofluid for application is in general in agglomerated state.
The slit viscometer was fabricated in this study by molding a polydimethylsiloxane (PDMS) channel on a glass substrate, with the pressure drop of the flow along the rec-tangular channel measured using a built-in manometer, for assessing the fluid viscosity. The magnetic field was applied to the flow by sandwiching the channel with two perma-nent magnetic plates, placed perpendicular or parallel to the vorticity of the flow, for stud-ying the effect of field orientation. The field strength can be adjusted by changing the dis-tance between the magnetic plates, and the volume flow rate can be adjusted through the setting of the volume flow rate of the syringe pump driving the flow. The main differences between the present device and those in the literature are that here the channel of the vis-cometers is smaller and the magnetic field is applied using permanent magnet plates, such that less test fluid is required and cooling can be avoided, respectively. The measurement using the present device was validated through the measurement using a modified Brookfield viscometer with the magnetic field generated by an electric coil surrounding the test section.
Measurements of Fe3O4-VPO nanofluids of different volume fractions (1.2% and 3%) under different shear rates (1.25 – 2.5 s-1), magnetic field strength (0 – 160 gauss) and magnetic field direction were performed. We found: (1) Viscosity can be enhanced sub-stantially through the application of a moderate magnetic field, for example, 50% and 300% increases were measured for a nanofluid of volume fraction 3% under 1.25 s-1 at 160 Gauss, when the field was applied parallel and perpendicular to the vorticity of the flow, respectively. (2) Shearing thinning of the viscosity of magnetic nanofluid is substan-tial, for example, the viscosity enhancement in (1) was reduced from 300% to 130% if the shear rate was increased from 1.25 s-1 to 2.5 s-1 when the field is applied perpendicular to the vorticity. (3) Aging effect is minor when the magnetic field is less than 100 Gauss, but shows moderate changes at a higher field strength. For example, twice of the viscosity increase was observed for a nanofluid of 1.2% volume fraction at 160 Gauss under 1.25 s-1.

並列關鍵字

Magnetic nanofluids ； Magnetoviscous effect ； slit viscometer ； Particle sizes

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