Deshawar Dharmansh, Chokshi Paresh
Department of Chemical Engineering, Indian Institute of Technology Delhi, New Delhi 110016, India.
Phys Rev E. 2021 Feb;103(2-1):023107. doi: 10.1103/PhysRevE.103.023107.
The linear stability of a jet propagating under an electric field is analyzed under nonisothermal conditions. The electrified jet of a Newtonian fluid is modeled as a slender filament, and the leaky dielectric model is used to account for the Maxwell stresses within the fluid. The convective heat transfer from high-temperature jet to the surroundings results in formation of thicker fibers owing to increased viscosity upon cooling. The jet exhibiting substantial thinning under the action of tangential electric field is examined for stability toward axisymmetric nonperiodic disturbances. This is in contrast to most prior studies which analyzed the stability of a cylindrical jet of uniform radius without thinning under extensional flow by examining only periodic disturbances. Two case studies of reference fluids differing in viscosity and electrical properties are examined. The spectrum of discrete growth rates for axisymmetric disturbances reveal qualitatively distinct instabilities for the two fluids. For a fluid with high electrical conductivity, the conducting mode driven by the coupling of surface charges and an external electric field is found to be the dominant mode of instability. On the contrary, for low conductivity materials, the surface-tension-driven capillary mode is found to be the most critical mode. Heat transfer from the jet to the surroundings tends to stabilize both types of instability mode. Under sufficiently strong heat transfer, the axisymmetric instability, which is believed to be responsible for producing nanofibers with diametric oscillations in electrospinning process, is suppressed. The stabilization is attributed to the enhancement of viscous stress in the thinning jet upon cooling. It is observed that the stabilization effect is relatively more pronounced in a thinning jet compared to the cylindrical jet of uniform radius. The effects of various material and process parameters on the stability behavior is also examined.
在非等温条件下分析了在电场作用下传播的射流的线性稳定性。将牛顿流体的带电射流建模为细长细丝，并使用漏电介质模型来考虑流体内的麦克斯韦应力。高温射流向周围环境的对流热传递由于冷却时粘度增加而导致形成更粗的纤维。研究了在切向电场作用下表现出显著变细的射流对轴对称非周期性扰动的稳定性。这与大多数先前的研究形成对比，后者仅通过检查周期性扰动来分析在拉伸流动下半径均匀的圆柱形射流的稳定性，而该射流不会变细。研究了两种粘度和电性能不同的参考流体的案例。轴对称扰动的离散增长率谱揭示了两种流体在定性上截然不同的不稳定性。对于高电导率的流体，发现由表面电荷与外部电场耦合驱动的传导模式是主要的不稳定模式。相反，对于低电导率材料，发现表面张力驱动的毛细模式是最关键的模式。射流向周围环境的热传递倾向于稳定这两种不稳定模式。在足够强的热传递下，被认为是在静电纺丝过程中产生具有直径振荡的纳米纤维的原因的轴对称不稳定性受到抑制。这种稳定归因于冷却时变细射流中粘性应力的增强。据观察，与半径均匀的圆柱形射流相比，变细射流中的稳定效果相对更明显。还研究了各种材料和工艺参数对稳定性行为的影响。
