Sauvage Laboratory for Smart Materials, School of Materials Science and Engineering, Harbin Institute of Technology (Shenzhen), Shenzhen 518055, China.
Department of Mechanical Engineering, National University of Singapore, Singapore 117575, Singapore.
Langmuir. 2023 May 16;39(19):6932-6945. doi: 10.1021/acs.langmuir.3c00759. Epub 2023 May 6.
Controlled colloidal levitation is key to many applications. Recently, it was discovered that polymer microspheres were levitated to a few micrometers in aqueous solutions in alternating current (AC) electric fields. A few mechanisms have been proposed to explain this AC levitation such as electrohydrodynamic flows, asymmetric rectified electric fields, and aperiodic electrodiffusiophoresis. Here, we propose an alternative mechanism based on dielectrophoresis in a spatially inhomogeneous electric field gradient extending from the electrode surface micrometers into the bulk. This field gradient is derived from electrode polarization, where counterions accumulate near electrode surfaces. A dielectric microparticle is then levitated from the electrode surface to a height where the dielectrophoretic lift balances gravity. The dielectrophoretic levitation mechanism is supported by two numerical models. One model assumes point dipoles and solves for the Poisson-Nernst-Planck equations, while the second model incorporates a dielectric sphere of a realistic size and permittivity and uses the Maxwell-stress tensor formulation to solve for the electrical body force. In addition to proposing a plausible levitation mechanism, we further demonstrate that AC colloidal levitation can be used to move synthetic microswimmers to controlled heights. This study sheds light on understanding the dynamics of colloidal particles near an electrode and paves the way to using AC levitation to manipulate colloidal particles, active or passive.
受控胶体悬浮是许多应用的关键。最近，人们发现聚合物微球在交流电（AC）电场中悬浮在几微米的水中。已经提出了几种机制来解释这种 AC 悬浮，例如电动力学流动、不对称整流电场和非周期性电渗流。在这里，我们提出了一种基于空间不均匀电场梯度中的介电泳的替代机制，该梯度从电极表面延伸到体相几微米处。这种电场梯度源自电极极化，其中反离子在电极表面附近积聚。然后，介电微球从电极表面上升到介电泳升力与重力平衡的高度。介电泳悬浮机制得到了两个数值模型的支持。一个模型假设点偶极子，并求解泊松-纳斯特-普朗克方程，而第二个模型则包含一个实际尺寸和介电常数的介电球体，并使用麦克斯韦应力张量公式求解电体力。除了提出一种合理的悬浮机制外，我们还进一步证明了 AC 胶体悬浮可用于将合成微游泳者移动到受控高度。这项研究阐明了理解电极附近胶体颗粒动力学的原理，并为使用 AC 悬浮来操纵胶体颗粒（主动或被动）铺平了道路。
