Department of Mathematics, Indian Institute of Technology Kharagpur, Kharagpur, India.
School of Mechanical and Materials Engineering, Washington State University, Pullman, USA.
Electrophoresis. 2021 Apr;42(7-8):920-931. doi: 10.1002/elps.202000343. Epub 2021 Feb 5.
Electrokinetic transport of an uncharged nonconducting microsized liquid droplet in a charged hydrogel medium is studied. Dielectric polarization of the liquid drop under the action of an externally imposed electric field induces a non-homogeneous charge density at the droplet surface. The interactions of the induced surface charge of the droplet with the immobile charges of the hydrogel medium generates an electric force to the droplet, which actuates the drop through the charged hydrogel medium. A numerical study based on the first principle of electrokinetics is adopted. Dependence of the droplet velocity on its dielectric permittivity, bulk ionic concentration, and immobile charge density of the gel is analyzed. The surface conduction is significant in presence of charged gel, which creates a concentration polarization. The impact of the counterion saturation in the Debye layer due to the dielectric decrement of the medium is addressed. The modified Nernst-Planck equation for ion transport and the Poisson equation for the electric field is considered to take into account the dielectric polarization. A quadrupolar vortex around the uncharged droplet is observed when the gel medium is considered to be uncharged, which is similar to the induced charge electroosmosis around an uncharged dielectric colloid in free-solution. We find that the induced charge electrokinetic mechanism creates a strong recirculation of liquid within the droplet and the translational velocity of the droplet strongly depends on its size for the dielectric droplet embedded in a charged gel medium.
研究了带电荷的水凝胶介质中不带电非导电微小型液滴的电动输运。在外部电场作用下，液滴的介电极化会在液滴表面引起非均匀电荷密度。液滴感应表面电荷与水凝胶介质中不可移动电荷之间的相互作用会产生电场力，使液滴在带电荷的水凝胶介质中运动。采用基于电动第一原理的数值研究。分析了液滴速度与其介电常数、体相离子浓度和凝胶不可移动电荷密度的关系。在存在带电荷的凝胶时，表面传导是显著的，这会产生浓度极化。考虑到介质的介电损耗，讨论了 Debye 层中反离子饱和对电场的影响。考虑到介电极化，采用了用于离子输运的修正的 Nernst-Planck 方程和用于电场的泊松方程。当水凝胶介质被认为不带电时，观察到不带电液滴周围存在四极涡旋，这类似于在自由溶液中不带电介电胶体周围的感应电荷电动渗流。我们发现，感应电荷电动机制在液滴内产生强烈的液体再循环，并且嵌入带电荷凝胶介质中的介电液滴的平移速度强烈取决于其尺寸。
