Squeeze-flow electroosmotic pumping between charged parallel plates

Abstract

In the present work, the squeezing flow between two charged parallel plates is theoretically investigated, with a provision of accounting for the electric double layer overlap effects. The electroviscous effects arising from the distortion of the electric double layer flow field are investigated in detail, for different strengths of the imposed plate motion. It is revealed that there can be a significant deviation between the predictions from the present model and those obtained by employing a classical Poisson-Boltzmann equation based model. This discrepancy can be attributed to some of the over-simplified assumptions associated with the standard models that might only remain valid for large separation distances between the two plates. Many of these simplified assumptions are found to hold inappropriate in case the squeezing flow occurs in such a narrow gap that the instantaneous liquid layer thickness becomes of the same order or less than the order of the characteristic electric double layer thickness. In such cases, there is likely to be a deficit of counterions within the bulk liquid due to an excess accumulation of those in the electrical double layer. On the other hand, there may occur a surplus of coions in the bulk liquid region due to a rejection of those in the electrical double layer. As a consequence of this presence of excess net charges in the bulk liquid region, strong electro-hydrodynamic interactions are likely to occur between the squeezing motion and the electroosmotic transport, which cannot be accurately captured by the classical theory.