Electrohydrodynamic phenomena

This work is a review article focused on exploring the interactions between external and induced electric fields and fluid motion, in the presence of embedded charges. Such interactions are generally termed electrohydrodynamics (EHD), which encompasses a vast range of flows stemming from multiscale physical effects. In this review article we shall mainly emphasize on two mechanisms of particular interest to fluid dynamists and engineers, namely electrokinetic flows and the leaky dielectric model. We shed light on the underlying physics behind the above mentioned phenomena and subsequently demonstrate the presence of a common underpinning pattern which governs any general electrohydrodynamic motion. Hence we go on to show that the seemingly unrelated fields of electrokinetics and the leaky dielectric models are indeed closely related to each other through the much celebrated Maxwell stresses, which have long been known as stresses caused in fluids in presence of electric and magnetic fields. Interactions between Maxwell Stresses and charges (for instance, in the form of ions) present in the fluid generates a body force on the same and eventually leads to flow actuation. We show that the manifestation of the Maxwell stresses itself depends on the charge densities, which in turn is dictated by the underlying motion of the fluid. We demonstrate how such inter-related dynamics may give rise intricately coupled and non-linear system of equations governing the dynamical state of the system. This article is mainly divided into two parts. First, we explore the realms of electrokinetics, wherein the formation and the structure of the so-called electrical double layer (EDL) is delineated. Subsequently, we review EDL’s relevance to electroosmosis and streaming potential with the key being the presence and absence of an applied pressure gradient. We thereafter focus on the leaky dielectric model, wherein the fundamental governing equations and its main difference with electrokinetics are described. We limit our attentions to the leaky dielectric motion around droplets and flat surfaces and subsequent interface deformation. To this end, through a rigorous review of a number of previous articles, we establish that the interface shapes can be finely tailored to achieve the desired geometrical characteristics by tuning the fluid properties. We further discuss previous studies, which have shown migration of droplets in the presence of strong electric fields. Finally, we describe the effects of external agents such as surface impurities on leaky dielectric motion and attempt to establish a qualitative connection between the leaky dielectric model and EDLs. We finish off with some pointers for further research activities and open questions in this field.by Aditya Bandopadhyay and Uddipta Ghos