Elecro-osmotic flows…what are they? A VERY brief intro

So my current research is concerned with simulating electro-osmotic flows (EOF), especially in really small diameter pipes and channels. I thought I would give a brief introduction to EOF, as unlike using pressure to drive a fluid through a pipe for example, EOF are very different. Instead, an electric field is used to drive the fluid.

However, simply applying an electric field at the ends of your pipe/channel will not guarantee an EOF… as a special structure, called the electric double layer, must first form at the interface between the fluid and substrate wall, shown in the following figure:

The green and red circles represent ions in the fluid, and the substrate wall is shown in grey. Once this layer is present, applying an electric field will give rise to an EOF, as seen in the next figure:

The difficulty in studying these systems stems from the ability to correctly model the interactions between the fluid-substrate interface. In order to do this effectively, the fluid can be treated discretely, which means that each atom/molecule of the fluid is to be represented. This is in contrast to CFD methods, where the fluid is treated as a whole continuous body. Unfortunately, this means that CFD methods cannot accurately capture the interactions occuring at the interface, which, as systems get smaller, start to dominate system behaviour.

So I use molecular dynamics (MD), which does treat the fluid discretely, to study fluid flow under electric fields! However, the drawbacks are that, MD simulations are generally much more computationally expensive than CFD, and are also limited to small spatial and temporal scales.

I hope that this brief introduction has helped in understanding what EOF is.