Electrohydrodynamics

Content sourced from Wikipedia, licensed under CC BY-SA 3.0.

Electrohydrodynamics (EHD) is the study of how electric forces interact with fluids. It combines electricity and fluid flow to see how applying an electric field can make a fluid move, change shape, or alter its properties.

The motion comes from the way electric fields act on charged particles and on polarized molecules in the liquid. This creates body forces inside the liquid that push it to flow. The main transport effects in EHD are:
- electrophoresis: charged particles migrate through a fluid under an electric field
- electrokinesis: movement of a fluid or its components in an electric field
- dielectrophoresis: polarized particles move in nonuniform electric fields
- electro-osmosis: liquid flows along charged surfaces when a field is applied
- electrorotation: tiny parts rotate under the field

In simple terms, EHD can turn electrical energy into motion (or heat) and, conversely, moving fluids or generating electricity with a field is possible.

How this is used: shaped electric fields can create pressure in dielectric liquids to produce flow. In fluids, this leads to controlled streams of liquid. If parts are allowed to move, the system can act like a tiny electric motor. Practical applications include air ionizers, EHD thrusters, and cooling systems that use electric fields to move heat or fluids.

EHD can also harvest energy: flowing liquid in a field can generate electricity, a concept explored in microfluidic devices where electric fields move fluids without moving parts. The flow rate from a single channel is small, but many channels in parallel can produce usable power.

Electrokinetic effects have a long history. Electrophoresis was observed in the early 1800s, and the so-called Biefeld–Brown effect in the 1920s described a related electric-force phenomenon. In micro- and nano-scale devices, these effects are especially useful for moving, mixing, and separating tiny amounts of fluid with electric fields.

At small scales, electrokinetic flows are usually smooth, but they can become unstable if the liquid’s conductivity varies widely, leading to rapid mixing or undesired dispersion. These electrokinetic instabilities are a topic of ongoing study and can be described as electric “Rayleigh-type” instabilities in the right conditions.

In short, EHD shows how electricity can directly control liquids, enabling a range of devices that move, mix, or generate signals and power at small scales.