Editing Electrohydrodynamics (section)

== Electrokinetic instabilities ==

The fluid flows in [[microfluidic]] and nanofluidic devices are often stable and strongly damped by viscous forces (with [[Reynolds number]]s of order unity or smaller). However, heterogeneous ionic conductivity fields in the presence of applied [[electric field]]s can, under certain conditions, generate an unstable flow field owing to '''electrokinetic instabilities (EKI)'''. Conductivity gradients are prevalent in on-chip electrokinetic processes such as preconcentration methods (e.g. field amplified sample stacking and [[isoelectric focusing]]), multidimensional assays, and systems with poorly specified sample chemistry.  The dynamics and periodic morphology of ''electrokinetic instabilities'' are similar to other systems with [[Rayleigh–Taylor instability|Rayleigh–Taylor]] instabilities. The particular case of a flat plane geometry with homogeneous ions injection in the bottom side leads to a mathematical frame identical to the [[Rayleigh–Bénard convection]].

EKI's can be leveraged for rapid [[Mixing (physics)|mixing]] or can cause undesirable dispersion in sample injection, separation and stacking. These instabilities are caused by a coupling of electric fields and ionic conductivity gradients that results in an electric body force. This coupling results in an electric body force in the bulk liquid, outside the [[double layer (interfacial)#Electrical double layers|electric double layer]], that can generate temporal, convective, and absolute flow instabilities. Electrokinetic flows with conductivity gradients become unstable when the [[electroviscous effects|electroviscous]] stretching and folding of conductivity interfaces grows faster than the dissipative effect of molecular diffusion.

Since these flows are characterized by low velocities and small length scales, the Reynolds number is below 0.01 and the flow is ''laminar''. The onset of instability in these flows is best described as an electric "Rayleigh number".