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Electrophoresis

Electrophoresis is the movement of electrically charged particles or fluid drops under the influence of an electric field. Electrophoresis has applications in areas like Bio-MEMS and electrohydrodynamic spraying.

In Bio-MEMS,electrophoresis provides a mechanism for separating large molecules such as DNA or proteins from a mixture. An electric current is passed through a gel containing a mix of protein molecules. Electromotive force (EMF) pushes or pulls the molecules through the gel matrix; the molecules move through the matrix at different rates depending on their mass/charge and get separated accordingly.

Related links:

Modeling and Characterization of Microfabricated Emitters: In Pursuit of Improved ESI-MS Performance

 

Taylor Cones: Electrohydrodynamic Spraying

In a classic paper, G.I. Taylor ("Disintegration of Water Drops in an Electric Field," Proc. R. Soc. Lon. A 280, 383 (1964)) described how electric fields can distort liquid drops and in some circumstances produce a thin liquid jet emanating from a drop. Now referred to as Taylor cones, these jet flows have found uses for a wide variety of applications that require a stream of more-or-less uniform droplets having an electric charge.

FLOW-3D CFD simulation of Taylor Cones modeled with electric potential
Taylor Cone flow simulation
results; Left colored by
electric potential, Right colored
by electric field magnitude

In the example given, an axisymmetric arrangement was used that consisted of a nozzle of inside diameter 7.0e-4m charged to 6000 volts. Downstream 25.0e-4m, a grounded plate is located perpendicular to the axis of the nozzle with a central hole of diameter 3.0e-4m. A flow of liquid into the bottom of the nozzle has a fixed axial velocity of 0.02m/s. The properties of the liquid are: density=827.0km/m3, viscosity 0.0081kg/m/s, dielectric constant=10.0, surface tension=0.0235N/m, and conductivity=8.05e-6S/m. The total time simulated was 0.03s. Initially, liquid was even with the top of the nozzle. The jet forms explosively at about t=0.017s and then slowly evolves into a smaller diameter jet as it approaches near steady conditions.

There are many other useful industrial applications for a system that produces small droplets of specified size; particularly if the droplets don't coalesce because of their electrical repulsion. Having a charge also means that these drops can be electrically deflected toward a target. This technology, for example, has been advantageously applied to paint spraying, atomization of fuels, printing, mass spectroscopy and a variety of spray drying processes. FLOW-3D can be used in evaluating such electro-spray-ionization systems.