(350b) Separation by Cyclic Electric Field-Flow Fractionation

In Electric Field-Flow Fractionation (EFFF) a lateral electric field is applied in a channel, which creates a lateral concentration gradient, and these gradients couple with the Poiseuille flow in the channel to separate particles on the basis of size and/or mobility. The magnitude of the current that flows on application of the field is a key parameter for separation because it is an indicator of the field experienced by the particles. If the applied field is unidirectional, the current rapidly decreases in time due to charging of the double layer. The rapid decrease results in poor separation. The separation could be improved by performing EFFF with cyclic electric fields. In Cyclic electric field flow fractionation (CEFFF), a periodic voltage, which can be either sinusoidal or square-wave, is applied in the lateral direction. The goals of our research are to understand the electrochemical response of CEFFF, i.e., understand the current-time response for a given time-dependent voltage and then to utilize this electrochemical response in a transport model to predict separation.

In this talk we will present the electrochemical response for a CEFFF device, i.e., a channel comprising of gold plated walls separated by a spacer. We will present the effect of magnitude of voltage and frequency and effect of salt concentration in the solution on the current-time relationship. Results show that the current decays with time as a sum of two exponential terms. Furthermore there is a residual current even at long times. In this talk we report our measurements and put forth a plausible explanation for the origin of the double exponential decay. Next we input the electrochemical data into the convection-diffusion equation and solve it by using a combination of analytical and numerical techniques to determine the mean velocity and the dispersion coefficient of molecules undergoing Poiseuille flow in a channel in presence of lateral cyclic electric fields. Finally, the mean velocities and the dispersion coefficients will be used to predict the separation efficiency of CEFFF devices.