Effect of Conductivity and Viscosity in the Velocity Characteristics of a fluid flow induced by nonuniform AC electric field in electrolytes on microelectrodes Pritesh Parikh, Astha Sethi, Samatha Benedict, Subimal Deb and Souri Banerjee Department of Physics, Birla Institute of Technology and Science- Pilani, Hyderabad Campus,India
THE BIG PICTURE Aim: Study the electrical properties of biomolecules suspended in solution on application of electric field ACEO Trapping of Molecules DEP ACEO and DEP: opposite in nature frequency dependent.
ELECTRO-OSMOTIC FLOW Caused by Coulomb force on net mobile charge In the process, it creates an electric double layer Fluid is pushed away from the gap between the electrodes towards the electrode surfaces [1] Green et al, Physical Review E,2002
NUMERICAL MODEL Coplanar parallel electrode (CPE) geometry was used and the phenomena of AC electro-osmosis was simulated using COMSOL Multiphysics Software. Symmetry line Gap between Electrodes : 25 m Electrolyte: KCl = 2.1x10-3 S/m = 0.001 Pa. s Double Layer ~ 25 nm
SIMULATION DETAILS Frequency of the applied electric potential : 1Hz to 80kHz Electric field was computed using Electric currents interface Fluid Flow was computed using the Creeping flow interface which makes use of the time dependent Navier Stokes equation The Debye layer was modeled as a capacitive boundary condition on the electrode surface
SIMULATION DETAILS The electro osmotic velocity was computed using the inbuilt equation(helmholtz Smoluchowski formula) System response studied for : 300 sinusoidal cycles Symmetry of electrodes: Only the right half electrode geometry simulated The time averaged velocity was computed at various distances along the electrode surface
Frequency 500Hz ELECTRIC POTENTIAL
FLUID FLOW PATTERN
Velocity(m/s) VELOCITY MAGNITUDE AT DIFFERENT DISTANCES ON ELECTRODE SURFACE 4.9E-04 4.4E-04 3.9E-04 d=13 d=17 3.4E-04 d=21 d=25 2.9E-04 2.4E-04 d=29 1.9E-04 1.4E-04 9.0E-05 4.0E-05-1.0E-05 1 10 100 1000 10000 100000 Frequency(Hz)
FLUID FLOW AT DIFFERENT FREQUENCIES Frequency 1000Hz
FLUID FLOW AT DIFFERENT FREQUENCIES Frequency 10000Hz
FLUID FLOW AT DIFFERENT FREQUENCIES Frequency 30000Hz Reversal of flow at high frequency
Velocity(m/s) OBSERVATIONS FROM PLOT 4.9E-04 4.4E-04 3.9E-04 The velocity maximum shift to lower frequencies for larger distances d=13 3.4E-04 d=17 2.9E-04 d=21 2.4E-04 d=25 1.9E-04 d=29 1.4E-04 9.0E-05 4.0E-05-1.0E-05 1 10 100 1000 10000 100000 Frequency(Hz) d: Distance from the edge of electrode
Velocity(m/s) SECOND PEAK 4.5E-05 4.0E-05 3.5E-05 3.0E-05 2.5E-05 2.0E-05 1.5E-05 1.0E-05 5.0E-06 d=13 d=17 d=21 d=25 d=29 0.0E+00 1000 10000 100000 Frequency(Hz)
Velocity(m/s) OBSERVATION FROM PLOT Another peak smaller by one order of magnitude was observed at higher 4.5E-05 frequencies 4.0E-05 3.5E-05 3.0E-05 2.5E-05 2.0E-05 1.5E-05 1.0E-05 5.0E-06 d=13 d=17 d=21 d=25 d=29 0.0E+00 1000 10000 100000 Frequency(Hz) The velocity max shift to higher frequencies for larger distances
Velocity(m/s) VARIATION IN CONDUCTIVITY 5.0E-04 4.5E-04 4.0E-04 3.5E-04 3.0E-04 2.5E-04 2.0E-04 sigma=.000513 sigma=.0021 sigma=.0086 1.5E-04 1.0E-04 5.0E-05 0.0E+00 1 10 100 1000 10000 100000 Frequency(Hz)
Velocity(m/s) OBSERVATION FROM PLOT 5.0E-04 4.5E-04 Shift in peak velocity to Higher frequencies at Larger conductivity values 4.0E-04 3.5E-04 3.0E-04 2.5E-04 2.0E-04 sigma=.000513 sigma=.0021 sigma=.0086 1.5E-04 1.0E-04 5.0E-05 0.0E+00 1 10 100 1000 10000 100000 Frequency(Hz)
Velocity(m/s) VARIATION IN VISCOSITY 1.8E-03 1.6E-03 1.4E-03 1.2E-03 1.0E-03 8.0E-04 6.0E-04 eta=.00025 eta=.001 eta=.004 4.0E-04 2.0E-04 0.0E+00 1 10 100 1000 10000 100000 Frequency(Hz)
Velocity(m/s) OBSERVATION FROM PLOT 1.8E-03 Shift in peak velocity Magnitude to lower values at higher viscosity values 1.6E-03 1.4E-03 1.2E-03 1.0E-03 8.0E-04 6.0E-04 eta=.00025 eta=.001 eta=.004 4.0E-04 2.0E-04 0.0E+00 1 10 100 1000 10000 100000 Frequency(Hz)
SUMMARY Observation of a weak second maxima in Velocity versus Frequency plots indicative of complete reversal of fluid flow Examination of the distance dependence of the two peaks. Observation of effect of conductivity and viscosity on electro osmotic velocity as a function of frequency
CONCLUSION The conductivity, viscosity of the electroylte and frequency of applied electric field affect the electro osmotic velocity. The direction of flow can be tuned by the frequency of applied electric field The magnitude of the peak velocity can be tuned by the viscosity The position of the peak velocity as a function of frequency can be tuned by the conductivity
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BOUNDARY CONDITIONS : Potential u: Normal velocity v: Tangential vel u ACE :Electro-osmotic vel
COMPARISION [1] Green et al, Physical Review E,2002