Droplet dynamics in non-uniform AC electric field
E.S. Batyrshin1, N.A. Gumerov1,2, C.-D. Ohl3, and I.S. Akhatov4
1
3
Center for Micro- and Nanoscale Dynamics of Dispersed Systems, Bashkir State University, Russia
2
Institute for Advanced Computer Studies, University of Maryland, USA
School of Physical and Mathematical Sciences, Division of Physics and Applied Physics, Nanyang Technological
University, Singapore
4
Center for Research, Education and Innovation, Skoltech Center for Hydrocarbon Recovery, Russia
Droplet motion
Motivation
I Electric field induced demulsification of crude oils
Frequency sweep (1 −→ 50)Hz, U0 = 45V
I Electrohydrodynamics of multiphase media
I Microscale technology
Experimental details
Setup
Three different frequency range can be identified:
Sample cell
I Large amplitude droplet oscillations between electrodes. Droplet makes a several oscillations for the period of the
electric field, fm 6= fe . Electrophoresis dominates droplet dynamics.
II Same as case “I”, but droplet oscillation fully synchronized with voltage, fm = fe
III Near electrode small amplitude oscillations. The frequency of oscillation equals to double voltage frequency fm = 2fe .
Dielectrophoresis becomes very important at the droplet dynamics.
The competition of the droplet electrophoresis and dielectrophoresis are coupled to the dynamics of droplet charging at the
electrodes and the charge leakage to the surrounding medium when the droplet moves away from electrodes.
Droplet deformation
∆dx = dx /d0 − 1 : the deformation of droplet.
The frequency of droplet deformation always
equal to double voltage frequency fd = 2fe .
Electric field
Strongly non-uniform electric field between
electrodes
E – arrows
|E| – colors
Voltage:
U = U0 sin(2πfe t),
The dependence of maximum deformation on voltage amplitude (fe = 500 Hz) and frequency (U0 = 150 V )
U0 = (0 . . . 150) V
fe = (1 . . . 104) Hz
Forces induced by the electric field:
Fep ∼ qE – electrophoretic force, q - induced charge of the droplet
Fdep ∼ ∇E2 – dielectrophoretic force (dipole approximation)
Video sequences (U0 = 45V )
- Droplet deformation occurs at the threshold voltage U0c
- The amplitude of deformation decreases with voltage frequency
Droplet motion and deformation during the period of electric field:
∆x - droplet displacement
dx - droplet diameter
Rupturing of droplet
The droplet size d0 = 50µm
is comparable with the gap
between electrodes.
Summary & outlook
fe = 15 Hz:
Large amplitude droplet motion. From electrode to electrode.
Charging and discharging of droplet. The frequency of motion fm = fe .
fe = 40 Hz:
Small amplitude droplet motion. Near electrode.
The frequency of motion fm = 2fe .
http://cmnd.bashedu.ru/
I Droplet charges/discharges at the electrodes. Oscillations of the
droplet are induced by electrophoresis and dielectrophoresis. The
contribution of each force depends on the voltage amplitude and
frequency. Competition of forces.
I mathematical model for droplet movement
that takes into account electrophoresis,
dielectrophoresis and time-depended charge
of the droplet
I Droplet deformation frequency are always double of voltage frequency.
Presumably, deformation are mostly driven by dielectrophoresis.
I clarifying the mechanism(s) of droplet
charging/discharging process.
batyrshine@mail.ru