# Equilibrium of a droplet suspended in an electric field

A droplet suspended in a fluid subjected to a uniform electric field deforms due to the competing effect of electrical forces and surface tension. If the electrification level is moderate an equilibrium shape is reached. It was observed experimentally that droplets deforms as prolate or oblate spheroids (i.e. larger elongation aligned with the external electric field or viceversa). The analytical analysis of O’Konski & Thacher (1953) unexpectedly predicted prolate forms whereas the experiments showed oblate ones (and viceversa).

It was the genius of Geoffrey Taylor (1966) who shed light on the problem. The work of O’Konski & Thacher assumed that both fluids (inner and outer) were perfect dielectrics, given the low conductivity of the fluids involved. Taylor realized that the conductivity could be very low but it was not zero, so that the charges could migrate through the “leaky” media, thus accumulating at the fluid interface and altering radically the pattern of electrical forces.

Moreover, approximating the electrostatic Maxwell equations with the Taylor–Melcher leaky dielectric model and assuming small deformation Taylor predicted recirculating velocities and provided analytical expressions for the radial and azimuthal velocities as functions of the dimensionless radius r and the azimuthal coordinate \theta. For r < 1 this gives, \displaystyle v_r = Ar(1-r^2)(3\cos^2 \theta -1) \quad \mathrm{and} \quad v_\theta = 3Ar/2 \left( 1-5r^2/3 \right)\cos 2\theta and for r \ge 1, \displaystyle v_r = A(r^{-4}-r^{-2})(3\cos^2 \theta -1) \quad \mathrm{and} \quad v_\theta = -Ar^{-4} \sin 2\theta with \displaystyle A = -\frac{9}{10} \frac{E_\infty^2 R_d \varepsilon_o}{ \mu_o} \frac{1}{1+\lambda}\frac{R-Q}{(R+2)^2}, where R, Q and \lambda are the ratio of inner to outer conductivity, permittivity and viscosity, respectively. E_\infty is the imposed electrid field, R_d the droplet radius, \varepsilon_o is the outer permittivity and \mu_2 is the outer viscosity. The electrical forces induces recirculations in both (viscous) fluids.

This test case is also discussed in Lopez-Herrera et al, 2011.

The problem is assumed to be axisymmetric.

#include "axi.h"
#include "navier-stokes/centered.h"
#include "ehd/implicit.h"
#include "ehd/stress.h"
#include "vof.h"
#include "tension.h"

We need to track the interface with the volume fraction field f. The viscosity is constant but the coefficients will vary due to the axisymmetric metric terms.

scalar f[], * interfaces = {f};
face vector muv[];

The maximum level of resolution, LEVEL, will be varied from 8 to 10.

int LEVEL = 8;

#define Ef 1.34 // External electric field
#define R0 0.1 // Radius of the droplet
#define F 50.
#define R 5.1 // Conductivity ratio
#define Q 10.0 // permittivity ratio
#define CMU 0.1 // Outer viscosity
#define theta (M_PI/4.)
#define LAM 1. // Viscosity ratio
#define VC (sq(Ef)*R0/CMU) // characteristic velocity
#define A (-9./10.*(R - Q)/sq(R + 2.)/(1. + LAM))

F is the conductivity of the outer medium. F has no influence on the steady solution but decreases the characteristic electrical relaxation time and consequently the electrical transient.

#define cond(T) (F*((1. - (T)) + R*(T)))
#define perm(T) ((1. - (T)) + Q*(T))

The electric potential is linear.

phi[top]   = dirichlet(Ef*x);
phi[left]  = dirichlet(Ef*x);
phi[right] = dirichlet(Ef*x);

We make sure there is no flow through the boundaries, otherwise the compatibility condition for the Poisson equation can be violated.

uf.n[left] = 0.;
uf.n[right] = 0.;
uf.n[top] = 0.;
uf.n[bottom] = 0.;

The domain spans [0:2]. We will compute only a quarter of the droplet, making use of axisymmetry and right-left symmetry. The surface tension coefficient is unity. The viscosity coefficients are variable.

int main() {
L0 = 2;
N = 1 << LEVEL;
f.sigma = 1.;
mu = muv;
run();
}

event init (t = 0) {

We initialize the volume fraction field corresponding to a circular interface of radius R0.

  fraction (f, sq(R0) - sq(x) - sq(y));

We initialize the electrical potential.

  foreach()
phi[] = Ef*x;
boundary ({phi});
}

Permittivity and electrical conductivity are face values and also incorporate the metric factors. The viscosity is constant but the viscosity coefficients need to incorporate the metric factors.

event properties (i++)
{
foreach_face() {
double ff = (f[] + f[-1])/2.;
epsilon.x[] = perm(ff)*fm.x[];
K.x[] = cond(ff)*fm.x[];
muv.x[] = CMU*fm.x[];
}
boundary ((scalar *){epsilon, K, muv});
}

## Convergence

We store the horizontal component of the velocity to check its convergence with time.

scalar un[];

event init_un (i = 0) {
foreach()
un[] = u.x[];
}

event error (i += 20; t <= 10.) {

We monitor the variation in the horizontal component of the velocity and the convergence of the multigrid solvers every 20 timesteps.

  double du = change (u.x, un);
fprintf (stdout, "%g %g %d %d %d %d %d %d %d %d\n", t, du,
mgp.i, mgp.nrelax, mgpf.i, mgpf.nrelax, mgu.i, mgu.nrelax,
mgphi.i, mgphi.nrelax);
fflush (stdout);

If the change is small enough (i.e. the solution has converged for this level of refinement), we increase the level of refinement. If the simulation has converged and the level of refinement is 10, we stop the simulation.

  if (i > 0 && du < 1e-5)
return (LEVEL++ == 10); /* stop */
}

## Results

At the end of the simulation we create two files: log (standard error) will contain the dimensionless radial and azimuthal velocities and their theoretical values as functions of the dimensionless radial coordinate along the line \theta= 45^o. vector.svg displays the velocity field, interface position and isopotential lines, as displayed by gfsview-batch.

event result (t = end) {
double h  = 0.35*L0/(2*99);
for (int i = 1; i <= 100; i++) {
double x = i*h, y = i*h, r = sqrt(sq(x) + sq(y))/R0;
double ux = interpolate (u.x, x, y)/VC; // dimensionless velocities
double uy = interpolate (u.y, x, y)/VC;
double vrt, vtt; // theoretical radial and azimuthal velocities;
if (r < 1.) {
vrt = A*r*(1. - sq(r))*(3.*sq(sin(theta)) - 1.);
vtt = 3*A/2*r*(1. - 5./3.*sq(r))*sin(2.*theta);
}
else {
vrt = A/sq(r)*(1/sq(r) - 1.)*(3.*sq(sin(theta)) - 1.);
vtt = - A*1./sq(sq(r))*sin(2.*theta);
}
fprintf (stderr, "%g %g %g %g %g\n", r,
(ux*x + uy*y)/(R0*r), vrt, (-uy*x + ux*y)/(R0*r), vtt);
}

FILE * fp = popen ("gfsview-batch2D taylor.gfv", "w");
output_gfs (fp);
fprintf (fp, "Save vectors.svg { format = SVG }\n");
pclose (fp);
}

The mesh is adapted according to interpolation errors on the volume fraction, charge density and velocity fields.

event adapt (i += 20) {
adapt_wavelet ({f, rhoe, u.x, u.y}, (double[]){1e-3, 1, 2e-4, 2e-4},
maxlevel = LEVEL);
}
set xlabel 'r'
set ylabel 'v'
plot 'log' u 1:2 notitle, 'log' u 1:3 w l t "v_r",\
'log' u 1:4 notitle, 'log' u 1:5 w l t "v_{/Symbol q}"

## Bibliography

 [konski1953] Chester T O’Konski and Henry C Thacher Jr. The distortion of aerosol droplets by an electric field. The Journal of Physical Chemistry, 57(9):955–958, 1953. [taylor1966] Geoffrey Ingram Taylor. Studies in electrohydrodynamics. i. the circulation produced in a drop by an electric field. Proc. R. Soc. Lond. A, 291(1425):159–166, 1966.