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Alternating current coaxial electrospray for micro-encapsulation

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Abstract

The present study examines the potential of low-frequency (< 1 kHz) alternating current (AC) electric field actuation for micro-encapsulation using coaxial electrospray. Ethanol, olive oil and glycerol fluid combinations have been used as working fluid. The amplitude of actuation has been varied in the range of 5.4–7.2 kV. Dye visualization of the Taylor cone and high-speed visualization of electrospray have been carried out. Confocal microscopy has been used to characterize the capsules structure. Current measurement has been used to quantify the net charge content of the capsules. The residual current carried by droplets is lower for AC actuation compared to that of DC actuation. DC actuation shows straight generatrix of Taylor cone while AC actuation shows cusp shape. Difference in charge accumulation on the interface of the core and shell liquid for the DC and AC actuation and the resulting Maxwell stress influence the curvature of Taylor cone. Bi-orthogonal decomposition is used to characterize the stability of the electrospray process. The cone jet is more stable at a higher frequency of AC actuation compared to low-frequency actuation. The minimum potential required for stable cone jet formation is lower for AC actuation compared to that of DC actuation. The Taylor cone length and cone angle are a function of actuation waveform, flow rates through the coaxial nozzle and actuation amplitude. The present study demonstrates that square wave AC actuation can successfully generate stable coaxial cone jet and capsules.

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Abbreviations

AC:

Alternating current

CEHDA:

Coaxial electrohydrodynamic atomization

DC:

Direct current

EHDA:

Electrohydrodynamic atomization

α :

Ratio of inner jet diameter to outer jet diameter

β :

Dielectric constant

\( \varepsilon \) :

Permittivity of fluid

\( \varepsilon_{0} \) :

Permittivity of free space

\( \gamma \) :

Surface tension

\( \gamma_{\text{eff}} \) :

Surface tension of mixture

\( \kappa \) :

Electrical conductivity

μ :

Viscosity

\( \rho \) :

Density of fluid

\( \theta_{\text{c}} \) :

Cone angle

B oe :

Electrical Bond number

B og :

Gravitational Bond number

d j,i :

Inner jet diameter

d j,o :

Outer jet diameter

d j :

Jet diameter

d ni,i :

Inner diameter of inner nozzle

d ni,o :

Outer diameter of inner nozzle

d no,i :

Inner diameter of outer nozzle

d no,o :

Outer diameter of outer nozzle

f :

Frequency of actuation signal

I :

Current

L :

Length scale

l :

Length scale of cone length

\(l'\) :

Length scale of droplet

L c :

Cone length

O h :

Ohnesorge number

Q :

Flow rate

Q i :

Inner fluid flow rate

Q 0 :

Outer fluid flow rate

t e :

Electrical relaxation time

t h :

Hydrodynamic time

V :

Applied potential

V 0 :

Characteristic velocity

V rms :

Root mean square voltage

W e :

Weber number

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Gupta, A., Panigrahi, P.K. Alternating current coaxial electrospray for micro-encapsulation. Exp Fluids 61, 29 (2020) doi:10.1007/s00348-019-2851-x

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