Equivalence of sessile droplet dynamics under periodic and steady electric fields

Muhamed Ashfak Kainikkara,Dipin S Pillai,Kirti Chandra Sahu

doi:10.1038/s41526-021-00176-2

Muhamed Ashfak Kainikkara, Dipin S Pillai + Show 1 more

Open Access

https://doi.org/10.1038/s41526-021-00176-2

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Abstract

The electrohydrodynamics of a sessile droplet under the influence of periodic and steady electric fields in microgravity conditions is theoretically investigated using an inertial lubrication model. Previous studies have revealed that a freely suspended spherical droplet with unequal conductivity and permittivity ratios exhibits distinct dynamics under periodic and equivalent steady forcing in the root mean-square sense. However, it is unclear when (if at all) such distinct dynamics occur for periodic and equivalent steady forcing in the case of sessile droplets. The equivalence between periodic and steady forcing is shown to be governed by the interfacial charge buildup, which further depends on the competition between the charge relaxation and forcing timescales. A circulation-deformation map is introduced for the sessile droplet that acts as a guideline to achieve electric field-induced wetting or dewetting as the case may be. We also demonstrate that a droplet may be rendered either more or less wetting solely by tuning the forcing frequency.

Highlights

The dynamics of sessile droplets under the influence of electric fields has fascinated researchers for decades due to their importance in a variety of microgravity applications[1,2] as well as technological operations ranging from electrostatic spraying, inkjet printing, medical diagnostics to microelectronics[3,4]
Despite significant progress in the field over the years, there are still considerable gaps in our understanding of the parities and disparities of a droplet’s dynamical response under alternating (AC) and direct (DC) electrical fields, owing to the difficulties in capturing the underlying physics as well as the assumptions made about the electrohydrodynamic properties of the fluids
A concern that emerges is what happens in the case of a sessile droplet under AC forcing for all possible combinations of timescales associated with charge relaxation and electric forcing, which can have significant ramifications in a variety of applications outlined above

Summary

INTRODUCTION

The dynamics of sessile droplets under the influence of electric fields has fascinated researchers for decades due to their importance in a variety of microgravity applications[1,2] as well as technological operations ranging from electrostatic spraying, inkjet printing, medical diagnostics to microelectronics[3,4]. The hydrodynamic flows inside a droplet under AC forcing were investigated by Lee et al.[17] They observed that low-frequency flow is caused by shape oscillation in conjunction with contact line oscillation, whereas high-frequency flow is caused by the electrothermal effect, which is triggered by electrical charge generated due to electrical conductivity and permittivity contrasts across the interface. The electrohydrodynamics of a sessile droplet is governed by the continuity and the Navier–Stokes equations with the the relaxation timescale becomes comparable or greater than the electrostatic Maxwell stress incorporated, which are given by forcing timescale. This phenomenon is explained via the distribution of surface charge formed at the interface and the associated.

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