Droplets in an Electric Field: Surface Rheology, Coalescence and Rebound

Author

Yu Han, CUNY City CollegeFollow

Date of Award

2022

Document Type

Dissertation

Department

Chemical Engineering

First Advisor

Charles Maldarelli

Second Advisor

Joel Koplik

Keywords

Droplet Deformation; Electrocoalescence; Surfactant; Surface Dilatational Viscosity; Asphaltenes; Langmuir Isotherm

Abstract

Electrocoalescence is the process in which pairs of conducting droplets suspended in a continuous dielectric (nonconducting) liquid phase are drawn together and merge upon the application of an electric field. The electric field polarizes each of the droplets in the field direction. The polarization causes the drops to deform and drives a dipolar attraction which forces them to approach each other and coalesce. Many technologies use electric fields to manipulate fluid dispersions. Electrocoalescence is an essential unit operation for separating water droplets in a crude oil. This water in oil emulsion is stabilized by surfactants, such as asphaltenes, resins and hydrophobic colloid particles indigenous to crude oil, adsorbed onto the water-oil interface forming an elastic monolayer. Electrically driven coalescence has to overcome the interfacial elasticity to merge.

We first study the electro-deformation of an isolated droplet. While surfactants lower the interface tension which facilitates electro-deformation, the monolayer elasticity resists deformation. High molecular weight surfactants, e.g. asphaltenes, with large dilatational viscosities, can potentially retard the deformation dynamics. We developed a boundary integral method that simulates the dynamic interfacial deformation of a perfectly conducting droplet in a dielectric in a uniform field. A range of initial surfactant surface concentrations are studied, with elasticity proportional to concentration. Equilibrium drop deformations, unaffected by surface viscosity, are strongly resisted by elasticity at high surface concentrations, and field strengths necessary for break-up increase with elasticity. We find that surface dilatational viscosity can extend the deformation time.

An identical droplet pair aligned parallel to an applied uniform electric field is the model problem for electrocoalescence. We developed an axisymmetric boundary integral method that simulates the dynamic interaction of a pair of perfectly conducting clean droplets in a dielectric. Before the contact of the droplet pair, a higher electric capillary number Ca^E (ratio between electric force to capillary force) with a larger initial separation results in a more significant deformation at the facing poles. To simulate the coalescence of the droplet pair, the interfaces are connected and reconstructed. Given the initial separation being 1 droplet radius, full coalescence is observed for lower Ca^E (< 0.1) and non-coalescence for higher Ca^E (>= 0.1). For non-coalescence (Ca^E = 0.1), the droplet pair rebounds with opposite charge and leaves a satellite droplet between them.

In applications of electrocoalescence, generally, there is a finite angle, ψ between the pair center line and applied uniform field. We developed a non-axisymmetric spectral boundary integral method that simulates the dynamic interaction of a pair of perfectly conducting clean droplets in a dielectric. The critical angle ψ^c, beyond which the pair initially repels, is weakly dependent on the initial separation. The pair will reorient towards the applied field direction hence reducing ψ, except for ψ = π/2. The effect of electro-deformation on the trajectory of the pair is investigated.

Our work on an isolated electro-deforming surfactant covered droplet and a clean droplet pair interaction in a uniform electric field gives a better understanding of the electrocoalescence process and facilitates its application.

Recommended Citation

Han, Yu, "Droplets in an Electric Field: Surface Rheology, Coalescence and Rebound" (2022). CUNY Academic Works.
https://academicworks.cuny.edu/cc_etds_theses/1068