Electrocoalescence of water drops in oil shear flow: Development of an experimental set up

Abstract

Present research aims at determining the conditions leading to electrocoalescence of water drops in oil flows and at characterizing the interplay between fluid dynamics and electric field in the mechanism. We describe here the build-up of an experimental setup designed to investigate the critical coalescence conditions in the case of two free water droplets in an oil shear flow under the action of an applied electric field. Drop pairs will be injected in a Poiseuille flow to study, by optical means, their relative motion, deformation and possible coalescence. Important part of this preliminary work concerns the injection of controlled charge free drop pairs using a newly developed EHD actuation technique. Second paper [1] deals with theoretical and numerical analyses in the same configuration.

Numerical study of non-Newtonian droplets electrocoalescence

“…Taylor [18] presented a function to classify deformations as prolate, oblate and spherical. Equation (10) shows this function [20]:…”

“…Yang et al [19] studied 3D phase field model in the electrohydrodynamic which included coalescence of four droplets in an electric field. Raisin et al [20] probed electrocoalescence of two water droplets in oil both experimentally and numerically. Kamali et al [21] analyzed leaky dielectric droplets generation in a co-flow device.…”

Field induced coalescence of two free water drops in a viscous dielectric fluid

“…Simulations were performed considering drops of tap water (ρ w = 1000 kg/m 3 , dynamic viscosity μ w = 1 mPa.s, dielectric constant ε r,w = 80, conductivity σ w ≥ 0.05 S/m) in a polybutene oil (ρ oil = 824 kg/m 3 , μ oil = 10.5 mPa.s, ε r, oil = 2.3, σ oil ≤ 10 -12 S/m) with interfacial tension γ on the order of 25 10 -3 N/m (fluids used in experiments [6,7]). For large separation s 0 between evenly sized drops (R 1 = R 2 = R 0 ), to a first approximation the time necessary for drops to get into contact depends only on the relative spacing s 0 /R 0 and not on R 0 (Fig.…”

“…Taking Davis expression p e0 = ε r,oil ε 0 (E 3 E 0 ) 2 /2 [10] corresponding to maximum field at the droplets surface and a fitted power law 1.87 (R 0 /s) 0.85 for the variation of E 3 [6], leads to the expression (with a factor β) : The temporal evolution of the spacing s then is : As the primary interest of electrocoalescence is to resolve the water-in-oil emulsions, we considered water droplets of small size (tens of micrometers) with an initial spacing s 0 ~ 1 µm and a polybutene oil (see [7]); the reference case is relative to R 0 = 20 µm, s 0 = 1 µm. A systematic study was conducted by varying the applied field.…”

A novel bench size model coalescer: dehydration efficiency of AC fields on water-in-crude-oil emulsions

“…State-ofthe-art CECs are in-line flow devices designed to increase the mean size of water droplets by merging and, therefore, to reduce the time required for their sedimentation under gravity. Though, the control and increase of their efficiency remain, to date, particularly challenging as the numerous phenomena involved in electrocoalescence are far from being fully understood [6].…”