Electrohydrodynamics of dispersed drops of conducting liquid : from drops deformation and interaction to emulsion evolution P. Atten G2Elab, (Dielectric.

Electrohydrodynamics of dispersed drops of conducting liquid : from drops deformation and interaction to emulsion evolution P. Atten G2Elab, (Dielectric Materials and Electrostatics team) UMR CNRS 5269 – Grenoble-INP – Université Grenoble Alpes 18 Février 2016

Electrohydrodynamics
Flow generation or modification under the action of electric forces (influenced by the flow) Volume force density : Coulomb force dielectric force electrostriction Surface force at interfaces interface between conducting and insulating fluids International Workshop EHD & Tribo-Electrostatics, Poitiers, Sept. 1-2, 2016

Two-phase systems Problems with surface charge Examples :
Charged drop instability (Rayleigh) Oblate or prolate deformation of drop (Taylor) Conducting drop in uniform field (Taylor) Wide variety of situations and problems concerning : planar interfaces, jets, meniscus drops, bubbles, emulsions, . . Electrocoalescence : drops in some warm clouds drops merging in lab-on-a-chip water-in-oil emulsions International Workshop EHD & Tribo-Electrostatics, Poitiers, Sept. 1-2, 2016 3 3

Water-in-oil emulsions
 extraction sites of crude oil : high pressure drop  fine water-in-oil emulsion  refineries, chemical plants Main concern here : petroleum industry chemical industry Separation of emulsions components due to gravitational force droplets size down to ~ 1 m  very slow settling w ~ 1 m/s for D = 10 m  use of demulsifiers  electric fields  electrocoalescence Techniques to promote drops coalescence and drop size increase International Workshop EHD & Tribo-Electrostatics, Poitiers, Sept. 1-2, 2016 4 4

Problems in Electrocoalescence
old  technique : moderate field (~ 500 V/cm), grids in very big tanks compact electrocoalescers : higher field (a few kV/cm) applied on flowing emulsion Practical problems : conditions for maximum efficiency ? compact electrocoalescers don’t work on all crude oils : why ? in some cases compact electrocoalescers are no more working after a few years : why ? International Workshop EHD & Tribo-Electrostatics, Poitiers, Sept. 1-2, 2016 5 5

Electrocoalescence in flowing emulsion
1st stage u Flow induced collision - ws ws 2nd stage E0 Electric force  pressure on oil film  oil expelled Film thinning 3rd stage coalescence (or not !) International Workshop EHD & Tribo-Electrostatics, Poitiers, Sept. 1-2, 2016 6 6

Outline of presentation
1. Field action on a single conducting drop 2. Field induced interaction of two conducting drops a) force between uncharged spheres b) static case : drops deformation and instability c) motion and deformation of two free drops 3. Coalescence and partial coalescence 4. Drops bouncing under strong field 5. Field action on water-in-oil emulsions International Workshop EHD & Tribo-Electrostatics, Poitiers, Sept. 1-2, 2016 7 7

Electrical Bond number Be
Considered problems equipotential surface charge density s =  E Drops of conducting liquid : insulating gas or pure liquid, crude oil (with surfactants) well defined interfacial tension T Suspending fluid : Interplay of capillary and electrical forces Electrical Bond number Be International Workshop EHD & Tribo-Electrostatics, Poitiers, Sept. 1-2, 2016 8 8

Uniformly charged drop
Stability study by Rayleigh : maximum drop charge : corresponding electric field at interface :  Becrit = 1 Instability when pes > pcap International Workshop EHD & Tribo-Electrostatics, Poitiers, Sept. 1-2, 2016 9 9

Conducting drop in uniform field
Stability analysis by Taylor (prolate spheroid)  critical value of applied uniform field : E0 no field at poles, with local field and curvature Becrit = 0.653 With other Be definition (E0 for field and drop radius R ) International Workshop EHD & Tribo-Electrostatics, Poitiers, Sept. 1-2, 2016 10 10

Conducting drop in uniform field
For E0 > (E0)crit interface disruption at nearly conical poles AC field of variable amplitude G. Berg, Sintef Energy Research, Trondheim - Norway Consortium project : "Electrocoalescence" International Workshop EHD & Tribo-Electrostatics, Poitiers, Sept. 1-2, 2016 11 11

Electrowetting metal insulating sheet V conducting drop Video taken by Mugele (from his web site) Electrically induced drop spreading up to a limit : interface disruption International Workshop EHD & Tribo-Electrostatics, Poitiers, Sept. 1-2, 2016 12 12

Field induced interaction of two drops
Various behaviours depending on : drops radii R1 and R2 drops spacing s applied field strength E0 and frequency f interfacial tension T drops charges Q1 and Q2 Basic case : drops without net charge Field action on drops : surface charge density at interface : s =  E non uniform s  interface deformation resultant electric force  drops motion International Workshop EHD & Tribo-Electrostatics, Poitiers, Sept. 1-2, 2016 14 14

Interaction force between two spherical drops small attraction force
 Large spacing : s > R2 dipole – dipole interaction force on drop # 2 ( ) attraction for ; tendency to align drops with E0 small attraction force International Workshop EHD & Tribo-Electrostatics, Poitiers, Sept. 1-2, 2016 15 15

Interaction force between two spherical drops strong attraction force
 Very small spacing : s << R2 strong attraction force asymptotic force expression exact expression of force on drop # 2 (Davis) F1, F2 and F3 : complicated series depending on s/R2 and R2/R1 empirical expression for aligned drops ( = 0) with R1 = R2 International Workshop EHD & Tribo-Electrostatics, Poitiers, Sept. 1-2, 2016 16 16

Deformation of anchored drops and instability
potential difference V  drops elongation deformation limited by instability critical conditions : scrit/s0 = 0.51 to 0.55 s0 = R0 qm = 3p/4 International Workshop EHD & Tribo-Electrostatics, Poitiers, Sept. 1-2, 2016 18 18

Experiments on anchored drops
Scrit International Workshop EHD & Tribo-Electrostatics, Poitiers, Sept. 1-2, 2016 19 19

Deformation and motion of two free drops numerical simulations
drops motion : s(t) alignment with E0 Attraction force between two drops  Drainage of oil between the two drops : viscous force ? numerical simulations Qualitative picture using Be Guess : Interface instability when Be ~ 1 at pole  Instability  contact between drops International Workshop EHD & Tribo-Electrostatics, Poitiers, Sept. 1-2, 2016 21 21

Two free drops under strong field
Case of Be0 not much smaller than 1 deformation increasing as spacing s decreases sinst not very small Drops shape at moment of rapidly increasing local deformation Results of numerical simulation using COMSOL Multiphysics™ International Workshop EHD & Tribo-Electrostatics, Poitiers, Sept. 1-2, 2016 22 22

Two closely spaced droplets without surfactants
Case of small R0 , weak field and s0 / R0 < ~ 0.3 pes << pcap (Be0 << 1)  negligible deformation and sinst << s0 Asymptotic case of no deformation, s0 / R0 << 1 and high enough oil Order of magnitude analysis + numerical simulations of oil film thinning  like Stokes formula ! ! with empirical formula International Workshop EHD & Tribo-Electrostatics, Poitiers, Sept. 1-2, 2016 23 23

Two closely spaced droplets with surfactants
Surfactants  interface between water and oil immobile Asymptotic case of no deformation, s0 / R0 << 1 and high enough oil viscosity with asymptotic expression of film thinning : and empirical formula for attraction force : with typical values : oil ~ 20 mPa.s,   F/m, E0 ~ 2 kV/cm International Workshop EHD & Tribo-Electrostatics, Poitiers, Sept. 1-2, 2016 24 24

Qualitative picture of field influence on coalescence (pure oil)
Interface instability : formation of a bump that extremely rapidly elongates  bridge between the two water drops Merging of the drops ? not always Exchange of charge  redistribution of field and electrostatic pressure slowing of coalescence dynamics partial coalescence drop bouncing after charge exchange Under higher and higher electric field International Workshop EHD & Tribo-Electrostatics, Poitiers, Sept. 1-2, 2016 26 26

Water droplet falling on a big water drop
Experiments performed by S.M. Helleso, Sintef Energy Research (Trondheim – Norway) in the framework of the collaborative project : "Electrocoalescence II" Grane crude oil (North Sea) Bipolar square voltage (1000 Hz, up to 3 kV) Near Infra-Red video-camera (up to 345 frames/s) Halogen lamp International Workshop EHD & Tribo-Electrostatics, Poitiers, Sept. 1-2, 2016 27 27

Droplet falling and coalescence
(oil = 25 mPa.s) T = 60°C E = 940 V/cm Diameter : 680 µm International Workshop EHD & Tribo-Electrostatics, Poitiers, Sept. 1-2, 2016 28 28

Field induced slowing of coalescence
oil = 24 mPa.s E = 250 V/cm t (ms) 5 10 15 20 25 E = 750 V/cm S.M. Helleso, P. Atten et al., Experimental study of electrocoalescence of water drops in crude oil using near-infrared camera, Experiments in Fluids (2015) International Workshop EHD & Tribo-Electrostatics, Poitiers, Sept. 1-2, 2016 29 29

Picture of partial coalescence
When upward electric force  interfacial tension force :  partial coalescence (part of falling drop does not merge) E0 Electrostatic pressure E0 International Workshop EHD & Tribo-Electrostatics, Poitiers, Sept. 1-2, 2016 30 30

Field induced partial coalescence
oil = 25 mPa.s T = 60°C E = 1330 V/cm Diameter : 710 µm T = 60°C E = 1660 V/cm Diameter : 750 µm International Workshop EHD & Tribo-Electrostatics, Poitiers, Sept. 1-2, 2016 31 31

Field induced partial coalescence
When electric force increases ( E > Ecrit) volume of “daughter” drop increases E = 1.33 kV/cm oil = 25 mPa.s E = 1.66 kV/cm S.M. Helleso, P. Atten et al., Experimental study of electrocoalescence of water drops in crude oil using near-infrared camera, Experiments in Fluids (2015) International Workshop EHD & Tribo-Electrostatics, Poitiers, Sept. 1-2, 2016 32 32

Non coalescence of drops
Limit case of partial coalescence at high field ? (oil = 62 mPa.s) T = 40°C E = 1.25 kV/cm Diameter : 720 µm No ! for crude oil at 40°C Influence of viscosity ? Possible role of interfacial film ? Similarities with observations in "pure" oils Threshold field of bouncing Mechanism ? International Workshop EHD & Tribo-Electrostatics, Poitiers, Sept. 1-2, 2016 34 34

Non coalescence of drops
Water with KCl (0.2 M/l) Drops radius ~ 0.8 mm Silicone oil (1000 cSt) Videos taken by Ristenpart & Bird (from their web site) International Workshop EHD & Tribo-Electrostatics, Poitiers, Sept. 1-2, 2016 35 35

Mechanism of drops bouncing
Dynamical process (case of uncharged drops)  instability of facing interfaces  thin jet (or ejection of droplet ?)  transitory bridge with charge exchange  repelling of drops before contact during bridging bouncing International Workshop EHD & Tribo-Electrostatics, Poitiers, Sept. 1-2, 2016 36 36

Non coalescence of charged drops  transitory bridge with charge exchange Water+ink drops radius  0.8 mm Silicone oil (50 cSt) Bridge lifetime ~ 1 s Y-M Jung & S. Kang A novel actuation method of transporting droplets by using electrical charging of droplet in a dielectric fluid, Biomicrofluidics, 3, (2009) International Workshop EHD & Tribo-Electrostatics, Poitiers, Sept. 1-2, 2016 37 37

Two drops on a PTFE plate; horizontal electric field (bipolar square voltage) Water drops radius ~ 1 mm Mineral oil (~ 20 cSt) G. Berg, Sintef Energy Research, Trondheim - Norway Consortium project : "Electrocoalescence" International Workshop EHD & Tribo-Electrostatics, Poitiers, Sept. 1-2, 2016 38 38

Drops bouncing : DC versus AC voltage
Behaviour after bouncing  DC field + perfectly insulating oil : spacing increases due to field action on charged drops DC field + slightly conducting oil : drops lose their charge  first increase, then decrease of spacing  AC field + oil : field action on charged drops : oscillating force component  spacing can strongly oscillate strong AC field : possible generation of very small droplets from ligament break-off  mist around the "contact " zone International Workshop EHD & Tribo-Electrostatics, Poitiers, Sept. 1-2, 2016 39 39

Drops bouncing : AC applied voltage
S. Ingebrigtsen, Sintef Energy Research, Trondheim - Norway Consortium project : "Electrocoalescence" International Workshop EHD & Tribo-Electrostatics, Poitiers, Sept. 1-2, 2016 40 40

Drop bouncing : DC versus AC voltage
Behaviour after bouncing  DC field + perfectly insulating oil : spacing increases due to field action on charged drops DC field + slightly conducting oil : drops lose their charge  first increase, then decrease of spacing  AC field + oil : field action on charged drops : oscillating force component  spacing can strongly oscillate strong AC field : possible generation of very small droplets from ligament break-off  mist around the "contact " zone International Workshop EHD & Tribo-Electrostatics, Poitiers, Sept. 1-2, 2016 41 41

Example : bouncing and mist generation
T = 40°C E = 1.5 kV/cm Diameter : 790 µm (oil = 62 mPa.s) International Workshop EHD & Tribo-Electrostatics, Poitiers, Sept. 1-2, 2016 42 42

Electrocoalescence in water-in-oil emulsions
 size distribution of water droplets given droplet : interaction with numerous neighbouring droplets but F  s-0.8  interaction with closest drop plays decisive role  AC field no electrolysis no limiting effect of electrode coatings International Workshop EHD & Tribo-Electrostatics, Poitiers, Sept. 1-2, 2016 44 44

Electrocoalescence in stagnant emulsions
Action of field  drops alignment with E droplets : motion and contact big drops : elongation, dispersion threshold field for dispersion scales with time evolution : International Workshop EHD & Tribo-Electrostatics, Poitiers, Sept. 1-2, 2016 45 45

Electric Field: 2.5 kV/cm Frequency: Hz Waveform: Sine Capture rate: fps Playback: 400 ms/s Frame Size: 2.5 x 2.5 mm Observations: Isotropic coalescence. Rapidly increasing drop-size. High coalescence efficiency Nytro 10X + 5 % water w. 3.5% NaCl % Span 80®. S. Ingebrigtsen, Sintef Energy Research, Trondheim - Norway Consortium project : "Electrocoalescence" International Workshop EHD & Tribo-Electrostatics, Poitiers, Sept. 1-2, 2016 46 46

Large Drop Introduced (D = 190 mm) Electric Field: 10 kV/cm Frequency: Hz Waveform: Bipolar Square Capture rate: fps Playback: 400 ms/s Frame Size: 2.5 x 2.5 mm Nytro 10X + 10% water w. 3.5% NaCl S. Ingebrigtsen, Sintef Energy Research, Trondheim - Norway Consortium project : "Electrocoalescence" International Workshop EHD & Tribo-Electrostatics, Poitiers, Sept. 1-2, 2016 47 47

Electric Field: 5.0 kV/cm Frequency: Hz Waveform: BSV Capture rate: fps Playback: 400 ms/s Frame Size: 2.5 x 2.5 mm Nytro 10X + 5 % water w. 3.5% NaCl % Span 80®. S. Ingebrigtsen, Sintef Energy Research, Trondheim - Norway Consortium project : "Electrocoalescence" International Workshop EHD & Tribo-Electrostatics, Poitiers, Sept. 1-2, 2016 48 48

E = 10 kV/cm, f = 500 Hz, waveform : Sine, frame size : 2.5 x 2.5 mm Nytro 10X + 10% water w. 3.5% NaCl capture rate : fps Playback: ms/s ms/s S. Ingebrigtsen, Sintef Energy Research, Trondheim - Norway Consortium project : "Electrocoalescence" Analogy and difference with electrorheological (ER) effect International Workshop EHD & Tribo-Electrostatics, Poitiers, Sept. 1-2, 2016 49 49

collisions Simple case of monodisperse drops in a shear flow u smin : statistical distribution drops collision rate depends on : shear rate drops concentration required time  tdrainage film thinning coalescence possible only if tdrainage < tproximity coalescence International Workshop EHD & Tribo-Electrostatics, Poitiers, Sept. 1-2, 2016 50 50

from : tdrainage < tproximity necessary condition for strong electrocoalescence of droplets (order of magnitude) with typical values : oil ~ 20 mPa.s, ~ 102 s-1,   F/m, smin /R ~ 10-1 E0 >~ 2 kV/cm comparison of tdrainage and tproximity should give probability of drops merging for each quasi-collision in numerical simulations "statistically steady" state of electrocoalesced emulsion : balance between coalescence of droplets and drops dispersion of biggest drops International Workshop EHD & Tribo-Electrostatics, Poitiers, Sept. 1-2, 2016 51 51

Conclusions 1 E < Ec : slowing down of merging
 In oil, process of drops approach until bridging : well understood and modeled  behaviour after contact depends on E, T, µoil, R1 and R2 E < Ec : slowing down of merging E > Ec : bouncing or partial coalescence Study to be performed : - influence of oil viscosity, interfacial tension and ratio R2/R1 - conditions for partial coalescence rather than bouncing International Workshop EHD & Tribo-Electrostatics, Poitiers, Sept. 1-2, 2016 52 52

Conclusions 2 stagnant emulsions : time evolution of emulsion
 For water emulsion in pure oil : stagnant emulsions : time evolution of emulsion qualitatively understood flowing emulsions : estimate of field necessary to have noticeable electrocoalescence rate  case of crude oils : open Preliminary study to be performed to characterize : visco-elastic properties of interfacial film due to surfactants, waxes, asphaltenes, . . . International Workshop EHD & Tribo-Electrostatics, Poitiers, Sept. 1-2, 2016 53 53

Acknowledgments Most of presented results, pictures and videos obtained by : P. Atten, J. Raisin and J.-L. Reboud (G2Elab) G. Berg, S.M. Helleso, S. Ingebrigtsen and L. Lungaard (Sintef Energy Research - Trondheim, Norway) Collaborative work and discussions continued for the past eigth years in the framework of the project : "Electrocoalescence – Criteria for an efficient process in real crude oil systems" co-ordinated by SINTEF Energy Research (contact : L. Lundgaard). Support by : The Research Council of Norway (contract no: /S30) Aker Solutions AS, BP Exploration Operating Company Ltd, Hamworthy Technology and Products AS, Shell Technology Norway AS, Petrobras, Saudi Aramco, Statoil ASA. International Workshop EHD & Tribo-Electrostatics, Poitiers, Sept. 1-2, 2016 54 54

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Fluid properties Procedures
Crude oil : from Grane field (North Sea) Water with 3.5 wt% NaCl Temperature (°C) 20 40 60 Crude oil viscosity (mPa/s) 208 62.5 25 Interfacial tension (mN/m) (after 10 mn) 22 15.5  10 Crude oil conductivity (S/m) 2 10-8 10-7 Procedures Droplet :   700 µm remaining 10 mn at needle tip before release Bipolar square voltage (1000 Hz, up to 3 kV) applied after droplet release NIR video camera (256 x 320 pixels) : up to 345 frames per second Experiments at 40°C and 60°C International Workshop EHD & Tribo-Electrostatics, Poitiers, Sept. 1-2, 2016 57 57

Threshold field for partial coalescence
(oil = 25 mPa.s) T = 60°C E = 1167 V/cm Diameter : 710 µm Threshold field such that : electric force compensates for interfacial tension force Order of magnitude estimate gives : Eth ~ kV/cm close to observation at 60 °C ( 1 kV/cm) Phenomena to investigate : experiments ? simulations ? International Workshop EHD & Tribo-Electrostatics, Poitiers, Sept. 1-2, 2016 58 58

Non coalescence of charged drops  marked deformation : angle > 31°  transitory bridge with charge exchange argument based on static conditions for the transitory bridge W.D. Ristenpart, J.C. Bird, A. Belmonte, F. Dollar & H.A. Stone Non-coalescence of oppositely charged drops Nature, 461 (17 sept. 2009), nature08294 International Workshop EHD & Tribo-Electrostatics, Poitiers, Sept. 1-2, 2016 59 59

Electrohydrodynamics of dispersed drops of conducting liquid : from drops deformation and interaction to emulsion evolution P. Atten G2Elab, (Dielectric.

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Electrohydrodynamics of dispersed drops of conducting liquid : from drops deformation and interaction to emulsion evolution P. Atten G2Elab, (Dielectric.

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