Electrophoretically mediated partial coalescence of a charged microdrop

Pillai, Rohit; Berry, Joseph D.; Harvie, Dalton J. E.; Davidson, Malcolm R.

doi:10.1016/j.ces.2016.07.022

Cited by 21 publications

(11 citation statements)

References 40 publications

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“…Their results include computations of tip streaming drops. Related numerical work includes [43][44][45]. A strong motivation for theoretical and numerical analysis of the electrokinetic models is their significance in microfluidic devices, for which electrokinetic techniques have been among the most important methods for the manipulation of drops and bubbles [46].…”

Section: Introductionmentioning

confidence: 99%

Deformation and stability of a viscous electrolyte drop in a uniform electric field

Wang

Siegel

2019

Phys. Rev. Fluids

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We study the deformation and breakup of an axisymmetric electrolyte drop which is freely suspended in an infinite dielectric medium and subjected to an imposed electric field. The electric potential in the drop phase is assumed small, so that its governing equation is approximated by a linearized Poisson-Boltzmann or modified Helmholtz equation (the Debye-Hückel regime). An accurate and efficient boundary integral method is developed to solve the low-Reynolds-number flow problem for the time-dependent drop deformation, in the case of arbitrary Debye layer thickness. Extensive numerical results are presented for the case when the viscosity of the drop and surrounding medium are comparable. Qualitative similarities are found between the evolution of a drop with a thick Debye layer (characterized by the parameter χ ≪ 1, which is an inverse dimensionless Debye layer thickness) and a perfect dielectric drop in an insulating medium. In this limit, a highly elongated steady state is obtained for sufficiently large imposed electric field, and the field inside the drop is found to be well approximated using slender body theory. In the opposite limit χ ≫ 1, when the Debye layer is thin, the drop behaves as a highly conducting drop, even for moderate permittivity ratio Q = ǫ 1 /ǫ 2 , where ǫ 1 , ǫ 2 is the dielectric permittivity of drop interior and exterior, respectively. For parameter values at which steady solutions no longer exist, we find three distinct types of unsteady solution or breakup modes. These are termed conical end formation, end splashing, and open end stretching. The second breakup mode, end splashing, resembles the breakup solution presented in a recent paper [R. B. Karyappa et al., J. Fluid Mech. 754, 550-589 (2014)]. We compute a phase diagram which illustrates the regions in parameter space in which the different breakup modes occur.

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Section: Introductionmentioning

confidence: 99%

Deformation and stability of a viscous electrolyte drop in a uniform electric field

Wang

Siegel

2019

Phys. Rev. Fluids

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“…In dynamic collisions the viscosity of the continuous phase becomes an important parameter [53,54]. Other important quantities that can affect dynamic drop and bubble coalescence are the presence of free charge, and in some practical applications the application of external electric fields [55][56][57][58].…”

Section: Introductionmentioning

confidence: 99%

Mapping coalescence of micron-sized drops and bubbles

Berry

Dagastine

2017

Journal of Colloid and Interface Science

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Emulsion formulation, solvent extraction and multiphase microfluidics are all examples of processes that require precise control of drop or bubble collision stability. We use a previously validated numerical model to map the exact conditions under which micron-sized drops or bubbles undergo coalescence in the presence of colloidal forces and hydrodynamic effects relevant to Brownian motion and low Reynolds number flows. We demonstrate that detailed understanding of how the equilibrium surface forces vary with film thickness can be applied to make accurate predictions of the outcome of a drop or bubble collision when hydrodynamic effects are negligible. In addition, we illuminate the parameter space (i.e. interaction velocity, drop deformation, interfacial tension, etc.) at which hydrodynamic effects can stabilise collisions that are unstable at equilibrium. Further, we determine conditions for which drop or bubble collisions become unstable upon separation, caused by negative hydrodynamic pressure in the film. Lastly, we show that scaling analyses are not applicable for constant force collisions where the approach timescale is comparable to the coalescence timescale, and demonstrate that initial conditions under these circumstances cannot be ignored.

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“…It is that balance that explains the emergence of the electric Bond number qE/𝛾a 0 . (Pillai et al 2017) and (Hamlin et al 2012) agree as far as the relevance of this parameter is concerned, but they report different scaling exponents. Our scaling exponent agrees with that of (Hamlin et al 2012), but an important difference to both papers is that in our case q is the average between daughter and parent droplet charge, whereas both works referred to define q as the daughter droplet charge.…”

Section: Resultsmentioning

confidence: 75%

“…Still, there are some similarities in the results. Roughly, in (Pillai et al 2017), the partial coalescence mechanism is described as a competition between electrostatic and capillary forces. When a droplet approaches the meniscus, it is already polarized by the electric field, where the part of the droplet that faces the meniscus (the "south pole") carries charges of a sign opposite to that of the charges at the meniscus.…”

Section: Resultsmentioning

confidence: 99%

“…We are only aware of one study that discusses the partial coalescence of micron-sized droplets under an electric field (Pillai et al., 2017). In this numerical study, equal properties of the droplet liquid and the oil phase were assumed, a marked difference to our studies where the oil had a viscosity more than three orders of magnitude higher than the droplet.…”

Section: Resultsmentioning

confidence: 99%

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Manipulation of single sub-femtolitre droplets via partial coalescence in a direct-current electric field

Shojaeian

Hardt

2021

Flow

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It is demonstrated how aqueous droplets with volumes down to the sub-femtolitre range can be manipulated, including the withdrawal of minute samples from the droplets. The underlying principle is that of partial coalescence with a liquid reservoir in an applied electric field. Upon partial coalescence, a droplet merges with a reservoir and reappears with a smaller diameter. The droplets studied here perform a reciprocating motion between two reservoirs during which their volume gets reduced. Manipulation of droplets with diameters down to 400 nm is reported. A similarity relation is derived expressing the ratio of droplet diameters before and after partial coalescence as a function of the ratio between electric and interfacial-tension forces. The presented scheme allows the withdrawal of minute samples from small droplets and could prove helpful in various applications where droplets are used as tiny reaction spaces or when the goal is to tailor the size of individual droplets.

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Electrophoretically mediated partial coalescence of a charged microdrop

Cited by 21 publications

References 40 publications

Deformation and stability of a viscous electrolyte drop in a uniform electric field

Deformation and stability of a viscous electrolyte drop in a uniform electric field

Mapping coalescence of micron-sized drops and bubbles

Manipulation of single sub-femtolitre droplets via partial coalescence in a direct-current electric field

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