pH-Tunable electrokinetic movement of droplets

Rashidi, Mansoureh; Benneker, Anne M.

doi:10.1039/d3sm00385j

Cited by 2 publications

(8 citation statements)

References 65 publications

(116 reference statements)

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“…The frequency of an applied oscillatory electric field is varied up to 10 MHz. − The concentration of the background aqueous solution is varied from low to high so that the Debye–Hückel parameter spans up to 100. It may be noted that the magnitude of the surface charge density of various types of fluid droplets including emulsion droplets often ranges up to 10 μC/cm 2 . ,, Besides, the polarity of the surface charge of the droplet may be positive or negative depending on the pH and concentration of the suspended aqueous medium. In our present study, we consider a typical surface charge density within the above-mentioned range in which the Debye–Hückel approximation holds true.…”

Section: Resultsmentioning

confidence: 99%

“…Recent studies suggest that various limiting forms of eq are found to be very important for the correct measurement of surface charge, surface potential, and hydrodynamic slip length for perfectly dielectric (under the limit of ϵ d → 0), perfectly conducting (under the limit of ϵ d → ∞) liquid droplets as well as hydrophobic rigid colloids. One of the limiting forms (under the limit of κa ≫ 1) (e.g., Smoluchowski limit) of expression was recently used by Rashidi and Benneker for the correct measurement of the surface potential as well as the surface charge density of dielectric pH-regulated liquid droplets suspended in electrolyte solution at high concentration. Besides, under the limit of ϵ d → ∞, the corresponding limiting form of eq was further used to measure the charge properties of sodium dodecyl sulfate-stabilized decane droplets .…”

Section: Methodsmentioning

confidence: 99%

“…However, such typical behavior is not possible for the dielectric droplets. In recent years, existing studies − on electrophoresis under static electric field have shown that nonzero Maxwell stress is induced due to the nonvanishing tangential component of the electric field across the surface of a dielectric and a polarizable liquid droplet possessing nonzero surface charge density. As a result, the dielectric droplet with uniform surface charge density under a static electric field shows unique fascinating behaviors.…”

Section: Introductionmentioning

confidence: 99%

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Dynamic Electrophoresis of a Hydrophobic and Dielectric Fluid Droplet

Chowdhury,

Mahapatra,

Ohshima

et al. 2023

Langmuir

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Dynamic electrophoresis is the foundation for electroacoustical measurements, in which the electroacoustical signals may be used to analyze the size and electrostatic charge of colloidal entities by means of the results for dynamic electrophoretic mobility. Thus, the electrophoresis under an alternating electric field is the key foundation for electroacoustic theory. In this article, we develop a tractable analytical theory for the dynamic electrophoresis of hydrophobic and dielectric fluid droplets possessing uniform surface charge density. The tiny fluid droplets possess charged mobile surfaces and have found widespread applications in our day-to-day life. For dielectric fluid droplets (e.g., oil–water emulsions), the tangential electric stress at the interface is nonzero, which significantly affects its electrohydrodynamics under an oscillatory electric field, which has, however, a negligible impact on the electrophoretic motion of conducting droplets (e.g., mercury droplets). Besides, the micro/nanoscale fluid droplets often show hydrophobicity when they are immersed in an aqueous medium, and the impact of the electric field on hydrophobic surfaces remains a research frontier in the chemical discipline. Whereas a number of approximate expressions for electrophoretic mobility have been derived for the conducting droplet, none of them are applicable to such generic hydrophobic fluid droplets with dielectric permittivity that is significantly lower than or comparable to that of an aqueous medium. In this work, within the Debye–Hückel electrostatic framework, we elaborate an original analytical expression of dynamic electrophoretic mobility for this generic dielectric fluid droplet with a hydrophobic surface considering that the droplet retains its spherical shape during its oscillatory motion. We further derived a set of simplified expressions for dynamic electrophoretic mobility deduced under several limiting cases. The results are further illustrated, indicating the impact of pertinent parameters.

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Section: Resultsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Dynamic Electrophoresis of a Hydrophobic and Dielectric Fluid Droplet

Chowdhury,

Mahapatra,

Ohshima

et al. 2023

Langmuir

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“…This has potential applications in emulsion destabilization and separations processes. 15 To describe the distribution of electrostatic potential in the EDL, the classic Poisson−Boltzmann (PB) theory assumes that the solvent's relative permittivity remains constant within the EDL. However, various studies have shown that the relative permittivity near a charged surface is lower than its bulk value.…”

Section: ■ Introductionmentioning

confidence: 99%

“…21 It has been indicated that varying permittivity within the EDL formed adjacent to a charged surface impacts the electrostatic potential distribution within the EDL and the effective thickness of the EDL itself. 17,21 Research findings suggest that this phenomenon becomes more prominent when the EDL thickness (λ) is approximately 1 nm and the surface potential of the substrate (ζ-potential) is around ∼10 −1 V. 17 Droplet EP has been thoroughly investigated through theoretical, 22−24 numerical, 25 and experimental 15,26,27 studies and have been reviewed recently. 28 Some studies have considered the effect of variable permittivity within the EDL on EP.…”

Section: ■ Introductionmentioning

confidence: 99%

Droplet Electrophoresis with Internal Free Ions: Effect of Permittivity Changes in the Electric Double Layer

Rashidi,

Panahi,

Pishevar

et al. 2024

Langmuir

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Droplet electrophoresis (EP) is of interest in biological systems, microfluidics, and separation techniques. We investigate EP of an oil droplet that contains free ions and is stabilized in an electrolyte solution through an amphoteric surfactant. The presence of mobile ions within the droplet leads to the creation of a distinct nonzero space charge density inside the droplet and consequently, formation of an inner EDL inside the droplet in addition to the traditionally considered outside EDL. While we assume the permittivity inside the inner EDL to remain constant, we consider both the case of constant and variable permittivity in the outer EDL. Our findings demonstrate a change in the droplet direction of motion in the electric field when transitioning from acidic to alkaline pH, regardless of permittivity and ionic strength in both oil and electrolyte. We further find a significant reduction in the magnitude of droplet velocity in the case of a variable permittivity due to reduction of the local space charge density within the EDL surrounding the droplet. When decreasing the viscosity ratio of the oil to the electrolyte, in all cases we find a reduction in droplet velocity. This decline is attributed mostly to the formation and strength of a vortex around the droplet. We finally demonstrate that with constant permittivity in the outer EDL, the variation in κa outer has a more significant effect on the droplet's EP velocity than altering κa inner . However, in cases where the body forces inside of the droplet dominate, minor changes in the outer electrolyte concentration have no influence on the droplet motion, which is relevant for biological colloids that can contain significant free internal charges. Our results are important for the manipulation of biological colloids, water and waste treatment such as lubricant removal from processing streams.

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pH-Tunable electrokinetic movement of droplets

Abstract: Manipulation and control of droplet motion in an electric field is of interest in biological systems, microfluidics and electrokinetic (EK) separation techniques. In this work, we show that the electrokinetic...

Cited by 2 publications

References 65 publications

Dynamic Electrophoresis of a Hydrophobic and Dielectric Fluid Droplet

Dynamic Electrophoresis of a Hydrophobic and Dielectric Fluid Droplet

Droplet Electrophoresis with Internal Free Ions: Effect of Permittivity Changes in the Electric Double Layer

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