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  • Featured in Physics
  • Open Access

Electrophoretic Retardation of Colloidal Particles in Nonpolar Liquids

Filip Strubbe*, Filip Beunis, Toon Brans, Masoumeh Karvar, Wouter Woestenborghs, and Kristiaan Neyts

  • Electronics and Information Systems, Ghent University, Sint-Pietersnieuwstraat 41, Ghent B-9000, Belgium and Center for Nano and Biophotonics (NB-Photonics), Ghent University, Sint-Pietersnieuwstraat 41, Ghent B-9000, Belgium

  • *Corresponding author. filip.strubbe@elis.ugent.be
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Phys. Rev. X 3, 021001 – Published 11 April, 2013

DOI: https://doi.org/10.1103/PhysRevX.3.021001

Abstract

We have measured the electrophoretic mobility of single, optically trapped colloidal particles, while gradually depleting the co-ions and counterions in the liquid around the particle by applying a dc voltage. This is achieved in a nonpolar liquid, where charged reverse micelles act as co-ions and counterions. By increasing the dc voltage, the mobility first increases when the concentrations of co-ions and counterions near the particle start to decrease. At sufficiently high dc voltage (around 2 V), the mobility reaches a saturation value when the co-ions and counterions are fully separated. The increase in mobility is larger when the equilibrium ionic strength is higher. The dependence of the experimental data on the equilibrium ionic strength and on the applied voltage is in good agreement with the standard theory of electrophoretic retardation, assuming that the bare particle charge remains constant. This method is useful for studying the electrophoretic retardation effect and charging mechanisms for nonpolar colloids, and it sheds light on previously unexplained particle acceleration in electronic ink devices.

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Synopsis

Stripping Away Confusion

Published 11 April, 2013

Removing the charged layer around a particle allows detailed measurements of its motion in an electric field.

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Popular Summary

Electrophoresis, the motion of charged particles in solution driven by an applied electric field, was discovered in 1807 and has, by now, been fairly well understood and broadly applied, as in DNA electrophoresis and electronic ink devices. It may seem that there is not much room left for new fundamental experiments in this rather mature field. In this paper, we present just such an experiment with optically trapped single latex spheres of micron size in a nonpolar solvent not typical in the traditional context of electrophoresis.

When a charged particle, such as the latex sphere we use, is dispersed in a solution, its charge attracts from the solution a cloud of ions of the opposite charge (what the experts call a “diffuse double layer”). When driven to move by an applied electric field, its motion is a result of three forces: the force coming from the electric field, a frictional force from the solution fluid, and a so-called electrophoretic retardation force that comes from the ion cloud that is driven to move by the same field, but in the opposite direction. The two centuries of development in electrophoresis notwithstanding, direct experimental measurements of the retardation force are very scarce. Measurements of this force through a continuous, controlled depletion of the ion cloud have not been done before. Our work fills this fundamental gap.

The central new idea underlying our experiment is that of stripping off the ion cloud in a continuous fashion to the limit of a “naked” charged latex particle by adding charged micelles formed by surfactant molecules to the solution. These micelles, which are nanometers in size, act as the counterions that form the ion cloud. Depleting them from the charged latex particles requires only the application of a very small electric field, because the charged micelles are not replaced as quickly by spontaneous generation as the atomic or molecular counterions present in traditional electrophoresis experiments. By optically trapping the latex particle and measuring its excursion in the trap at the application of an additional small ac field—an already established method—as the ion cloud is gradually reduced and finally fully stripped, we can then determine the electrophoretic retardation force and its dependence on the ion cloud.

While our experimental measurements do not change the theory of electrophoresis, they make an important addition to our fundamental understanding of electrophoresis. Already, they explain the previously unexplained particle acceleration seen in electronic ink devices. And our method should become useful for studying the electrophoretic retardation effect in colloidal systems.

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Phys. Rev. X 3, 021001– Published 11 April, 2013
  • Received 17 June 2011
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This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

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