Like-charge attraction in confinement: myth or truth?

Abstract

It is general wisdom that likely charged colloidal particles repel each other when suspended in liquids. This is in perfect agreement with mean field theories being developed more than 60 years ago. Accordingly, it was a big surprise when several groups independently reported long-ranged attractive components in the pair potential U(r) of equally charged colloids. This so-called like-charge attraction (LCA) was only observed in thin sample cells while the pair-interaction in unconfined suspensions has been experimentally confirmed to be entirely repulsive. Despite considerable experimental and theoretical efforts, LCA remains one of the most challenging mysteries in colloidal science. We experimentally reinvestigate the pair-potential U(r) of charged colloidal particles with digital video microscopy and demonstrate that optical distortions in the particle's images lead to slightly erroneous particle positions. If not properly taken into account, this artefact pretends a minimum in U(r) which was in the past misleadingly interpreted as LCA. After correcting optical distortions we obtain entirely repulsive pair interactions which show good agreement with linearized mean field theories. 82,70.Dd, A controversial debate in colloidal science has been launched in 1994 when Kepler and Fraden reported an unusual long-ranged attractive component in the pair potential of charged colloidal particles 1 . This so-called like-charge attraction (LCA) was only observed in thin cells (typical sample height H < 10µm) while the pair-interaction in bulk suspensions has been experimentally confirmed to be entirely repulsive in agreement with Poisson-Boltzmann theories 2-4 . Therefore, it was speculated that the confining plates are responsible for the deviations from theory. Soon after its initial observation LCA was also observed by other authors 5-11 which then provoked considerable theoretical interest in this phenomenon. In the meantime it has been rigorously proven that the observed attraction can not be explained within the framework of mean field theories, irrespective of whether the particles are suspended in bulk or in confinement [12][13][14] . Several other approaches beyond PoissonBoltzmann have been proposed as the origin of confinement-induced attraction. While correlation effects of the electrolyte indeed can lead to a short-ranged attraction of negatively charged colloids, none of the existing theories can account for the long-ranged attraction as observed in experiments 15,16 . Accordingly, even after more than ten years of considerable research, LCA is still one of the most challenging unsolved mysteries in colloidal science. In this study we reinvestigate the pair potential U(r) of charged colloidal particles in thin sample cells. Contrary to previous experiments where U(r) was derived from the pair correlation function g(r) in semi-dilute suspensions, here we determine the pair potential directly by measuring the probability distribution of two silica spheres. This is of particular advantage because it avoids the...

Charged colloidal particles in a charged wedge: do they go in or out?

“…We emphasize that-in order to explain particle accumulation in the wedge-one does not need to invoke a like-charge attraction picture [18]. In fact, the linear screening theory is based on like-charge repulsion.…”

“…One may use an understanding of the mechanisms to fabricate microfluidic devices to guide colloidal particles and/or to separate mixtures of the latter. The second motivation has to do with the still controversial discussion about like-charge attraction [18]. If charged particles accumulate in the wedge cusp it is tempting to investigate whether the underlying mechanism has to do with like-charge attraction.…”

Tailoring of Phononic Band Structures in Colloidal Crystals

“…1). Particle positions were determined for different combinations of interference pattern intensities fU 1 0 ; U 2 0 g from sequences consisting of several thousand images using digital video microscopy at an acquisition rate of 2 frames per second [9]. From these data we finally obtained the particle trajectoriesr t x t; y t with 1; .…”

“…The particles interact via a screened Coulomb potential r / Z 2 expÿr=r with Z 10 000 the renormalized surface charge and ÿ1 300 nm the screening length. Both values were determined according to a procedure described in [9]. As the sample cell we used a cuvette made of fused silica with 200 m spacing between the top and bottom plate which was connected to a standard closed deionization circuit to maintain stable ionic conditions during the measurements [10].…”

Hydrodynamics on Charged Superparamagnetic Microparticles in Water Suspension: Effects of Low-Confinement Conditions and Electrostatics Interactions

“…However, the calculated hydrodynamic effects do not seem to explain the experimental minimum on the potential (Grier & Han (2004); Han & Grier (2003)). Other authors argue that this kind of studies should be more rigorous in the analysis of errors when extracting data from the images (Savin & Doyle (2005;2007); ) and other authors claim that the effect on the electrostatic potential may be an artefact (Baumgart et al (2006)) that occurs because of a incorrect extraction of the position of the particles (Gyger et al (2008)). Polin et al (2007) realized that some minimums in the electrostatic potential can be eliminated by measuring the error on the displacement of the particles.…”