Like-charged particles at liquid interfaces

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Original entry: Nefeli Georgoulia, APPHY 226, Spring 2009

Overview

Authors: Part 1 - Mischa Megens, Joanna Aizenberg. Part 2 - M. G.Nikolaides, A. R. Bausch, M. F.Hsu, A.D.Dinsmore, M. P. Brenner, C.Gay, D. A.Weitz

Source: Narture, Vol. 424, August 2003

Supporting Reference: <math>^1</math> M. G.Nikolaides, A. R. Bausch, M. F.Hsu, A.D.Dinsmore, M. P. Brenner, C.Gay, D. A.Weitz, Electric-field-induced capillary attraction between like-charged particles at liquid interfaces, Nature, Vol 420, 2002

Soft Matter key words: capillary attraction, surface tension, electrostatic force

Abstract

This publication is actually a brief communication between two scientific parties. The topic is the attraction between micro-meter sized particles absorbed at aqueous interfaces. On the first part, M. Megens and J. Aizenberg argue that the attractive force arising between the two particles cannot be attributed to capillary attraction alone. On the second part M.G. Nikolaides et al. refute this argument by presenting novel experimental data.

Soft Matter Snippet

Fig.1 : M. G.Nikolaides, A. R. Bausch, M. F.Hsu, A.D.Dinsmore, M. P. Brenner, C.Gay, D. A.Weitz

This article offers insight into the forces acting between charged particles, floating on a water surface. It also proposes some stipulations on how attraction might arise between these particles. Essentially, the authors are investigating the micrometer-equivalent of the Cheerios effect. And although the attraction between millimeter-sized particles can be attributed to buoyancy, another mechanism has to account for the attraction between micrometer-sized particles. Nikolaides et al.<math>^{[1]}</math> match experimental data with a model in which capillary force accounts for the attraction. According to them, the origin of capillary forces is electrostatic: dipolar electric fields induce surface charges that distort the liquid interface between the particles. The resulting dipolar interaction causes repulsion, while the interfacial distortion causes capillary attraction. The capillary attraction is:

<math>F = \frac{P^2}{16 \pi \epsilon_0 \alpha_w^4} \frac{\epsilon_{oil}}{\epsilon_{water}}</math>

While the potential is:

<math>U = \frac{F^2}{2 \pi \gamma} log\frac{r}{r_0} + \frac{P^2}{4 \pi \epsilon_0r^3} \frac{2 \epsilon_{oil}}{\epsilon^2_{water}}</math>

Where the first term is the capillary attraction and the second is the dipolar repulsion.

Fig.1 : M.Megens & J.Aizenberg


However, Megens et al. argue that this model fails to take into account that force F acts on floating particles as well as the liquid interface and therefore cannot be the one causing the attraction. They propose instead, that the capillary attraction between particles is due to the dimple overlap of the particles, which reduces the total surface area of the liquid. However, they argue that the potential of that attraction decays as 1/r^3 , i.e. its range is too short to account for the attraction.