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Electric-field-induced capillary attraction between like-charged particles at liquid interfaces

Abstract

Nanometre- and micrometre-sized charged particles at aqueous interfaces are typically stabilized by a repulsive Coulomb interaction. If one of the phases forming the interface is a nonpolar substance (such as air or oil) that cannot sustain a charge, the particles will exhibit long-ranged dipolar repulsion1; if the interface area is confined, mutual repulsion between the particles can induce ordering2 and even crystallization3,4. However, particle ordering has also been observed in the absence of area confinement5, suggesting that like-charged particles at interfaces can also experience attractive interactions6. Interface deformations are known to cause capillary forces that attract neighbouring particles to each other, but a satisfying explanation for the origin of such distortions remains outstanding7,8. Here we present quantitative measurements of attractive interactions between colloidal particles at an oil–water interface and show that the attraction can be explained by capillary forces that arise from a distortion of the interface shape that is due to electrostatic stresses caused by the particles' dipolar field. This explanation, which is consistent with all reports on interfacial particle ordering so far, also suggests that the attractive interactions might be controllable: by tuning the polarity of one of the interfacial fluids, it should be possible to adjust the electrostatic stresses of the system and hence the interparticle attractions.

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Figure 1: Interfacial colloidal particle ordering induced by repulsive interactions.
Figure 2: Scatter plot showing positions of a seven-particle hexagonal crystallite on a water droplet of 24 µm radius.
Figure 3: Secondary interparticle potential minimum derived from experimental observations.
Figure 4: Sketch of the equipotential lines at the fluid interface and the resulting distortion of the oil–water interface.

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Acknowledgements

We thank B. Berge and T. M. Squires for discussions. We gratefully acknowledge support from the NSF, the Materials Research Science and Engineering Center through the auspices of the NSF and the Division of Mathematical Sciences. A.B. acknowledges the support from the Emmy Noether-Program of the DFG.

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Correspondence to D. A. Weitz.

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Nikolaides, M., Bausch, A., Hsu, M. et al. Electric-field-induced capillary attraction between like-charged particles at liquid interfaces. Nature 420, 299–301 (2002). https://doi.org/10.1038/nature01113

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