1. Could you explain the significance of your article to the non-specialist?

Two-dimensional arrays of colloidal particles, which have found applications in materials science, optics, medicine, etc., can be produced at an interface under the action of attractive lateral capillary forces. Such forces can be induced by gravity and wetting, but it is also possible for an electric-field-induced capillary attraction to exist, which could be significant even for nanoparticles. To check whether such attraction does exist, we compared the motion of charged and uncharged particles toward each other at a liquid interface. The experiments really indicate the existence of additional attraction between two like-charged particles. It exceeds the electrostatic repulsion between the particles and leads to a noticeable acceleration of their motion.

2. What has motivated you to conduct this work?

The idea about the action of electric-field-induced capillary attraction engendered great interest in view of the potential significance of this effect for the nano-world. In some purely theoretical works, controversial results have been obtained: repulsive, attractive of negligibly small capillary interaction, depending on the used model assumptions/approximations. In our opinion, the problem could be solved step-by-step, by combination of theory and experiment. In preceding studies we started with single particles: The existence of the electrodipping force and the electric meniscus deformation was unequivocally established and quantitatively interpreted. The present paper represents the next step: the transition from a single particle to pairs of particles.

3. Where do you see this work developing in the future?

In theoretical aspect, a quantitative theory of the electric-field induced capillary attraction should be developed which compares well with the experiment. (The theory presented in the present manuscript is related to the determination of the acting forces from the particle motion, but it does not give a physical explanation of the established excess attractive force). Such a complete theory could provide a general picture of the effect and will clarify the significance of this effect in the vast range from nanometer to millimeter sized particles. In experimental aspects, the occurrence of this effect with other systems should be also examined. Especially, experiments with smaller particles could bring interesting new results. The effect of electrocapillary forces on the production of two-dimensional arrays of particles is another broad field for further development.

4. Are there any particular challenges facing future research in this area?

The physicochemical origin of the surface charges on the particle/oil interface should be established. Agreement between theory and experiment in the field of the electric-fieldinduced capillary forces has to be achieved. The significance of these forces for submicrometer particles (nanoparticles) should be investigated. The electrocapillary forces could be employed for the creation of structured surfaces with desired properties, and their application for production of new materials and devices.