Effect of electric-field-induced capillary attraction on the motion of particles at an oil–water interface

Abstract

Here, we investigate experimentally and theoretically the motion of spherical glass particles of radii 240-310 microm attached to a tetradecane-water interface. Pairs of particles, which are moving toward each other under the action of lateral capillary force, are observed by optical microscopy. The purpose is to check whether the particle electric charges influence the particle motion, and whether an electric-field-induced capillary attraction could be detected. The particles have been hydrophobized by using two different procedures, which allow one to prepare charged and uncharged particles. To quantify the hydrodynamic viscous effects, we developed a semiempirical quantitative approach, whose validity was verified by control experiments with uncharged particles. An appropriate trajectory function was defined, which should increase linearly with time if the particle motion is driven solely by the gravity-induced capillary force. The analysis of the experimental results evidences for the existence of an additional attraction between two like-charged particles at the oil-water interface. This attraction exceeds the direct electrostatic repulsion between the two particles and leads to a noticeable acceleration of their motion.

Interaction between like-charged particles at a liquid interface: Electrostatic repulsion vs. electrocapillary attraction

“…Electric-field-induced attraction that prevails over the electrostatic repulsion was established (both experimentally and theoretically) in the case of not-too-small floating particles, for which the interfacial deformation due to gravity is not negligible [34,35]. If the surface charge is anisotropically distributed (this may happen at low surface charge density), the electric field produces a saddle-shaped deformation in the liquid interface near the particle, which is equivalent to a ''capillary quadrupole".…”

“…(1.1) accounts for the fact that the dipolar field occupies only the upper half-space (the nonpolar fluid), whereas the electric field in the aqueous phase is screened by the ions in water. The electric charges that create the electric field can be located at the particle/water interface [22,29,[36][37][38] and/or on the particle/nonpolar-fluid interface [9,10,[25][26][27]34,35,[39][40][41][42][43]. Eq.…”

“…Attraction between like-charged particles of radii 200-300 lm floating at an oil/water interface was experimentally detected on the background of the gravity-induced lateral capillary force [34]. This attraction was interpreted as a hybrid capillary interaction between a gravity-induced capillary charge and a capillary where L is the center-to-center distance between the two particles; r c is the radius of their contact lines; e n is the dielectric constant of the nonpolar fluid (air, oil); p d is an effective dipole moment.…”

Buckling of particle-laden interfaces

“…Because a number of forces are at play at any given time (gravity, surface tension, Coulombic attraction and repulsion, electrodipping, van der Waals, etc.) and they depend on a number of factors including the distance between the particles [8], it is natural to expect that the nature of these interactions can change as the packing of particles is modified.…”

Self-assembly of repulsive interfacial particles via collective sinking

“…A particularly striking feature of these self-assembled colloidal crystals is that the constituent colloids maintain an equilibrium separation that can be several times larger than the particle diameter 5 : this feature is particularly useful in particle lithography since it allows the fabrication of patterns without particle-particle contact, which can cause cross-talk in various applications 4 . At a fundamental level, this separation clearly indicates that the particles are repulsive; detailed studies show that the charge on the particles is, in fact, separated so that the colloids behave as electrical dipoles 6,7 or even quadrupoles 8,9 .…”