Interfacial rheometry of polymer at a water–oil interface by intra-pair magnetophoresis

Abstract

We describe an interfacial rheometry technique based on pairs of micrometer-sized magnetic particles at a fluid–fluid interface. The particles are repeatedly attracted and repelled by well-controlled magnetic dipole–dipole forces, so-called interfacial rheometry by intra-pair magnetophoresis (IPM). From the forces (∼pN), displacements (∼μm) and velocities (∼μm s⁻¹) of the particles we are able to quantify the interfacial drag coefficient of particles within a few seconds and over very long timescales. The use of local dipole–dipole forces makes the system insensitive to fluid flow and suited for simultaneously recording many particles in parallel over a long period of time. We apply IPM to study the time-dependent adsorption of an oil-soluble amino-modified silicone polymer at a water–oil interface using carboxylated magnetic particles. At low polymer concentration the carboxylated particles remain on the water side of the water–oil interface, while at high polymer concentrations the particles transit into the oil phase. Both conditions show a drag coefficient that does not depend on time. However, at intermediate polymer concentrations data show an increase of the interfacial drag coefficient as a function of time, with an increase over more than three orders of magnitude (10⁻⁷ to 10⁻⁴ N s m⁻¹), pointing to a strong polymer-polymer interaction at the interface. The time-dependence of the interfacial drag appears to be highly sensitive to the polymer concentration and to the ionic strength of the aqueous phase. We foresee that IPM will be a very convenient technique to study fluid–fluid interfaces for a broad range of materials systems.