Rotating Micro-Spheres for Adsorption Monitoring at a Fluid Interface

TL;DR

This study introduces a novel method using rotating magnetic microspheres at fluid interfaces to monitor adsorption and desorption dynamics, providing insights into early-stage processes and effects of additives.

Contribution

The paper presents a new experimental approach for real-time monitoring of particle adsorption/desorption at fluid interfaces using rotating magnetic microspheres.

Findings

Method detects early desorption of particles.

Addition of salts or surfactants affects adsorption and bonding.

Estimates of adsorption/desorption constants are obtained.

Abstract

Hypothesis: A broad range of phenomena, such as emulsification and emulsion stability, foam formation or liquid evaporation, are closely related to the dynamics of adsorbing colloidal particles. Elucidation of the mechanisms implied is key to a correct design of many different types of materials. Experiments: Microspheres forced to rotate near a fluid interface exhibit a roto-translational hydrodynamic mechanism that is hindered by capillary torques as soon as the particles protrude the interface. Under these conditions, the time evolution in the ratio of moving spheres provides a direct description of the adsorption kinetics, while microscopy monitoring of particle acceleration\deceleration informs about the adsorption\desorption dynamics. In this work, the proposed strategy is applied at an air/water interface loaded with spherical magnetic particles negatively charged, forced to rotate by the action of a rotating magnetic field. Findings: The proposed method enables the adsorption/desorption dynamics to be followed during the earliest phase of the process, when desorption of a small fraction of particles is detected, as well as to estimate approximated values of the adsorption/desorption constants. The results obtained show that the addition of a monovalent salt or a cationic (anionic) surfactant promotes (inhibits) both adsorption and formation of permanent bonds between particles.