Measuring Inter-Particle Forces at an Interface With Optical Tweezers and a Long Working-Distance Objective Lens

Abstract

In this effort, an optical tweezers setup is used to measure the forces between polystyrene microspheres on the interface between mineral oil and water. Colloidal interaction between particles on an interface is more complicated than their interaction in bulk. Knowing the forces between colloidal particles on an oil-water interface is important in order to improve aggregation and emulsion models and understand many phenomena in fluid dynamics and rheology. A two beam trap is used to control the distance between two interfacial particles. A long working-distance objective allows for the interface to be significantly far from the coverslip and for an extremely sensitive force measurement. The forces were found to follow a combination of Coulomb’s law and dipole-dipole interaction, but for much less charge than is specified by the manufacturer to be on the surface of the microspheres. These measurements will aid in the creation of models that can predict interfacial colloid phenomena.