Probing interfacial dynamics and mechanics using submerged particle microrheology. II. Experiment

Abstract

A non-contact microrheological technique to probe the mechanics of the air/water interface is explored. Polystyrene spheres dissolved in water are trapped with an optical tweezer near the free surface of water, allowing the response functions of the particles to be measured as a function of the distance from the air/water interface. These measurements show that at the surface, the imaginary part of the response function increases by approximately 30% from the Stokes value measured in the bulk. As the particle is moved away from the surface via an optical trap, the response function returns to the bulk value. The method is tested by comparing the response function of particles near a rigid wall to the theory developed by Fax'en. A newly developed hydrodynamic theory is used to explain the results at the free interface through a calculation of the linear response function as a function of depth. These results show a range of sensitivity that can be utilized to study the microrheology of a Langmuir monolayer without distorting its structure.