Micro-rheology near fluid interfaces

Using optical trapping, we have measured anisotropic and distance-dependent mobility of a colloidal particle near high surface tension fluid-fluid interfaces. These experimental results show that the parallel mobility is enhanced near a high surface tension liquid-gas surface, which is consistent with hydrodynamic predictions for a surface that does not support a stress, and that the parallel mobility is suppressed near a high surface tension liquid-liquid surface, consistent with a no-slip solid boundary. We demonstrate the potential for this optical technique to be applied to soft interfaces by predicting the normalized PSD using recent hydrodynamic predictions on particle mobility near soft surfaces.