HYDRODYNAMIC MAPPING OF 2-DIMENSIONAL ELECTRIC-FIELDS IN MONOLAYERS

We have measured the 2D dipolar electric fields generated by surface d ensity fluctuations in Langmuir monolayers using optical microscopy to monitor the motion of micron-size, electrically charged, particles tr apped at the air-water interface. The particle velocity is directly pr oportional to the local electric field gradient. Quantitative agreemen t with the theory is demonstrated for charged polystyrene latex partic les interacting with liquid condensed domains of pentadecanoic acid. T ypical velocities are of order 0.1-10 mu m/s, corresponding to forces in the 10(-15)-10(-12) N range.