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.