  Nanoparticle manipulations require a careful analysis of the forces at play.
Unfortunately, traditional force measurement techniques based on the particle
velocity do not provide a sufficient resolution, while balancing approaches
involving counteracting forces are often cumbersome. Here, we demonstrate that
a nanoparticle dielectrophoretic response can be quantitatively studied by a
straightforward visual delineation of the dielectrophoretic trapping volume. We
reveal this volume by detecting the width of the region depleted of gold
nanoparticles by the dielectrophoretic force. Comparison of the measured widths
for various nanoparticle sizes with numerical simulations obtained by solving
the particle-conservation equation shows excellent agreement, thus providing
access to the particles physical properties, such as polarizability and size.
These findings can be further extended to investigate various types of
nano-objects $-$ including bio- and molecular aggregates $-$ and offer a robust
characterization tool that can enhance the control of matter at the nanoscale.
