  We compare the behavior of a new two-dimensional aqueous colloidal model
system with a simple numerical treatment. To the first order the attractive
interaction between the colloids induced by an in-plane rotating ac electric
field is dipolar, while the charge stabilization leads to a shorter ranged,
Yukawa-like repulsion. In the crystal-like 'rafts' formed at sufficient field
strengths, we find quantitative agreement between experiment and Monte Carlo
simulation, except in the case of strongly interacting systems, where the well
depth of the effective potential exceeds 250 times the thermal energy. The
'lattice constant' of the crystal-like raft is located approximately at the
minimum of the effective potential, resulting from the sum of the Yukawa and
dipolar interactions.The experimental system has display applications, owing to
the possibility of tuning the lattice spacing with the external electric field.
Limitations in the applied field strength and relative range of the
electrostatic interactions of the particles result in a reduction in tunable
lattice spacing for small and large particles, respectively. The optimal
particle size for maximizing the lattice spacing tunability was found to be
around 1000 nm.
