This paper reports the results of experimental studies of the dynamics of p
articles in a dense quasi-two-dimensional colloidal liquid. We find that at
high density, near close packing but still in the liquid phase, the spatia
l configurations of the particles in the colloidal liquid consist of small
ordered domains separated by disordered boundaries. There are frequent exch
anges of particles between the ordered and disordered domains, so the lifet
ime of a particular ordered domain is short and the state of the system is
ergodic. The motion of a particle in an ordered domain is constrained but f
ully two dimensional. The motion of a particle in a disordered boundary has
considerable one-dimensional file-server character. By virtue of exchanges
of particles between the ordered and disordered domains, the time dependen
ce of the particle displacement has mixed character. We find that the parti
cle dynamics in the dense quasi-two-dimensional colloidal liquid can be cha
racterized with three simultaneous competing relaxation processes, each of
which contributes to the mean square displacement of a particle, W(t). For
an interval that is shorter than the time between collisions, W(t) is linea
r in time; this behavior results from particle displacements within a cage
of fluctuating neighbors. At somewhat longer time W(t) has a sublinear depe
ndence on time. We associate this temporal behavior with a file-server-like
contribution to W(t), arising from correlated motion in the disordered bou
ndary regions. At very long time we find that there are contributions to W(
t) from infrequent large displacements, of the order of a particle diameter
in length. The large displacements are associated with density fluctuation
s that fluidize the surroundings of a particle. These large displacements a
re not ballistic; rather the particle diffuses rapidly enough to generate a
large displacement per unit time. The common occurrence of structural hete
rogeneity in simulation studies of systems with different particle-particle
interactions and different equations of motion suggests that fluctuations
that generate local order in a dense two-dimensional liquid may be a ubiqui
tous feature of that class of systems. The results of the experiments repor
ted in this paper suggest that current theories of relaxation processes in
a dense quasi-two-dimensional liquid do not adequately take into account th
e class of fluctuations that generates transient ordered domains in the liq
uid; nor do these theories adequately account for the cooperative relaxatio
n of those domains. (C) 2001 American Institute of Physics.