Structure, phase behavior, and dynamics of colloidal systems characterized by strong, short- and moderate-ranged attractions: a computational study
Attractions between colloidal particles are often so strong that non-equilibrium
behavior results. However, dissolved non-adsorbing polymer can be added
to give a weak attraction between particles so that equilibrium phase transitions
appear at moderate polymer concentrations. At higher polymer concentrations
and small polymer-colloid size ratios non-equilibrium effects like
gelation occur, for which a complete understanding is lacking.
Monte Carlo and Monte Carlo-like computer simulations have been used to
investigate the role of many-body effects and the structures that colloidal
particles adopt under influence of a polymer-induced depletion attraction.
The phase diagram proves difficult to determine for these systems by direct
application of the Gibbs ensemble Monte Carlo method, especially for small
polymer-colloid size ratios that correspond to short-range attractions. However,
a sequential equilibration scheme is shown be able to give equilibrated
fluid-fluid coexistence data where usual application of the method fails. The
dynamics of colloidal particles along this fluid-fluid coexistence curve is studied
by a Brownian dynamics algorithm, corrected for the use of a large time
step. The dynamics slows down as the particle and polymer concentrations
are increased, but the systems appear to reach equilibrium for the cases studied.
This is in contrast to what is found by applying mode coupling theory; it
predicts glass-like transitions already at modest polymer concentrations for
short-range attractive systems, which is an issue that is investigated to some
extent. In addition, a number of approximate theories have been developed
and tested against the results from the computer simulations.