Computer Simulations of Colloidal Systems
Stephen A. Barr (Princeton University)
Colloidal particles are important for many applications, including
inks, paints, pharmaceuticals and as precursors for advanced
materials. By understanding colloidal interactions better we can
improve properties of these systems and find new applications. To
this end I use computer simulations to study colloids in three
different systems: charged colloids with multivalent counterions,
colloidal particles being pushed by an advancing ice front, and
surfaces with explicit ionizable sites. In systems of like-charged
colloids, an effective attraction has been observed under certain
conditions, which is not predicted by mean field theories. However, a
new theory, called strong coupling theory, has been introduced which
does predict an attraction and makes predictions about the conditions
required for this to occur. I use a novel Monte Carlo method to
quantitatively test these predictions. I also examine the structure
of colloidal particles undergoing freeze casting. In this process, an
aqueous suspension of solid particles is cast into a mold and
subsequently frozen. The growing ice crystals generate ice fronts that
concentrate the suspended particles in the intervening space. For
this system I focus on how details of the freezing process affect the
resulting solid structure. Finally, I consider ionizable surfaces.
Surfaces typically become charged though a titration process where
surface groups can become ionized based on their dissociation constant
and the pH of the solution. In this work I use a Monte Carlo method to
treat this process explicitly in a system with two planar surfaces in
a salt solution. I find that the surface charge density changes as the
surfaces come close to contact due to interactions between the
ionizable groups on each surface. Close to contact, I also find the
force between the surfaces is significantly different than for a
uniformly charged system.
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