Sara Hashmi, Yale University

Complex fluids are everywhere: they include suspensions, emulsions,
dispersions, and foams.  A few examples in daily life include
cosmetics, pharmaceuticals, cleaning supplies, and most biological
materials.  A common theme among these everyday materials:
characteristics and dynamic properties on the macroscopic scale arise
from microstructure and constituent properties on the nano- and
microscale.  In this talk, I will present an in-depth study of the
influence of nanoparticle properties on bulk suspension
characteristics.  As a case study, we will investigate suspensions of
asphaltenes, naturally occurring molecules found in petroleum, which
can cause clogging in even the largest pipelines.  A combination of
various molecular and colloidal scale measurements reveals asphaltene
molecular assembly, nanoparticle surface chemistry, and aggregation
dynamics.  The aromatic chemistry of asphaltenes, along with trace
heteroatoms and metals, imbues asphaltene nanoparticles with surface
charge.  Given the non-polar nature of the background alkane solvents,
these charged moieties readily destabilize, aggregate, and phase
separate out of solution.  However, electrostatic forces in oil are
long-range, enabling repulsion between nanoparticles with appropriate
surface chemistry.  Controlling nano- and micro-scale characteristics
of asphaltenes using surfactant additives enables control over bulk
suspension dynamics.  In particular, we will see how colloidal surface
charge, growth and aggregation influence macroscopic dynamics of
sedimentation and deposition.