Joost de Graaf Anisotropic Nanocolloids - Universiteit Utrecht

212 Summary
our understanding of phase transitions and critical phenomena in two-dimensional
(2D) systems, but also has great potential for use in industry. Possible applications
include the encapsulation of drugs in emulsion droplets for medical purposes and
the stabilization of foams and emulsions, which are relevant to the food industry.
• Crystal-structure prediction for colloidal systems and the related phase behaviour.
Although there has been a tremendous increase in the ability to control the ways
in which colloids and nanoparticles self-assemble, there are still many unanswered
questions with regards to determining the specific particle properties that result in a
desired structure. In particular, predicting crystal structures based only on knowledge
of the interactions between particles has proven very challenging. However, an
efficient ab initio way to predict crystal structures holds the key to designing new
materials with predetermined properties and is therefore highly sought after.
• The ion distribution around charged particles suspended in a dielectric medium. In
many colloidal suspensions electrostatic interactions play an important role and it is
therefore important to characterise the nature of such interactions using theory and
simulations. However, even for systems containing only homogeneously charged
spherical colloids studying the physical properties by theory or by simulations is
difficult due to the long range of the Coulomb interactions. These long-range interactions
coupled with the presence of mobile ions that screen the colloid’s bare
charge present a complex many-body problem, which cannot be easily unravelled to
yield effective colloid-colloid interactions. For anisotropic charge distributions the
complexity of the problem increases significantly.
In Chapter 2 we described the numerical technique of triangular tessellation, by which
the surface areas and line length that are associated with a plane-particle intersection
can be approximated. Our method allowed us to determine the free-energy of adsorption
for a single shape-anisotropic colloid with homogeneous surface properties adsorbed at
a flat interface in the Pieranski approximation. We established that prolate ellipsoids
and spherocylinders absorb perpendicular to the interfacial normal. For prolate cylinders
there can also be a metastable adsorption parallel to this normal. We continued our investigation
in Chapter 3 where we considered the free energy in more detail and introduced
simple dynamics to analyse the process of a particle attaching to the interface and relaxing
to its equilibrium position and orientation. When there are metastable adsorption
configurations, we showed that the orientation of a colloid at its initial contact with the
interface has a strong influence on its final orientation. Within the confines of our model
this resulted in an unexpectedly large domain of stability for the metastable configuration
of relatively long cylindrical particles. We even encountered situations for which
a particle (short cylinder) passed through the interface unhindered, despite there being
deep minima in the free energy of adsorption that would ordinarily give rise to strong
binding to the interface. Finally, in Chapter 4 we extended the triangular-tessellation
technique to numerically determine the free energy of adsorption for a nonconvex colloidal
particle with surface patterning. We showed that the equilibrium orientation of
a truncated cube falls into one of three distinct categories we found; which of the three
depends on the details of the contact-angle pattern. We also considered plane-particle