One way to prepare anisotropic colloidal particles is to start with isotropic (spherical) particles and assemble them into clusters. When the particles assemble under equilibrium conditions -- such that they can rearrange dynamically -- we find that the resulting clusters tend to be asymmetric. I will show that the asymmetry is a result of entropy, specifically rotational entropy, which is related to the number of permutations of particles. Symmetric structures such as octahedra and icosahedra have fewer permutations and are therefore highly unfavorable.  The tendency toward asymmetry may complicate the self-assembly of larger scale structures, since asymmetric clusters show slower rotational diffusion. I will discuss how we can measure this rotational diffusion using holographic microscopy, and will also discuss a new way to make large quantities of quasi-symmetric tetrahedral clusters using a non-equilibrium assembly method called random parking.

Causes and consequences of asymmetry in colloidal clusters

246th National Meeting (2013)

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