Phase behaviour and gravity-directed self assembly of hard convex spherical caps

Abstract

We investigate the phase behaviour and self-assembly of convex spherical caps using Monte Carlo simulations. This model is used to represent the main features observed in experimental colloidal particles with mushroom-cap shape [Riley et al., Langmuir, 2010, 26, 1648. The geometry of this non-centrosymmetric convex model is fully characterized by the aspect ratio χ * defined as the spherical cap height to diameter ratio. We use NPT Monte Carlo simulations combined with free energy calculations to determine the most stable crystal structures and the phase behaviour of convex spherical caps with different aspect ratios. We find a variety of crystal structures at each aspect ratio, including plastic and dimer-based crystals; small differences in chemical potential between the structures with similar morphology suggest that convex spherical caps have the tendency to form polycrystalline phases rather than crystallising into a single uniform structure. With the exception of plastic crystals observed at large aspect ratios (χ * > 0.75), crystallisation kinetics seem to be too slow, hindering the spontaneous formation of ordered structures. As an alternative, we also present a study of directing the self-assembly of convex spherical caps via sedimentation onto solid substrates. This study contributes to show how small changes to particle shape can significantly alter the self-assembly of crystal structures, and how a simple gravity field and a template can substantially enhance the process.

Role of local assembly in the hierarchical crystallization of associating colloidal hard hemispheres

“…3 missing in the previous works [28], as by reducing the free energy of our crystal phase by 1 2 ln 2k B T per particle, we obtain the same phase boundaries as in Ref. [24].…”

“…These are substantially higher than the values obtained in Ref. [24], and the reason is the last term of Eq. 3 missing in the previous works [28], as by reducing the free energy of our crystal phase by 1 2 ln 2k B T per particle, we obtain the same phase boundaries as in Ref.…”

“…We consider a system of N colloidal hard hemispheres, which at high density can crystallize into an FCC 2 crystal [22][23][24]. To control the formation of local structures, i.e.…”

Phase Behavior of Bowl‐Shaped Colloids: Order and Dynamics in Plastic Crystals and Glasses

“…The phase behavior of bowl‐shaped (or cap‐shaped) particles has been addressed extensively with computer simulations . Marechal et al showed in experiments supported by simulations that entropy can drive stacking of indented colloids at the mesoscale .…”

Packing, entropic patchiness, and self-assembly of non-convex colloidal particles: A simulation perspective

“…When the indentation in the bowl becomes shallow, a different aggregation phenomenon is observed. For example, contact lens-like particles modelled as non-convex spherical caps, show multiple phases behavior, ranging from a liquid crystalline nematic phase to cluster-like aggregates 105,106 depending on the curvature of the particles, with the aggregates being remarkably different to the structures observed in convex-spherical caps 102,[107][108][109] .…”