Identification and classification of clusters of dipolar colloids in an external field

TL;DR

This study investigates how dipolar colloids form various cluster structures under different external field strengths, revealing a rich variety of configurations near the transition between isotropic and dipolar interactions.

Contribution

The paper introduces a comprehensive analysis of lowest energy clusters of dipolar colloids, combining basin-hopping optimization with topological classification in bulk simulations.

Findings

Identification of diverse cluster structures including polytetrahedral, octahedral, and spiral forms.

Observation of non-rigid clusters near the transition to string formation.

Demonstration of the methodology's effectiveness in bulk systems as a function of dipole strength.

Abstract

Colloids can acquire a dipolar interaction in the presence of an external electric field. At high field strength, the particles form strings in the field direction. However at weaker field strength, competition with isotropic interactions is expected. One means to investigate this interplay between dipolar and isotropic interactions is to consider clusters of such particles. We have therefore identified, using the GMIN basinhopping tool, a rich library of lowest energy clusters of a dipolar colloidal system where the dipole orientation is fixed with respect to the z-axis, and the dipole strength is varied for m--membered clusters of 7 <= m <=13. In the regime where the isotropic and dipolar interactions are comparable, we find elongated polytetrahedral, octahedral and spiral clusters as well as a set of non--rigid clusters which emerge close to the transition to strings. We further implement a search algorithm which identifies these minimum energy clusters in bulk systems using the topological cluster classification [J. Chem. Phys. 139 234506 (2013)]. We demonstrate this methodology with computer simulations which shows instances of these clusters as a function of dipole strength.