Segregation of "isotope" particles within colloidal molecules

Area of Science:

• Colloid and surface science
• Statistical mechanics
• Soft matter physics

Background:

• Spherical particle clusters, termed "colloidal molecules," mimic molecular structures with particles as atoms and interactions as bonds.
• Isomers represent different equilibrium structures adopted by these colloidal molecules.
• This study introduces "colloidal isotopes"—particles of identical size but varying bonding energies—to further the analogy with atomic isotopes.

Purpose of the Study:

• To investigate the positional behavior of colloidal isotopes within self-assembled colloidal molecules.
• To develop a statistical mechanical model predicting and controlling isomer formation and isotope placement.
• To demonstrate the high degree of control achievable in self-assembled colloidal structures.

Main Methods:

• Fabrication of 2D colloidal molecules using polystyrene and silica microspheres.
• Utilizing depletion interactions to bind the microspheres.
• Employing optical microscopy and particle tracking to analyze ensembles of 4- and 5-particle molecules.
• Developing a statistical mechanical model incorporating rotational entropy and differential interaction potentials.

Main Results:

• Colloidal isotopes were observed to segregate to specific positions within different colloidal molecule isomers.
• The statistical mechanical model accurately predicted these segregation patterns.
• The model provides a method for optimizing isomer yield and directing isotope placement.

Conclusions:

• The positional preferences of colloidal isotopes are governed by their differing interaction energies and the rotational entropy of the isomers.
• Self-assembled colloidal structures can be precisely controlled, even with isotropic interactions, by strategic doping with colloidal isotopes.
• This work offers a pathway to design and synthesize complex colloidal architectures with tailored properties.