The Polymer Society of Korea

Abstract

Fractional crystallization exploits subtle solubility differences for selective separation. Although colloids are insoluble, they display crystallization and melting governed by interparticle interactions. We present a thermodynamic method to separate colloidal particles by size using depletion forces. By tuning interactions to Mie-type potentials, particles crystallize when the well depth exceeds ~3kBT. Larger particles experience stronger depletion attraction and crystallize first, while smaller ones remain fluid. Using this principle, we develop a sequential fractional crystallization protocol—crystallization, melting, and recrystallization—achieving high-precision fractionation of colloids with <10% size differences. Near the crystallization threshold, each species forms its own lattice to maximize coordination and enthalpic gain. At stronger attractions, particles form glassy alloys that subsequently relax into metastable crystalline alloys.