How soft is too soft? Tuning order and disorder in dimeric core–soft colloids with bond flexibility

We employ molecular dynamics simulations to explore how internal flexibility affects phase transitions in soft-matter systems composed of dimers interacting via a core-softened potential with two characteristic length scales. Monomers are connected by harmonic springs with varying stiffness, allowing us to tune the dimer rigidity from highly flexible to nearly rigid. Flexible dimers reproduce the behavior of monomeric systems, displaying well-defined body-centered cubic (BCC) and hexagonal close-packed (HCP) crystalline phases separated by a narrow amorphous region. As the bond stiffness increases, this amorphous phase gives way to a coexistence region between BCC and HCP structures. In the rigid limit, amorphous regions reemerge and expand, and high-density systems fail to crystallize completely, instead forming mixed phases with HCP-like and disordered local environments. This transition arises from geometric frustration: rigid dimers are unable to adjust their internal configuration to optimize local packing, thereby suppressing crystallization and promoting amorphization. Our findings reveal that bond flexibility is a key control parameter governing structural organization in core-softened colloidal and molecular systems, offering insights for the design of tunable soft materials.