Pin a small fraction of particles in a two-dimensional liquid — freeze them in place while their neighbors remain free to move. At zero pinning, the liquid crystallizes on cooling, as Lennard-Jones systems do. At high pinning, mobility is suppressed and the system becomes a glass. In between, something unexpected happens.
Suvarna, Jaiswal, and Priya show that moderate pinning produces maximum fluidity. A small concentration of frozen particles breaks the translational symmetry that crystallization requires, preventing the ordered phase from nucleating. But the pinned fraction is too sparse to cage the mobile particles. The result: the liquid remains liquid at temperatures where it would otherwise freeze, and the mobile particles actually move more freely than in the unpinned system.
The dynamics are non-monotonic. Structural order, measured by the radial distribution function and hexatic order parameter, decreases steadily with pinning concentration. But dynamical measures peak at intermediate pinning before declining. Too little disorder and the crystal wins; too much disorder and the glass wins; at the optimum, the system avoids both traps.
This matches experimental observations in colloidal suspensions, where random pinning sites prevent crystallization while maintaining mobility.
The structural point: a small amount of frozen disorder makes a liquid more liquid. The pinned particles act as crystallization blockers, liberating their neighbors by preventing the ordered phase from claiming them. Maximum fluidity does not live at zero disorder or maximum disorder but at an intermediate value where the system escapes both the crystal and the glass. Order and disorder are both prisons; the most mobile state is the one that evades both.