  Biomolecular condensates are membraneless compartments in the cell that are
involved in a wide diversity of biological processes. These phase-separated
droplets usually exhibit a viscoelastic mechanical response. A behavior
rationalized by modeling the complex molecules that make up a condensate as
stickers and spacers, which assemble into a network-like structure. The proper
functioning of biocondensates requires precise control over their composition,
size, and mechanical response. For example, several neurodegenerative diseases
are associated with dysfunctional condensates that solidify over a long period
of time (days) until they become solid. A phenomenon usually described as
aging. The emergence of such a long timescale of evolution from microscopic
events, as well as the associated microscopic reorganization leading to aging,
remains mostly an open question. In this article, we explore the connection
between the mechanical properties of the condensates and their microscopic
structure. We propose a minimal model for the dynamic of stickers and spacers,
and show that entropy minimization of spacers leads to an attractive force
between stickers. Our system displays a surprisingly slow relaxation toward
equilibrium, reminiscent of glassy systems and consistent with the
liquid-to-solid transition observed. To explain this behavior, we study the
clustering dynamic of stickers and successfully explain the origin of glassy
relaxation.
