Soft and stiff normal modes in floppy colloidal square lattices*

Floppy microscale spring networks are widely studied in theory and simulations, but no well-controlled experimental system currently exists. In this talk, I will show that square lattices consisting of flexibly linked colloid-supported lipid bilayers functionalized with DNA linkers act as microscale floppy spring networks. We extract the normal modes of these floppy lattices by inverting the particle displacement correlation matrix, showing the emergence of a spectrum of soft modes with low effective stiffness in addition to stiff modes that derive from linker interactions. Evaluation of the softest mode, a uniform shear mode, reveals that shear stiffness decreases with lattice size. Experiments match well with Brownian particle simulations and we develop a theoretical description based on mapping interactions onto linear response to describe the modes. Our results reveal the importance of entropic steric effects, and can be used for developing reconfigurable materials at the colloidal length scale.