The vibrational density of states of a disordered gel model

TL;DR

This study explores the vibrational density of states in a disordered gel model formed by patchy colloidal particles, revealing how network structure influences vibrational modes and low-frequency excitations.

Contribution

It provides a detailed analysis of the vibrational density of states in a disordered gel, linking spectral features to network structure and rotational-translational coupling.

Findings

Pronounced low-frequency peak in vDOS increases with chain length.

Spectral features can be explained by small network structures and linear chains.

Low-frequency modes arise from coupling between rotations and translations.

Abstract

We investigate the vibrational density of states (vDOS) in harmonic approximation of a binary mixture of colloidal patchy particles with two and three patches for different relative compositions $x_2$. At low temperature, this system forms a thermo-reversible gel, i.e. a fully bonded network of chains of two-patches particles, in which the branching points are provided by three-patches particles. For all the compositions we find in the vDOS a pronounced peak at low frequency whose height grows on increasing the fraction of two-functional particles, or equivalently with the average length of the chains. To identify the various spectral features, we compare the vDOS of the whole system with the one of small representative structures of the network and with the vDOS of a long linear chain of two-patches particles and we find that these structures are indeed able to rationalize the various peaks in the vDOS of the full system. At large $x_2$ the vDOS of the gel and of the long chain show remarkable similarities. Analyzing the dispersion relations and the spectrum of the linear chain we show that the excess of low frequency modes, the analog of the boson peak in glassy disordered systems, arises from the strong coupling between rotations and translations.