  We experimentally and numerically examine stress-dependent electrical
transport in granular materials to elucidate the origins of their universal
dielectric response. The ac responses of granular systems under varied
compressive loadings consistently exhibit a transition from a resistive plateau
at low frequencies to a state of nearly constant loss at high frequencies. By
using characteristic frequencies corresponding to the onset of conductance
dispersion and measured direct-current resistance as scaling parameters to
normalize the measured impedance, results of the spectra under different stress
states collapse onto a single master curve, revealing well-defined
stress-independent universality. In order to model this electrical transport, a
contact network is constructed on the basis of prescribed packing structures,
which is then used to establish a resistor-capacitor network by considering
interactions between individual particles. In this model the
frequency-dependent network response meaningfully reproduces the experimentally
observed master curve exhibited by granular materials under various normal
stress levels indicating this universal scaling behaviour is found to be
governed by i) interfacial properties between grains and ii) the network
configuration. The findings suggest the necessity of considering contact
morphologies and packing structures in modelling electrical responses using
network-based approaches.
