A link between anomalous viscous loss and the boson peak in soft jammed solids

Understanding the mechanical properties of soft jammed solids that consist of densely packed particles, such as foams and emulsions, requires insights into the microscopic origins of linear viscoelasticity—how a solid responds to an infinitesimal deformation. Here we perform microrheology experiments on concentrated emulsions and measure the storage and loss moduli for a wide range of frequencies. We applied a linear response formalism for microrheology to a soft sphere model that undergoes the jamming transition. We find that the theory quantitatively explains the experiments. Our analysis reveals that the anomalous viscous loss seen in emulsions results from the boson peak, which is a universal vibrational property of amorphous solids and reflects the marginal stability in soft jammed solids. We show that the anomalous viscous loss is universal in systems with various interparticle interactions as it stems from the universal boson peak; it even survives below the jamming density at which thermal fluctuation is pronounced and the dynamics becomes inherently nonlinear.