Seminar - SCM

The macroscopic response of soft materials often emerges from the organization and dynamics of their constituents. In some systems, the collective behavior of the constituents can be described through an effective continuum, while in others, their organization and cooperative dynamics become important to understand the material response. In this talk, I will explore these two limits using dilute magnetic colloidal liquids and dense emulsions as experimental model systems.
 

I will first consider dilute suspensions of superparamagnetic nanoparticles in a carrier medium, under external magnetic fields. Using their collective magnetic response, we create and control liquid–liquid interfaces through magnetic confinement. These interfaces act as mobile, adaptive and externally tunable hydrodynamic boundaries, showing strongly reduced flow resistance, reversible flow-induced deformation, relaxation and instability. These experiments show how magnetic confinement can introduce characteristic length and time scales that control the dynamics and mechanical response of an adaptive liquid–liquid interface.
 

I will then turn to dense, jammed emulsions, where the organization of individual droplets becomes central to the macroscopic response. Using emulsions with precisely controlled droplet-size distributions, and combining rheology, laser diffraction and droplet-scale velocimetry, we study how droplet organization affects structure, local flow and the overall rheological response. By comparing ultra-monodisperse emulsions with conventional polydisperse emulsions and controlled bidisperse systems, we find that droplet-size distribution strongly affects dynamics and response. In particular, ultra-monodisperse repulsive emulsions can develop heterogeneous flow and shear bands under homogeneous applied stress. Extensive tests establish the robustness of this behavior. More broadly, these observations raise questions about the length and time scales associated with cooperative droplet dynamics, and how these dynamics ultimately control the flow of jammed soft materials.
 

Together, these two systems show how interactions and organization at smaller scales can lead to new dynamics and macroscopic responses in soft materials.