Physics Colloquium: Colloidal physics in a drop

Dr. Laurence Ramos, Laboratoire Charles Coulomb (L2C), University Montpellier, CNRS, Montpellier, France

Abstract: Drop evaporation is relevant to a variety of applications, including surface patterning, spray drying, and virus survival in aerosols. The presence of colloidal particles in the evaporating drop eventually leads to the formation of a dense shell that may undergo a shape instability. We have developed a unique multispeckle light scattering set-up to probe with a space- and time-resolution the microscopic dynamics of nanoparticles (NPs) confined in a drop that evaporates in a controlled fashion. Thanks to this set-up we measure the evolution of the thickness of the shell and the NP volume fraction in the drop while it dries. These unprecedented experimental data point out the flaws of previous assumptions regarding the volume fraction of the shell, and question a liquid-to-solid transition as the physical mechanism that triggers the shell instability. Based on the measurements of the microscopic dynamics of the NP in the shell, we propose instead that a glass transition of the NP confined in the shell is at the origin of the shell instability.

Finally, using the same set-up, we can probe the space-dependence of the microscopic dynamics of free-standing gels during their isotropic compression. We compare the dynamics of fractal colloidal gels and polymer gels, and discuss the results in the light of gel toughness.