Thursday 7 April 2022

Laurence Ramos
Laboratoire Charles Coulomb (L2C), Univ. Montpellier, CNRS, Montpellier, France
Colloidal physics in a drop
Seminar room 1 (FORTH) and online. Please visit the link.

Drop evaporation is relevant to a variety of applications, including surface patterning, spray drying, and virus survival in aerosols. It is important to characterize the spatial distribution of the species confined in the drop at it evaporates and to understand which physical processes govern this distribution and in turn the fate of the drop. Current time-and space-resolved experimental characterizations are scarce and most works aiming at predicting whether a drop of colloidal suspension will evaporate isotropically or buckle are based on crude assumptions for the formation and features of a dense shell. To rationalize the behavior of evaporating drops, we have developed a unique multispeckle light scattering set-up to probe with a space and time resolution the microscopic dynamics of nanoparticles confined in a drop that evaporates in a controlled fashion. We perform experiments at different Peclet numbers Pe, defined as the ratio between the time for a drop to fully evaporate and the time for a particle to diffuse over a distance equal to the drop radius. We show that, depending on Pe, the nanoparticles remain homogeneously distributed or accumulate at the periphery of the drop as it evaporates. We measure the time evolution of the thickness of the shell, and of the particle concentration in the shell. We show that the particle concentration in the shell increases with time and Pe, and reaches random close-packing only for the largest Pe investigated. For Pe>10, the drop becomes unstable. This finding is rationalized thanks to the measurements of the microscopic dynamics of the nanoparticles in the shell: when the shell is concentrated and thick, the dynamics of the nanoparticles is controlled by the overall strain rate of the drop, indicating that on the timescale imposed by the evaporation rate, the shell behaves as a solid-like material.

In the second part of the talk, I will present preliminary results on free standing fractal colloidal gels. Using our novel set-up, we probe the space-dependence of the microscopic dynamics of gels during their isotropic compression, and compare the dynamics with the one obtained with polymer gels and with colloidal suspensions. The role of connectivity and the concept of brittleness and ductility in gels will be discussed.

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