Paper Number
SC36
Session
Suspensions and Colloids
Title
Colloidal physics in a drop
Presentation Date and Time
October 11, 2022 (Tuesday) 5:25
Track / Room
Track 1 / Sheraton 4
Authors
- Milani, Matteo (Laboratoire Charles Coulomb, CNRS and University of Montpellier)
- Cipelletti, Luca (Laboratoire Charles Coulomb, CNRS-University of Montpellier)
- Ramos, Laurence (Laboratoire Charles Coulomb, CNRS-University of Montpellier)
Author and Affiliation Lines
Matteo Milani, Luca Cipelletti and Laurence Ramos
Laboratoire Charles Coulomb, CNRS-University of Montpellier, Montpellier, France
Speaker / Presenter
Ramos, Laurence
Keywords
experimental methods; colloids; gels; suspensions
Text of Abstract
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 overcome these limitations, 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, and 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. Finally, using the same set-up, we 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.