Résumé
Gravity affects the flow and the behavior at rest of complex fluids and flows by inducing sedimentation, drainage, and interfacial deformation, which can mask more fundamental physical processes. On Earth, standard countermeasures against gravity come with limitations in terms of possible formulations or the quality and homogeneity of the applied strain. Alternatively, one can run these experiments in free-fall or in orbit where the effects of gravity are canceled i.e., under microgravity conditions. In this short review, we present several European Space Agency-led projects of interest to rheologists that leverage microgravity environments: FAST/FASTER , which studies interfacial rheology using Capillary Pressure Tensiometers; SMD-FOAM, fundamental experiments on foam coarsening ; SMD-PASTA and SEEDS, aimed at understanding the dynamics of emulsion droplets, its evolution and microrheology using Diffusing Wave Spectroscopy; results associated with studies on granular materials in microgravity, including AstEx and CompGran; COLIS, which examines the thermally driven perturbation of soft colloidal glasses monitored by Dynamic Light Scattering; BIOMICS/KRABS, investigating the aggregation and migration of soft particles and red blood cells under flow. Microgravity experiments allowed for high-precision measurements of interfacial viscoelasticity, detailed studies of emulsion aging and drop coalescence, observed plastic rearrangements in colloidal glasses, characterized the dynamics under flow of red blood cell analogues, and rationalized the impact of gravity on convection and fluidization in agitated granular matter. Collectively, these experiments demonstrate the complementarity and the relevance of ESA's microgravity platforms in expanding the frontiers of soft matter rheology.