Rheology in space—A review of past and present ESA experiments

Gravity affects the flow and the behavior at rest of complex fluids 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 where the effects of gravity are canceled, i.e., under microgravity conditions. In this Short Review, we present several European Space Agency (ESA)-led projects of interest to rheologists that leverage microgravity environments, including interfacial rhe ology experiments using capillary pressure tensiometers; Fundamental experiments on the coarsening of foams and the intrinsic dynam ics and microrheology of emulsion droplets; Multiple granular materials investigations on the transition to jamming, fluidized beds, quasistatic and dilute granular flows; Experiments examining the thermally driven perturbation of soft colloidal glasses; And studies of the aggregation dynamics and migration of soft particles and red blood cells under flow. The microgravity conditions offered by the ESA platforms enabled high-precision measurements of interfacial viscoelasticity; revealed plastic rearrangements in colloidal glasses; detected roaming bubbles in foams, and underlined the progressive arrest of droplet motion in emulsions; uncovered margination effects in blood cell analogues under flow; 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