Alessandro Martinelli

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 rheology experiments using capillary pressure tensiometers; Fundamental experiments on the coarsening of foams and the intrinsic dynamics 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.

We employ small-angle x-ray and dynamic light scattering to investigate the microscopic structure and dynamics of dense suspensions of ultra-low crosslinked (ULC) poly(N-isopropylacrylamide) microgels. By probing the supercooled and glassy regimes, we characterize the relationship between the structure and dynamics as a function of effective volume fraction φ and probed length scale. We demonstrate that ULC microgels act as fragile glass formers whose dynamics are governed solely by φ. In contrast, the microscopic structure depends on the specific combination of microgel number density and swelling state that defines φ. We identify an anomalous glassy regime where relaxation times are orders of magnitude faster than predicted by supercooled extrapolations and show that in this regime, dynamics are partly accelerated by laser light absorption. Finally, we show that the microscopic relaxation time measured for different φ’s and at various scattering vectors may be rationalized by a remarkable “time-length scale superposition principle” analogous to the time-temperature superposition used to scale onto a master curve rheology or dielectric relaxation data of molecular systems. Remarkably, we find that the resulting master curve also applies to a different microgel system [Nigro et al., Macromolecules 53, 1596 (2020)], suggesting a general dynamical behavior of polymeric particles.

The yield point marks the beginning of plastic deformation for a solid subjected to sufficient stress, but it can alternatively be reached by x-ray irradiation. We characterize this latter route in terms of thermodynamics, structure and dynamics for a series of GeSe₃ chalcogenide glasses with different amount of disorder. We show that a sufficiently long irradiation at room temperature results in a stationary and unique yielding state, independent of the initial state of the glass. The glass at yield is more disordered and has higher enthalpy than the annealed glass, but its properties are not extreme: they rather match those of a glass instantaneously quenched from a temperature 20% higher than the glass-transition temperature. This is a well-known, key temperature for glass-forming liquids which marks the location of a dynamical transition, and it is remarkable that different glasses upon irradiation head all there.

Colloidal Solids (COLIS) is a state-of-the-art light scattering setup developed for experiments onboard the International Space Station (ISS). COLIS allows for probing the structure and dynamics of soft matter systems on a wide range of length scales, from a few nm to tens of microns, and on time scales from 100 ns to tens of hours. In addition to conventional static and dynamic light scattering, COLIS includes depolarized dynamic light scattering, a small-angle camera, photon correlation imaging, and optical manipulation of thermosensitive samples through an auxiliary near-infrared laser beam, thereby providing a unique platform for probing soft matter systems. We demonstrate COLIS through ground tests on standard Brownian suspensions, and on protein, colloidal glasses, and gel systems similar to those to be used in future ISS experiments.

We crossed two femtosecond extreme ultraviolet (EUV) pulses on a 100 nm thick amorphous membrane of SiO2, generating transient gratings (TGs) of light intensity with 84 nm spatial periodicity. The EUV TG excitation gave rise to the efficient generation of Lamb waves (LWs) and of a temperature grating, whose dynamics was studied at two different initial sample temperatures, 50 and 300 K. The short penetration depth of the EUV excitation pulses turned into a strong non-uniformity in the actual temperature as a function of the depth from the sample surface. At the lowest temperature, the LW frequencies presented a sizable shift in time due to the thermal equilibration along the membrane thickness. The analysis of the EUV TG waveforms allowed us to determine the decay time of the thermal grating and the sound attenuation coefficient, both found in reasonable agreement with the literature. The results show how EUV TG can provide information of non-equilibrium thermoelastic dynamics in thin membranes transparent to optical radiation.

Synchrotron-radiation-based techniques are a powerful tool for the investigation of materials. In particular, the availability of highly brilliant sources has opened the possibility to develop techniques sensitive to dynamics at the atomic scale such as X-ray photon correlation spectroscopy (XPCS). XPCS is particularly relevant in the study of glasses, which have been often investigated at the macroscopic scale by, for example, differential scanning calorimetry. Here, we show how to adapt a Flash calorimeter to combine XPCS and calorimetric scans. This setup paves the way to novel experiments requiring dynamical and thermodynamic information, ranging from the study of the crystallization kinetics to the study of the glass transition in systems that can be vitrified thanks to the high cooling rates reachable with an ultrafast calorimeter.