Introduction:
Exploring the behavior of granular gases in microgravity holds an immense scientific and practical significance, advancing our understanding of physics, engineering, and space exploration. The ensembles of inelastically colliding macroscopic particles offer unique perspectives on the basic laws of multiparticle physics. While most previous experiments and theoretical investigations were focused on monodisperse ensembles of simple particles [1,2], more realistic studies have to take into account that such systems may be polydisperse and consist of complex-shaped grains. We have made a step towards the study of complex granular gases by investigating the heating and cooling properties of a mixture of rod-shaped particles. In addition, we present first results on a granular gas of non-convex 3D crosses (JACKS).
Methods:
The tracked colored and “background” particles are put in the cuboid container with two vibrating side walls. To achieve the microgravity state, the experiments were performed at ZARM drop tower in Bremen. For JACKS, longer periods of microgravity are achieved in ESRANGE sounding rocket flight. Resulting two-view videos are analyzed with help of custom AI-aided software package [3]. In addition, DEM simulations are performed on GPUs, where rods are represented as single spherocylinders, while JACKS particles can be modelled as their clumps.
Results:
For the granular gas mixture of rods, cooling dynamics follows Haff’s law [4], while the ratio of the average kinetic energy between components remains constant during both heating and cooling stages [5]. Interestingly, the simulations show relatively high amount of energy corresponding to rotation around long rod axes, which was not confirmed by the experiment so far.
For non-convex JACKS particles, clustering is immediately observed during both heating and cooling, see Fig.1. Clustering happens at relatively low packing fractions in comparison to spherical particles and rods, and seems to be increasingly pronounced for higher particle numbers.
Conclusion:
The study of granular gases of rods and 3D crosses has proved that they demonstrate immediately evident but highly non-trivial emergent phenomena. Fundamental features of such systems were confirmed, i.e., Haff’s cooling law for rods and their mixtures and tendency towards clustering for non-convex particles. At the same time, we have observed surprising features which require thorough investigation, e.g. the partition of energy between DOFs for the rod-like particles and dependence of granular temperature ratio of the mixture components on their relative sizes. For the 3D crosses, the intensity of clustering was even higher than expected.

Granular fluids, as defined by a collection of moving solid particles, is a paradigm of a dissipative system out of equilibrium. Inelastic collisions between particles is the source of dissipation, and is the origin of a transition from a gas to a liquid-like state. This transition can be triggered by an increase of the solid fraction. Moreover, in compartmentalized systems, this condensation is driving the entire granular fluid into a Maxwell demon phenomenon, localizing most of the grains into a specific compartment. Classical approaches fail to capture these phenomena, thus motivating lots of experimental and numerical works. Herein, we demonstrate that Onsager's variational principle is able to predict accurately the coexistence of gas/liquid states in granular systems, opening ways to model other phenomena observed in such dissipative systems like segregation or the jamming transition.

Typical emulsions are constituted by fine dispersions of microscopic droplets in a matrix liquid, common in natural and synthetic products and serving several technologies or processes. Emulsions are intrinsically unstable and their kinetic stability warranted by the presence of amphiphilic species, such as surfactants, polymers, proteins and even particles. These species segregate at the droplet interfaces and warrant the kinetic stability of these droplets against the typical destabilisation mechanisms, such as, droplet-droplet coalescence and Ostwald ripening. The action of these species on the destabilising mechanism, even known from a general point of view, it is not sufficiently detailed to allow for an on-design formulation, so that emulsion technology relies still of semi-empirical models and practices.
Aiming at overcoming these limitations, warranting a more efficient utilisation of resources in emulsion-related technologies and products, the relations between interfacial adsorption layer properties and the mechanisms of destabilisation of emulsions have been the subject of many studies. On ground these studies are complicated by the presence of buoyancy, inducing creaming or sedimentation of the droplets, which results strongly coupled with the above mechanisms.
Microgravity conditions offers therefore a unique environment to study the evolution of emulsions under the sole effects of the mechanisms responsible for emulsion destabilisation, such as droplet coalescence and Ostwald ripening, allowing deriving more clear correlations with the features of adsorption layers at the droplet interface.
With above aims, the project EDDI (Emulsion Dynamics and Droplet Interface) has been proposed to ESA by an international partnership, In the envelop of the project, two campaigns of experiments (PASTA-1 and PASTA-2) have been succesfully performed between 2022 and 2014, using the ESA FSL-Soft Matter Dynamics facility, onboard the ISS. More experiments are planned in 2025, within a recently approved project (SEEDS). Here we present the concepts underlying the EDDI project and the PASTA experiments, also providing some overview of the results.
Besides obvious terrestrial applications, the results of these studies can also serve the optimisation and development of technologies where emulsions are utilised in reduced-gravity conditions, such as space vehicles and planetary environments.

Following the major current focus on the return to the Moon to further explore and eventually inhabit our closest celestial neighbor, it has never been more important to understand how to manipulate the abundantly available resources found on the lunar surface. In such a context, new methods to manage and transport the lunar regolith, based on the application of vibrations, are being investigated. This research builds on an existing line of inquiry dealing with the “vibro-fluidization” of monodisperse granular media constituted by particles of all the same size and shape. Being made of particles of differing sizes and shape, lunar regolith is a multidisperse material, which adds a new level of complexity to such an endeavor.

Two different scenarios were investigated in the framework of this research, one dealing with the vibrationally-driven behavior of regolith encapsulated in closed containers and another one concerned with its response in terms of mass flow rate when flowing through the orifice of a (vibrated) hopper configuration. Moreover, this study was articulated in a parametric investigation involving several disjoint influential parameters, namely, the frequency and amplitude of the applied vibrations, the composition (in terms of particle size distribution) of the considered lunar regolith simulant and even the inclination of the shaking direction (vertical, horizontal, and inclined with respect to gravity).

The experiments conducted in the closed configuration have shown that lunar regolith can indeed enter a fluidized state under the influence of vibrations. This effect has been quantified by using the vertical distance between vibrationally induced peaks and valleys (starting from a distribution of lunar regolith having a perfectly horizontal top surface). For a fixed amplitude of vibrations, this distance displays a non-monotonic dependence on the related shaking frequency in the range of 30 – 50 Hz . These phenomena also display a complex relationship with the particle average size. Most notably, similar trends are observed when the open (hopper) configuration is considered. In this case, at a fixed amplitude, there exist peak values of mass flow rate at values of specific frequency very close to those maximizing particle convective motion in the closed container.

Different metrics (the peak-to-valley vertical distance and the mass flow rate in the closed and open system cases, respectively) have confirmed that a non-monotonic relationship exists between the intensity of applied vibrations, in terms of frequency and amplitude, and the degree of fluidization of lunar regolith.

Describing the ageing of emulsions is a major challenge for both fundamental and industrial applications. When looking more specifically at the contribution of the phenomena occurring at interfaces, a basic problem is the influence of gravity drainage. It tends to mask the phenomenology associated with capillarity, such as ripening, flocculation or droplet coalescence. The aim of the ISS EDDI-PASTA experiment is to investigate emulsions into microgravity conditions for sufficiently long periods of time to allow such phenomena to be described.
This work focuses on recent experiments performed using the ESA Soft Matter Dynamics (SMD) facility with MCT (Medium Chain Triglyceride, supplied by IOI Oleo) oil-in-water emulsions, stabilized with a non-ionic surfactant (C12EO21, supplied by Nikko Chemicals). Emulsions are generated onboard in small sample cells (3.7 mL) using a magnetically coupled mobile piston. For the aims of the study, we used 12 interchangeable sample cells housed in the SMD carousel, with oil volume fraction between 20% and 50% and surfactant concentrations covering about one decade below the cmc.
The aging dynamics of the emulsions is analyzed with optical microscopy in transmission mode (see figure). Images are acquired using the so-called Overview Camera of the SMD facility. The focal plane is located about a millimeter inside the emulsion. The emulsification procedure and the high volume fraction of oil produce emulsions with a large number of droplets acting like thick lenses. Images are therefore subject to important optical distortion, making it impossible to identify individual droplets. However, their contrast is sufficient to perform gray levels analysis (see figure).
To describe the temporal evolution of the emulsion, the idea is to identify a specific family of contours for each image (in black on the figure) which give rise to a typical pattern. As evolution of the emulsions is slow, successive images exhibit a good coherence making this pattern easy to follow as time runs. Any events or modifications occurring anywhere inside the emulsions therefore result in a number of changes in this pattern that can be easily spotted and counted.
In the transient phase, the evolution of the emulsions shows phases of relative calm punctuated by bursts of activity during which tens of correlated events occur. Their amplitude tends to decrease with time, indicating a gradual relaxation of the emulsions towards a quieter evolutionary phase. The role of MCT-oil volume fraction and surfactant concentration in the overall statistics of these bursts are investigated.

Introduction

Intuitively, granular gases are rather simple multi-particle systems, where grains only interact by dissipative collisions. However, this subtle difference to atomic gases causes large physical effects, many of them predicted in theory or simulation. One of them is the continuous loss of energy, when particles in an initially excited state are left without external energy supply, the “granular cooling”, first described by P. Haff [1]. Experimental confirmation of its scaling law followed recently for rods [2,3], translational motion of spheres [4] and ellipsoids [5], with significant deviations from the predicted time scales. A second striking effect is the absence of equipartition of kinetic energy between translational and rotational degrees of motion, experimentally proven for rods [2], and between different components in a granular gas mixture [3]. The detection of rotations of spheres is more complicated and requires, e.g., patterning of the particles in optical measurements, see Fig. 1. We present 3D experiments accompanied by simulations, both analyzing the rotations and translations during granular cooling.

Methods

We analyze the statistics of velocities, rotational velocities, positions and energy in a granular gas of spheres. We performed experiments with centimeter-sized elastic, rough rubber balls in the ZARM drop tower. The granular gas is first excited by vibrating walls of the container, and consecutively, the excitation is switched off and the actual measurement period starts. Rotations of the spheres can be tracked using specific coloring and patterns. We employ custom AI-aided software for particle detection, adapted from [6]. DEM simulations of granular gases of rough spheres are performed on GPUs.

Results

As expected, the distributions of the velocity components at the beginning of, but also during later stages of cooling are non-Gaussian with fast grains overrepresented. The initial excess in energy in excitation direction decays after few collisions. Thereafter, the granular cooling follows Haff’s predictions. Angular velocities possess a non-Maxwell distribution. Equipartition of energy is violated. In simulations, we observe similar behavior. There, however, we can compare the cooling from a truly homogeneous random initial state to a mimicked experiment.

Conclusion

Granular gases in 3D, even for simple particle types, are still an under-researched field of experiments. AI aided solutions are important for data evaluation. Open questions in particular related to experiments remain, e.g., regarding realistic contact models or the time scales of cooling. We present a first experimental / simulation study considering all degrees of freedom in 3D granular gases of spheres.

Introduction.
Emulsions are colloidal dispersions of two or more immiscible liquids, very important in several processes and products, (e.g. oil industry, lubrication and detergency, with serious concerns about environment preservation…) and as enabling technologies for space exploration (food, pharma detergency, smart materials). Technological advances are hindered by the lack of fundamental knowledge on such complex systems. We report results about the evolution of oil in water emulsion formulated with a non-ionic surfactant. The investigation, performed in weightlessness conditions onboard the ISS, allows to isolate intrinsic evolution processes from creaming/sedimentation typically dominant in terrestrial conditions, as shown in the figure.
Methods.
The main experimental technique employed was Diffusing Wave Spectroscopy (DWS) [1-4], implemented in the ESA Soft Matter Dynamics [5] facility onboard the International Space Station. To overcome strong limitations of some traditional interpretation schemes, concerned f.i. with shadowing important dynamical features, we applied an original interpretation scheme [6] based on optical Monte Carlo simulations. The analysis was supported by ground-based characterizations of initial drop size distribution and of properties of the oil/water interface in the presence of surfactant.
Results.
By detailed analysis of the DWS correlation function, we obtained information on the evolution of mean drop radius and of the droplet dynamics, which features a Brownian stationary regime punctuated by transient ballistic accelerations.
Based on this, we identified different de-emulsification mechanisms and tentatively assessed their relative importance as a function of the properties of the droplet population and of the interfaces. At early stage of the emulsion life, in all emulsions investigated, the drop population evolves mainly via interaction between small droplets. Nevertheless, in some emulsions a transition to a different regime is found at later times, in which the dominant interaction is between small and large droplets. This regime is particularly important at late stage of de-emulsification.
Conclusions.
These findings shed a new light on the phenomena relevant to emulsion stability, with potential impact not only on industrial processes on earth, but also as enabling technology for long flights and space colonies.