@article{WangHarthPuzyrevetal.2022, author = {Wang, Jing and Harth, Kirsten and Puzyrev, Dmitry and Stannarius, Ralf}, title = {The effect of obstacles near a silo outlet on the discharge of soft spheres}, series = {New Journal of Physics}, volume = {24}, journal = {New Journal of Physics}, publisher = {IOP}, doi = {10.1088/1367-2630/ac8bea}, year = {2022}, language = {en} } @article{MissaouiHarthTritteletal.2022, author = {Missaoui, Amine and Harth, Kirsten and Trittel, Torsten and Klopp, Christoph and Stannarius, Ralf and Lacaze, Emanuelle}, title = {Shape instabilities of islands in smectic films under lateral compression}, series = {Soft Matter}, volume = {16}, journal = {Soft Matter}, publisher = {Royal Society of Chemistry}, issn = {1744-6848}, doi = {10.1039/D2SM00144F}, pages = {3193 -- 3205}, year = {2022}, language = {en} } @article{KloppTrittelHarthetal.2022, author = {Klopp, Christoph and Trittel, Torsten and Harth, Kirsten and Stannarius, Ralf}, title = {Smectic free-standing films under fast lateral compression}, series = {Soft Matter}, journal = {Soft Matter}, publisher = {Royal Society of Chemistry}, issn = {1744-683X, 1744-6848}, doi = {10.1039/D1SM01207J}, pages = {146 -- 155}, year = {2022}, abstract = {Smectic films in a collapsing catenoid develop spontaneous wrinkling patterns like solid sheets. Smectic freely-suspended films can wrinkle like solid sheets. This has been demonstrated earlier with shape-fluctuating smectic bubbles. Here, we exploit the collapse of smectic catenoid films with a central equatorial film to expose the latter to rapid lateral compression. Wrinkle formation is observed in the planar film and the thickness dependence of the undulation wavelength is measured. In addition to the central film, its border undergoes an undulation instability as well.}, language = {en} } @article{StannariusHarth2023, author = {Stannarius, Ralf and Harth, Kirsten}, title = {The structure of disintegrating defect clusters in smectic C freely suspended films}, series = {Soft Matter}, journal = {Soft Matter}, number = {19}, publisher = {Royal Society of Chemistry}, doi = {https://doi.org/10.1039/d3sm00808h}, pages = {6109 -- 6115}, year = {2023}, abstract = {Disclinations or disclination clusters in smectic C freely suspended films with topological charges larger than one are unstable. They disintegrate, preferably in a spatially symmetric fashion, into single defects with individual charges of +1, which is the smallest positive topological charge allowed in polar vector fields. While the opposite process of defect annihilation is well-defined by the initial defect positions, disintegration starts from a singular state and the following scenario including the emerging regular defect patterns must be selected by specific mechanisms. We analyze experimental data and compare them with a simple model where the defect clusters adiabatically pass quasi-equilibrium solutions in one-constant approximation. It is found that the defects arrange in geometrical patterns that correspond very closely to superimposed singular defect solutions, without additional director distortions. The patterns expand by affine transformations where all distances between individual defects scale with the same time-dependent scaling factor proportional to the square-root of time.}, language = {en} } @article{KloppTrittelHarthetal.2023, author = {Klopp, Christoph and Trittel, Torsten and Harth, Kirsten and Stannarius, Ralf}, title = {Coalescence of biphasic droplets embedded in free standing smectic A films}, series = {Soft Matter}, volume = {20}, journal = {Soft Matter}, number = {5}, publisher = {Royal Society of Chemistry}, issn = {1744-683X}, doi = {10.1039/d3sm01549a}, pages = {1036 -- 1046}, year = {2023}, abstract = {We investigate micrometer-sized flat droplets consisting of an isotropic core surrounded by a nematic rim in freely suspended smectic A liquid-crystal films. In contrast to purely isotropic droplets which are characterized by a sharp edge and no long-range interactions, the nematic fringe introduces a continuous film thickness change resulting in long-range mutual attraction of droplets. The coalescence scenario is divided in two phases. The first one consists in the fusion of the nematic regions. The second phase involves the dissolution of a thin nematic film between the two isotropic cores. The latter has many similarities with the rupture of thin liquid films between droplets coalescing in an immiscible viscous liquid. Coalescence of biphasic droplets in free-standing smectic films is controlled in part by the rupture of a thin nematic membrane separating the isotropic droplet cores.}, language = {en} } @article{TrittelKloppHarthetal.2022, author = {Trittel, Torsten and Klopp, Christoph and Harth, Kirsten and Stannarius, Ralf}, title = {Stability and Rupture of Liquid Crystal Bridges under Microgravity}, series = {Crystals}, volume = {12}, journal = {Crystals}, number = {8}, publisher = {MDPI}, doi = {https://doi.org/10.3390/cryst12081092}, pages = {16}, year = {2022}, language = {en} } @article{KloppTrittelStannarius2024, author = {Klopp, Christoph and Trittel, Torsten and Stannarius, Ralf}, title = {Coarsening of quasi two-dimensional emulsions formed by islands in free-standing smectic films}, series = {ChemPhysChem}, volume = {25}, journal = {ChemPhysChem}, number = {12}, publisher = {Wiley}, doi = {10.1002/cphc.202400166}, pages = {1 -- 12}, year = {2024}, abstract = {We study the coarsening behavior of assemblies of islands on smectic A freely suspended films in ISS microgravity experiments. The islands can be regarded as liquid inclusions in a two-dimensional fluid in analogy to liquid droplets of the discontinuous phase of an emulsion. The coarsening is effectuated by two processes, predominantly by island coalescence, but to some extend also by Ostwald ripening, whereby large islands grow at the expense of surrounding smaller ones. A peculiarity of this system is that the continuous and the discontinuous phases consist of the same material. We determine the dynamics, analyze the self-similar aging of the island size distribution and discuss characteristic exponents of the mean island growth.}, language = {en} } @article{KloppStannarius2024, author = {Klopp, Christoph and Stannarius, Ralf}, title = {Rupture of thin nematic walls between isotropic droplets of the same mesogenic material}, series = {Liquid Crystals}, volume = {51}, journal = {Liquid Crystals}, number = {5}, publisher = {Taylor \& Francis}, doi = {10.1080/02678292.2024.2325619}, pages = {783 -- 797}, year = {2024}, abstract = {We prepared isotropic droplets surrounded by nematic fringes in smectic A freely suspended films. During their coalescence, a thin nematic film is formed that acts as a barrier and separates the isotropic cores of these droplets. This film ruptures in the course of the droplet merging process. The observation of the film dynamics provides an exceptional opportunity to study the rupture of a thin liquid barrier embedded in a surrounding viscous liquid. A detailed dynamic characterisation of the nematic walls is performed. Qualitatively similar observations have been reported in 3D for oil drops coalescing in water and vice versa, where the rupture of an intermediate thin fluid film retards the merging process. The peculiarity of the system reported here is that the material of the droplets and the membrane is identical.}, language = {en} } @article{PuzyrevTorstenHarthetal.2024, author = {Puzyrev, Dmitry and Torsten, Torsten and Harth, Kirsten and Stannarius, Ralf}, title = {Cooling of a granular gas mixture in microgravity}, series = {npj microgravity}, volume = {10}, journal = {npj microgravity}, number = {1}, publisher = {Springer}, address = {Brandenburg an der Havel}, doi = {10.1038/s41526-024-00369-5}, pages = {1 -- 9}, year = {2024}, abstract = {Granular gases are fascinating non-equilibrium systems with interesting features such as spontaneous clustering and non-Gaussian velocity distributions. Mixtures of different components represent a much more natural composition than monodisperse ensembles but attracted comparably little attention so far. We present the observation and characterization of a mixture of rod-like particles with different sizes and masses in a drop tower experiment. Kinetic energy decay rates during granular cooling and collision rates were determined and Haff's law for homogeneous granular cooling was confirmed. Thereby, energy equipartition between the mixture components and between individual degrees of freedom is violated. Heavier particles keep a slightly higher average kinetic energy than lighter ones. Experimental results are supported by numerical simulations.}, language = {de} } @article{TrittelPuzyrevStannarius2024, author = {Trittel, Torsten and Puzyrev, Dmitry and Stannarius, Ralf}, title = {Platonic solids bouncing on a vibrating plate}, series = {Physical Review E}, volume = {109}, journal = {Physical Review E}, number = {3}, publisher = {APS}, doi = {10.1103/PhysRevE.109.034903}, year = {2024}, abstract = {The energy transfer between bouncing particles and rigid boundaries during impacts is crucially influenced not only by restitution coefficients of the material but also by particle shapes. This is particularly important when such particles are mechanically agitated with vibrating plates. Inertial measurement units are able to measure all acceleration and rotational velocity components of an object and store these data for subsequent analysis. We employ them to measure the dynamics of cubes and icosahedra on vibrating plates to study the efficiency of energy transfer into the individual degrees of freedom (DOFs) of the excited object. The rotational DOFs turn out to be much less excited than the vertical translational motion. Most remarkably, there is only little difference between the two Platonic solids in both the absolute energies and the energy partition ratios.}, language = {en} } @article{TrittelPuzyrevHarthetal.2024, author = {Trittel, Torsten and Puzyrev, Dmitry and Harth, Kirsten and Stannarius, Ralf}, title = {Rotational and translational motions in a homogeneously cooling granular gas}, series = {npj microgravity}, volume = {10}, journal = {npj microgravity}, number = {1}, publisher = {Springer}, doi = {10.1038/s41526-024-00420-5}, pages = {1 -- 6}, year = {2024}, abstract = {A granular gas composed of monodisperse spherical particles was studied in microgravity experiments in a drop tower. Translations and rotations of the particles were extracted from optical video data. Equipartition is violated, the rotational degrees of freedom were excited only to roughly 2/3 of the translational ones. After stopping the mechanical excitation, we observed granular cooling of the ensemble for a period of three times the Haff time, where the kinetic energy dropped to about 5\% of its initial value. The cooling rates of all observable degrees of freedom were comparable, and the ratio of rotational and translational kinetic energies fluctuated around a constant value. The distributions of translational and rotational velocity components showed slight but systematic deviations from Gaussians at the start of cooling.}, language = {en} } @article{KloppTrittelStannarius2024, author = {Klopp, Christoph and Trittel, Torsten and Stannarius, Ralf}, title = {Coarsening of Quasi Two-Dimensional Emulsions Formed by Islands in Free-Standing Smectic Films}, series = {CHEMPHYSCHEM}, journal = {CHEMPHYSCHEM}, publisher = {Wiley}, issn = {1439-4235}, doi = {10.1002/cphc.202400166}, pages = {1 -- 13}, year = {2024}, abstract = {We study the coarsening behavior of assemblies of islands on smectic A freely suspended films in ISS microgravity experiments. The islands can be regarded as liquid inclusions in a two-dimensional fluid in analogy to liquid droplets of the discontinuous phase of an emulsion. The coarsening is effectuated by two processes, predominantly by island coalescence, but to some extend also by Ostwald ripening, whereby large islands grow at the expense of surrounding smaller ones. A peculiarity of this system is that the continuous and the discontinuous phases consist of the same material. We determine the dynamics, analyze the self-similar aging of the island size distribution and discuss characteristic exponents of the mean island growth. Smectic islands on a thin freely suspended smectic film form a quasi two-dimensional emulsion, without a liquid subphase. We describe the coarsening dynamics that involves island coalescence and Ostwald ripening.}, language = {en} } @article{SataStannariusPuzyrev2025, author = {Sata, Sai Preetham and Stannarius, Ralf and Puzyrev, Dmitry}, title = {Criteria for dynamical clustering in permanently excited granular gases: comparison and estimation with machine learning approaches}, series = {Granular Matter}, volume = {27}, journal = {Granular Matter}, number = {4}, publisher = {Springer}, doi = {10.1007/s10035-025-01560-5}, year = {2025}, abstract = {When granular gases in microgravity are continuously excited mechanically, spatial inhomogeneities of the particle distribution can emerge. At a sufficiently large overall packing fraction, a significant share of particles tend to concentrate in strongly overpopulated regions, so-called clusters, far from the excitation sources. This dynamical clustering is caused by a complex balance between energy influx and dissipation. The mean number density of particles, the geometry of the container, and the excitation strength influence cluster formation. A quantification of clustering thresholds is not trivial. We generate 'synthetic' data sets by Discrete Element Method simulations of frictional spheres in a cuboid container and apply established criteria to classify the local packing fraction profiles. Machine learning approaches that predict dynamic clustering from known system parameters on the basis of classical test criteria areoposed and tested. It avoids the necessity of complex numerical simulations.}, language = {en} } @article{HarthTrittelMohammadietal.2025, author = {Harth, Kirsten and Trittel, Torsten and Mohammadi, Mahdieh and Puzyrev, Dmitry and Enezz, Mohammad and Cruz Hidalgo, Raul and Stannarius, Ralf}, title = {Granular gases of rough spheres: Velocity, rotation and collision statistics from in-situ measurements, optical data and simulations}, series = {EPJ Web of Conferences}, volume = {340}, journal = {EPJ Web of Conferences}, editor = {Alam, M. and Das, S.L. and Bose, M. and Murthy, T.G. and Chikkadi, V. and Singh, A. and Luding, S.}, publisher = {EDP Sciences}, doi = {10.1051/epjconf/202534003012}, year = {2025}, abstract = {Granular gases are not only of interest in fundamental physics, but they can also serve as a test ensembles for the validity of collision models employed in (loose) granular matter. The theoretical literature mainly addresses spheres under ideal conditions and simulations allow full access to all particle parameters, but experiments cannot fulfill these idealizations. We investigate granular gases of soft, rough spheres by combining microgravity experiments and adjusted simulations. We introduce Smart Particles with embedded autarkic micro-sensors for in-situ measurements of rotation rates and accelerations. Additionally, we extract 3D positions, translations and orientations of the particles from stereoscopic video data using Machine-Learning based algorithms. We address the partition of kinetic energy between the degrees of freedom, the angular and translational velocity as well as collision statistics. A simulation is adjusted to experiment parameters, showing good agreement of translational motion, but qualitative differences in the decay of rotational kinetic energy.}, language = {en} } @article{StannariusWangPongoetal.2025, author = {Stannarius, Ralf and Wang, Jing and Pong{\´o}, Tivadar and Fan, Bo and B{\"o}rzs{\"o}nyi, Tam{\´a}s and Hidalgo, Ra{\´u}l Cruz}, title = {Forces on a sphere suspended in flowing granulate}, series = {EPJ Web of Conferences}, volume = {340}, journal = {EPJ Web of Conferences}, editor = {Alam, M. and Das, S.L. and Bose, M. and Murthy, T.G. and Chikkadi, V. and Singh, A. and Luding, S.}, publisher = {EDP Sciences}, doi = {10.1051/epjconf/202534002019}, year = {2025}, abstract = {We investigate the forces of flowing granular material on an obstacle. A sphere suspended in a discharging silo experiences both weight of the overlaying layers and drag of the surrounding moving grains. In experiments with frictional hard glass beads, the force on the obstacle was found to be practically flow-rate independent. In contrast, flow of nearly frictionless soft hydrogel spheres added drag forces which increased with the flow rate until reaching saturation at high flow speeds. The total force grew quadratically with the obstacle diameter in the soft, low friction material, while it grew much weaker, nearly linearly with the obstacle diameter, in the bed of hard, frictional glass spheres. In addition to the drag, obstacles embedded in the flowing hydrogel spheres experience a weight force from the top as if immersed in a hydrostatic pressure profile, but negligible counter-forces from below. In contrast, the frictional hard particles create a strong pressure gradient near the upper surface of the obstacle. Numerical simulations provide additional information that is difficult to access experimentally. They reproduce the experimental results and give hints for the origin of the different force contributions. The results have considerable practical importance for the discharge of storage containers with large objects suspended in flowing granular material.}, language = {en} } @article{PreethamSataStannariusPuzyrev2025, author = {Preetham Sata, Sai and Stannarius, Ralf and Puzyrev, Dmitry}, title = {Machine learning for prediction of dynamical clustering in granular gases}, series = {EPJ Web of Conferences}, volume = {340}, journal = {EPJ Web of Conferences}, editor = {Alam, M. and Das, S.L. and Bose, M. and Murthy, T.G. and Chikkadi, V. and Singh, A. and Luding, S.}, publisher = {EDP Sciences}, doi = {10.1051/epjconf/202534012012}, year = {2025}, abstract = {Continuously excited dense granular gases in microgravity can develop spatial inhomogeneities of the particle distribution. Dynamical clustering is a phenomenon where a significant share of particles concentrate in strongly overpopulated regions. It is caused by a complex interplay between the energy influx and dissipation in collisions. The overall packing fraction, container geometry, and excitation parameters influence the gas-cluster transition. We perform Discrete Element Method (DEM) simulations for frictional spheres in a cuboid container and apply statistical criteria to the packing fraction profiles. Machine learning (ML) methods are used to study the dependence of the gas-cluster transition on system parameters. It is a promising alternative to predict the state of the system without the need for the time-consuming DEM simulations. We identify the best models for predicting the dynamical clustering of frictional spheres in a specific experimental geometry.}, language = {en} } @article{PuzyrevTrittelHarthetal.2025, author = {Puzyrev, Dmitry and Trittel, Torsten and Harth, Kirsten and Sata, Sai Preetham and Stannarius, Ralf and Cruz Hidalgo, Ra{\´u}l}, title = {AI-aided visual data analysis for granular gases: Complex particles, high density, and other challenges}, series = {EPJ Web of Conferences}, volume = {340}, journal = {EPJ Web of Conferences}, editor = {Alam, M. and Das, S.L. and Bose, M. and Murthy, T.G. and Chikkadi, V. and Singh, A. and Luding, S.}, publisher = {EDP Sciences}, doi = {10.1051/epjconf/202534012013}, year = {2025}, abstract = {Microgravity experiments with three-dimensional (3D) granular gases, i.e., ensembles of freemoving macroscopic particles which collide inelastically, produce large amounts of stereo video footage which require processing and analysis. The main steps of data treatment are particle detection, 3D matching and tracking in stereoscopic views, and quantification of ensemble statistical properties such as, e.g. mean kinetic energy or collision processes. Frequent overlapping or clustering of particles and their complicated movement patterns require smart software solutions. In recent years, Artificial Intelligence/Machine Learning (AI/ML) methods were successfully used for analysis of granular systems. We have applied such techniques to the granular gases of rod-like particles and developed a software tool which enables a full cycle of semi-automatic experimental data analysis. The approach is now tested on more complex, non-convex particles, shaped as 3D crosses (hexapods). Another challenge is optical analysis of dense granular gases, where individual particles cannot be tracked. We present a preliminary result of application of an ML method for number density profiles extraction in VIP-Gran experiment with dense ensemble of rod-like particles.}, language = {en} }