20-EPN2-094: Miniature Wind Vanes on Mars – Extracting Wind Flow Properties From Close-up Images of Sand Grains

Visit by Sebastiaan de Vet and Andreas Zafiropoulosof TU Delft (Netherlands) to TA2.4 Planetary Environment Facilities (PEF), AU (Denmark)
Dates of visit: 22-26 January 2024

Report summary: Aeolian sediments are ubiquitous at the surface of Mars where they are found in active sand dunes and lithified aeolian outcrops. These features form a geological record of past and present interactions between the surface and atmosphere of an evolving planet. A deeper understanding of their formation and context can be obtained by studying these deposits at the grain-scale level. From terrestrial studies, we known that airborne orientation of non-spherical grains due to long-axis streamlining to the wind flow is considered to occur almost instantaneously after entrainment grains by saltation. Grain orientation in sediment fabrics therefore harbours valuable information on near-surface wind flow and particle mobility. A key focus of this project was to establish how atmospheric pressure and above-threshold wind conditions modulate the observed grain orientation signal. Well-rounded aeolian sand for the wind tunnel experiments was collected from the active drift sand geotope at ‘Bedafse Bergen’, located in SE Netherlands.

At the Aarhus Wind Tunnel Facility, we performed several experiments under various atmospheric pressures from ambient sea-level conditions to Mars-analogue pressures. Close-up images were obtained from the sediment fabric after various runs at 120% and 150% above threshold conditions. A date pipeline has been developed and is being refined for processing of imagery and quantifying the grain orientation to ultimately disentangle effects of airborne streamlining of grains and impact randomisation at various wind speeds and atmospheric pressures.

22-EPN3-073: Emergence of Ice Ripples by Sublimation at Various Wind Velocities and Air Pressure

Visit by Sabrina Carpy of the Laboratoire de Planétologie et Géosciences (France) and Philippe Claudin of the Laboratoire de Physique et Mécanique des Milieux Hétérogènes (France), to TA2.4 Planetary Environment Facilities (PEF), AU (Denmark)
Dates of visit: 04-08 March 2024

Report summary: Sublimation waves are the result of the sublimation of icy substrates under turbulent winds. They are expected to be found on many planetary surfaces with volatile ices in contact with an atmosphere. They are periodic, linear and transverse bedforms of different scales (centimeter to decameter) depending on their dry windy environment [Bordiec et al, 2020]. Apart from the BIA’s [Weller, 1969; Mellor and Swithinbank, 1989; Bintanja et al, 2001] and on the walls of ice caves [Curl, 1966], very few field measurements have been collected on sublimation waves. There is no extensive morphological and kinetic data of the development of these bedforms and there are no systematic measurements of the bedform’s wavelength change when continuously varying the wind velocity and fluid viscosity. This is why we have performed a series of experiments in the AWTS-II on carbon dioxide ice under a dry air atmosphere.

After initial difficulties with the ice slab creation and surface conditions, we managed to track the evolution of the substrate height throughout the sublimation phase using laser profilometry. We observed emergent morphologies that could be traces of the linear instability, but whether they correspond to the expected sublimation waves remains to be confirmed by detailed data analysis. Also, an unforeseen phenomenon was observed where transverse waves of frost on leading edges of the ice slab would migrate downstream as the aerodynamic forces on the growing frost would break off flakes. We are developing a model to link this frost coverage to the local friction velocity distribution.

22-EPN3-095: Behaviour of Saline Liquid Droplets in Wind Tunnel Conditions Relevant to the Plumes of Enceladus

Visit by Mark Fox-Powell of the Open University (UK), to TA2.4 Planetary Environment Facilities (PEF), AU (Denmark)
Dates of visit: 19-23 February 2024

Report summary:

The main aim of this project was to investigate the freezing of droplets of saline water injected at high velocity into low pressure conditions, in order to better understand the formation history of salt-rich ice grains encountered by Cassini in the plumes of Saturn’s moon Enceladus. Our objectives were to (1) investigate freezing times of sub-millimetric droplets under pressures representative of the subsurface icy vents at Enceladus; (2) investigate the tendency for ice grains to fragment into smaller grains on impact with solid surfaces; and (3) identify the effects of dissolved salts on both factors. Experiments were conducted in the Aarhus large wind tunnel. Liquids were sprayed into a range of pressures spanning 0.1 – 6 mbar. Spray droplets were imaged with a high-speed camera as they impacted an aluminium target plate, and velocity distributions of droplets were recorded using a laser doppler velocimeter. Real-time observations indicated a strong pressure dependence on freezing times, with droplets at the lowest pressures freezing in less than one second. We observed that the presence of salts enabled droplets to remain liquid longer at lower pressures. In ongoing work, we are analysing high speed footage to quantify the proportions of frozen vs. liquid droplet impacts, and fragmenting vs. non-fragmenting frozen grains, and how these proportions vary across all experiments. These data will allow us to predict likely freezing behaviour of similar droplets in the Enceladus plumes, leading to new understanding of the processes controlling ice grain formation at Enceladus.

22-EPN3-099: Formation of Impact Ripples Induced by Different Flow Rates Under Martian Pressure and Temperature

Visit by Aurore Collet and Sabrina Carpy of the Laboratoire de Planétologie et Géosciences (France), to TA2.4 Planetary Environment Facilities (PEF), AU (Denmark)
Dates of visit: 22 May – 2 June 2023

Report summary: The conditions for the formation of aeolian ripples depend on the surface characteristics (particle flux, surface roughness, nature of the substrate) and the atmosphere (pressure, temperature, viscosity). The understanding of their formation mechanisms is a real challenge to characterise the interaction between the atmosphere and the substrate of different planetary bodies. Constraining the mechanisms would also allow to go back to past information as on Mars where the atmosphere was not as tenuous as today (Mischna, Lee, et Richardson 2012). A method to link their morphological characteristics (wavelength, amplitude) to the physical characteristics (pressure, temperature, viscosity, particle flux) and verify the associated flow regime (transition, equilibrium) (Selmani et al. 2018) is to use scaling law.

There is currently a significant lack of experimental data to understand the influence of the different physical parameters on the initial formation mechanism and the development of the ripples. On Mars, the formation of ripples under a tenuous atmosphere is still poorly constrained, despite that recent works have attempted to tackle this question (Lapotre et al. 2016; Sullivan et al. 2020). The study of the variation of the flow velocity on a given material (silica sand), can be experimented in a wind tunnel under a Martian atmosphere to understand the formation and the development of the aeolian ripples. A pioneer study in the Martian Chamber formed aeolian ripples with a quartz grain size of 125 μm (Andreotti et al. 2021) but natural ripples generally have a distributed grain size as on Mars where aeolian ripples of centimeter-scale wavelength have been detected on an active dune, with a grain size range of 45 – 500 μm (Ehlmann et al. 2017). Moreover, numerical modelling indicates a link between ripple wavelength, grain flow and the erosion rate of the system (Lester et al. 2022). The interaction between flow velocity, wavelength and erosion rate can be studied in space and time using photogrammetry.

20-EPN2-123: Controls on the Resuspension of Dusts of Different Mineral Composition in Air Flows

Visit by Merren Jones and Steve Covey-Crump of the University of Manchester (UK), to TA2.4 Planetary Environment Facilities (PEF), AU (Denmark)
Dates of visit: 11-15 September 2023

Report summary: Mineral dusts in the atmosphere cause a wide variety of damage to aircraft engines resulting in significant economic cost to the engine manufacturers and airline operators. The nature of damage is dependent on the mineralogy of the dust, and so there is currently a major drive to understand the controls on the composition of atmospheric dusts. We are currently planning a field campaign in central Australia to sample airborne sands and dusts together with surficial deposits, and the present study was designed to inform our sampling strategy and to help constrain the interpretation of our anticipated findings.

We performed wind tunnel experiments to examine detachment thresholds of seven minerals with different density, grain shape, and surface properties. The particle size variation (50-500 μm) of the detachment threshold is well-described by existing semi-empirical models but highlights the need to incorporate shape and adhesion properties into these models. In a further set of experiments, we used a new suction sampler to collect sand and dust particles in transport through the full depth of the flow (175 mm) to examine dust emission from the surface of a sand bed under conditions where the sand particles are saltating, and vertical particle size and compositional sorting developed within and above the saltating layer of a polymineralic sand. Provisional results show a good correlation between collected sample mass and mass-flux estimated from a laser opacity system throughout the depth of flow, and show strong vertical particle size and compositional sorting.

22-EPN3-076: Aeolian Ripples Development under Martian Atmospheric Conditions

Visit by Simone Silvestro and Giuseppe Mongelluzzo of INAF (Italy), to TA2.4 Planetary Environment Facilities (PEF), AU (Denmark)
Dates of visit: 16-20 October 2023

Report summary: In our experiments in the Planetary Environment Facility in Aarhus we obtained, two superimposed ripple patterns on monodisperse (90-150) glass beads in CO₂ air. The presence of two distinct sets of aeolian sand ripples in unimodal sand suggests two formational mechanisms, impact for the cm-scale ripples and hydrodynamic for the decimetre scale set. We also run the experiment for natural 125µm-sand for terrestrial and Martian pressures (and below). Two patterns might be visible, but not very clearly, suggesting that the shape of the grains is playing a role in the formation of the hydrodynamic pattern. We also found a potential inverse relationship between impact ripple wavelength and pressure that we are currently investigating.

22-EPN3-060: A new apparatus for measuring the electrical charge of volcanic ash particles

Visit by Allan Fries, University of Geneva (Switzerland), to TA2.4 Planetary Environment Facilities (PEF), AU (Denmark)
Dates of visit: 02-11 August 2023

Report summary: The electrical charge carried by volcanic ash particles is known to play a crucial role in the formation of aggregates, which modulate the atmospheric transport and deposition rate of volcanic ash. However, direct field measurements of these charges remain rare, and more investigations are required to better comprehend both charge generation and aggregation mechanisms. This study aimed at building and testing a portable device to quantify the charge of individual volcanic ash particles and aggregates using a Through-Type Faraday Cage (TTFC) connected to a charge amplifier circuit capable of detecting the very small voltages induced by charged particles entering the TTFC.

After building the apparatus, 187 tests were performed, using different quantities and types of particles, as well as varying the measurement technique. First results show that the newly built device is capable of measuring charges down to 0.1 pC and that measurements agree well with alternative estimations obtained by directly integrating the current generated in the TTFC over time.

Beside the charge, results also suggest that the device can be used to obtain the particle settling velocity, based on the duration of the voltage signal. Few additionnal tests are now required to detect smaller particles (i.e., smaller charges) and the instrument will then be used for field analysis and laboratory experiments.

Read the full scientific report, with kind permission from Allan Fries.

20-EPN-061: CO₂ Ice Crystals Formation Under Conditions in the Martian Polar Regions: Influence of Substrate Properties and Temperature Gradient

Visit by Ganna Portyankina, DLR (Germany), to TA2.4 Planetary Environment Facilities (PEF), AU (Denmark)
Dates of visit: 30 April – 01 October 2023 (two visits)

Report summary: The main goal of most recent tests conducted at the Planetary Environment Facilities at Aarhus University was to condense CO₂ from the chamber’s atmosphere under Martian conditions onto a specially designed set of different surface materials (martian regolith simulant, glass beads of various sizes, dust). We investigated ranges of temperatures and pressures and observed the texture of the created CO₂ ice. Our goal was to determine if CO₂ deposits over regolith/glass beads/dust differently compared to brushed aluminium. We have observed that various properties of substrate did not considerably alter the deposition morphologies of CO₂ observed in our previous work. Most importantly, we find that under conditions usual for Martian polar areas in fall and winter, CO₂ ice always deposits as a translucent slab.  Under deviating conditions, i.e. colder temperatures and lower pressures, CO₂crystals assume different shapes including opaque slab and highly porous multi-crystalline. Such CO₂crystalline morphologies require further investigations, because of their relevance to icy satellite surfaces as well as CO₂ cloud formation.

Read the full scientific report, with kind permission from Ganna Portyankina.

20-EPN-027: Characterisation of the response of an Optical Particle Counter in a simulated Martian environment

Visit by Giuseppe Mongelluzzo and Gabriele Franzese, INAF (Italy), to TA2.4 Planetary Environment Facilities (PEF), AU (Denmark)
Dates of visit: 05-09 September 2022

Report summary: The characterisation of dust is paramount in the understanding of Martian climatology. Dust significantly influences Mars global climate, interacting with the incoming solar radiation, altering the atmospheric temperature budget. Local and global dust storms can cover the planet for weeks, influencing the correct functioning of scientific instruments on the surface (The Rover “Opportunity” is the most famous example). The dynamics of dust lifting is strictly related to the wind velocity field, so its characterisation would provide important information on the characteristics of Martian winds.

The proposing team has developed an Optical Particle Counter (OPC) aimed at providing direct measurements of grain concentration and size distribution on Mars, which would be the first ever accomplished outside of Earth. The instrument is able to detect dust grains in the 0.4-20 μm diameter range. Both the breadboard and the flight model versions of the instrument have been tested in Martian environment, showing good performances.

The trip to the Planetary Environment Facilities at the University of Aarhus has allowed the verification that the instrument is correctly able to sample dust grains in its upper sensible range (up to 20 µm in grain diameter) also in presence of winds up to 20 m/s. A sandbed with embedded dust has also been created inside the Martian wind tunnel, allowing the simulation of natural saltation conditions. The instrument has been able to retrieve dust grains in all simulated conditions, both for monodispersed calibrated dust samples and for polydisperse samples.  

Read the full scientific report, with kind permission from Giuseppe Mongelluzzo.

20-EPN2-050: Turbulent suspensions of volcanic ash: an experimental simulation for eruptive and resuspension ash plumes

Visit by Jacopo Taddeucci and Elisabetta Del Bello, Istituto Nazionale di Geofisica e Vulcanologia, Rome (Italy), to TA2.4 Planetary Environment Facilities (PEF), AU (Denmark)
Dates of visit: 07-11 November 2022

Report summary: Ash Injection and settling experiments have been carried out using the environmentally controlled recirculating wind tunnel facility at Aarhus University, in order to understand the processes controlling deposition and segregation of ash from volcanic plumes at stratospheric altitudes.

Ash particles erupted from the Campi Flegrei volcano (Italy) and smaller than 63 micron were injected in the wind tunnel either from the upwind extremity in the presence of wind (1 m/s) or from the side in the absence of wind. The atmospheric pressure in the wind tunnel was systematically varied to simulate the corresponding elevation in the atmosphere from 10 to 50 km. The vertical and horizontal velocity of the particles was measured, as well as the plume opacity, proxy for particle concentration over time.

Settled particles were sampled at different times during the experiments and then analysed for their abundance and size distribution. Both the opacity measurements and the number of particles sampled over time display the decay of particle concentration over time in the suspended plume. The rate of decay is strongly dependent on the atmospheric elevation in a nonlinear way, with modest changes from 10 to 20 km elevation and much larger changes for higher elevations.

From these data we will retrieve experimentally the settling velocity of volcanic ash particles at a range of elevations that is of interest for both aviation and climate modelling implications.

Read the full scientific report, with kind permission from Jacopo Taddeucci.

20-EPN-054: Understanding large aeolian ripples on Mars through wind tunnel experiments

Visit by Simone Silvestro, INAF Osservatorio Astronomico di Capodimonte (Italy), and Hezi Yizhaq, Midreshet Ben-Gurion (Israel), to TA2.4 Planetary Environment Facilities (PEF), AU (Denmark)
Dates of visit: 04-08 April 2022

Report summary: In our experiments in the Planetary Environment Facility in Aarhus we obtained, for the first time, two superimposed ripple patterns on monodisperse sand beads in CO₂ air.

The presence of two distinct sets of aeolian sand ripples in unimodal sand suggests two formational mechanisms. Morphological characteristics such as straight crests and regular spacing point toward an impact mechanism to be responsible for the formation of the smaller (cm-scale) ripples. Conversely, the higher sinuosity of the larger (decimeter) ripples suggest a different type instability (hydrodynamic) at work. We also detect an increase in sizes for the ripples with decreasing pressure which is currently under investigation.

Collectively, our work seem to confirm the hydrodynamic nature hypothesised for the large Martian ripples.

Read the full scientific report, with kind permission from Simone Silvestro.

20-EPN2-051: Evaluation of physical parameters influencing the ice particle transportby wind in a Martian-like environment

Visit by Clémence Herny, University of Bern (Switzerland) to TA2.4 Planetary Environment Facilities (PEF), AU (Denmark)
Dates of visit: 16-20 May 2022

Report summary: The transport of ice by wind plays a major role in the surface mass balance of polar caps. Ice can be redistributed by wind due to (1) transport of ice particles and/or (2) transport of water vapour associated with sublimation/condensation. On Mars, although the low atmospheric density is less favourable for the transport of particles than on Earth, both dust and sand have been observed to be transported by wind. Despite ice aeolian landforms have been observed at the surface of the North Polar Cap of Mars, ice particle transport has not been directly observed on the Martian surface. Similarly, no laboratory studies of snow/ice particle transport under Martian-like conditions have been attempted thus far due to the complexity of the material.

In this study we propose to perform experiments in the environmental wind tunnel AWTSII at Aahrus University to study the ice particle transport in a wind-flow under Martian-like pressure and temperature conditions. The threshold shear velocity, which is a critical physical parameter for particle transportation, is determined by analysing the images of the samples acquired during the experimental runs as the wind speed was increased. The influence of ice grain sizes, pressure and temperature are investigated. Results will give constraints on the plausibility of wind driven ice particle transport on Mars.

20-EPN2-023: FLUME-ET: Fluidisation of mass flows by metastable volatiles on extra-terrestrial bodies

Visit by Lonneke Roelofs and Tjalling de Haas, Utrecht University (Netherlands) to TA2.4 Planetary Environment Facilities (PEF), AU (Denmark)
Dates of visit: 03-28 October 2022

Report summary: Martian gullies are alcove-channel-fan systems which have been hypothesised to be formed by the action of liquid water and brines, the effects of sublimating CO₂ ice or a combination of these processes. Recent activity and new flow deposits in these systems have shifted the leading hypothesis from water-based flows to CO₂-driven flows. This shift in thinking is supported by the low availability of atmospheric water under present Martian conditions and the observation that gully activity occurs at times when CO₂ ice is present.

We recently performed novel experiments in the Mars chamber at the Open University in the UK that have shown that this hypothesis holds; sediment can be mobilised and fluidised by sublimating CO₂ ice under Martian atmospheric pressure. However, if these experiments work on a larger scale and if these granular-gas flows are able to erode the underlying surface and can explain the formation of Martian gully systems over the long term remained unknown. Therefore, we conducted an additional series of experiments in a larger flume that test the capacity of CO₂-driven granular flows under Martian atmospheric conditions to erode sediment.

These experiments were conducted in a 4 m long flume in the Aarhus Mars Simulation Wind Tunnel. Our experiments show that CO₂-driven granular flows can erode loose sediment under a range of different slopes and CO₂-ice fractions. The results also show that incorporation of warmer sediment increases fluidisation of the mixture, reflected by an increase in gas pore pressure in the flow. These results thus prove that morphological evolution in the gully systems on Mars can be explained by CO₂-driven granular flows.

20-EPN-048: Effect of disperse grain size distributions on the aeolian remobilisation of volcanic ash

Visit by Allen Fries, University of Geneva (Switzerland) to TA2.4 Planetary Environment Facilities (PEF), AU (Denmark)
Dates of visit: 13-17 June 2022

Report summary: Thanks to the collaboration between the University of Geneva and the University of Aarhus through the Europlanet Research program we performed a set of experiments on the remobilisation of volcanic particles. Since removal processes of volcanic particles are relatively poorly characterised, these experiments represent a unique opportunity to study the dynamics of aeolian processes. The obtained results provide valuable information on the threshold friction velocities (i.e. wind friction velocity above which particles start to detach from deposits) of different ash compositions, fundamental for modelling and forecasting remobilisation events.

A total of 32 experiments were performed by using a setup composed of a sample plate (i.e., bed of volcanic ash exposed to gradually increasing wind friction velocities) from which particles were removed by wind and imaged using various techniques simultaneously (i.e. microscopes, and webcam)). These direct observations were combined with multiple particle collection methods to study the characteristics of remobilised particles (i.e. sediment traps and adhesive papers). A complete set of half-phi grainsize classes (from 0 to 500 μm) from 4 volcanoes were analysed. 

Preliminary results show a relation between the threshold friction velocity and the grainsize, in agreement with erosion theories. In addition, these experiments illustrate variations in threshold friction velocities as a function of magma composition: lighter particles (i.e. rhyolite) are easier to remobilise than denser particles (i.e. basalt). These results are pioneering, systematically quantifying threshold friction velocities of volcanic ash for wide grainsize and composition ranges for the first time. 

Read the full scientific report, with kind permission from Allen Fries.

20-EPN-053: Investigation of the electrical properties of volcanic ash

Visit by Eduardo Rossi, University of Geneva (Switzerland) to TA2.4 Planetary Environment Facilities (PEF), AU (Denmark)
Dates of visit: 11-17 June 2022

The set of experiments within the 20-EPN-053 project represents a preliminary step towards the characterisation of the electrical properties of volcanic ash in a collaboration between the University of Geneva and the Aarhus University. This topic is fundamental to better understand aggregates formation in a volcanic eruption and improve model forecasting.

The main setup consists of a horizontal wind tunnel where a recirculating flow allows volcanic ash of different sizes (e.g. from 1 μm to 500 μm, sieved in intervals of half-phi on the Krumbein scale) and compositions (e.g. from basaltic to andesitic) to be resuspended and collide together. Single particles and aggregates are filmed during their motion in the wind tunnel by means of a High Speed Camera (HSC) placed crosswise to the main flow direction. In addition to this, a set of four Optical Particle Counters (OPCs) are located at different heights downwind to the flow (i.e. 5 cm, 10 cm, 25 cm, 50 cm) with the goal of capturing differences in particle population for very fine ash (i.e. <40μm) due to electrostatic phenomena.

The primary goal is to detected particle trajectories that will be later used to quantify the bulk charge carried by single particles by means of an inversion of the equation of motion. The secondary goal is to understand if OPCs can be used combined with the wind tunnel facility to reveal a change in particle population on the recorded histograms that can be associated with aggregation processes due to the electrostatic force.

Read the full scientific report, with kind permission from Eduardo Rossi.

20-EPN-069: Exomars Dust Sensor 22 Characterisation

Visit by Andrés Russu Berlanga, Carlos III University of Madrid (Spain) to TA2.4 Planetary Environment Facilities (PEF), AU (Denmark)
Dates of visit: 29 November 21 – 3 December

The Dust Sensor (DS’22) is designed to measure the parameters that determine in situ the size distribution of suspended dust on the surface of Mars. The sensor module is composed of an IR source and two IR detectors based on Lead Sulphide (PbS) and Lead Selenide (PbSe) active elements. These materials are defined by the integration of two spectral filters, band 1 operates in the range 1-3 µm (PbS), and Band 2 operates in the range 3-5 µm (PbSe). The Dust Sensor is part of the Radiation and Dust Monitor (RDM), one of the atmospheric devices of the METEO instrument that will be launched on the Exomars’22 mission led by ESA,

The use of the DENMARK – AU Planetary Environmental Facility is a unique opportunity to obtain experimental measurements in a reproduced condition found at the surface of Mars. The tests campaign has been developed for wind speeds of 2, 7, and 13 m/s; DS’22 has been tested in a temperature range between 5 and -55 °C, and three different positions in relation to the wind direction have been tested. The test campaign has approximately 100 independent campaigns where 5 different types of dust have been used, with various particle distributions.

Thanks to all this information, it has been done a characterization of DS’22 and will help to determine the dust distributions that will be observed during the mission.

Read the full scientific report, with kind permission from Andrés Russu Berlanga.

20-EPN2-112: Aeolian saltation at Martian pressures and below

Visit by Philippe Claudin, PMMH – ESPCI – CNRS, Paris (France) to TA2.4 Planetary Environment Facilities (PEF), AU (Denmark)
Dates of visit: 30 May – 3 June 2022

Understanding the conditions required for initiating and sustaining sand motion on Mars is important for determining wind strengths required for mobilizing widespread ripples and dunes. Our previous experimental campaign in the planetary laboratory facility of Aarhus has provided evidence for a lower than expected transport threshold and for the emergence of impact ripples at Martian-like pressures. This time, we have made use of a new grain injector set-up at the entrance of the bed, which allowed us to trigger saltation with grains impacting with the granular surface, and therefore to mimic an effective longer bed. Even with a relatively low injection rate, we were able to reach a saturated sediment flux at the bed outlet, characterised by a neutral bed (no erosion nor deposition), whereas a purely erosive regime is always observed at vanishing injection. We could measure this saturated flux varying wind velocity in Martian conditions and found that transport can be sustained at even lower values than previously reported. With the analysis of the erosion profile along the bed, we shall furthermore be able to extract the saturation length. Finally, we recorded movies of grain motion close to the bed with a high-speed camera, which will allow us to study the properties of grain trajectories in the saturated state.

Read the full scientific report, with kind permission from Philippe Claudin.