Holly Capelo | University of Bern (original) (raw)

Papers by Holly Capelo

Monthly Notices of the Royal Astronomical Society, Apr 21, 2021

ABSTRACTAccording to current theories of the formation of stellar systems, comets belong to the o... more ABSTRACTAccording to current theories of the formation of stellar systems, comets belong to the oldest and most pristine class of bodies to be found around a star. When approaching the Sun, the nucleus shows increasing activity and a pressure increase inside the material causes sublimated and trapped gas molecules to stream away from their regions of origin towards the surface. The present work studies two essential mechanisms of gas transport through a porous layer, namely the Darcy and the Knudsen flow. Gas flow measurements are performed in the laboratory with several analogue materials, which are mimicking dry cometary surface properties. In this first series of measurements, the aim was to separate gas transport properties from internal sources like local sublimation or release of trapped gases. Therefore, only dry granular materials were used and maintaining a low temperature environment was unnecessary. The gas permeability and the Knudsen diffusion coefficient of the sample materials are obtained, thereby representing the relative importance of the respective flow mechanism. The experiments performed with air at a stable room temperature show that the grain size distribution and the packing density of the sample play a major role for the permeability of the sample. The larger the grains, the bigger the permeability and the Knudsen diffusion coefficient. From the latter, we estimated effective pore diameters. Finally, we explain how these parameters can be adapted to obtain the gas flow properties of the investigated analogue materials under the conditions to be expected on the comet.

The Astronomical Journal, 2012

Sublimation of volatile materials causes a pressure build-up in the surface laye... more Sublimation of volatile materials causes a pressure build-up in the surface layers of comets, which then leads to outgassing and to the ejection of particles. To investigate this behavior in more detail, an experimental setup was developed as part of the CoPhyLab campaign [1]. With this setup different comet analogue samples are exposed to an artificial Sun to simulate cometary activity. The first experiments are conducted with pure water ice samples only since water ice comprises the bulk of volatile materials in most comets [2] and pure samples provide a well comprehensible starting point to model the development of the experiments. Thanks to our experiments we were able to observe, for the first time, that the sublimation of pure water ice (ice without any additional components, or impurities) can eject solid grains. Hence, the ejection of water ice grains does not require the presence of a more volatile species as suggested by [3]. In our experiments, the trajectories of the grains were recorded with a high-speed camera and subsequently evaluated with respect to their motion behavior using a particle tracking routine (Fig. 1). <img src="" alt="" /> Fig.1:&#160;&#160; Superimposed image of all &#8220;well-tracked&#8221; particle trajectories of one experiment over the observed sample surface. The ejected particles possess terminal speeds of about 1 m/s. Further, we found that the terminal particle velocity, as well as the total activity, depends directly on the insolation. Moreover, the observed trajectories were modeled by using gas-drag laws to derive an estimate for the sublimation pressure inside the samples [4]. The knowledge gained from these experiments will be used both in a larger experimental setup [5] and in further experiments including mixtures of water ice with dust and with super-volatiles such as CO2. &#160; References [1] Gundlach, B., &#8220;CoPhyLab: recent and future experiments - an overview&#8221;, 2020. doi:10.5194/epsc2020-218. [2] Bockel&#233;e-Morvan, D., &#8220;An Overview of Comet Composition&#8221;, in The Molecular Universe, 2011, vol. 280, pp. 261&#8211;274. doi:10.1017/S1743921311025038. [3] A'Hearn, M. F., &#8220;EPOXI at Comet Hartley 2&#8221;, Science, vol. 332, no. 6036, p. 1396, 2011. doi:10.1126/science.1204054. [4] Capelo, H. L., &#8220;Observation of aerodynamic instability in the flow of a particle stream in a dilute gas&#8221;, Astronomy and Astrophysics, vol. 622, 2019. doi:10.1051/0004-6361/201833702. [5] Kreuzig, C., &#8220;The CoPhyLab comet-simulation chamber&#8221;, Review of Scientific Instruments, vol. 92, no. 11, 2021. doi:10.1063/5.0057030. &#160;

&lt;ol&gt;1. INTRODUCTION&lt;/ol&gt; &lt;p&gt;Pla... more &lt;ol&gt;1. INTRODUCTION&lt;/ol&gt; &lt;p&gt;Planet formation in protoplanetary discs is a process whereby the primitive solids that are initially of microscopic scale, must be converted into larger objects such as pebbles (mm-cm size), planetesimals, and eventually planets. It has been acknowledged that drifting pebbles play an important role in the core accretion scenario by triggering streaming instabilities or by aiding the growth of planetary cores. Moreover, ice lines of volatile species such as water seem to be promising sites for this process. At the water ice line, the higher surface energy of ice promotes coagulation and the sublimated vapor can diffuse outward in the disk and deposit onto pebbles, allowing fast growth [1][2]. These processes can then trigger streaming instabilities or core formation through gravitational collapse of small bodies. However, very little is known about the evolution of these objects&amp;#8217; cohesive properties and volatile content as they drift and encounter various conditions throughout the disc. Investigating the morphology, chemistry, and physical processes of pebbles close to the ice line is essential to have an insight into the overall evolution of small bodies in the early phases of protoplanetary discs. We are interested in studying the different outcomes of the sublimation of an icy pebble, to understand how its optical properties are changing over time and if the dust aggregates survive to the sublimation process. In the Laboratory for Outflow Studies of Sublimating Materials (LOSSy) at the University of Bern [3], we are researching optical and physical properties of ice-dust mixtures with relevance for protoplanetary discs and planet formation, with focus on the role of ice sublimation in changing these properties.&lt;/p&gt; &lt;ol&gt;2. EXPERIMENTAL SETUP&lt;/ol&gt; &lt;p&gt;Two new methods for the preparation of ice-dust aggregate with mm-size have been developed. The first method (Pebble-A) uses an inclined superhydrophobic surface with dust on it; a mm-size droplet of distilled water rolls on it and collects the dust, then it falls into liquid nitrogen. The second method (Pebble-B) exploits the capillary forces between water and solid grains: a droplet impinges a dust bed and penetrates it forming a wet aggregate, which is then sunk in liquid nitrogen. PAs have generally ~50%wt of ice and the dust is mainly accumulated close to the surface of the pebble, while the core has more ice. PBs have lower amounts of ice (~15%wt) and the dust grains are connected through ice films. Different types of dust with relevance for protoplanetary discs are used, such as olivine, pyroxene, corundum, serpentine, CI and CR asteroids simulants [4]. Humic acid (HA) is used as an organic compound, simulating complex organics that can be found on comets.&lt;/p&gt; &lt;p&gt;The SCITEAS-2 (Simulation Chamber for Imaging the Temporal Evolution of Analogue Samples version 2.0) vacuum chamber provides a low-pressure and low-temperature environment for the sublimation of icy samples, and a hyperspectral measurement of the sample over time in the VIS and NIR [5]. The pressure inside the chamber is kept low (around 10&lt;sup&gt;-6 &lt;/sup&gt;mbar) through a turbomolecular vacuum pump, and a He-cryocooler guarantees a temperature at the base of the sample of&amp;#160; ~120 K. A fast sublimation of the icy pebbles is achieved by letting the temperature evolve freely up to room temperature, while the vacuum pump pumps out the vapor formed in the chamber. The overall process lasts around 20 hours.&lt;/p&gt; &lt;ol&gt;3. NOTABLE RESULTS&lt;/ol&gt; &lt;p&gt;Ice sublimation, gravity, grain size distribution of the dust, type of dust, ice content, and presence of organics concur all together to determine the sublimation outcome of the icy pebble. Is it going to disrupt to dust or will it maintain its shape? We list here some interesting preliminary observations:&lt;/p&gt; &lt;ul&gt;- ice sublimation is detectable in the VIS and NIR reflectance spectra of the pebbles. Although the disappearance of the ice absorption bands provides information on ice content at the surface of the pebble only, the core could still contain ice;&lt;/ul&gt; &lt;ul&gt;- PAs seem to disrupt more easily with respect to PBs made of the same dust, due to the higher amount of ice that is embedded in the core, which pushes the particles away when sublimating;&lt;/ul&gt; &lt;ul&gt;- in PAs, the presence of HA mixed with mineral powders prevents disruption, which is observed in the absence of HA (Fig.1);&lt;/ul&gt; &lt;ul&gt;- in PBs made of pyroxene, different grain sizes result in different outcomes: PBs made of dust with grain sizes bigger than 100 microns disrupt, while smaller grain sizes are more efficient in maintaining…

arXiv (Cornell University), Nov 2, 2021

Icy pebbles may play an important role in planet formation close to the water ice line of protopl... more Icy pebbles may play an important role in planet formation close to the water ice line of protoplanetary discs. There, dust coagulation is more efficient and re-condensation of vapor on pebbles may enhance their growth outside the ice line. Previous theoretical studies showed that disruption of icy pebbles due to sublimation increases the growth rate of pebbles inside and outside the ice line, by freeing small silicate particles back in the dust reservoir of the disc. However, since planet accretion is dependent on the Stokes number of the accreting pebbles, the growth of planetesimals could be enhanced downstream of the ice line if pebbles are not disrupting upon sublimation. We developed two experimental models of icy pebbles using different silicate dusts, and we exposed them to low-temperature and low-pressure conditions in a vacuum chamber. Increasing the temperature inside the chamber, we studied the conditions for which pebbles are preserved through sublimation without disrupting. We find that small silicate particles (< 50 µm) and a small quantity of ice (around 15 % pebble mass) are optimal conditions for preserving pebbles through sublimation. Furthermore, pebbles with coarse dust distribution (100 − 300 µm) do not disrupt if a small percentage (10 − 20 % mass) of dust grains are smaller than 50 µm. Our findings highlight how sublimation is not necessarily causing disruption, and that pebbles seem to survive fast sublimation processes effectively.

We present the early results from a novel experiment to study a particle-laden flow, under a para... more We present the early results from a novel experiment to study a particle-laden flow, under a parameter regime relevant to the conditions in planet-forming systems. We investigate the gas-particle interactions to identify the presence of and details regarding the streaming instability, which is theoretically predicted to aid the coalescence of small dust grains to form planetesimals-the macroscopic objects that will eventually interact gravitationally and become planets. We vary properties of the system such as dust-togas ratio, relative particle-gas velocity and gas pressure, for comparison to numerical simulations of protoplanetary disks. Experimentally calibrated numerical calculations of the particle motion within the instability regions will be used to model the evolution of protoplanetary disks at the scale of small dust grains, representing an unprecedented precision in our understanding of these difficult to study systems.

NOAO Proposal, Feb 1, 2013

Polarization phase curves of asteroids and other small airless bodies are influenced by the compo... more Polarization phase curves of asteroids and other small airless bodies are influenced by the compositional and physical properties of their regolith. The mixing of minerals composing the regolith influences the negative polarization at small phase angles because it changes the multiple scattering properties of the medium. This work aims to demonstrate experimentally how the mixing effect influences the polarization phase curve at small phase angles for different mineralogies relevant for asteroids, and to determine how different aggregate sizes affect the negative polarization. We prepared a set of binary and ternary mixtures with different common minerals on asteroids and one set of the same mixture with different aggregate sizes. We measured their reflected light at 530 nm with full Stokes polarimetry at phase angles ranging from 0.8° to 30°. The mixing effect of the mixtures with both bright and dark minerals significantly changes the behavior of the phase curves in terms of minimum polarization, phase angle of the minimum, and inversion angle with respect to the mineral components that are mixed together. The changes in phase curve could explain the polarization observation of particular classes of asteroids (F and L class) and other asteroids with peculiar polarization curves or photometric properties. Furthermore, we demonstrate that the negative polarization is invariant to the presence of dust aggregates up to centimeter sizes.

Review of Scientific Instruments

The field of planetary system formation relies extensively on our understanding of the aerodynami... more The field of planetary system formation relies extensively on our understanding of the aerodynamic interaction between gas and dust in protoplanetary disks. Of particular importance are the mechanisms triggering fluid instabilities and clumping of dust particles into aggregates, and their subsequent inclusion into planetesimals. We introduce the timed Epstein multi-pressure vessel at low accelerations, which is an experimental apparatus for the study of particle dynamics and rarefied gas under micro-gravity conditions. This facility contains three experiments dedicated to studying aerodynamic processes: (i) the development of pressure gradients due to collective particle–gas interaction, (ii) the drag coefficients of dust aggregates with variable particle–gas velocity, and (iii) the effect of dust on the profile of a shear flow and resultant onset of turbulence. The approach is innovative with respect to previous experiments because we access an untouched parameter space in terms of...

Polarization phase curves of asteroids and other small airless bodies are influe... more Polarization phase curves of asteroids and other small airless bodies are influenced by the compositional and physical properties of their regolith. The mixing of minerals composing the regolith influences the negative polarization at small phase angles as it changes the multiple scattering properties of the medium. In recent years, two new asteroid classes have been identified by their polarization phase curve at small phase angles [1][2]. The F-class shows an inversion angle (i.e. the phase angle at which the polarization becomes positive) relatively small to other asteroid families (about 14-16&#176;), and the L-class (the so-called &#8220;Barbarians&#8221; after the prototype of this class (234) Barbara) show very high inversion angles, about 25-30&#176;. The interpretation of these phase curves is challenging, and it has been proposed that the polarization properties of Barbarians are directly correlated to their surface mineralogy, in particular to the presence of spinel-bearing calcium-aluminum inclusions in a dark matrix [3][4]. The F-class peculiar negative polarization has been interpreted as the result of a homogeneous composition of the surface [1]. We know that the surface mineralogy is directly influencing the polarization phase curve of asteroids. In fact, asteroids tend to group in their respective spectral classes when their minimum of polarization is plotted versus &#160;[5]. It has been experimentally demonstrated that a mixture of minerals with different albedos can deepen the negative polarization with respect to the polarization phase curves of the single minerals [6][7], and we call this effect the &#8220;mixing effect&#8221;. A systematic study on the &#8220;mixing effect&#8221;, however, is lacking in the literature. The mixing of bright ad dark minerals has been studied in polarization only for a few minerals, and it has been used to explain the polarization behavior of F-type asteroids and trans-Neptunian objects [8]. Our work aims to demonstrate experimentally how the &#8220;mixing effect&#8221; influences the polarization phase curve at small phase angles for different mineralogies relevant for asteroids and to determine how different aggregate sizes affect the negative polarization. We prepared a set of binary and ternary mixtures with different common minerals on asteroids and one set of the same mixture with different aggregate sizes. We measured their reflected light at 530 nm with full-Stokes polarimetry at phase angles ranging from 0.8&#176; to 30&#176;. The mixing effect of those mixtures with both bright and dark minerals significantly changes the behavior of the phase curves in terms of minimum polarization, phase angle of the minimum, and inversion angle with respect to the mineral components that are mixed together (Fig. 1). Some of the binary mixtures show a deepening of the negative polarization when a low reflectance mineral is mixed with a high reflectance mineral. The inversion angle is affected similarly, increasing respect to the inversion angle of the single minerals. Interestingly, some binary mixtures with high and low reflectance minerals show no deepening of the negative polarization (e.g. spinel and graphite). In general, the polarization phase curve changes significantly every time that two or more minerals are mixed together. Furthermore, we find that the presence of aggregates up to cm-size does not affect the negative polarization. Our binary mineral mixtures can explore large areas of the space (Fig. 2). Our results show that F-class asteroids could be not as homogeneous as previously thought, since similar reflectance, &#160;and can be obtained by a mixture with 1:3 mass ratio of bright and dark minerals (forsterite and graphite). Barbarians lay in a region that is not explored by our binary mixtures. Nonetheless, we observe that our binary mixtures have a higher inversion angle with increasing contrast of the two end-members (pure Mg-spinel excluded). Other studies found similar results for mixing bright and dark materials: [6] found that the inversion angle increases by 3&#176; compared to pure fine silicates when adding 10% of 10 nm soot, and [7] found that a 1:1 mixture of sub-micron MgO and Fe2O3 shows a 9&#176; higher inversion angle with respect to the highest inversion angle of the end-members (MgO). In our sample, the largest increase in inversion angle is given by a 1:1 mixture of silica and magnetite, with an excess of 1&#176; compared to the inversion angle of pure silica. We…

The planetary Ice Laboratory has been developed at the University of Bern to mea... more The planetary Ice Laboratory has been developed at the University of Bern to measure various reflectance properties of analogues for small bodies and planetary surfaces with a special focus on icy surfaces. Its core facilities are a set of devices to produce well-characterized and reproducible analogue samples and a set of instruments designed to measure the reflectance properties of the samples over different spectral ranges, different geometrical configurations, either in total light or sorted by polarisation state. We use the laboratory to work on a range of scientific questions, mostly related to current Solar System missions and the interpretation of remote-sensing data but also occasionally on telescopic observations of Solar System objects as well as distant exoplanets and planet-forming circum-stellar discs. The experimental datasets collected are then provided to public databases for use by the community. Some of our most recent investigations include: - Visible and near-infrared reflectance spectra of analogues for the icy moons of the outer Solar System. We have collected new data with salty ice samples (Fig. 1), before and after irradiation by electrons, which can be compared with reflectance data measured by previous missions. The dataset will also help with the preparation of the JUICE and Europa Clipper missions. - Simulations of cometary activity, in the framework of the CoPhyLab project led by TU Braunschweig, with IWF Graz, MPS and DLR as partners. We develop quantitative methods to derive the ice-to-dust ratio from reflectance data in order to study the sublimation of dust ice mixtures prepared as cometary analogues. The results help interpreting the data from previous missions such as Rosetta and preparing future missions such as Comet Interceptor. - Spectral reflectance properties of different types of icy and dry Martian analogues, most notably in relation with the quantitative analysis of images returned by the CaSSIS imager of Exomars TGO. These measurements are performed in a simulation chamber equipped to simulate the Martian conditions of composition, temperature and pressure and we look at the samples with an imager equipped with the spare bandpass filters from CaSSIS (Fig. 2). - The linear polarisation properties of pure and mixed minerals as analogues for the surfaces of asteroids with implications for the interpretation of measured phase curves in terms of composition. (see also Spadaccia et al., this conference). - Assessment of the potential of circular spectropolarimetry as a biosignature in reflected light for Solar System objects and exoplanets. - Measurements of artificial samples for comparison with numerical simulations. With this presentation, we will provide an overview of the main objectives of the overall project, detail the methodology used, and present some of the results from the most recent studies mentioned here. &#160; <img src="" alt="" /> Fig. 1: Blueish salty (NaCl) ice particles (~67&#181;m) produced after the freezing in LN2 of salty solutions. Particles freeze from the exterior to the interior, forming a thin mantle of pure crystalline ice around a core of amorphous hyper saline ice. After the LN2 has evaporated, ice and salt in the core slowly crystallize as the temperature increases, generating Rayleigh scattering. &#160; <img src="" alt="" /> Fig. 2: The SCITEAS-2 simulation chamber configured to simulate the cold surface and atmosphere at the Martian poles and equipped with a microscope and a multispectral imager with the spare bandpass filters from CaSSIS.

Astronomy & Astrophysics

Context. Polarization phase curves of asteroids and other small airless bodies are influenced by ... more Context. Polarization phase curves of asteroids and other small airless bodies are influenced by the compositional and physical properties of their regolith. The mixing of minerals composing the regolith influences the negative polarization at small phase angles because it changes the multiple scattering properties of the medium. Aims. This work aims to demonstrate experimentally how the mixing effect influences the polarization phase curve at small phase angles for different mineralogies relevant for asteroids, and to determine how different aggregate sizes affect the negative polarization. Methods. We prepared a set of binary and ternary mixtures with different common minerals on asteroids and one set of the same mixture with different aggregate sizes. We measured their reflected light at 530 nm with full Stokes polarimetry at phase angles ranging from 0.8° to 30°. Results. The mixing effect of the mixtures with both bright and dark minerals significantly changes the behavior of t...

Monthly Notices of the Royal Astronomical Society

The CoPhyLab (Cometary Physics Laboratory) project is designed to study the physics of comets thr... more The CoPhyLab (Cometary Physics Laboratory) project is designed to study the physics of comets through a series of earth-based experiments. For these experiments, a dust analogue was created with physical properties comparable to those of the non-volatile dust found on comets. This ‘CoPhyLab dust’ is planned to be mixed with water and CO2 ice and placed under cometary conditions in vacuum chambers to study the physical processes taking place on the nuclei of comets. In order to develop this dust analogue, we mixed two components representative for the non-volatile materials present in cometary nuclei. We chose silica dust as a representative for the mineral phase and charcoal for the organic phase, which also acts as a darkening agent. In this paper, we provide an overview of known cometary analogues before presenting measurements of eight physical properties of different mixtures of the two materials and a comparison of these measurements with known cometary values. The physical pro...

This project includes all the images taken from the camera of the sublimation of icy pebbles insi... more This project includes all the images taken from the camera of the sublimation of icy pebbles inside the vacuum chamber. The images are taken before and after the sublimation of icy, and any changes in projected area of pebbles are hints of disruption due to sublimation and/or gravity.

EPSC-DPS Joint Meeting 2019, Sep 1, 2019

We present the early results from a novel experiment to study a particle-laden flow, under a para... more We present the early results from a novel experiment to study a particle-laden flow, under a parameter regime relevant to the conditions in planet-forming systems. We investigate the gas-particle interactions to identify the presence of and details regarding the streaming instability, which is theoretically predicted to aid the coalescence of small dust grains to form planetesimals - the macroscopic objects that will eventually interact gravitationally and become planets. We vary properties of the system such as dust-to-gas ratio, relative particle-gas velocity and gas pressure, for comparison to numerical simulations of protoplanetary disks. Experimentally calibrated numerical calculations of the particle motion within the instability regions will be used to model the evolution of protoplanetary disks at the scale of small dust grains, representing an unprecedented precision in our understanding of these difficult to study systems.

Introduction: Terraces are common on all known cometary surfaces, but their origins are not well ... more Introduction: Terraces are common on all known cometary surfaces, but their origins are not well understood. The formation of terraces on comets has been attributed to wide range of processes from accretionary to evolutionary. It is noteworthy, however, that terraces have been observed on comets over a broad range of scales, from millimeters to tens of meters. We have therefore begun to examine the role of sublimation, a ubiquitous occurrence on comets that occurs at all scales, in forming terraced terrains. In particular, we are exploring the possibility that backwasting along sublimation fronts can lead to multi-scale terracing by examining the morphologies produced in laboratory ice experiments. Background: Terraces were first identified on comet 9/P Tempel 1 [1, 2], observed on 103P/Hartley 2 [e.g., 3], and are clearly evident on comet 67P/Churyumov-Gerasimenko (C-G) [e.g., 4], the three comets for which we have the highest resolution images. Proposed formation mechanisms for cometary terraces include low-velocity impacts during the original accretion from smaller bodies (i.e., cometesimals) [5], collisions that led to the formation of bilobal comets [6], repeated cycles of dust deposition, and biproducts of sublimation [7]. It is possible that all of these processes play a role in the formation of some terraces on some comets. However, terracing occurs on multiple comets, both binary (e.g., C-G and Hartley 2) and not (e.g., Tempel 1). Therefore, some of the proposed mechanisms cannot explain all the observations. Instead, we look to an explanation that can occur at all scales and on all comets. Fractal Morphology: As shown in Fig. 1, terraces exhibit a fractal morphology and occur at scales from tens of meters [e.g., 2] to millimeters [e.g., 8, 9]. Note, that the self-similarity of terraces at all scales is accentuated near zero phase where shadowing is minimized. This fractal morphology requires a scaleindependent and widespread formation process [7]. Although the bulk of cometary activity is now known to arise from only a small fraction of a comet's surface [2, 10, 11], low-level sublimation is nonetheless common and must occur even at the scale of grains. Backwasting of Sublimation Fronts: We are therefore exploring the possibility that terracing, particularly on cm to mm scales, can be produced from backwasting of sublimation fronts. In particular, we are

Monthly Notices of the Royal Astronomical Society, Apr 21, 2021

The Astronomical Journal, 2012

arXiv (Cornell University), Nov 2, 2021

NOAO Proposal, Feb 1, 2013

Review of Scientific Instruments

Astronomy & Astrophysics

Monthly Notices of the Royal Astronomical Society

EPSC-DPS Joint Meeting 2019, Sep 1, 2019

We present the early results from a novel experiment to study a particle-laden flow, under a para... more We present the early results from a novel experiment to study a particle-laden flow, under a parameter regime relevant to the conditions in planet-forming systems. We investigate the gas-particle interactions to identify the presence of and details regarding the streaming instability, which is theoretically predicted to aid the coalescence of small dust grains to form planetesimals - the macroscopic objects that will eventually interact gravitationally and become planets. We vary properties of the system such as dust-to-gas ratio, relative particle-gas velocity and gas pressure, for comparison to numerical simulations of protoplanetary disks. Experimentally calibrated numerical calculations of the particle motion within the instability regions will be used to model the evolution of protoplanetary disks at the scale of small dust grains, representing an unprecedented precision in our understanding of these difficult to study systems.