Soil granular dynamics on-a-chip: fluidization inception under scrutiny

Spatiotemporal Mineral Phase Evolution and Arsenic Retention in Microfluidic Models of Zerovalent Iron-Based Water Treatment

Arsenic (As) is a toxic element, and elevated levels of geogenic As in drinking water pose a threat to the health of several hundred million people worldwide. In this study, we used microfluidics in combination with optical microscopy and X-ray spectroscopy to investigate zerovalent iron (ZVI) corrosion, secondary iron (Fe) phase formation, and As retention processes at the pore scale in ZVI-based water treatment filters. Two 250 μm thick microchannels filled with single ZVI and quartz grain layers were operated intermittently (12 h flow/12 h no-flow) with synthetic groundwater (pH 7.5; 570 μg/L As(III)) over 13 and 49 days. Initially, lepidocrocite (Lp) and carbonate green rust (GRC) were the dominant secondary Fe-phases and underwent cyclic transformation. During no-flow, lepidocrocite partially transformed into GRC and small fractions of magnetite, kinetically limited by Fe(II) diffusion or by decreasing corrosion rates. When flow resumed, GRC rapidly and nearly completely transformed back into lepidocrocite. Longer filter operation combined with a prolonged no-flow period accelerated magnetite formation. Phosphate adsorption onto Fe-phases allowed for downstream calcium carbonate precipitation and, consequently, accelerated anoxic ZVI corrosion. Arsenic was retained on Fe-coated quartz grains and in zones of cyclic Lp-GRC transformation. Our results suggest that intermittent filter operation leads to denser secondary Fe-solids and thereby ensures prolonged filter performance.

Future challenges on focused fluid migration in sedimentary basins: Insight from field data, laboratory experiments and numerical simulations

In a present context of sustainable energy and hazard mitigation, understanding fluid migration in sedimentary basins – large subsea provinces of fine saturated sands and clays – is a crucial challenge. Such migration leads to gas or liquid expulsion at the seafloor, whichmay be the signature of deep hydrocarbon reservoirs, or precursors to violent subsea fluid releases. If the former may orient future exploitation, the latter represent strong hazards for anthropic activities such as offshore production, CO$_2$ storage, transoceanic telecom fibers or deep-sea mining. However, at present, the dynamics of fluid migration in sedimentary layers, in particular the upper 500 m, still remains unknown in spite of its strong influence on fluid distribution at the seafloor. Understanding the mechanisms controlling fluid migration and release requires the combination of accurate field data, laboratory experiments and numerical simulations. Each technique shall lead to the understanding of the fluid structures, the mechanisms at stake, and deep insights into fundamental processes ranging from the grain scale to the kilometers-long natural pipes in the sedimentary layers.Here we review the present available techniques, advances and challenges still open for the geosciences, physics, and computer science communities.

Microfluidics as an Emerging Platform for Exploring Soil Environmental Processes: A Critical Review

Investigating environmental processes, especially those occurring in soils, calls for innovative and multidisciplinary technologies that can provide insights at the microscale. The heterogeneity, opacity, and dynamics make the soil a "black box" where interactions and processes are elusive. Recently, microfluidics has emerged as a powerful research platform and experimental tool which can create artificial soil micromodels, enabling exploring soil processes on a chip. Micro/nanofabricated microfluidic devices can mimic some of the key features of soil with highly controlled physical and chemical microenvironments at the scale of pores, aggregates, and microbes. The combination of various techniques makes microfluidics an integrated approach for observation, reaction, analysis, and characterization. In this review, we systematically summarize the emerging applications of microfluidic soil platforms, from investigating soil interfacial processes and soil microbial processes to soil analysis and high-throughput screening. We highlight how innovative microfluidic devices are used to provide new insights into soil processes, mechanisms, and effects at the microscale, which contribute to an integrated interrogation of the soil systems across different scales. Critical discussions of the practical limitations of microfluidic soil platforms and perspectives of future research directions are summarized. We envisage that microfluidics will represent the technological advances toward microscopic, controllable, and in situ soil research.

The effect of channel aspect ratio on air entrapment during imbibition in soil-on-a-chip micromodels with 2D and 2.5D pore structures

We developed a low-cost method for fabricating "soil-on-a-chip" micromodels with 2D and 2.5D pore structures by stacking layers made with a conventional low-cost tabletop CNC router followed by tape bonding. The pore structure was extracted from an X-ray micro-computed tomography scanning image of a medium-grain sandstone sample. The imbibition experiments performed in the 2D and 2.5D micromodels showed the trends of the residual saturation versus capillary number (Ca). The channels showed opposing trends for low-aspect-ratio 2D and high-aspect-ratio 2.5D micromodels. As the channel aspect ratio increased, the location of air entrapment changed from dead-end pores to transport pores. The sizes of trapped air bubbles in the transport pores decreased as the injection flow rates increased. To show the relationship between the air trapped size and Ca, we derived equations that described the competition between the bulk menisci and the corner flow in the channels for different Ca based on the "supply principle." The relative contributions of the piston displacement and corner film flow, which were dependent on the cross-sectional shapes of the pores and Ca, determined the size and location of the air bubbles trapped in the 2.5D micromodel.

Brownian Granular Flows Down Heaps

We study the avalanche dynamics of a pile of micrometer-sized silica grains in water-filled microfluidic drums. Contrary to what is expected for classical granular materials, avalanches do not stop at a finite angle of repose. After a first rapid phase during which the angle of the pile relaxes to an angle θ_{c}, a creep regime is observed where the pile slowly flows until the free surface reaches the horizontal. This relaxation is logarithmic in time and strongly depends on the ratio between the weight of the grains and the thermal agitation (gravitational Péclet number). We propose a simple one-dimensional model based on Kramers' escape rate to describe these Brownian granular avalanches, which reproduces the main observations.

A large size-selective DNA nanopore with sensing applications

Transmembrane nanostructures like ion channels and transporters perform key biological functions by controlling flow of molecules across lipid bilayers. Much work has gone into engineering artificial nanopores and applications in selective gating of molecules, label-free detection/sensing of biomolecules and DNA sequencing have shown promise. Here, we use DNA origami to create a synthetic 9 nm wide DNA nanopore, controlled by programmable, lipidated flaps and equipped with a size-selective gating system for the translocation of macromolecules. Successful assembly and insertion of the nanopore into lipid bilayers are validated by transmission electron microscopy (TEM), while selective translocation of cargo and the pore mechanosensitivity are studied using optical methods, including single-molecule, total internal reflection fluorescence (TIRF) microscopy. Size-specific cargo translocation and oligonucleotide-triggered opening of the pore are demonstrated showing that the DNA nanopore can function as a real-time detection system for external signals, offering potential for a variety of highly parallelized sensing applications.

Direct observation of Thermomyces lanuginosus lipase diffusional states by Single Particle Tracking and their remodeling by mutations and inhibition

Lipases are interfacially activated enzymes that catalyze the hydrolysis of ester bonds and constitute prime candidates for industrial and biotechnological applications ranging from detergent industry, to chiral organic synthesis. As a result, there is an incentive to understand the mechanisms underlying lipase activity at the molecular level, so as to be able to design new lipase variants with tailor-made functionalities. Our understanding of lipase function primarily relies on bulk assay averaging the behavior of a high number of enzymes masking structural dynamics and functional heterogeneities. Recent advances in single molecule techniques based on fluorogenic substrate analogues revealed the existence of lipase functional states, and furthermore so how they are remodeled by regulatory cues. Single particle studies of lipases on the other hand directly observed diffusional heterogeneities and suggested lipases to operate in two different modes. Here to decipher how mutations in the lid region controls Thermomyces lanuginosus lipase (TLL) diffusion and function we employed a Single Particle Tracking (SPT) assay to directly observe the spatiotemporal localization of TLL and rationally designed mutants on native substrate surfaces. Parallel imaging of thousands of individual TLL enzymes and HMM analysis allowed us to observe and quantify the diffusion, abundance and microscopic transition rates between three linearly interconverting diffusional states for each lipase. We proposed a model that correlate diffusion with function that allowed us to predict that lipase regulation, via mutations in lid region or product inhibition, primarily operates via biasing transitions to the active states.