A microfluidic chip for geoelectrical monitoring of critical zone processes.

We miniaturize geoelectrical acquisition using advanced microfabrication technologies to investigate coupled processes in the critical zone. We focus on the development of the complex electrical conductivity acquisition with the spectral induced polarization (SIP) method on a microfluidic chip equipped with electrodes. SIP is an innovative detection method that has the potential to monitor biogeochemical processes. However, due to the lack of microscale visualization of the processes, the interpretation of the SIP response remains under debate. This approach at the micrometer scale allows working in well-controlled conditions, with real-time monitoring by high-speed and high-resolution microscopy. It enables direct observation of microscopic reactive transport processes in the critical zone. We monitor the dissolution of pure calcite, a common geochemical reaction studied as an analog of the water-mineral interactions. We highlight the strong correlation between SIP response and dissolution through image processing. These results demonstrate that the proposed technological advancement will provide a further understanding of the critical zone processes through SIP observation.

Microfluidic flow-through reactor and 3D Raman imaging for in situ assessment of mineral reactivity in porous and fractured porous media.

An in-depth understanding of dissolution and precipitation of minerals in porous and fractured porous media and the complex feedback on the transport of fluids is essential for various subsurface applications. In this context, we developed a novel non-destructive "lab-on-chip" approach for quantitative in situ assessments of mineralogical changes in porous media. Our experimental approach involves a microfluidic flow-through reactor of reactive homogeneous and heterogeneous (fractured) porous media coupled with high-resolution imaging. Here, the reactive medium consists of compacted celestine grains seeded in a reservoir within the microfluidic chip. This medium reacts with a barium chloride solution injected into the microreactor at a constant flow rate, leading to the dissolution of celestine and growth of barite. Various seeding processes of the mineral grains allow the creation of homogeneous reactive porous media or the introduction of large heterogeneities such as fractures. Hence, our approach enables high-resolution investigations of reactive transport in fractured porous media. The use of confocal Raman spectroscopic techniques enables the spatio-temporal visualization of the mineral transformation at the pore-scale in two- and three-dimensions. Moreover, advanced pore-scale modelling correlates the hydrological heterogeneities to the geochemical observations in the micro-reactor, which explains the observed discrepancies between homogeneous and heterogeneous reactive media. Eventually, the proposed methodology can be applied to other chemical systems to provide new insights into hydro-geochemical coupling in porous and fractured porous media as well as high-fidelity datasets to benchmark reactive transport codes that are currently under development.

Pore-scale visualization and characterization of viscous dissipation in porous media.

HYPOTHESIS: The effects of mutual transfer of momentum between two immiscible flowing fluids in porous media are not well understood nor predictable yet. From considerations at the pore-scale, it should be possible to determine whether and to what extent interfacial viscous coupling effects are significant. EXPERIMENTS: We visualize the velocity distributions inside immobile globules of wetting phase (water) while a non-wetting phase (oil) is injected. We investigate viscous coupling effects and their relationship with the viscosity ratio and the capillary number. FINDINGS: Four regimes of viscous dissipation are identified: (i) a regime for which the fluid-fluid interface acts as a solid wall; (ii) a regime where the wetting phase is dragged in the direction of the imposed flow; (iii) and (iv) two regimes for which the trapped globule of water shows a recirculating motion due to the shear stress at the oil/water interface. We demonstrate the significant role of the lubricating effect and of the topology of the pore space on the magnitude of viscous dissipation. Importantly, for a viscosity ratio close to one and low capillary number, we demonstrate that viscous coupling effects should be incorporated into the existing Darcy's law formulation for two-phase flow in porous media.

Measurements and simulation of liquid films during drainage displacements and snap-off in constricted capillary tubes.

When a wetting liquid is displaced by air in a capillary tube, a wetting film develops between the tube wall and the air that is responsible for the snap-off mechanism of the gas phase. By dissolving a dye in the wetting phase it is possible to relate a measure of the absorbance in the capillary to the thickness of liquid films. These data could be used to compare with cutting edge numerical simulations of the dynamics of snap-off for which experimental and numerical data are lacking. Drainage experiments in constricted capillary tubes were performed where a dyed wetting liquid is displaced by air for varying flow rates. We developed an optical method to measure liquid film thicknesses that range from 3 to 1000µm. The optical measures are validated by comparison with both theory and direct numerical simulations. In a constricted capillary tube we observed, both experimentally and numerically, a phenomenon of snap-off coalescence events in the vicinity of the constriction that bring new insights into our understanding and modeling of two-phase flows. In addition, the good agreement between experiments and numerical simulations gives confidence to use the numerical method for more complex geometries in the future.

Creation of a dual-porosity and dual-depth micromodel for the study of multiphase flow in complex porous media.

Silicon-based microfluidic devices, so-called micromodels in this application, are particularly useful laboratory tools for the direct visualization of fluid flow revealing pore-scale mechanisms controlling flow and transport phenomena in natural porous media. Current microfluidic devices with uniform etched depths, however, are limited when representing complex geometries such as the multiple-scale pore sizes common in carbonate rocks. In this study, we successfully developed optimized sequential photolithography to etch micropores (1.5 to 21 µm width) less deeply than the depth of wider macropores (>21 µm width) to improve the structural realism of an existing single-depth micromodel with a carbonate-derived pore structure. Surface profilimetry illustrates the configuration of the dual-depth dual-porosity micromodel and is used to estimate the corresponding pore volume change for the dual-depth micromodel compared to the equivalent uniform- or single-depth model. The flow characteristics of the dual-depth dual-porosity micromodel were characterized using micro-particle image velocimetry (µ-PIV), relative permeability measurements, and pore-scale observations during imbibition and drainage processes. The µ-PIV technique provides insights into the fluid dynamics within microfluidic channels and relevant fluid velocities controlled predominantly by changes in etching depth. In addition, the reduction of end-point relative permeability for both oil and water in the new dual-depth dual-porosity micromodel compared to the equivalent single-depth micromodel implies more realistic capillary forces occurring in the new dual-depth micromodel. Throughout the imbibition and drainage experiments, the flow behaviors of single- and dual-depth micromodels are further differentiated using direct visualization of the trapped non-wetting phase and the preferential mobilization of the wetting phase in the dual-depth micromodel. The visual observations agree with the relative permeability results. These findings indicate that dual-porosity and dual-depth micromodels have enhanced physical realism that is pertinent to oil recovery processes in complex porous media.

Going beyond 20 µm-sized channels for studying red blood cell phase separation in microfluidic bifurcations.

Despite the development of microfluidics, experimental challenges are considerable for achieving a quantitative study of phase separation, i.e., the non-proportional distribution of Red Blood Cells (RBCs) and suspending fluid, in microfluidic bifurcations with channels smaller than 20 µm. Yet, a basic understanding of phase separation in such small vessels is needed for understanding the coupling between microvascular network architecture and dynamics at larger scale. Here, we present the experimental methodologies and measurement techniques developed for that purpose for RBC concentrations (tube hematocrits) ranging between 2% and 20%. The maximal RBC velocity profile is directly measured by a temporal cross-correlation technique which enables to capture the RBC slip velocity at walls with high resolution, highlighting two different regimes (flat and more blunted ones) as a function of RBC confinement. The tube hematocrit is independently measured by a photometric technique. The RBC and suspending fluid flow rates are then deduced assuming the velocity profile of a Newtonian fluid with no slip at walls for the latter. The accuracy of this combination of techniques is demonstrated by comparison with reference measurements and verification of RBC and suspending fluid mass conservation at individual bifurcations. The present methodologies are much more accurate, with less than 15% relative errors, than the ones used in previous in vivo experiments. Their potential for studying steady state phase separation is demonstrated, highlighting an unexpected decrease of phase separation with increasing hematocrit in symmetrical, but not asymmetrical, bifurcations and providing new reference data in regimes where in vitro results were previously lacking.

Velocimetry of red blood cells in microvessels by the dual-slit method: effect of velocity gradients.

The dual-slit is a photometric technique used for the measurement of red blood cell (RBC) velocity in microvessels. Two photometric windows (slits) are positioned along the vessel. Because the light is modulated by the RBCs flowing through the microvessel, a time dependent signal is captured for each window. A time delay between the two signals is obtained by temporal cross correlation, and is used to deduce a velocity, knowing the distance between the two slits. Despite its wide use in the field of microvascular research, the velocity actually measured by this technique has not yet been unambiguously related to a relevant velocity scale of the flow (e.g. mean or maximal velocity) or to the blood flow rate. This is due to a lack of fundamental understanding of the measurement and also because such a relationship is crucially dependent on the non-uniform velocity distribution of RBCs in the direction parallel to the light beam, which is generally unknown. The aim of the present work is to clarify the physical significance of the velocity measured by the dual-slit technique. For that purpose, dual-slit measurements were performed on computer-generated image sequences of RBCs flowing in microvessels, which allowed all the parameters related to this technique to be precisely controlled. A parametric study determined the range of optimal parameters for the implementation of the dual-slit technique. In this range, it was shown that, whatever the parameters governing the flow, the measured velocity was the maximal RBC velocity found in the direction parallel to the light beam. This finding was then verified by working with image sequences of flowing RBCs acquired in PDMS micro-systems in vitro. Besides confirming the results and physical understanding gained from the study with computer generated images, this in vitro study showed that the profile of RBC maximal velocity across the channel was blunter than a parabolic profile, and exhibited a non-zero sliding velocity at the channel walls. Overall, the present work demonstrates the robustness and high accuracy of the optimized dual-slit technique in various flow conditions, especially at high hematocrit, and discusses its potential for applications in vivo.