Session Z23: Porous Media Flows: Immiscible Displacements

Diffusiophoretic transport of colloids in periodic porous media

Colloids can move in the presence of solute gradients; this phenomenon is known as diffusiophoresis. Recent studies have demonstrated the utility of diffusiophoresis in manipulating and steering colloids using simple microfluidic geometries and in the absence of any background flows. Yet, it remains a question whether diffusiophoresis could play an important role in more complex environments such as in porous media, where background fluid flows are often present. Here, combining experimental observations and numerical simulations of microfluidic channels patterned with an ordered lattice of obstacles, we study the competition between phoretic and convective migration of colloids. 

    

Quantifying the Flow Dynamics of Water-Air Flow in Dual-Porosity Micromodels Using Micro-PIV

Multiphase flow in porous media is central to a broad range of natural and engineering applications, including oil recovery, CO₂ storage, and critical zone science. Many of these porous solid matrices display multi-scale variability in pore structure and physical properties such as porosity and permeability. For instance, in critical zone, soil is often viewed as a hierarchical organization: primary particles form aggregates, which in turn form macroaggregates, effectively leading to a dual-porosity medium. The resultant multi-scale flow dynamics and inter-/intra-aggregate interaction in this system are recognized to control numerous processes, such as water and gaseous transport. However, the underlying physics is not well understood from a fluid mechanics perspective. To that end, the multi-phase flow of air and water is investigated using a novel 2D dual-porosity microfluidic device, with a focus on the multi-scale interaction and the role of corner film flows. The microfluidic device, constructed in a glass-silicon-glass architecture, offers precise structure and excellent optical access. The micro-PIV measurement provides valuable insight into the flow dynamics at both micro- and macro-scales simultaneously enabled by an innovative dual-magnification imaging system.   

Impact of Medium Wettability in Geometrical Valving Based on Immiscible Displacement in Porous Media

Controlling the flow and valving in microfluidic devices where external small fluctuations can trigger a cascade of unpredicted events is a challenging task in experiments. Based on concepts of capillary displacement in porous media, we design a microfluidic valve that consists of arrays of parallel triangles. Flow is allowed in one direction and prohibited in the opposite direction due to capillary effects. Previous simulations suggest a strong valving effect over a considerable range of Capillary numbers. Here, we control the wettability of the medium and study the applicability of this geometrical valve for different physical conditions experimentally. Our results show a strong effect of medium wettability on valving between water and air even at small capillary numbers. A hydrophobic medium inhibits flow over a large range of imposed pressure on the water while an extremely hydrophilic medium almost never valves. Interestingly, in the regime of intermediate wettability, we observe efficient valving over a considerable range of contact angles.   

Effect of Capillary Number on Drainage from Microscale Sinusoidal Pores

Understanding the dynamics of fluid transport and trapping within micro-scale pores is important for a range of environmental, industrial, and research applications. Prior work has used porous media micromodels containing 2D or 3D networks of obstacles to investigate drainage of a trapped phase being displaced by another immiscible fluid. Here, we explore liquid drainage/entrapment in microscale pores in a quasi-1D system at a range of capillary numbers with water as a displacing fluid, fluorinated oil as the (wetting) trapped phase, and PDMS microfluidics patterned with sinusoidal pores as the solid matrix. We find that higher velocity flow leads to a greater amount of oil trapped within each pore. This is in contrast with findings from 2D and 3D micromodels in which the availability of multiple flow pathways leads to less drainage at lower flow rates. We also discuss the role of geometry and aspect ratio on oil drainage, and show two limiting behaviors. At lower flow rates and greater aspect ratio between pore wavelength to amplitude, oil is completely displaced. At larger flow rates, there is a geometry-dependent critical flow rate at which the trapped oil phase forms a continuous thin film across the geometry rather than snapping-off into discrete pockets within the pores.   

Similarity characteristics in the growth of radial fingering in a Hele-Shaw

We conduct nonlinear simulations based on a boundary integral method to describe the radial growth of viscous fingers in a Hele-Shaw. The system is governed by two dimensionless parameters, the viscosity contrast, A, and an effective surface tension, B. Most numerical investigations of the problem have analyzed the case in which the viscosity contrast is large, fixed at A=1. Experimentally, this corresponds to having air as the displacing fluid. By exploring a wide range of parametric values, we find that for smaller values of the viscosity contrast, which are typical of the displacement between two immiscible liquids, the effective surface tension behaves as a similarity parameter. As such, similar morphological patterns are obtained as B-values are increased, but these patterns are larger and take longer to develop than their lower-value counterparts. These numerical results are compared to experiments.

    

Impact of interfacial adsorption of nanoparticles on transport properties

We study the transport and adsorption of nanoparticles in partially saturated porous media experimentally. Using fluorescent microscopy in two-dimensional model porous media we measure the distribution and breakthrough of nanoparticles at small flow rates. Our results show that at large Peclet numbers, nanoparticles primarily follow the major flow paths while they tend to diffuse close to the interfaces and solid structure at small Peclet numbers. Comparing the breakthrough of nanoparticles in a fully saturated porous medium with a medium partially saturated with a second immiscible fluid shows that by increasing the number of interfaces a smaller number of nanoparticles are discharged from the medium. The impact of interfacial adsorption of nanoparticles strongly depends on the nanoparticle charges and wettability of the medium.   

An Experimental Study of Multiphase Flow in Deforming Porous Media Subject to Dissolution

Multi-phase flow in porous media is pervasive in natural and engineering processes such as enhanced oil recovery (EOR) and carbon capture and sequestration (CCS). From a fundamental fluid mechanics perspective, such a flow system has also been an excellent example problem to study instability and percolation theories. To make this problem even more challenging and intriguing, dissolution of the solid matrix can occur in many scenarios, effectively modifying the physical and hydrological properties of the solid structures, which in turn provides a feedback to the flow. However, our fundamental understanding of this coupling effect is still limited. To that end, multiphase flow of aqueous acid and gas is studied using novel calcite-based microfluidic channels. The microfluidic devices used in the experiments were fabricated in calcite using photolithography and wet etching, which offers precise control over their structural and chemical properties and unaberrated optical access to the microscale flow. These experiments provide a unique picture of the flow dynamics in such a complex system as well as develop correlations between pore-scale flow and dissolution rates under various flow conditions.   

Immiscible fluid displacement in a porous medium

Understanding the physics of immiscible fluid displacement within porous media is of great

importance in many fields of study, for example, geological CO2 sequestration, hydrology, and enhanced oil recovery. In these applications, it is important to understand the fundamental laws of immiscible fluid displacement in the complex pore network to optimize the fluid entrapment depending on the application. In this study, we investigate experimentally the fluid entrapment process in an idealized model of the porous media called pore-doublet model for a fluid pair of silicone oil (as the wetting fluid) and water (as the non-wetting fluid). Specifically, we study the drainage process of non-wetting fluid displacing the wetting fluid for a Weber number (based on the height of the channel) of 0.0003 to 0.03 for three different pore geometries. It was found that increasing the Weber number leads to increased entrapped volume of the wetting fluid in the pore doublet model. However, beyond a critical Weber number of ~0.007, the displacing fluid pierces through the pockets of the entrapped fluid leading to an instability of the trapped fluid. For these cases, the trapped oil is washed off almost completely. The findings from this study will be useful not only for understanding the fundamental laws of these complex flows, but also for improving the accuracy of predictive numerical models for which resolution of contacts, thin films and contact line motions is an ongoing challenge.

    

Going with the flow: colloidal dynamics at moving immiscible fluid interfaces

The presence of colloidal particles in geological media from diffusion of naturally occurring sediment and migration of microplastics can alter immiscible fluid displacement patterns that are central to enhanced oil recovery (EOR) and groundwater remediation. To understand the impact of deposited colloidal particles on immiscible flows, we visualize the interactions between colloidal deposits and moving immiscible fluid droplets in microchannels and develop a pore network model to predict the macro-scale implications of this process. As an immiscible fluid interface passes over particles, we observe that they strongly adsorb to it due to the influence of capillary forces exerted by the fluid interface as it impinges on particles. We see that the surface coverage of the interface increases with time as the fluid droplet traverses the channel, eventually reaching a finite “carrying capacity,” after which it must continually slough off excess particles. Two-dimensional simulations reveal striking deviations from immiscible fluid displacement via standard capillary fingering. Surprisingly, under certain deposition and erosion conditions, the immiscible fluid explores a larger pore space volume than what it can access through capillary fingering alone, establishing patterns that we term “erosion-enhanced fingering.”

    

Breakup of gas bubbles during solution-gas-drive in porous media using experiments and simulations

Solution-gas-drive is one of the main recovery mechanisms for oil reservoirs. This mechanism is initiated after the reservoir pressure drops below the bubble point, and gas molecules start to nucleate inside the pores. Our experiments on microfluidic chips show that growing bubbles undergo breakup (Rayleigh-Plateau) instability if some amount of pressure gradient is sustained between both ends of the porous medium. In this regime, we observe that exsolved gas bubbles are swept out of the system by the flowing liquids due to the pressure gradient. To understand the conditions leading to this breakup instability, we built a three-dimensional direct numerical simulation model of the multiphase flow in a representative T-junction geometry, where we observe bubbles breaking up experimentally. Using this numerical model we performed a parameter sweep that includes pressure gradient, length scale of the geometry along with various material properties including surface tension, contact angle, viscosity, and density. As a result of this effort, we built a phase diagram using two dimensionless numbers including the relevant parameters to evaluate if a given set of conditions would result in bubble breakup.