Low Viscosity Ratio Research Articles

A numerical investigation to assess changes to displacement front and by-passed zones employing kinetic interface-sensitive tracer

An important aspect in groundwater remediation is to understand changes of multiphase fluid front morphology and stagnant regions on macro scale. However, the prediction of those changes during two-phase flow remains a challenging task due to the interplay of various physical factors. Recent laboratory experiments have demonstrated tracers' ability to predict deformation in the front of a two-phase flow system by utilizing a new reactive tracer known as, the kinetic interface sensitive tracer (KIS). This research employs a reactive transport model coupled with a macro-scale two-phase flow model to numerically analyse how viscosity ratio, capillary number, and heterogeneities on the tracer's signal and its impact the frontal deformation. One homogeneous and two heterogeneous types of porous media are considered. The background porous medium is a fine-grained, low-permeability medium, with a coarser, high-permeability lenses, generating heterogeneous material properties. The high-permeability lenses account for 25 % of the total model area and are arranged in either periodic or random patterns. The findings are evaluated using four parameters (effective front length, swept area, front roughness, and transition zone length). The flow patterns dominating the shape of the front are characterized by the viscous and capillary forces i.e. capillary number and the viscosity ratio between the two fluids. The results show that changes in flow regimes can be quantified using effective front length, thus employing the effective front length the viscous fingering regions can be quantified. Furthermore, front roughness and transition zone length are extracted and their relevance to the by-passed zones is presented. The slope of the reactive KIS tracer breakthrough curve, plotted on a phase diagram, can also be used to predict the existence of the by-passed zones for a low viscosity ratio. Finally, changes in front roughness and transition zone length induced by the inclusions are correlated to the slope of the KIS tracer BTC. The findings of this study can contribute to a better understanding of the impact of different flow regimes on the KIS tracer breakthrough signals and the linkages between the tracer signals and the front sizes.

Asymmetric droplet splitting in a T-junction under a pressure difference

In this paper, the phase-field multiphase lattice Boltzmann method is employed to simulate droplet breakup in a T-junction under different outlet pressures. Three behaviors of droplet breakup: non-breakup (flow pattern I), breakup with tunnels (flow pattern II), and breakup with permanent obstruction (flow pattern Ⅲ) are identified. The evolution of morphological characteristics of droplet breakup is quantitatively characterized, based on which the asymmetric splitting mechanisms and the influencing factors are clarified. Additionally, the factors influencing the droplet splitting volume ratio (VII/VI) are elucidated. The results indicate that there is a non-linear relationship between the VII/VI and the flow rate ratio. Moreover, the curve depicting the final VII/VI versus the initial droplet length exhibits a V-shape and has a minimum value. A conclusion is drawn that the Capillary number mainly influences flow pattern II, with the final VII/VI decreasing as the Capillary number increases. Additionally, for flow pattern III, the final VII/VI increases linearly with rising droplet size at low viscosity ratios, whereas it decreases linearly at high viscosity ratios. The growing outlet pressure difference enlarges the flow difference between the two branches, leading to an increase in the final VII/VI.

Does dispersed phase inertia affect the shape of sheared emulsion droplets?

Inertial effects on sheared emulsion droplets are a topic of scientific and industrial interest for several applications from processing to microfluidics. Most of the literature have addressed so far the role of inertia of the continuous phase, which is known to affect shear-induced droplet deformation and migration at values of the Reynolds number of the external fluid Rec > 1. However, less attention has been paid to the case of inertial effects inside the droplets, corresponding to values of the Reynolds number of the droplet fluid Red > 1. Such a case is especially relevant when the viscosity ratio λ between the droplet and the external fluid is ≪ 1, which is typical of water-in-oil emulsions where the low values of droplet viscosity can result in Red > 1, while Rec < 1 due to the larger oil viscosity. Here, we focus on the effect of droplet inertia under shear flow at λ ≪ 1 by high-speed video microscopy experiments in a microcapillary and by numerical simulations based on a front-tracking finite-difference method. The results unveil the droplet's three-dimensional shape under shear flow at low viscosity ratios and show that droplet inertia tends to increase droplet deformation and orientation along the flow direction and to form two vortices inside the droplets even at small Rec. The latter findings are at variance with the case of external fluid inertia, where droplets become more aligned with the velocity gradient direction.

Numerical Simulation of the Stability of Low Viscosity Ratio Viscoelastic Lid-Driven Cavity Flow Based on the Log-Conformation Representation (LCR) Algorithm

Numerical Simulation of the Stability of Low Viscosity Ratio Viscoelastic Lid-Driven Cavity Flow Based on the Log-Conformation Representation (LCR) Algorithm

NUMERICAL MODELING OF BUBBLE DYNAMICS USING INTERFACE CAPTURING METHOD

In the present numerical study, a dynamics of single gas bubble (circular in 2D and spherical in 3D) rising in a stagnant viscous liquid due to the buoyancy is presented using various volume of fluid (VOF) based computational fluid dynamics (CFD) solvers such as commercial Converge and Star CCM+, and open source OpenFOAM<sup>®</sup> platform. To capture the interface dynamics, either an interpolated curved interface based on the high-resolution interface framework or a mass conservative VOF approach with a planar sharp interface based geometric reconstruction of the piecewise-linear interface calculation (PLIC) scheme was used. Both qualitative and quantitative analysis of air bubble rising upward inside the quiescent water column at ratios of low density, ρ<sub>r</sub> = 10 and high density, ρ<sub>r</sub> = 1000 are simulated and evaluated similar to report by Hysing et al. The proposed numerical models can simulate a wide range of density and viscosity ratios. In this study, a robustness and accuracy of the solvers are evaluated and comparative study between open source OpenFOAM<sup>®</sup> solver with commercial solvers such as Converge and Star CCM+. Based on the present numerical results, the gas bubble base undergoes severe deformations for the high density ratio, ρ<sub>r</sub> = 1000 and high viscosity ratio, μ<sub>r</sub> = 100 compared to low density ratio, ρ<sub>r</sub> = 10 and low viscosity ratio, μ<sub>r</sub> = 10. Any of the solvers can be used to simulate complex multiphase flow situations encountered in many industrial applications.

Effects of viscosity ratio and surface wettability on viscous fingering instability in rectangular channel

This paper explores the impact of viscosity ratio and surface wettability on immiscible viscous fingering instability within a rectangular channel. Numerical investigations are conducted across a range of viscosity ratios (VR) from 0.0009 to 0.5 and wall wettability ([Formula: see text]) from 15° to 150°. The volume of fluid (VOF) model is employed to track the development of finger-shaped instability at the fluid interface. Our results indicate that higher viscosity ratios lead to increased displacement efficiency. Additionally, we find the formation of necking at low VR, which diminishes at higher VR values. The finger-shaped pattern splits into two parts at a wettability of 15°; beyond this threshold, no such splitting occurs. Furthermore, a transition from hydrophilic to superhydrophobic wettability abolishes necking, resulting in enhanced displacement efficiency. Notably, as wettability shifts from hydrophilic to super hydrophobic, instability shifts toward the left side. These findings hold relevance for applications in drug delivery, clinical processes and oil recovery.

Study on effect of viscosity on thickness of boundary layer in tight reservoir

It has become a consensus that viscosity has an important impact on boundary layer of tight reservoirs. However, there are few quantitative studies on the effect of viscosity on boundary layer by core experiments. NMR analysis based on high-speed centrifugal gas-water and gas-oil displacement and low-temperature adsorption experiment were implemented in this study on typical tight reservoir parallel cores of Ordos and Bohai Bay basins. Total bound water/oil saturation, specific surface area and ratio of micro pore of the reservoir quantitatively were finally obtained, which were used for calculating irreducible water/oil film thickness of reservoir and analyzing influence of viscosity on movable oil and boundary layer. Research show that, movable oil saturation of core saturated high viscosity oil is 23.92% and 18.61% lower than that of core saturated low viscosity oil and water respectively, and effective seepage space is greatly reduced. The lower permeability reservoir has, the greater reduction movable space with high viscosity oil of reservoir owns. The average bound water film thickness of Ordos and Bohai Bay basin cores is 17.74 nm and 16.99 nm, respectively. The average bound oil film boundary of core in Ordos Basin is 20.72 nm. The average thickness of high viscous oil film of core in Bohai Bay basin is 38.10 nm, and the thickness of low viscosity oil film is 18.80 nm. The oil-water viscosity ratio in Ordos Basin is small, and oil-water film thickness is close, indicating that fluid viscosity has little influence on boundary layer thickness of target reservoir. The low viscosity oil-water viscosity ratio in Bohai Bay basin is small, and oil-water film thickness is close. The thickness of high viscosity oil film is more than twice that of low viscosity oil film and water film. When the oil viscosity is higher than 8.9 mPa s, thickness of boundary layer will be greatly affected, leading to a significant reduction of effective seepage throat, a significant increase of seepage resistance and development difficulty.

Preparation of PP/PC light‐diffusing materials with UV‐shielding property

AbstractIn this study, we presented a one‐step melt‐blending method to fabricate polypropylene (PP)/polycarbonate (PC) light‐diffusing materials. A promising balance between haze and transmittance was obtained by the addition of small amount of PC to the matrix. On the basis of Mie scattering theory and morphology analysis, the correlation between processing conditions and light‐diffusing properties was built. Low rotor speed and high viscosity ratio were beneficial to optical properties of PP/PC composites. To extend the advanced application, two general anti‐ultraviolet additives nanoscale titanium dioxide (Nano‐TiO2) and 2‐(2‐hydroxy‐5‐methylphenyl) benzotriazole (UV‐P) were added to impart UV‐shielding property to the composites. Their specific influences on the morphology and comprehensive properties of PP/PC composites were studied. It was found that the introduction of Rayleigh scattering raised by Nano‐TiO2 could strengthen the scattering effect and cause a significant reduction in light transmittance, while UV‐P had little effect on the optical properties of the light‐diffusing composites. In summary, this study prepared the PP/PC composites with high‐performance light scattering and UV‐shielding properties applying a facile blending method, which showed a potential application in flexible lightings and LCD displays.

Head-On Collision of Dissimilar Viscosity Drops.

The head-on collision of drops is governed by the interfacial tension, viscosity, and inertia of the impacting drops. Earlier studies show that depending on the relative magnitude of these forces, the outcome of a head-on collision of two identical drops of the same liquid is likely to culminate in coalescence or reflexive separation. In this study, the head-on collision of drops of miscible liquids having dissimilar viscosity has been investigated numerically. As the two drop liquids are miscible, it is anticipated that the average viscosity of the two liquids will replicate the transition boundaries of coalescence and reflexive separation for a single fluid. However, numerical simulations reveal that this is true only for low-viscosity ratios. A high-viscosity ratio creates asymmetric flow; hence, the average viscosity does not accurately represent the local viscous effect. The asymmetric flow also facilitates the pinch-off of a thread without the separation of a satellite. The present investigation reveals that viscosity contrast leads to two additional outcomes of the head-on collision of drops: encapsulation and crossing separation. We have built a phase diagram identifying the outcome of a head-on collision of dissimilar viscosity drops on the viscosity ratio (μr)-Weber number (We) plane based on the results of approximately 450 simulations.

Pore scale insights into the role of inertial effect during the two-phase forced imbibition

Fluid inertia could significantly alter the flow paths of fluids in multiphase flow. This study aims to quantify the role of the inertia effect in fluid flow through porous media using the modified color-gradient lattice Boltzmann method. Specifically, we focused on understanding the effects of fluid inertia on two-phase distribution, pore scale events, displacement patterns, transient behaviors of fluids, and the consumption of thermodynamic energy. Results show that the effect of fluid inertia on the two-phase distribution and the final invading wetting saturation is trivial under a low viscosity ratio. However, as the viscosity ratio increases, the inertia effect becomes more pronounced and cannot be ignored in the displacement process. This observation can also be found from the dependence of the invading saturation on the injected volume when the viscosity ratio and the Ohnesorge (Oh) number are varied. In addition, we found that the contribution from the post-breakthrough stage to the total invading saturation is increasingly significant with the increasing viscosity ratio, and its contribution varies depending on the fluid inertia for a given viscosity ratio. We also used the pressure difference between the inlet and outlet to identify the direction of energy conversion in multiphase flow. It is found that for the high viscosity ratio, there is a positive relationship between the dissipated energy and the invading saturation, and both exhibit a non-monotonic change with increasing Oh. However, for low viscosity ratios, fluid inertia does not significantly affect forced imbibition. In addition, the nonwetting phase becomes less connected with the injected wetting fluid, and there exists an almost linear relationship between the invading wetting saturation and the Euler characteristic, irrespective of the Oh numbers for a given viscosity ratio. The mechanism of inertial effect is that the local fluid velocity can be several orders of magnitude larger than the mean velocity despite the low capillary number, which leads to frequent release of surface energy and notably alters the flow paths, particularly under the unfavorable displacement pattern.

Monomethyl branched-chain fatty acids enriched bacterial oil production by furan aldehydes tolerant halophile Lentibacillus salarius strain BPIITR using non-detoxified sugarcane bagasse

The structural diversity of monomethyl branched-chain fatty acids (mBCFAs) expanded their application in biolubricants, biofuels for enhancing cold flow and thermo-oxidative properties. Current study focuses on mBCFAs production from sugarcane bagasse hydrolysate in biorefinery approach with halophilic Lentibacillus salarius BPIITR. Halophilic bacterium exhibited tolerance towards furan aldehydes up to 150 mM in minimal medium and produced 3.40 ± 0.13 and 2.47 ± 0.15 gL−1 lipid rich in mBCFAs, in xylose and glucose rich non-detoxified hydrolysate, respectively at bench-scale bioreactor. In addition, 2,5-furandicarboxylic acid and 2-furancarboxylic acids were co-produced as value-added products up to 41.34 ± 4.73 and 59.84 ± 5.17 mM, respectively. The biosynthesized bacterial oil exhibited onset oxidation temperature of 319.5 °C and low temperature viscosity ratio of 2.92. The accumulated lipid was rich in triacylglycerol content more than 67 % with 12-methyl tetradecanoic acid as major fatty acid.

Effects of injection directions and boundary exchange times on adaptive pumping in heterogeneous porous media: Pore-scale simulation

Adaptive pumping, changing pumping rates or exchanging injection and extraction wells, is an enhancement of traditional Pump-and-Treat (P&T) technology. Since most previous studies on adaptive pumping are conducted through field-scale simulations, the mechanism behind it is not fully understood. An in-depth investigation of the pore-scale remediation mechanism of adaptive pumping is undoubtedly helpful in combining it with other decontamination methods to further enhance the remediation efficiency. In this study, coupling the Cahn–Hilliard phase field method and the Navier-Stokes equations, the dynamic displacement process in a heterogeneous porous medium is obtained. The effects of initial injection direction, boundary exchange times, and displacement regimes on the interface evolution and the remediation efficiency are systematically investigated. The results present that a significant increase in phase interface area is the most critical remediation mechanism for adaptive pumping. The effects of injection directions and boundary exchange times on remediation performance are mainly determined by the differences in pore connectivity and flow parameters. Higher pore connectivity under high and low viscosity ratios inhibits and promotes remediation performance, respectively. At high viscosity ratios, the residual oil morphology in the matrix after adaptive pumping is similar to that obtained by positive pumping with the opposite initial injection direction. The improvement in remediation performance of adaptive pumping is more significant under low viscosity ratio conditions. These results provide new pore-scale insights into the remediation mechanism of adaptive pumping, which contribute to the design and application of innovative remediation methods.

Pore-Scale Numerical Simulation of CO2–Oil Two-Phase Flow: A Multiple-Parameter Analysis Based on Phase-Field Method

A deep understanding of the pore-scale fluid flow mechanism during the CO2 flooding process is essential to enhanced oil recovery (EOR) and subsurface CO2 sequestration. Two-phase flow simulations were performed to simulate the CO2 flooding process based on the phase-field method in this study. Two-dimensional models with random positions and sizes of grains of circular shape were constructed to reproduce the topology of porous media with heterogeneous pore size distributions in the reservoir rock. A multiple-parameter analysis was performed to investigate the effects of capillary number, viscosity ratio, wettability, density, gravity, interfacial tension, and absolute permeability on the two-phase fluid flow characteristics. The results indicated that when the capillary number and viscosity ratio were large enough, i.e., log Ca = −3.62 and log M = −1.00, the fingering phenomenon was not obvious, which could be regarded as a stable displacement process. CO2 saturation increased with the increase in the PV value of the injected CO2. Once the injected CO2 broke through at the outlet, the oil recovery efficiency approached stability. Two types of broken behaviors of the fluids were observed during the wettability alternation, i.e., snap-off and viscous breakup. Snap-off occurred when capillary forces dominated the fluid flow process, while viscous breakup occurred with a low viscosity ratio. With a low capillary number, the flooding process of the injected CO2 was mainly controlled by the capillary force and gravity. With the decrease in the interfacial tension between the fluids and the increase in the permeability of the porous media, the recovery of the displaced phase could be enhanced effectively. In the mixed-wet model, with the increase in the percentage of the nonoil-wetted grains, the intersecting point of the relative permeability curve moved to the right and led to a higher oil recovery.

Prediction of spontaneous imbibition with gravity in porous media micromodels

In this work, theoretical modelling, quasi-three-dimensional (quasi-3D) simulations and micromodel experiments are conducted to study spontaneous imbibition with gravity in porous media micromodels. By establishing the force balance governing the spontaneous imbibition process, we develop a theoretical model for predicting the imbibition length against time in a rectangular capillary. The theoretical model is then extended to the prediction of a compact displacement process in a micromodel by using an equivalent width, which is derived by analogising the micromodel to a rectangular capillary. By simulating spontaneous imbibition in a rectangular capillary with various aspect ratios ( $\varepsilon$ ), we show that the application condition of the quasi-3D method is $\varepsilon \leqslant 1/3$ . Next, we simulate spontaneous imbibition in micromodels with various geometries and flow conditions. Fingering and compact displacement are identified for varying viscosity ratios and gravitational accelerations. At low (high) viscosity ratio of wetting to non-wetting fluids, an upward (downward) gravity can promote the stability of the wetting front, favouring the transition from fingering to compact displacement. In addition, we find that the depth-oriented interface curvature dominates the capillary effect during the imbibition, and such a mechanism is considered by introducing an equivalent contact angle into the theoretical model. With the help of equivalent width and contact angle, the theoretical model is shown to provide satisfactory prediction of the compact displacement process. Finally, a micromodel experiment is presented to further verify the developed theoretical model and the quasi-3D simulation.

Competition between main meniscus and corner film flow during imbibition in a strongly wetting square tube

Imbibition in a strongly wetting square tube with corner flow can be described by an interacting capillary bundle model, where the first sub-capillary describes the main meniscus flow while the others describe corner film flow. In this work, an advanced modified interacting capillary bundle model (MICBM) is developed to simulate imbibition dynamics in a strongly wetting square tube incorporating the viscous coupling effect. The flow conductances of each sub-capillary are obtained from two-phase lattice Boltzmann simulations with different viscosity ratios considering the viscous coupling effect. After verifying its accuracy, MICBM is used to analyze the imbibition dynamics of main meniscus and corner film flows under different conditions. The results show that, with increasing viscosity ratio between wetting and non-wetting fluids and increasing driving force, the wetting corner film development tends to be less significant compared with main meniscus flow. Interestingly, the corner film length first increases then decreases when the wetting fluid is less viscous than the non-wetting fluid in a long square tube. A phase diagram of dimensionless corner film length versus driving force and viscosity ratio is proposed to characterize the competition between main meniscus and corner film flow. Gravity and smaller contact angle make the corner film development more obvious. In addition, the tube length and width are shown to influence the interaction between main meniscus and corner film flow for a low viscosity ratio. The underlying physics of the above phenomena are explained in detail.

Identification and Mapping of Three Distinct Breakup Morphologies in the Turbulent Inertial Regime of Emulsification—Effect of Weber Number and Viscosity Ratio

Turbulent emulsification is an important unit operation in chemical engineering. Due to its high energy cost, there is substantial interest in increasing the fundamental understanding of drop breakup in these devices, e.g., for optimization. In this study, numerical breakup experiments are used to study turbulent fragmentation of viscous drops, under conditions similar to emulsification devices such as high-pressure homogenizers and rotor-stator mixers. The drop diameter was kept larger than the Kolmogorov length scale (i.e., turbulent inertial breakup). When varying the Weber number (We) and the disperse-to-continuous phase viscosity ratio in a range applicable to emulsification, three distinct breakup morphologies are identified: sheet breakup (large We and/or low viscosity ratio), thread breakup (intermediary We and viscosity ratio > 5), and bulb breakup (low We). The number and size of resulting fragments differ between these three morphologies. Moreover, results also confirm previous findings showing drops with different We differing in how they attenuate the surrounding turbulent flow. This can create ‘exclaves’ in the phase space, i.e., narrow We-intervals, where drops with lower We break and drops with higher We do not (due to the latter attenuating the surrounding turbulence stresses more).

X-Ray Imaging of Immiscible Fluid Fingering Patterns in a Natural High Porosity Rock

This paper presents the development of a laboratory scale apparatus and first experimental results on the characterization of fingering patterns of immiscible fluids in a porous rock (Fontainebleau sandstone), using three dimensional full-field measurements from x-ray tomography. The few existing studies that have extended experimental investigation of immiscible fluid flow from 2D to 3D have been primarily interested in the pore scale or performed on idealized porous media. While the heterogeneities inherent to natural rocks are known to play an important role on subsurface fluid flow regimes, a limited number of studies have approached the problem of characterizing the time resolved 3D multiphase flow in these material, at the mesoscale. The series of experiments reported in this paper has been performed at a low viscosity ratio, water invasion into oil as the defending fluid, and different capillary numbers (1.8 orders of magnitude). The results illustrate the qualitative transition in the flow regime, from capillary fingering to viscous fingering. While a full quantitative characterization of geometrical features of fluid fingers will require further technical refinements, a qualitative understanding can be already gathered from the results presented herein.

Study of Rayleigh-Taylor instability in viscosity-stratified fluid layers

The Rayleigh-Taylor instability in viscosity varying fluid layers is studied. A heavier fluid layer is superimposed on a lighter fluid layer. Both the fluid layers are bounded between two parallel isothermal horizontal walls. Out of the two walls, one wall is isothermally heated, while the other wall is isothermally cooled. The Rayleigh-Taylor instability is studied for axisymmetric configuration. Viscosities of both the fluids are considered to be varying with temperature. The effect of viscosity ratio, temperature ratio, heating location, Prandtl number and Weber number on the instability is studied. When the viscosity of fluid layers vary with temperature, the configuration becomes more unstable compared to that for constant viscosity fluid layers. For varying viscosity fluid layers the spike undergoes large deformations. At lower viscosity ratios, mushroom shaped spike is formed with elongated skirt structure. Whereas, at higher viscosity ratios, spike with fluid column structure is formed. It is found that increasing viscosity ratio shows stabilizing effect. Increasing temperature ratio is found to show destabilizing effect with complex spike structure formation at high temperature ratios. In top wall heating configuration, formation of mushroom shaped spike and skirt of the spike occurs earlier in a thinner and elongated form compared to that of bottom wall heating configuration. Prandtl number showed insignificant effect on the instability. For the parameter values of the study, when Weber is lower than 55 the configuration is stable and does not undergo instability. Overall, surface tension has shown stabilizing effect on the instability.

Inertial Effect on Oil/Water Countercurrent Imbibition in Porous Media from a Pore-Scale Perspective

Summary The color-gradient lattice Boltzmann (LB) method is used to investigate the inertial effect on oil/water countercurrent imbibition characteristics in a matrix-fracture system. The interplay between capillarity, fluid inertia, and viscous force during the imbibition under different viscosity ratios is delineated. Pore-scale dynamics, the interfacial front morphology, and oil recovery under the influence of fluid inertia are also elucidated. Additionally, we study the energy conversion during the imbibition displacement from the perspective of energy balance. Finally, the application of the theoretical scaling model is discussed based on the simulated data. Results show that the pore-scale events involved mainly consist of cooperative pore filling, oil expelled from large pores, and the motion of jetting-like oil clusters under high viscosity ratios. The curve of pressure difference between the fracture inlet and outlet vs. imbibition time can be regarded as a signal to discern the imbibition regime, which is taken together with the energy conversion analysis could further determine how capillarity, external pressure, and viscous dissipation contribute to water imbibition. Capillary force dominates in the cases of low viscosity ratios, and the majority of the surface energy is dissipated. The external pressure becomes increasingly significant and even governs the countercurrent imbibition as the viscosity ratio increases. Furthermore, the oil recovery, interfacial area, and fractal dimension of the nonwetting phase strongly rely on the Ohnesorge (Oh) number when the viscosity ratio is low. In contrast, the inertial effect can be neglected in the cases of high viscosity ratios. Besides, the relationship between the simulated imbibition recovery and imbibition time follows the theoretical scaling model as the external pressure is trivial. The comparable exponents fitted from different Oh numbers reveal that the inertial effect does not alter the imbibition dynamics. In sum, fluid inertia only affects the local fluid behaviors and thus the imbibition oil recovery when the viscosity ratio is low. These results could provide important implications for a range of energy-related and environmental applications, such as the evaluation of fracturing fluids loss, oil recovery by water huff n puff, microfluidic devices, and hydrological sciences.

Numerical study of droplet thermocapillary migration behavior on wettability-confined tracks using a three-dimensional color-gradient lattice Boltzmann model

Thermocapillary migration describes the phenomenon whereby liquid droplets move from warm to cold regions on a nonuniformly heated hydrophilic surface. Surface modifications can be applied to manipulate this migration process. In the present study, a three-dimensional color-gradient lattice Boltzmann model is used to investigate the droplet migration behavior on a series of wettability-confined tracks subject to a uniform temperature gradient. The model is validated by simulating the thermocapillary-driven flow with two superimposed planar fluids in a heated microchannel and the capillary penetration of a wetting fluid in a capillary tube. An in-depth study of the wettability-confined tracks confirms the capacity to manipulate the droplet migration process, that is, the wettability-confined tracks can accelerate thermocapillary migration compared with a smooth surface. The effects of changes in the viscosity ratio and interfacial tension are investigated, and it is found that a lower viscosity ratio and larger interfacial tension cause the droplet to migrate faster. Moreover, a systematic study of the track vertex angle is conducted, and the mechanism through which this parameter influences the droplet migration is analyzed. Then the effect of the track wettability on droplet migration is explored and analyzed. Finally, a serial wettability-confined track is designed to realize long-distance droplet migration, and the narrow side width of the connection region is found to play a key role in determining whether the droplets can migrate over long distances. The results provide some guidance for designing tracks that enable precise droplet migration control.