Deformation Of Microgels Research Articles

Deformable Microgel for Enhanced Oil Recovery in High-Temperature and Ultrahigh-Salinity Reservoirs: How to Design the Particle Size of Microgel to Achieve Its Optimal Match with Pore Throat of Porous Media

Summary Conformance control treatment in high-temperature and ultrahigh-salinity reservoirs for easing water/gas channeling through high-permeability zones has been a great challenge. In this work, we propose a deformable microgel that can be used at more than 373.15 K and ultrahigh-salinity conditions (total dissolved solids > 200 kg/m3, Ca2+ + Mg2+ > 10 kg/m3) and present a method for choosing the suitable particle size of the microgel to achieve an optimal match with the pore throat of the core. First, the particle size distribution of the microgel was analyzed to decide d50, d10, and d90 (diameter when cumulative frequency is 50, 10, and 90%, respectively). Coreflooding experiments were conducted under different permeability conditions from 20 to 900 md to investigate the migration and plugging patterns of the microgel by analyzing and fitting injection pressure curves together with the change in the morphology of the produced microgel analyzed by a microscope. The migration and plugging patterns were divided into three patterns: complete plugging; plugging—passing through in a deformation or broken state—deep migration; and inefficient plugging—smoothly passing through—stable flow. The second pattern can be further divided into three subpatterns as strong plugging, general plugging, and weak plugging. Finally, on the basis of five patterns, we build a quantitative matching relation between the particle size distribution of microgel and the pore-throat size of cores by defining three matching coefficients α = d10/d, β = d50/d, γ = d90/d (d is the average pore-throat diameter). The effectiveness of this quantitative matching relation was verified by evaluating the plugging ability (residual resistance factor) in a post-waterflooding process after the injection of 1.5 pore volume (PV) of microgel. For a strong permeability heterogeneity, the strong plugging is believed to be the expected pattern. The particles size and the pore-throat size should meet the following relationship: 1 < α < 2, 2 < β < 4, 4 < γ < 6. In this scenario, the deformable microgel particles could achieve both an effective plugging and a deep migration. The quantitative matching relation with multiple matching coefficients determined based on the particle size distribution might help to choose suitable particles more precisely in comparison to the method based on one matching coefficient (mostly, the ratio of the average diameter of particles to the average pore-throat diameter). In addition, the method itself to build a quantitative matching relation according to particle size distribution can be used for designing different particle-type conformance control agents for profile control and water shutoff treatment in field applications.

Behavior and mechanics of dense microgel suspensions

Suspensions of soft and highly deformable microgels can be concentrated far more than suspensions of hard colloids, leading to their unusual mechanical properties. Microgels can accommodate compression in suspensions in a variety of ways such as interpenetration, deformation, and shrinking. Previous experiments have offered insightful, but somewhat conflicting, accounts of the behavior of individual microgels in compressed suspensions. We develop a mesoscale computational model to probe the behavior of compressed suspensions consisting of microgels with different architectures at a variety of packing fractions and solvent conditions. We find that microgels predominantly change shape and mildly shrink above random close packing. Interpenetration is only appreciable above space filling, remaining small relative to the mean distance between cross-links. At even higher packing fractions, microgels solely shrink. Remarkably, irrespective of the single-microgel properties, and whether the suspension concentration is changed via changing the particle number density or the swelling state of the particles, which can even result in colloidal gelation, the mechanics of the suspension can be quantified in terms of the single-microgel bulk modulus, which thus emerges as the correct mechanical measure for these type of soft-colloidal suspensions. Our results rationalize the many and varied experimental results, providing insights into the relative importance of effects defining the mechanics of suspensions comprising soft particles.

Understanding transport of an elastic, spherical particle through a confining channel

The transport of soft particles through narrow channels or pores is ubiquitous in biological systems and industrial processes. On many occasions, the particles deform and temporarily block the channel, inducing a built-up pressure. This pressure buildup often has a profound effect on the behavior of the respective system; yet, it is difficult to be characterized. In this work, we establish a quantitative correlation between the built-up pressure and the material and geometry properties through experiments and mechanics analysis. We fabricate microgels with a controlled diameter and elastic modulus by microfluidics. We then force them to individually pass through a constrictive or straight confining channel and monitor the pressure variation across the channel. To interpret the pressure measurement, we develop an analytical model based on the Neo-Hookean material law to quantify the dependence of the maximum built-up pressure on the radius ratio of the elastic sphere to the channel, the elastic modulus of the sphere, and two constant parameters in the friction constitutive law between the sphere and the channel wall. This model not only agrees very well with the experimental measurement conducted at large microgel deformation but also recovers the classical theory of contact at small deformation. Featuring a balance between accuracy and simplicity, our result could shed light on understanding various biological and engineering processes involving the passage of elastic particles through narrow channels or pores.

Deformation of Microgels at Solid-Liquid Interfaces Visualized in Three-Dimension.

Solid-liquid interfaces play an important role for functional devices. Hence, a detailed understanding of the interaction of soft matter objects with solid supports and of the often concomitant structural deformations is of great importance. We address this topic in a combined experimental and simulation approach. We investigated thermoresponsive poly(N-isopropylmethacrylamide) microgels (μGs) at different surfaces in an aqueous environment. As super-resolution fluorescence imaging method, three-dimensional direct stochastical optical reconstruction microscopy (dSTORM) allowed for visualizing μGs in their three-dimensional (3D) shape, for example, in a "fried-egg" conformation depending on the hydrophilicity of the surface (strength of adsorption). The 3D shape, as defined by point clouds obtained from single-molecule localizations, was analyzed. A new fitting algorithm yielded an isosurface of constant density which defines the deformation of μGs at the different surfaces. The presented methodology quantifies deformation of objects with fuzzy surfaces and allows for comparison of their structures, whereby it is completely independent from the data acquisition method. Finally, the experimental data are complemented with mesoscopic computer simulations in order to (i) rationalize the experimental results and (ii) to track the evolution of the shape with changing surface hydrophilicity; a good correlation of the shapes obtained experimentally and with computer simulations was found.

Effect of Charge Groups Immobilized in Hydrogel Microspheres during the Evaporation of Aqueous Sessile Droplets.

In contrast to conventional dispersions of solid microspheres, dilute dispersions containing soft hydrogel microspheres (microgels) exhibit unique drying behavior due to their selective adsorption at the air/water interface of sessile droplets. So far, the impact of the size, chemical composition, and softness (degree of cross-linking) of microgels has been investigated. In the present study, we present the impact of charged groups introduced in the microgels on the adsorption and assembly behavior of these microgels at the air/water interface using a series of microgels with different amounts and distribution of charged groups. A series of experiments under different conditions (pH value and ionic strength) afforded information that clarified the adsorption, interpenetration, and deformation behavior of such charged microgels at the air/water interface. The results indicate that the adsorption and the deformation of charged microgels at the air/water interface are suppressed by the presence of charged groups. Moreover, charged microgels adsorbed at the interface are more dynamic and not highly entangled with each other; i.e., even though the more dynamic charged microgels are arranged at the interface, these arranged structures are disrupted upon transferring onto the solid substrates. Our findings of this study can be expected to promote the further development of applications, e.g., foams and emulsions stabilized by microgels, that crucially requires an in-depth understanding of the adsorption behavior of charged microgels at the air/water interface such as coatings.

Transport mechanism of deformable micro-gel particle through micropores with mechanical properties characterized by AFM

Deformable micro-gel particles (DMP) have been used to enhanced oil recovery (EOR) in reservoirs with unfavourable conditions. Direct pore-scale understanding of the DMP transport mechanism is important for further improvements of its EOR performance. To consider the interaction between soft particle and fluid in complex pore-throat geometries, we perform an Immersed Boundary-Lattice Boltzmann (IB-LB) simulation of DMP passing through a throat. A spring-network model is used to capture the deformation of DMP. In order to obtain appropriate simulation parameters that represent the real mechanical properties of DMP, we propose a procedure via fitting the DMP elastic modulus data measured by the nano-indentation experiments using Atomic Force Microscope (AFM). The pore-scale modelling obtains the critical pressure of the DMP for different particle-throat diameter ratios and elastic modulus. It is found that two-clog particle transport mode is observed in a contracted throat, the relationship between the critical pressure and the elastic modulus/particle-throat diameter ratio follows a power law. The particle-throat diameter ratio shows a greater impact on the critical pressure difference than the elastic modulus of particles.

Impacts of Size and Deformability of β-Lactoglobulin Microgels on the Colloidal Stability and Volatile Flavor Release of Microgel-Stabilized Emulsions.

Emulsions can be prepared from protein microgel particles as an alternative to traditional emulsifiers. Prior experiments have indicated that smaller and more deformable microgels would decrease both the physical destabilization of emulsions and the diffusion-based losses of entrapped volatile molecules. The microgels were prepared from β-lactoglobulin with an average diameter of 150 nm, 231 nm, or 266 nm; large microgels were cross-linked to decrease their deformability. Dilute emulsions of 15–50 μm diameter were prepared with microgels by high shear mixing. Light scattering and microscopy showed that the emulsions prepared with larger, untreated microgels possessed a larger initial droplet size, but were resistant to droplet growth during storage or after acidification, increased ionic strength, and exposure to surfactants. The emulsions prepared with cross-linked microgels emulsions were the least resistant to flocculation, creaming, and shrinkage. All emulsion droplets shrank as limonene was lost during storage, and the inability of microgels to desorb caused droplets to become non-spherical. The microgels were not displaced by Tween 20 but were displaced by excess sodium dodecyl sulfate. Hexanol diffusion and associated shrinkage of pendant droplets was not prevented by any of the microgels, yet the rate of shrinkage was reduced with the largest microgels.

Compressive resistance of granular-scale microgels: From loose to dense packing

Microgels are deformable and compressible particles that can be packed to concentrations that exceed the random close packing limit of hard spheres. For reaching high packing levels, one has to overcome the resistance to compression of the system. This resistance potentially originates from many different phenomena (thermal agitation effects, surface interactions, microgel deformation, interpenetration, water expulsion) that depend on the microgel properties (size, ionic charge, structure, softness). Here, we investigate granular-scale dextran-based microgels with different native water contents. The resistance to compression of the suspensions is measured through the variation of the osmotic pressure with packing concentration. In parallel, we characterize the structure of the packings in terms of polymer heterogeneity, microgel deformation, and average size using confocal microscopy. We find that all microgel suspensions resist compression in the same manner; however, the mechanisms involved clearly depend on the actual degree of compression. In the loose packing regime, the resistance originates mainly from the resistance of the microgels to their own deformation, with no or negligible deswelling; the osmotic pressure rises abruptly with concentration in analogy to compressed emulsion droplets. In the second and dense packing regime, the microgels necessarily have to expel water to withstand compression. The resistance of the packing is then similar to that of a continuous gel of the same polymer. Importantly, we find that structural macro-voids are still observable in these systems; the presence of which needs to be taken into account when modeling the osmotic resistance.

Study of Poly(N-isopropylacrylamide-co-acrylic acid) (pNIPAM) Microgel Particle Induced Deformations of Tissue-Mimicking Phantom by Ultrasound Stimulation.

Poly(N-isopropylacrylamide) (pNIPAm) microgels (microgels) are colloidal particles that have been used extensively for biomedical applications. Typically, these particles are synthesized in the presence of an exogenous cross-linker, such as N,N'-methylenebis(acrylamide) (BIS); however, recent studies have demonstrated that pNIPAm microgels can be synthesized in the absence of an exogenous cross-linker, resulting in the formation of ultralow cross-linked (ULC) particles, which are highly deformable. Microgel deformability has been linked in certain cases to enhanced bioactivity when ULC microgels are used for the creation of biomimetic particles. We hypothesized that ultrasound stimulation of microgels would enhance particle deformation and that the degree of enhancement would negatively correlate with the degree of particle cross-linking. Here, we demonstrate in tissue-mimicking phantoms that using ultrasound insonification causes deformations of ULC microgel particles. Furthermore, the amount of deformation depends on the ultrasound excitation frequency and amplitude and on the concentration of ULC microgel particles. We observed that the amplitude of deformation increases with increasing ULC microgel particle concentration up to 2.5 mg/100 mL, but concentrations higher than 2.5 mg/100 mL result in reduced amount of deformation. In addition, we observed that the amplitude of deformation was significantly higher at 1 MHz insonification frequency. We also report that increasing the degree of microgel cross-linking reduces the magnitude of the deformation and increases the optimal concentration required to achieve the largest amount of deformation. Stimulated ULC microgel particle deformation has numerous potential biomedical applications, including enhancement of localized drug delivery and biomimetic activity. These results demonstrate the potential of ultrasound stimulation for such applications.

Deformation and Stability of Core–Shell Microgels at Oil/Water Interface

This paper investigates the behavior of P(NIPAM-co-AAc)@PTFMA core–shell microgels at the decane/water interface. The microgels were deposited at the interface to form a monolayer film, and the film’s compression behavior was measured using Langmuir trough. Typical compression isotherm embodies four regimes, weak interaction between microgels in regime I, viscoelastic deformation in regime II, elastic deformation of microgels with thin shell while still viscoelastic deformation with thick shell in regime III. Minor desorption of microgels takes place in regime III and massive in regime IV. The critical interfacial pressure for desorption of microgels is identified in the range of 43–45 mN/m, independent of the shell thickness. It shows that the deformability of the surrounding part rather than the mean deformation of the microgels dominates their stability at the interface. These results illustrate the behavior of microgels at the interface under loading, and deepen the understanding of the stability of m...

Deswelling and deformation of microgels in concentrated packings

Increasing the particle density of a suspension of microgel colloids above the point of random-close packing, must involve deformations of the particle to accommodate the increase in volume fraction. By contrast to the isotropic osmotic deswelling of soft particles, the particle-particle contacts give rise to a non-homogeneous pressure, raising the question if these deformations occur through homogeneous deswelling or by the formation of facets. Here we aim to answer this question through a combination of imaging of individual microgels in dense packings and a simple model to describe the balance between shape versus volume changes. We find a transition from shape changes at low pressures to volume changes at high pressures, which can be explained qualitatively with our model. Whereas contact mechanics govern at low pressures giving rise to facets, osmotic effects govern at higher pressures, which leads to a more homogeneous deswelling. Our results show that both types of deformation play a large role in highly concentrated microgel suspensions and thus must be taken into account to arrive at an accurate description of the structure, dynamics and mechanics of concentrated suspensions of soft spheres.

Effect of internal architecture on microgel deformation in microfluidic constrictions.

The study of how soft particles deform to pass through narrow openings is important for understanding the transit of biological cells, as well as for designing deformable drug delivery carriers. In this work, we systematically explore how soft microparticles with various internal architectures deform during passage through microfluidic constrictions. We synthesize hydrogel particles with well-defined internal structure using lithography-based UV polymerization in microfluidic channels (stop-flow lithography). Using this in situ technique, we explore a range of 2D particle architectures and their effect on particle deformation. We observe that particles undergo buckling of internal supports and reorient at the constriction entrance in order to adopt preferred shapes that correspond to minimum energy configurations. Using finite element simulations of elastic deformation under compression, we accurately predict the optimal deformation configuration of these structured particles.

Electrostatic Swelling Transitions in Surface-Bound Microgels

Herein, electrostatic swelling transitions of poly(ethyl acrylate-co-methacrylic acid) microgels covalently bound to silica surfaces are investigated. Confined at a solid surface, microgel swelling is anisotropically hindered and the structure is flattened to an extent dictated by pH and microgel composition. Microgel deformation under applied load is also shown to depend on microgel charge density, with the highest deformation observed at intermediate charge densities. Two modes of microgel deformation under load were observed, one elastic and one viscoelastic, related to polymer strand deformation and displacement of trapped water, respectively. Results on polymer strand dynamics reveal that the microgels are highly dynamic, as the number of strand-tip interaction points increases 4-fold during a 10 s contact time. Furthermore, finite element modeling captures these effects qualitatively and shows that stress propagation in the microgel network decays locally at the rim of contact with a solid interface or close to the tip probe. Taken together, the results demonstrate a delicate interplay between the surface and microgel which determines the structure and nanomechanical properties of the latter and needs to be controlled in applications of systems such as pH-responsive surface coatings in biomaterials.

Biopolymer-based particles as stabilizing agents for emulsions and foams

The effectiveness of biopolymer-based particles as stabilizing agents of emulsified and aerated systems is attributable to a pair of contributory mechanisms — Pickering stabilization by adsorbed particles at the oil–water or air–water interface, and gel-like structuring of particles between droplets or bubbles. This review describes recent advances in the stabilization of emulsions and foams by edible particles of nanoscale and microscale dimensions. The biopolymer ingredients involved include common food proteins such as whey protein, soy protein and gelatin, and polysaccharides such as starch, cellulose and chitin. The diverse morphologies of the biopolymer-based entities range from rigid structures like dense spheres and polygonal crystals, through to semi-flexible fibres and deformable microgels. The clear message to be inferred from these studies is that a broadly consistent range of mechanistic behaviour has emerged — albeit with some basic questions remaining that offer exciting opportunities for future research.

Persulfate initiated ultra-low cross-linked poly(N-isopropylacrylamide) microgels possess an unusual inverted cross-linking structure.

Cross-linking density and distribution are decisive for the mechanical and other properties of stimuli-sensitive poly(N-isopropylacrylamide) microgels. Here we investigate the structure of ultra-low cross-linked microgels by static light scattering and scanning force microscopy, and show that they have an inverted cross-linking structure with respect to conventional microgels, contrary to what has been assumed previously. The conventional microgels have the largest polymer volume fraction in the core from where the particle density decays radially outwards, whereas ultra-low cross-linked particles have the highest polymer volume fraction close to the surface. On a solid substrate these particles form buckled shapes at high surface coverage, as shown by scanning force micrographs. The special structure of ultra-low cross-linked microgels is attributed to cross-linking of the particle surface, which is exposed to hydrogen abstraction by radicals generated from persulfate initiators during and after polymerization. The particle core, which is less accessible to the diffusion of radicals, has consequently a lower polymer volume fraction in the swollen state. By systematic variation of the cross-linker concentration it is shown that the cross-linking contribution from peroxide under typical synthesis conditions is weaker than that from the use of 1 mol% N,N'-methylenebisacrylamide. Soft deformable hydrogel particles are of interest because they emulate biological tissues, and understanding the underlying synthesis principle enables tailoring the microgel structure for biomimetic applications. Deformability of microgels is usually controlled by the amount of added cross-linker; here we however highlight an alternative approach through structural softness.

Platelet-like particles for wound-triggered hemostasis in patients at high risk for bleeding during cardiac surgery

Event Abstract Back to Event Platelet-like particles for wound-triggered hemostasis in patients at high risk for bleeding during cardiac surgery Riley Hannan1, Byungwook Ahn2, 3, 4, Wilbur Lam3, 4, Andrew Lyon5, Nina Guzzetta3, 4, Thomas H. Barker2, 4 and Ashley Brown6 1 University of Virginia, United States 2 Georgia Institute of Technology, Biomedical Engineering, United States 3 Children’s Healthcare of Atlanta, United States 4 Emory University, United States 5 Chapman University, Schmid College of Science and Technology, United States 6 North Carolina State University and University of North Carolina at Chapel Hill, Biomedical Engineering, United States Introduction: Neonates born with congenital cardiac defects often undergo complex corrective surgery requiring cardiopulmonary bypass (CPB) within a few days following birth. CPB is known to contribute significantly to surgery associated bleeding through induction of massive hemodilution, thrombocytopenia and platelet dysfunction; these bleeding risks are exacerbated in neonates due to preexisting platelet dysfunction and an immature coagulation system. CPB greatly hinders platelet function; therefore, materials that recapitulate platelet hemostatic functions could be utilized as an alternative to transfusion of adult blood products for augmentation of clotting in patients post-CPB. We have recently developed platelet-like-particles (PLPs) created from highly deformable microgels conjugated to molecularly evolved wound-targeting motifs which enable specific and high affinity binding to the provisional matrix protein fibrin[1]. These PLPs are capable of recapitulating a number of functions of natural platelets, including augmentation of clotting of adult plasma in vitro, decreased bleeding times in vivo in rodent models of traumatic injury, specific homing to injury sites and clot contraction. Here, we characterize the ability of PLPs to augment clotting in CPB-patient plasma samples in vitro and in vivo in a rat model of impaired platelet activity. Materials and Methods: Ultra-low crosslinked (ULC) poly(N-isopropylacrylamide) (pNIPAM) microgels containing 95% pNIPAM and 5% acrylic acid (AAc) were produced through precipitation polymerization. PLPs were produced by conjugating fibrin-specific single domain variable fragment antibodies to ULC microgels through EDC/NHS coupling. Clotting was assessed in vitro in patient platelet poor plasma (PPP) samples collected from neonatal cardiac surgery patients at baseline, post-CPB (+/- PLPs) and post transfusion through an endothelialized microfluidics device that mimics the microenvironment of microcirculation[2]. Fibrin microarchitecture was analyzed using confocal microscopy. Clotting was assessed in vivo in a rodent platelet deficiency model, which mimics the gross dysfunction of platelets in pediatric cardiac patients. To induce platelet hyporeactivity, male Sprague Dawley rats received a daily oral gavage of aspirin (5 mg/kg) for seven days. On the seventh day, bleeding times following induction of a femoral vein injury were assessed in animals receiving intravenous administration of PLPs or saline. Results and Discussion: In vitro clotting experiments in an endothelialized microfluidic device demonstrate minimal clotting in post-CPB PPP alone, while the addition of PLPs induces significant fibrin formation (Figure 1A). To determine the effect of PLPs on fibrin network microstructure, confocal microscopy was performed; incorporation of PLPs into post-CPB PPP results in a denser fibrin network compared to post-CBP PPP alone or samples collected post-transfusion (Figure 1B). In an in vivo model of platelet hyporeactivity, PLPs significantly decreased bleeding following injury compared to control animals (Figure 1C). Conclusions: PLPs enhance clot formation in neonatal patient samples in vitro and decrease bleeding times in vivo in a rodent model of platelet dysfunction. These results highlight the utility of PLPs for treatment of postsurgical bleeding in coagulopathic neonatal patients. 1R21EB019068, NIH/NIBIB

Rheology of emulsion-filled alginate microgel suspensions

Emulsion filled polysaccharide gels can be used as carrier systems of lipophilic bioactives in the food, pharmaceutical and cosmetics industry. This carrier system can exist either as bulk or discrete gel systems. In this study the rheological properties of discrete emulsion filled alginate microgel suspension was examined as a function of volume fraction (ϕ) and oil content. Fine emulsion (220nm) was encapsulated within alginate microgels (mean size 36.2–57.8μm) by using the impinging aerosol technique. The microgels (containing 0–77% w/w oil total solids basis) produced were estimated to have particle modulus in the range of 150–212Pa. An increase in oil content in the microgels led to more deformable microgels due to the reduction in gel density. The deformability of microgels influenced the bulk modulus and apparent viscosity of the concentrated suspension. At the same suspension volume fraction (ϕ), suspensions with more deformable microgels exhibited a lower bulk modulus. We also showed that the Carreau and Cross models were adequate in predicting the flow behaviour of the concentrated emulsion filled microgel suspension.

Alginate droplets pre-crosslinked in microchannels to prepare monodispersed spherical microgels

Abstract Monodispersed spherical calcium alginate microgels (denoted as Ca-ALG) with uniform and controllable size were prepared by using a microfluidics technique. Herein, an aqueous solution of sodium alginate (denoted as Na-ALG) was used as a dispersed phase, and the liquid paraffin containing CaCl 2 nanoparticles formed in situ was used as a continuous phase. Na-ALG droplets were first pre-crosslinked by CaCl 2 diffused from the oil phase in the microchannel, and then crosslinked completely in the collection bath containing an aqueous solution of CaCl 2 . In comparison with conventional external crosslinking, this method can avoid deformation of microgels which usually occurs in the process of external crosslinking. Due to the pre-crosslinking of Na-ALG droplets in the microchannel, the spherical morphology of the microgels could not be affected by the preparation conditions, such as types of microfluidic devices, flow rate ratios of the continuous phase and dispersed phase, size of the droplets, collection height and so on. In addition, the size of the spherical Ca-ALG microgels can be well controlled from 40 to 700 μm in diameter by changing the dimension of the microchannels and the flow rate ratios of the dispersed phase and continuous phase. Furthermore, such spherical Ca-ALG microgels can improve their movement and packing state in a microchannel, which is beneficial for their use as embolic materials for interventional therapy.

Bulk and interfacial stresses in suspensions of soft and hard colloids

We explore the influence of particle softness and internal structure on both the bulk and interfacial rheological properties of colloidal suspensions. We probe bulk stresses by conventional rheology, by measuring the flow curves, shear stress versus strain rate, for suspensions of soft, deformable microgel particles and suspensions of near hard-sphere-like silica particles. A similar behaviour is seen for both kinds of particles in suspensions at concentrations up to the random close packing volume fraction, in agreement with recent theoretical predictions for sub-micron colloids. Transient interfacial stresses are measured by analyzing the patterns formed by the interface between the suspensions and their solvent, due to a generalized Saffman–Taylor hydrodynamic instability. At odds with the bulk behaviour, we find that microgels and hard particle suspensions exhibit vastly different interfacial stress properties. We propose that this surprising behaviour results mainly from the difference in particle internal structure (polymeric network for microgels versus compact solid for the silica particles), rather than softness alone.

Conformation and plugging properties of crosslinked polymer microspheres for profile control

Abstract Shape, size, and deformation of cross-linked polymer microspheres were investigated using scanning electron microscopy (SEM), microscopy and laser diffracting measurements experiments. Capillary flow experiment, micro-visual model, nuclear-pore film filtration experiment and sand-packed model were utilized to explore the plugging mechanism of microspheres in porous media. The results demonstrated that the microspheres typically had a spherical original shape with a size ranging from a few microns to tens of microns. When the microspheres were dispersed in water, they became larger due to swelling, at a range of 10–100 μm. The microspheres can plug porous medium and there is certain matching relation between the size of microspheres and the size of reservoir pore, and they can be adsorbed, accumulated and bridged in the pore-throat to play a role of plugging. Microspheres are soft, flexible, deformable microgel particles and they will be deformed under the pressure, entering deep into the reservoir when sufficient pressure is supplied, with the ultimate purpose of the deep profile control. Moreover, the microsphere can deform to entry the small capillaries and the microsphere with a smaller deformation preferably migrates in capillaries under a certain displacement pressure.