Microgel Suspensions Research Articles

Inner structure of adsorbed ionic microgel particles.

Microgel particles of cross-linked poly(NIPAM-co-acrylic acid) with different acrylic acid contents are investigated in solution and in the adsorbed state. As a substrate, silicon with a poly(allylamine hydrochloride) (PAH) coating is used. The temperature dependence of the deswelling of the microgel particles was probed with atomic force microscopy (AFM). The inner structure of the adsorbed microgel particles was detected with grazing incidence small angle neutron scattering (GISANS). Small angle neutron scattering (SANS) on corresponding microgel suspensions was performed for comparison. Whereas the correlation length of the polymer network shows a divergence in the bulk samples, in the adsorbed microgel particles it remains unchanged over the entire temperature range. In addition, GISANS indicates changes in the particles along the surface normal. This suggests that the presence of a solid surface suppresses the divergence of internal fluctuations in the adsorbed microgels close to the volume phase transition.

Revisit to phase diagram of poly(N-isopropylacrylamide) microgel suspensions by mechanical spectroscopy

Microgels are soft particles that can be deformed and compressed, which would induce intriguing phase behaviors at high packing fractions. Poly(N-isopropylacrylamide) (PNIPAM) microgels, with a lower critical solution temperature (LCST) of 33 °C, have attracted considerable interests as model colloids, since the volume of them and the interaction between the microgels can be tuned precisely by temperature. In this work, the linear viscoelastic properties of PNIPAM microgel suspensions have been investigated using mechanical spectroscopy. A particular attention is focused on the phase behaviors at high concentrations. With increasing concentration the system undergoes a repulsive glass-to-gel transition below the LCST, while, as temperature is raised across the LCST, the system undergoes a gel-to-attractive glass transition. A mechanism of these transitions for the microgels is proposed based on the directional interaction between the particles. In moderate concentration or de-swelling microgels the interaction is isotropic leading to the glass phase, while in concentrated and deformed microgels the interaction is directional leading to the gel phase. Our results enrich the current understanding of the phase transition in microgel systems and shed new light on the phase diagram of colloidal suspensions in general.

Effects of particle softness on shear thickening of microgel suspensions.

A series of microgel particles composed of a polystyrene (PS) core and a thermo-sensitive poly(N-isopropylacrylamide) (PNIPAM) shell with different shell thicknesses were investigated to elucidate the effect of microgel softness on its shear thickening behavior. Since the softness of the microgels increases with decreasing temperature through the volume phase transition effect of PNIPAM shell, the measured softness parameter, n, which is derived from the Zwanzig-Mountain equation, was used to measure and describe the combined influences of temperature and shell thickness. Confocal microscopy is used to investigate the interaction potential between microgel particles with different softness parameters. According to the obtained results, the softness parameter can provide an estimate for the shear thickening behavior of microgel suspensions, at least semi-quantitatively.

Smart Profile Control by Salt-Reversible Flocculation of Cationic Microgels and Polyacrylamide

A flocculation system is detailed to deal with the size matching problem between pore throats and microgels for profile modification in high permeable formation. This system consists of partially hydrolyzed polyacrylamide (HPAM) and cationic microgels synthesized by inverse suspension polymerization with acrylamide and methylacryloxylethyl trimethyl ammonium chloride as monomers. By adding HPAM into cationic microgel suspensions, flocculation occurs because of electrostatic attraction. Then it reversibly restructures from bulk to an insulated microgel particle at a critical salt concentration (CSC). In the profile modification test, this flocculation system diverts nearly 100% chase water into the low permeable tube compared to 20% by using microgels only. The performance improvement testifies the mechanism that HPAM flocculates around the cationic microgels to make up the interstitials between microgels and irregular pore throats. This flocculation plug can be removed by water with salinity above CSC due to the desorption of HPAM on cationic microgels.

The jamming and unjamming transition in poly(N-isopropylacrylamide) microgel suspensions

The jamming and unjamming transition in thermo-sensitive poly(N-isopropylacrylamide) (PNIPAm) microgel suspensions was observed by rheology with three independent variables of temperature T, microgel weight fraction w, and the shear stress σ. The effective volume fraction φeff, depending on both T and w, was adopted to interpret this transition. In the jamming state, the plateau modulus G′p followed the same power law to φeff with varying T and w, indicating that the change of the interaction potential with temperature was insignificant. However, the volume fraction φc at the jamming transition decreased with increasing w because higher concentration of particles led to stronger repulsive interaction. The shear-unjamming transition occurred as the shear stress exceeded the yield stress σy. A jamming phase diagram for the microgel suspension was established in a [1/w, σRh3/(kBθ), T] coordinate. The nonlinear viscoelasticity was observed by the large amplitude oscillatory shear (LAOS) analysis within the shear-unjamming region, which suggested that the shear-unjamming state under high strain would not be equivalent in rheology to the unjamming state appeared at high temperature or low concentration without shearing.

Particulate and continuum mechanics of microgel pastes: effect and non-effect of compositional heterogeneity

Microgels are deformable colloids that can be packed by external compression; such packing transforms a suspension of loose microgels into a viscoelastic paste with mechanical properties controlled by the elasticity of the constituent particles. We aim to understand how the presence of microgel particles with different individual elastic moduli affects this interplay in heterogeneous microgel packings. We do this by preparing microgel pastes that contain both soft, loosely cross-linked and stiff, densely cross-linked microgel particles and probe their shear elasticity. We consider particle packing fractions that cover the range from particles at the onset of contact to particles that are strongly packed, deformed, and deswollen to investigate the transition from a particulate suspension to a macrogel-type system. These studies reveal that the elasticity of heterogeneous microgel suspensions at low packing is due to the response of the soft, easily deformable microgel particles alone, whereas at high packing both soft and stiff microgels linearly add to the paste elasticity. This fundamental difference is due to the fundamentally different origin of elasticity at different microgel packing; whereas the soft particle interaction potential dominates the suspension mechanics at low microgel packing, rubber-like elasticity that equally reflects both soft and stiff contributions governs the mechanics of the same samples at high microgel packing.

Electrochemical sensor based on chlorohemin modified molecularly imprinted microgel for determination of 2,4-dichlorophenol

A newly designed molecularly imprinted polymer (MIP) was synthesized and successfully utilized as a recognition element of an amperometric sensor for 2,4-dichlorophenol (2,4-DCP) detection. The MIP with a well-defined structure could imitate the dehalogenative function of the natural enzyme chloroperoxidase for 2,4-DCP. Imprinted sensor was fabricated in situ on a glassy carbon electrode surface by drop-coating the 2,4-DCP imprinted microgel suspension and chitosan/Nafion mixture. Under optimized conditions, the sensor showed a linear response in the range of 5.0–100μmolL−1 with a detection limit of 1.6μmolL−1. Additionally, the imprinted sensor demonstrated higher affinity to target 2,4-DCP over competitive chlorophenolic compounds than non-imprinted sensor. It also exhibited good stability and acceptable repeatability. The proposed sensor could be used for the determination of 2,4-DCP in water samples with the recoveries of 96.2–111.8%, showing a promising potential in practical application.

Local mobility and microstructure in periodically sheared soft particle glasses and their connection to macroscopic rheology

Oscillatory shear is a widely used characterization technique for complex fluids but the microstructural changes that produce the material response are not well understood. We apply a recent micromechanical model to soft particle glasses subjected to oscillatory flow. We use particle scale simulations at small, intermediate and large strain amplitudes to determine their microstructure, particle scale mobility, and macroscopic rheology. The macroscopic properties computed from simulations quantitatively agree with experimental measurements on well-characterized microgel suspensions, which validate the model. At the mesoscopic scale, the evolution of the particle pair distribution during a cycle reveals the physical mechanisms responsible for yielding and flow and also leads to quantitative prediction of shear stress. At the local scale, the particles remain trapped inside their surrounding cage below the yield strain and yielding is associated with the onset of large scale rearrangements and shear-induced diffusion. This multiscale analysis thus highlights the distinct microscopic events that make these glasses exhibit a combination of solid-like and liquid-like behavior.

Effective electrostatic interactions arising in core-shell charged microgel suspensions with added salt

The mixture formed by charged (ionic) microgels in the presence of 1:1 added salt, with explicit consideration of a core-shell structure of the microgel particles, is studied. By solving numerically the three-component Ornstein-Zernike integral equations, the counter- and coion penetration inside the microgel network and the resulting effective microgel-microgel electrostatic interaction are calculated. This is done in the limit of very low microgel concentration, so that the resulting pair-wise effective potential is not affected by many-body particle-particle interactions. The ion-ion, microgel-ion, and microgel-microgel correlations are all treated within the Hypernetted-Chain approximation. The results obtained clearly show that the addition of salt to the microgel suspension has a deep impact on the screening of the bare charge of the particles, confirming an already well-known result: the strong reduction of the effective charge of the microgel occurring even for diluted electrolyte concentrations. We show that this effect becomes more important as we increase the shell size of the particle and derive a semi-empirical model for the effective charge as a function of the electrolyte concentration and the shell extension. The resulting microgel-microgel effective pair potential is analysed as a function of the shell extension and salt concentration. In all cases the interaction is a soft potential when particles overlap. For non-overlapping distances, our theoretical results indicate that microgel particles can be regarded as hard spherical colloids bearing an effective charge given by the net charge inside the particle and the microgel-microgel interaction shows a Yukawa-like behaviour as a function of the interparticle distance. It is also observed that increasing the bare-charge of the microgel induces a strong microgel-counterion coupling in the limit of very low electrolyte concentrations, which cannot be justified using linearized theories. This leads to an even more important adsorption of counterions inside the microgel network and to a reduction of the microgel-microgel effective repulsion.

Disentangling glass and jamming physics in the rheology of soft materials

The shear rheology of soft particles systems becomes complex at large density because crowding effects may induce a glass transition for Brownian particles, or a jamming transition for non-Brownian systems. Here we successfully explore the hypothesis that the shear stress contributions from glass and jamming physics are `additive'. We show that the experimental flow curves measured in a large variety of soft materials (colloidal hard spheres, microgel suspensions, emulsions, aqueous foams) as well as numerical flow curves obtained for soft repulsive particles in both thermal and athermal limits are well described by a simple model assuming that glass and jamming rheologies contribute linearly to the shear stress, provided that the relevant scales for time and stress are correctly identified in both sectors. Our analysis confirms that the dynamics of colloidal hard spheres is uniquely controlled by glass physics while aqueous foams are only sensitive to jamming effects. We show that for micron-sized emulsions both contributions are needed to successfully account for the flow curves, which reveal distinct signatures of both phenomena. Finally, for two systems of soft microgel particles we show that the flow curves are representative of the glass transition of colloidal systems, and deduce that microgel particles are not well suited to studying the jamming transition experimentally.

Bio-ink for on-demand printing of living cells.

Drop-on-demand bioprinting allows the controlled placement of living cells, and will benefit research in the fields of tissue engineering, drug screening and toxicology. We show that a bio-ink based on a novel microgel suspension in a surfactant-containing tissue culture medium can be used to reproducibly print several different cell types, from two different commercially available drop-on-demand printing systems, over long printing periods. The bio-ink maintains a stable cell suspension, preventing the settling and aggregation of cells that usually impedes cell printing, whilst meeting the stringent fluid property requirements needed to enable printing even from many-nozzle commercial inkjet print heads. This innovation in printing technology may pave the way for the biofabrication of multi-cellular structures and functional tissue.

CO2-triggered liquid–solid switching through a jamming mechanism

The concept of jamming is introduced into a CO2-responsive microgel suspension. The particle size of the microgel changes with the pH of the solvent and the rheological properties of the suspension are responsive to the triggering of CO2. At a certain concentration of the microgel, the microgel suspension transforms from a liquid into a solid-like material as CO2 is bubbled into it, and it returns to a liquid after the CO2 is blown away with N2. The CO2-triggered jamming is rapid and reversible, leading to many interesting applications of the system.

Role of interparticle attraction in the yielding response of microgel suspensions

We use rheological measurements to examine the yielding behavior of a microgel system spanning the range from soft jammed glassy suspensions dominated by inter-particle repulsion to colloidal gels produced by attractive interactions. Under repulsive conditions, the suspensions display a prototypical soft glassy yielding response in which the shear loss modulus exhibits a single peak on increasing strain during the crossover from elastic to viscous behavior. By contrast, under fully attractive conditions the colloidal gel displays a more complex yielding, with two distinct peaks in the loss modulus in the vicinity of the yield strain. It is apparent that the gels yield initially by network rupture, followed by shear induced densification which leads to the formation of compact clusters. We show that the second peak in the loss modulus is consistent with the subsequent breakup of these dense clusters. We quantitatively map the steady progression from simple glassy yielding to the more complex gel response on increasing attraction strength by the evolution of peak locations, magnitudes and frequency dependencies. Notably, the peak locations diverge as the network becomes more fragile and spatially heterogeneous with increasing attraction strength. There is little frequency dependence in the peak positions, but the amplitude of the second yielding peak shows a non-monotonic dependence with a maximum near 5 rad s−1. Time-resolved measurements and decreasing strain sweeps highlight pronounced differences in the reversibility of the network rupture and cluster breakup processes. Correspondingly, the linear viscoelastic properties of the gel are strongly dependent on mechanical history whereas the glass exhibits no such dependence.

Elasticity of compressed microgel suspensions

We describe the elasticity of compressed microgel suspensions starting from a core--shell model for the single particle. The mechanical properties of the inner core and of the outer corona are described via the mean field Flory theory, and via the Alexander--de Gennes model for polymer brushes, respectively. The model successfully reproduces experimental measures of the elastic shear modulus up to a constant factor that we rationalize in the jamming perspective.

Heat induced formation of beta-lactoglobulin microgels driven by addition of calcium ions

Stable suspensions of spherical protein particles (microgels) can be formed by heating beta-lactoglobulin solutions in the presence of calcium ions. The conditions for the calcium induced microgel formation were studied at different pH between 5.8 and 7.5 and different protein concentrations between 5 and 100 g/L after heating at 85 °C for 15 h. The results showed that a critical molar ratio of calcium to proteins (R) is needed to form microgels independent of the protein concentration. R decreases with decreasing pH. The microgels have a hydrodynamic radius ranging from 100 to 300 nm and their internal protein concentration ranges from 0.2 to 0.45 g/mL. The amount of calcium bound to the proteins was determined and the results suggest that the crucial parameter for microgel formation is the net charge density of the native proteins. The microgel suspensions are stable in a narrow range of R but the microgels aggregate at higher calcium concentrations.

Origin of de-swelling and dynamics of dense ionic microgel suspensions

A direct consequence of the finite compressibility of a swollen microgel is that it can shrink and deform in response to an external perturbation. As a result, concentrated suspensions of these particles exhibit relaxation dynamics and rheological properties which can be very different with respect to those of a hard sphere suspension or an emulsion. We study the reduction in size of ionic microgels in response to increasing number of particles to show that particle shrinkage originates primarily from steric compression, and that the effect of ion-induced de-swelling of the polymer network is negligible. With increasing particle concentration, the single particle dynamics switch from those typical of a liquid to those of a super-cooled liquid and finally to those of a glass. However, the transitions occur at volume fractions much higher than those characterizing a hard sphere system. In the super-cooled state, the distribution of displacements is non-gaussian and the dependence of the structural relaxation time on volume fraction is describable by a Volger-Fulcher-Tammann function.

Phase and non-equilibrium behaviour of microgel suspensions as a function of particle stiffness

We investigate the phase and non-equilibrium behavior of suspensions comprised of swollen, ionic microgels as a function of particle stiffness. We use visual inspection of the samples, rheology and UV-visible spectroscopy to determine how the system phase changes with particle concentration for particles with different crosslinker content. Our results indicate that stiff particles exhibit all three phases observed in hard sphere suspensions: liquid, crystal and glass; however, the boundaries separating one phase from the other are different from the corresponding boundaries in hard sphere suspensions. In particular, the width of the liquid–crystal phase-coexistence region increases with decreasing particle stiffness. For particles with intermediate stiffness, the crystal phase disappears and the microgel suspension transitions from a liquid to a glassy state at certain particle concentration. For even softer particles, no glassy state is observed. Instead the system remains liquid within the experimentally accessed concentration range. These results emphasize the richness in behaviour that is brought about when deformable and compressible objects are considered instead of the more usual hard colloidal particles.

Stick-slip control of the Carbopol microgels on polymethyl methacrylate transparent smooth walls

Microgel suspensions such as Carbopol are commonly used as viscoplastic fluid models in fluid mechanics experiments. Polymethyl methacrylate (PMMA) walls are also frequently used in these experiments since they enable flow visualizations via their transparency properties. When sheared with smooth surfaces, the microgels slip on the surfaces and an apparent motion of the fluid occurs below the bulk yield stress τy. This motion stops at a sliding yield stress τs. The occurrence of this apparent slip below τy can significantly modify the flow features. This article presents a PMMA treatment to inhibit the slip of the Carbopol microgels. The method is simple to perform, independent of the treatment conditions and stable over time.

How do soft particle glasses yield and flow near solid surfaces?

We use fluorescence microscopy and particle tracking velocimetry to image the motion of concentrated emulsions and microgel suspensions near solid surfaces. The local deformation involves a combination of slip and bulk flow, which are found to be controlled by surface forces. With smooth surfaces, two slip mechanisms are identified depending on whether particle–wall interactions are repulsive or weakly attractive. In the former case, the materials yield uniformly and the local rheology can be mapped on the macroscopic flow curve. In the latter case, yielding is non-uniform which reveals a continuous distribution of states from the immediate vicinity of the smooth surface to the bulk of the material. The effect of the surface is long-ranged and decays exponentially with the distance, which can be described by a non-local fluidity model. Our results establish a link between surface forces, lubrication and yielding in soft glassy or jammed materials and open new routes to manipulate their flow through the surface chemistry of the confining boundaries.

Bulk and shear moduli of compressed microgel suspensions

We independently determine the bulk and shear moduli of compressed microgel suspensions and the bulk modulus of individual microgel particles and find that the elastic behavior of the suspension reflects the degree of compression of the particles. This feature, which is distinct from other soft materials such as emulsions or foams, can give rise to an unusually large difference between the bulk and shear moduli of the suspension. Our results extend our understanding of soft materials to systems based on compressible objects, opening up possibilities for engineering materials with drastically different responses to shear and compression.