Session Q51: Gels, Complex Fluids and Vesicles

Linear Viscoelasticity and Swelling of Polyelectrolyte Complex Coacervates

The addition of near equimolar amounts of poly(diallyldimethylammonium chloride) to poly(isobutylene-alt-maleate sodium), results in formation of a polyelectrolyte complex coacervate. Zeta-potential titrations conclude that these PE-complexes are nearly charge-neutral. Swelling and rheological properties are studied at different salt concentrations in the surrounding solution. The enhanced swelling observed at high salt concentration suggests the system behaves like a polyampholyte gel, and weaker swelling at very low salt concentrations implies polyelectrolyte gel behavior. Linear viscoelastic oscillatory shear measurements indicate that the coacervates are viscoelastic liquids and that increasing ionic strength of the medium weakens the electrostatic interactions between charged units, lowering the relaxation time and viscosity. We use the time-salt superposition idea recently proposed by Spruijt, et al., allowing us to construct master curves for these soft materials. Similar swelling properties observed when varying molecular weights. Rheological measurements reveal that PE-complexes with increasing molecular weight polyelectrolytes form a network with higher crosslink density, suggesting time-molecular weight superposition idea.   

Multicomponent effects in diffusion within microemulsions

Holographic interferometry was used to monitor transport of hydrophobic solutes in systems containing nanometer-scale microemulsion droplets. In this technique, variations in the refractive index between a reference time and a later time are monitored via interference fringes that are formed with the help of a holographic plate. The refractive index change is driven by imposed concentration differences of either the solute (at constant surfactant concentration), or of the surfactant (at constant solute concentration). We find that, especially for hydrophobic solutes, the transport kinetics cannot be interpreted by using a pseudobinary approximation. Multicomponent interaction effects must be taken into account even at micelle concentrations as low as 6{\%}. By performing multiple experiments with different initial concentration gradients, and extending earlier analyses of the experimental interference fringes, the multicomponent effects can be resolved, yielding results for all the relevant diffusion coefficients.   

Determining the structure and properties of complex coacervate crosslinked triblock copolymer hydrogels

The mechanical properties and structures of functionalized P(AGE-b-EO-b-AGE) hydrogels utilizing complex coacervation as a physical crosslink have been studied. The effects of variables such as polymer concentration, salt concentration, pH, stoichiometric ratios and temperature have been investigated by rheology and SAXS. It was found that the organization of the cores has a very strong effect on the mechanical properties. This can be observed as the storage modulus increases significantly between 15 and 16 wt{\%} corresponding to a transition from a disordered gel to a BCC structure. Another dramatic change is observed when the storage modulus drops between 25 and 30 wt{\%} as the hexagonal structure becomes predominant. Just as polymer concentration causes changes in structure and thus the properties, salt concentration has a similar effect due to the electrostatic nature of the hydrogels. As salt is added, the electrostatic interactions in the cores are screened until they are weak enough that the polymers are dissolved into the matrix. The mechanical properties and the physical nature of the crosslinks lead to the possibility of these gels being used as an injectable drug delivery system.   

Density mode microrheology in polyacrylamide gels

In passive microrheology the viscoelastic properties of soft materials are deduced by observing thermal fluctuations of tracer particles embedded within the material, and the response function obtained from the spectrum of thermally excited modes is then related to the viscoelastic shear modulus. This approach is valid for single-component, isotropic, incompressible materials. However, for heterogeneous materials, such as hydrogels, a more comprehensive approach is needed. We measure the equilibrium density fluctuations of a cross-linked polymer gel swollen in a solvent and compare them to the predictions of the `two--fluid' model of the dynamics of polymer gels. We will describe a direct method of extracting the longitudinal response function of a soft material based on the temporal and spatial correlations of density fluctuations of fluorescent markers, called density mode microrheology (DMM). We will also present results of applying DMM to fluorescent polyacrylamide gels in an aqueous solvent of varying viscosity and comparing them with parameters obtained from conventional macrorheology.   

Effect of Polymer Molecular Weight and Synthesis Temperature on Structure and Dynamics of Microgels

Environmentally-sensitive microgels have been synthesized under varying conditions to study the dependences on polymer molecular weight (M$_{W})$ and synthesis temperature (T$_{syn})$. The dynamics and structure of the synthesized microgels below and above the LCST of the polymer (T$_{c}\sim $41$^{o}$C) were studied using dynamic and static light scattering spectroscopy. All microgels exhibit a volume phase transition above the LCST of the polymer and undergo a reversible 15-50-fold volume shrinkage. The size distribution, structure, deswelling ability, and temperature response of microgels strongly depend on synthesis conditions. T$_{syn}$ dependence was studied with 1000kDa polymer. Increasing $\Delta $T = T$_{syn}$ -- T$_{C}$ yields smaller microgels with a smaller swelling ratio up to $\Delta $T = 8.5$^{o}$C, after which the trend is reversed. The amphiphilic nature of the polymer may explain this trend. T$_{syn}$ also affects the structure of microgels; low T$_{syn}$ yields elongated particles, while high T$_{syn}$ microgels are more spherical. Polymer M$_{W}$ directly effects microgel polydispersity and temperature response. While microgels synthesized with 1000kDa polymer are relatively monodisperse, synthesis with low M$_{W}$ polymers (80-370kDa) yields systems with a large population (R$_{h} \quad \sim $1000nm) precipitating out of solution and a smaller population (R$_{h}$ $\sim $300nm) staying in suspension. M$_{W}$ also influences the temperature response of microgels; high M$_{W}$ microgels show a gradual shrinkage with increasing temperature while low M$_{W}$ microgels display a delayed and sudden shrinkage at high temperatures.   

Gelation and state diagram for a model nanoparticle system with adhesive hard sphere interactions

We provide the first comprehensive state diagram of thermoreversible gelation in a model nanoparticle system from dilute concentrations to the attractive driven glass. We show the temperature dependence of the interparticle potential is related to a surface molecular phase transition of the brush layer using neutron reflectivity (NR) and small-angle neutron scattering (SANS) [1]. We establish the temperature dependence of the interparticle potential using SANS, dynamic light scattering (DLS), and rheology. The potential parameters extracted from SANS suggest that, for this system, gelation is an extension of the Mode Coupling Theory (MCT) attractive driven glass line (ADG) to lower volume fractions and follows the percolation transition. Below the critical concentration, gelation proceeds without competition for phase separation [2]. These results are used to develop a complete state diagram for the sticky hard sphere reference system. \\[4pt] [1] A.P.R. Eberle, N.J. Wagner, B. Akgun, S.K. Satija, Langmuir \textbf{26} 3003 (2010).\\[0pt] [2] A.P.R. Eberle, N.J. Wagner, R. Castaneda-Priego, Phys. Rev. Let. \textbf{105704} (2011).   

Hydrodynamics and Rheology of Active Liquid Crystals

Active liquid crystals such as swimming bacteria, active gels and assemblies of motors and filaments are active complex fluids. Such systems differ from their passive counterparts in that particles absorb energy and generate motion. They are interesting from a more fundamental perspective as their dynamic phenomenons are both physically fascinating and potentially of great biological significance. In this talk, I will present a continuum model for active liquid crystals and analyze the behavior of a suspension subjected to a weak Poiseuille flow. Hydrodynamics, stability and rheology will also be discussed.   

Electrostatics-driven assembly of uni-lamellar catanionic facetted vesicles

Nature utilizes shape to generate function. Organelle and halophilic bacteria wall envelopes, for example, adopt various polyhedral shapes to compartmentalize matter. The origin of these shapes is unknown. A large variety of shell geometries, either fully faceted polyhedra or mixed Janus-like vesicles with faceted and curved domains that resemble cellular shells can be generated by coassembling water-insoluble anionic (--1) amphiphiles with high valence cationic (+2 and +3) amphiphiles. Electron microscopy, X-ray scattering, theory and simulations demonstrate that the resulting faceted ionic shells are crystalline, and stable at high salt concentrations. The crystallization of the co-assembled single tail amphiphiles is induced by ionic correlations, and modified by the solution pH. This work promotes the design of faceted shapes for various applications and improves our understanding of the origin of polyhedral shells in nature.   

Modeling controlled release from responsive microgel capsules

We introduce a coarse-grained computational method that explicitly captures the release of nanoparticles and macromolecules from responsive microgel capsules. The model is based on the dissipative particle dynamics. Our simulations reveal that not only swelling, but also deswelling of hollow microcapsules can be harnessed for controlled release. We show that the release from swollen capsules is diffusion driven, whereas the release from deswelling gel capsules occurs due to the flow of encapsulated solvent that is expelled from the shrinking capsule interior. The latter hydrodynamic release is burst-like and continues only during capsule deswelling. We find that deformable polymer chains that can easily penetrate thorough membrane pores are released in larger amounts from deswelling capsules, than nanoparticles that are filtered out by shrinking membrane pores. Our simulations further demonstrate that the inclusion of rigid microrods inside deswelling capsules mitigates the membrane pore closing, and, in this fashion, provides an effective method for regulating the rate of hydrodynamic release of nanoparticles.   

Thermoresponsive microcapsules for controlled release of hydrophilic cargo

Thermoresponsive microcapsules that collapse upon increasing the temperature above their lower critical solution temperature (LCST) such as poly(N-isopropyl acrylamide) (PNIPAM) capsules are well known. However, capsules consisting of thermoresponsive polymers that possess an upper critical solution temperature (UCST) and therefore swell upon increasing the temperature above their UCST are scarce. We will present a microfluidic method to assemble thermoresponsive poly([2-(methacryloyloxy)-ethyl]-dimethyl-[3-sulfopropyl-ammoniumhzdroxide) (PMEDSH) microcapsules that have UCST. These capsules are in a collapsed state at room temperature and become highly water permeable upon increasing the temperature above the UCST. To simultaneously allow for encapsulation of hydrophilic cargo and enable the water based polymerization reaction of the PMEDSH shell, these microcapsules are assembled as water/water/oil emulsions using capillary microfluidic devices. The resulting PMEDSH microcapsules are envisaged as delivery vehicles and microreactors that allow for temperature induced controlled release of hydrophilic cargo. .   

A Nano Engineered Membrane for Oil-Water Separation

Oil and water separation is an extremely costly problem in the petroleum industry. Pumping the complete emulsion to the surface requires substantially more power than pumping the oil alone. A membrane that can efficiently separate oil from water at the source would revolutionize this process. To this end a novel, layered, hierarchical thermoplastic membrane was fabricated with both nanoscale and microscale features. Modifying the length scales involved in fabrication of the membrane yields interesting and non-obvious implications. Under certain regimes, the microscale features independently control the membrane's permeability, while the microscale features control only the membrane's breakthrough pressure. By operating in this regime, separation efficiencies can be realized that are otherwise unattainable by conventional membranes. Taking it a step further, chemical treatments have been used to achieve higher hydrophobicity for the membrane by lowering the surface energy of the membrane surface. Although this research focused on oil-water separation, the results have implications for other multiphase systems and hold for many other filtration and separation technologies including in lab-on-chip devices and micro/nanofluidic devices.   

Tether formation on a settling vesicle

When submitted to a point-like force, a phospholipid vesicle (a lipid membrane enclosing a drop) is known to develop a narrow tether. This tether formation is reminiscent of drop pinch-off, but the peculiar properties of the vesicle interface prevents the apparition of a finite-time singularity. It is shown that a settling vesicle may develop such tethered shapes, with hydrodynamic stresses acting as the pulling force. These shapes are studied numerically and theoretically, and continuous families of stationary tethered shapes are found, depending on two control parameters. Dynamics of formation is studied and it is shown that changing the initial condition can lead to complex transients, with formation of pearls onto the tether.   

Electrohydrodynamic instabilities of biomimetic bilayer membranes

Living cells actively maintain electrochemical potentials across their membranes, which regulates cell migration, motility, and development. In this presentation, we focus on the effect of an external electric field on membrane dynamics. We present a physical model for the dynamic coupling between transmembrane potential and deformation of biomimetic membranes. We perform linear stability analysis to clarify and quantify the effects of the lipid bilayer properties (conductivity and capacitance), and asymmetry in the embedding electrolyte solutions, on membrane deformation.   

Polyoxometalate (POM) Nanocluster-Induced Phase Transition and Structural Disruption in Lipid Bilayers.

Polyoxometalate (POM) nanoclusters that are transition metal oxygen clusters with well defined atomic coordination structures have recently emerged as new and functional nanocolloidal materials used as catalysts, anti-cancer medicines, and building blocks for novel functional materials. However, their implications to human health and environment remain poorly investigated. In this work, we examine the interaction of highly charged anionic POM nanocluters with lipid bilayers as a model cell membrane system. It is observed that upon the adsorption of anionic POMs, lipid dynamics is significantly suppressed and lipid bilayers are disrupted with resultant pore and budding-like structural formation. Direct calorimetric experiment of POM interaction with lipid bilayers of varied lipid compositions confirms the POM-induced fluid-to-gel phase transition in lipid bilayers, due to strong electrostatic interaction between POM nanocluster and lipid head groups.   

Liquid-to-solid transition in suspensions of swollen microgels

We investigate the phase and non-equilibrium behavior of suspensions comprised of swollen, ionic microgels as a function of particle stiffness. We find that stiff particles exhibit all three phases observed in hard sphere suspensions, liquid, crystal and glass. 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. Interestingly, for microgels with intermediate stiffness, we find that the bulk modulus of individual particles seems to control the mechanical properties of the microgel suspension in the overpacked regime, emphasizing the relevance of being compressible.