Session C36: Soft Colloids: From Single Particle Properties to Bulk Phase Behavior and Dynamics

Superresolution Microscopy of the Volume Phase Transition of pNIPAM Microgels

Hierarchical polymer structures such as pNIPAM microgels have been extensively studied for their ability to undergo significant structural and physical transformations that can be controlled by external stimuli such as temperature, pH or solvent composition. Here we discuss in-situ three-dimensional superresolution microscopy of dye-labeled submicron sized PNiPAM microgels [1]. We use direct STochastic Optical Reconstruction Microscopy (dSTORM) to study the internal density distribution and the particle-to-particle variability of the volume phase transition. Moreover we discuss the potential of this technique towards future applications to more complex architectures for example microgel with anisotropic shape or ones that are doped or decorated with nanoparticles. [1] G. M. Conley, S. No¨jd, M. Braibanti, P. Schurtenberger, and F. Scheffold, submitted.   

Swelling of Superabsorbent Poly(Sodium-Acrylate Acrylamide) Hydrogels and Influence of Chemical Structure on Internally Cured Mortar

Superabsorbent hydrogel particles show promise as internal curing agents for high performance concrete (HPC). These gels can absorb and release large volumes of water and offer a solution to the problem of self-dessication in HPC. However, the gels are sensitive to ions naturally present in concrete. This research connects swelling behavior with gel-ion interactions to optimize hydrogel performance for internal curing, reducing the chance of early-age cracking and increasing the durability of HPC. Four different hydrogels of poly(sodium-acrylate acrylamide) are synthesized and characterized with swelling tests in different salt solutions. Depending on solution pH, ionic character, and gel composition, diffrerent swelling behaviors are observed. As weight percent of acrylic acid increases, gels demonstrate higher swelling ratios in reverse osmosis water, but showed substantially decreased swelling when aqueous cations are present. Additionally, in multivalent cation solutions, overshoot peaks are present, whereby the gels have a peak swelling ratio but then deswell. Multivalent cations interact with deprotonated carboxylic acid groups, constricting the gel and expelling water. Mortar containing hydrogels showed reduced autogenous shrinkage and increased relative humidity.   

Dynamics and filtration of microgel suspensions

Microgel suspensions exhibit interesting transport properties determined by direct and hydrodynamic interactions. Using an annulus model to account for solvent permeability, we calculate the diffusion function and sedimentation coefficient of PNiPAM microgel suspensions, in excellent agreement with experimental results [1]. Moreover, an extension of our precise analytic methods to long-time properties including viscosity and self-diffusion coefficient is presented, with results compared to simulation and experimental data. The predicted transport properties are an important ingredient to the modeling of convective-diffusive transport in membrane ultrafiltration of permeable particles. The efficiency of the separation process depends on hydrodynamic boundary conditions, membrane properties and particle interactions. We calculate the concentration polarization layer and permeate flux at different operating conditions for cross-flow ultrafiltration of non-ionic [2] and ionic [3] microgels. Small microgel permeability already affects the filtration significantly [2]. \begin{enumerate} \item J. Riest, T. Eckert, W. Richtering, G. N\"{a}gele, \textit{Soft Matter} \textbf{11}, 2821 (2015) \item R. Roa, E.K. Zholkovskiy, G. N\"{a}gele, \textit{Soft Matter} \textbf{11}, 4016 (2015) \item R. Roa, J. Riest, G. N\"{a}gele \textit{et al.}, \textit{to be submitted} (2015) \end{enumerate}   

Soft particles with anisotropic interactions

Responsive colloids such as thermo- or pH-sensitive microgels are ideal model systems to investigate the relationship between the nature of interparticle interactions and the plethora of self-assembled structures that can form in colloidal suspensions. They allow for a variation of the form, strength and range of the interaction potential almost at will. While microgels have extensively been used as model systems to investigate various condensed matter problems such as glass formation, jamming or crystallization, they can also be used to study systems with anisotropic interactions. Here we show results from a systematic investigation of the influence of softness and anisotropy on the structural and dynamic properties of strongly interacting suspensions. We focus first on ionic microgels.\footnote{ P. Holmqvist, P.S. Mohanty, G. Nägele, P. Schurtenberger, and M. Heinen, Phys. Rev. Lett. 109, 048302 (2012)} \footnote{ J. Riest, P. Mohanti, P. Schurtenberger, and C. N. Likos, Z. Phys. Chem. 226, 711 (2012)} Due to their large number of internal counterions they possess very large polarisabilities, and we can thus use external electrical ac fields to generate large dipolar contributions to the interparticle interaction potential. This leads to a number of new crystal phases, and we can trigger crystal-crystal phase transitions through the appropriate choice of the field strength.\footnote{ S. Nöjd, P. S. Mohanty, P. Bagheri, A. Yethiraj and P. Schurtenberger, Soft Matter 9, 9199 (2013)} \footnote{ P. S. Mohanty, P. Bagheri, S. Nöjd, A. Yethiraj and P. Schurtenberger, Phys. Rev. X 5, 011030 (2015)} We then show that this approach can be extended to more complex particle shapes \footnote{ J. J. Crassous, A. M. Mihut, L. K. Månsson, and P. Schurtenberger, Nanoscale 7, 15971-15982 (2015).} \footnote{ Linda K. Månsson, Jasper N. Immink, Adriana M. Mihut, Peter Schurtenberger, and Jérôme J. Crassous, Faraday Discussions 181, 49 (2015).} in an attempt to copy nature’s well documented success in fabricating complex nanostructures such as virus shells via self assembly.\footnote{ J. J. Crassous, A. M. Mihut, E. Wernersson, P. Pfleiderer, J. Vermant, P. Linse, and P. Schurtenberger, Nature Communications 5:5516 doi: 10.1038/ncomms6516 (2014)}   

Glass transition and jamming in soft microgel suspensions: Relationship between alpha relaxation times and elastic energy scales

Glassy and jammed states of soft colloidal matter combine several open questions -- how are glassy and jammed states differentiated from one another and from equilibrium states of dense suspensions, and how should particle ``softness'' be quantified? We combine light scattering and rheological measurements of well-characterized soft microgel particles at various packing fractions and degrees of swelling to answer these questions. We identify several regimes of liquid, supercooled, glassy, and jammed behavior that correlate with an increasing elastic energy scale due to interparticle interactions. When this energy scale increases above k$_{\mathrm{B}}$T, the entropically-driven glassy state gives way to a jammed state dominated by elastic interactions.   

Swelling, Compressibility, and Phase Behavior of Soft Ionic Microgels

Soft colloids have inspired great attention recently for their rich and tunable materials properties. Particular interest has focused on microgels -- microscopic cross-linked polymer gel particles that, when dispersed in water, become swollen and can acquire charge through dissociation of counterions. Electrostatic interparticle interactions strongly influence the structure and thermodynamics of ionic microgel suspensions*. Permeability to solvent molecules and small ions creates a competition between elastic and electrostatic forces that determines equilibrium particle sizes. Swelling can be controlled by adjusting temperature, pH, and salt concentration, with applications to chemical/biosensing and targeted drug delivery. By combining molecular dynamics and Monte Carlo simulation with Poisson-Boltzmann theory of electrostatics and Flory-Rehner theory of swollen polymer networks, we investigate swelling and compressibility of ionic microgel particles and implications for thermodynamic phase behavior of bulk suspensions at concentrations approaching and exceeding hard-sphere close packing. Predictions for particle size and osmotic pressure are compared with available experimental data. \\[1ex] *M. M. Hedrick, J. K. Chung, and A. R. Denton, J. Chem. Phys. 142, 034904 (2015).   

Structure and Hydration of Highly Branched, Monodisperse Phytoglycogen Nanoparticles

Monodisperse phytoglycogen nanoparticles are a promising, new soft colloidal nanomaterial with many applications in the personal care, food, nutraceutical and pharmaceutical industries. These applications rely on exceptional properties that emerge from the highly branched structure of phytoglycogen and its interaction with water, such as extraordinarily high water retention, and low viscosity and exceptional stability in water. The structure and hydration of the nanoparticles was characterized using small angle neutron scattering (SANS) and quasielastic neutron scattering (QENS). SANS allowed us to determine the size of the nanoparticles, evaluate their radial density profile, quantify the particle-to-particle spacing, and determine their water content. The results show clearly that the nanoparticles are highly hydrated, with each nanoparticle containing 250\% of its mass in water, and that aqueous dispersions approach a jamming transition at $\sim$ 25\% (w/w). QENS experiments provided an independent and consistent measure of the high level of hydration of the particles.   

High Deformability and Particle Size Distribution of Monodisperse Phytoglycogen Nanoparticles Revealed By Atomic Force Microscopy Imaging

We have used atomic force microscopy (AFM) imaging in water to determine the volume of hydrated monodisperse phytoglycogen nanoparticles adsorbed onto mica surfaces. By significantly reducing the interaction between the AFM tip and the ``sticky'' nanoparticles, we were able to obtain high quality images. We found that the adsorbed particles are highly deformed, forming pancake-like objects on the hydrophilic mica surface. By measuring the distribution of particle volumes, we calculated the average effective spherical radius of the hydrated particles, and compared this value with that measured in solution using small angle neutron scattering. These measurements illustrate the distinct advantages of AFM imaging over other imaging techniques, namely the ability to measure the height of objects in a liquid environment.   

Rheology of Dilute Aqueous Dispersions of Monodisperse Phytoglycogen Nanoparticles

The viscosity of dilute colloidal dispersions is well described by the Einstein relation, which is linear in the volume fraction of the particles. For hard spheres, this allows the calculation of the specific volume of the spheres [1]. For soft colloidal particles, the analysis of the data can be complicated by the uptake of the solvent by the particles. We have measured the concentration dependence of the zero shear viscosity of dilute aqueous dispersions of monodisperse phytoglycogen nanoparticles, which absorb a large amount of water (each nanoparticle contains about 250\% of its mass in water). By using values of the particle size and the hydrated and dehydrated molecular weights determined using neutron scattering, we can interpret the measured viscosity-concentration data in terms of the Einstein relation to obtain the particle density and corresponding volume fraction of the dispersions. [1] J.C. van der Werff et al., Phys. Rev. A {\bf 39}, 795 (1989).   

Ligand-Driven Phase Separation in Binary Particle Brush Materials

The tethering of polymer chains to the surface of nanoparticles (to form so-called `particle brush materials') has emerged as an effective means to enable the bottom-up assembly of one-component hybrid materials with controlled microstructure and improved mechanical stability as well as novel optical or acoustic properties. The polymer-like interactions and response of these particle-brush materials suggest intriguing new opportunities to control structure formation in multicomponent particle mixtures. This contribution will demonstrate that polymer-ligand interactions can drive phase separation processes in mixed particle systems that share analogies to those of regular binary polymer blends. The role of particle size, density and degree of polymerization of tethered chains as well as the interaction parameter between the distinct tethered chains on the mechanism and kinetics of phase separation processes in mixed particle brush systems will be discussed. Ligand-driven phase separation will be shown to enable the efficient fabrication of monochromatic domain structured in mixed quantum dot systems that might find application in next generation quantum dot-enabled LEDs.   

Colloidal models for anisotropic particles

Nanoparticles (NP) self-assembly is often thought as an equivalent to crystallization of atoms, where NP pairs exhibits effective potentials similar to atomic interactions. For the usual spherical NPs, this potential is only dependent on the distance between the particles due to symmetry. Use of anisotropic NPs provide an analog to atomic orbitals, leading to anisotropic effective potentials, which can be used to obtain new crystal lattices. \paragraph{} We express the effective potential of anisotropic NPs as the overlap between two functions, each of which is only dependent on the position and orientation of a single particle. Using a Fourier method, this contribution is expended into spherical harmonics and directly calculated in molecular dynamics simulations, reminiscent of energy calculations in quantum mechanics. \paragraph{} Using the effective potential of two spherical DNA-grafted NPs, we show an approximate method to obtain the Fourier components of an anisotropic shape, as well as the resulting simulations.   

Mesoscale simulation of asphaltene aggregation

Asphaltenes constitute a heavy aromatic crude oil fraction that can aggregate and precipitate out of solution. Association is thought to proceed hierarchically according to the Yen-Mullins model, but the molecular mechanisms and pathways remain poorly understood. In this study, we perform molecular dynamics simulations of the aggregation of hundreds of asphaltenes over microseconds using the coarse-grained Martini force field. We identified a hierarchical self-assembly mechanism consistent with Yen-Mullins model, but the details of which are strongly dependent on asphaltene molecular structure. Monomeric asphaltenes first self-assemble into 1-D rod-like nanoaggregates, followed by the formation of clusters of nanoaggregates. At high concentrations, asphaltenes with short aliphatic side chains assemble into a percolating network with the binding of 1-D rods. Conversely, molecules with more and longer side chains cannot efficiently stack, producing a fractal network of 1-D rods suspended in a sea of interpenetrating aliphatic side chains. Our results provide the first molecularly-detailed validation of the full Yen-Mullins hierarchy, and are in good agreement with recent computational and experimental studies.   

Optical Characterization of Temperature- and Composition-Dependent Microstructure in Asphalt Binders

We introduce noncontact optical microscopy and optical scattering to characterize asphalt binder microstructure at temperatures ranging from 15 to 85$^\circ$C for two compositionally different asphalt binders. We benchmark optical measurements against rheometric measurements of the magnitude of the temperature-dependent bulk complex shear modulus $|G^*(T)|$. The main findings are: (1) Elongated (~5 x 1 $\mu$m), striped microstructures (known from AFM studies as "bees" because they resemble bumble-bees) are resolved optically, found to reside primarily at the surface, and do not reappear immediately after a single heating-cooling cycle. (2) Smaller (~1 $\mu$m$^2$) microstructures with no observable internal structure (hereafter dubbed “ants”), are found to reside primarily in the bulk, to persist after multiple thermal cycles and to scatter light strongly. Optical scattering from "ants" decreases to zero with heating from 15 to 65$^\circ$C, but recovers completely upon cooling back to 15$^\circ$C, albeit with distinct hysteresis. (3) Rheometric measurements of $|G^*(T)|$ reveal hysteresis that closely resembles that observed by optical scatter, suggesting that thermally-driven changes in microstructure volume fraction cause corresponding changes in $|G^*(T)|$.