Phase behavior of colloid-polymer mixtures with unary or binary depletants

Submicron particles suspended in solutions containing surfactants, polymers, micelles, or other species are widely used in materials shaping and forming processes, including three-dimensional printing and nanocomposite processing, and in technical applications as paints, coatings, inks, and drilling muds. These applications require control over suspension rheology and microstructure, which are affected by interactions between the different constituents. Practically, constituents of high dispersity in size or molecular weight are inexpensive and hence widely used; fundamentally, the effects of size dispersity on suspension properties remain poorly understood when the particles exhibit attractive as well as repulsive interparticle interactions. As simple models of practical suspensions we formulate mixtures of submicron poly(methyl methacrylate) (PMMA) particles suspended in solutions of non adsorbing polystyrene polymers, which generate a controlled entropic depletion attraction between the particles. Here, I will discuss studies in which we investigate the effect of polymer dispersity on the phase behavior, microstructure, and rheology of colloid-polymer mixtures. We added unary or binary mixtures of polystyrene as the depletant to suspensions of charged PMMA particles. The structure and dynamics of the particles were compared over three sets of samples with various mixtures of two different polystyrenes whose size varied by an order of magnitude. The structure and dynamics were nearly independent of depletant dispersity if the polymer concentration was represented as a sum of normalized concentrations of each species. Near the transition region between a fluid of clusters and an interconnected gel at intermediate volume fractions, partitioning of polymers in a binary mixture into colloid-rich and polymer-rich phase leads to a slightly different gelation pathway. Hence this work suggests that polymers of high dispersity, which are more affordable than uniformly distributed polymers, can be used for applications requiring certain final structures if all polymers in the distribution are small compared to the particles and if the desired phase behavior is far from non-equilibrium boundaries. It also suggests the ability to tune the final polymer concentration by mixing polymers of different sizes to control particle phase behavior in solution. Please click Additional Files below to see the full abstract

Tunable Assembly of Gold Nanorods in Polymer Solutions to Generate Controlled Nanostructured Materials

Gold nanorods grafted with short chain polymers are assembled into controlled open structures using polymer-induced depletion interactions and structurally characterized using small angle x-ray scattering. When the nanorod diameter is smaller than the radius of gyration of the depletant polymer, the depletion interaction depends solely on the correlation length of the polymer solution and not directly on the polymer molecular weight. As the polymer concentration increases, the stronger depletion interactions increasingly compress the grafted chains and push the gold nanorods closer together. By contrast, other structural characteristics such as the number of nearest neighbors and fractal dimension exhibit a non-monotonic dependence on polymer concentration. These parameters are maximal at intermediate concentrations, which are attributed to a crossover from reaction-limited to diffusion-limited aggregation. The control over structural properties of anisotropic nanoscale building blocks demonstrated here will be beneficial to designing and producing materials \emph{in situ} with specific direction-dependent nanoscale properties and provides a crucial route for advances in additive manufacturing

Transport of nanoparticles in polyelectrolyte solutions as a model of polymer nanocomposite processing

The transport properties of nanoparticles in complex confined media play a significant role in the processing of advanced polymer nanocomposites. In polymer nanocomposite processing, nanoparticles suspended in polymer resins must be efficiently dispersed to obtain optimal mechanical, electrical, thermal, and/or optical properties and eliminate stress concentrators. Here, we investigate the diffusion and dispersion of nanoparticles in solutions of unentangled polyelectrolytes as a model system in which to understand the coupling between particle and polymer dynamics in flow conditions representative of those encountered during nanocomposite processing. First, we measure the long-time quiescent diffusivity of fluorescent polystyrene nanoparticles in dilute and semidilute solutions of partially hydrolyzed polyacrylamide. At short time scales, the particles exhibit subdiffusive behavior, as characterized by sublinear scaling of the mean-square displacement with time. On long time scales, the particles exhibit Fickian diffusion from which the diffusivities are extracted. Whereas diffusivities of the large particle agree with predictions using the Stokes-Einstein equation and bulk zero-shear viscosity, the smaller particles diffuse much faster than predicted. To capture the particle- and polymer-size dependence of the long-time diffusivity, we propose a model in which particles diffuse in a matrix with relaxing constraints caused by the diffusion of polymer segments. We derive an effective length scale that collapses the long-time diffusivities onto a single curve and cleanly captures a smooth crossover to bulk behavior when the particles are much larger than the polymer chains. Furthermore, the effective length scale controls the crossover time scale between subdiffusive and Fickian behavior for all particle sizes and polymer concentrations. These results demonstrate that polymer mobility controls the size-dependent deviations from Stokes-Einstein behavior for nanoparticles diffusing in unentangled polymer solutions Second, we visualize transport of nanoparticles suspended in Newtonian and non-Newtonian solutions through porous media using confocal microscopy. We flow nanoparticles suspended in glycerol-water mixtures or in solutions of unentangled polyelectrolytes through porous media of varying pore size. Both the longitudinal and transverse distributions of normalized velocities scale onto master curves, which are independent of solution viscoelasticity, pore size, and flow properties. Although the polymer elasticity may affect the time over which the dispersion coefficients approach asymptotic limits, the long-time (asymptotic) dispersion coefficients of nanoparticles in both the longitudinal and transverse directions scale onto master curves as a function of the Péclet number characterizing the flows, again independent of viscoelasticity, pore size, and flow properties. These results suggest that flow through rigid porous media, such as matrix fibers in certain polymer nanocomposites, may “break” the non-Newtonian characteristics of a complex polymeric solution

Bacteria Use Type IV Pili to Walk Upright and Detach from Surfaces

1. Department of Bioengineering, California Nano Systems Institute,University of California, Los Angeles, CA 90024, USA. 2. Department of Chemical and Biomolecular Engineering, University of Houston, Houston, TX 77204, USA. 3. Department of Materials Science and Engineering, University of Illinois, Urbana-Champaign, IL 61801, USA. 4. Department of Civil Engineering and Geological Sciences, University of Notre Dame, Notre Dame, IN 46556, USA.Bacterial biofilms are structured multicellular communities involved in a broad range of infections. Knowing how free-swimming bacteria adapt their motility mechanisms near surfaces is crucial for understanding the transition between planktonic and biofilm phenotypes. By translating microscopy movies into searchable databases of bacterial behavior, we identified fundamental type IV pili–driven mechanisms for Pseudomonas aeruginosa surface motility involved in distinct foraging strategies. Bacteria stood upright and “walked” with trajectories optimized for two-dimensional surface exploration. Vertical orientation facilitated surface detachment and could influence biofilm morphology.Center for Nonlinear Dynamic

Increasing Binding Efficiency via Reporter Shape and Flux in a Viral Nanoparticle Lateral-Flow Assay

To identify factors controlling the performance of reporter particles in a sensitive lateral-flow assay (LFA), we investigated the effect of the flux and shape of filamentous bacteriophage (phage) on the performance of phage LFAs. Phage of three different lengths and diameters were modified with biotin and AlexaFluor 555 as binding and read-out elements, respectively. The binding efficiencies of the functionalized phage were tested in a fibrous glass LFA membrane modified with avidin. The total binding rate, quantified using real-time particle counting and particle image velocimetry, decreased monotonically with the average bulk flux of phage through the membrane. At the pore scale, more phage bound in regions with faster local flow, confirming that both average and local flux increased binding. The number of bound phage increased with the aspect ratio of the phage and scaled with the phage surface area, consistent with a binding interaction controlled by the number of recognition elements on the surface. Together, these results indicate that increasing the likelihood that recognition elements on the surface of phage encounter the fibers enhances the assay binding efficiency and suggests one origin for the improved performance of nonspherical phage reporters

Evaluation of Air Pollution Tolerance Index and Anticipated Performance Index of Selected Plant Species

This study reports a combination of two indices, air pollution tolerance index (APTI) and anticipated performance index (API) as viable tools for selecting suitable plants for pollution abatement program. Leaf samples of 6 plant species; Mangifera indica, Araucaria heterophylla, Elaeis guineensis, Syzygium malaccense, Acacia auriculiformis, and Chrysophyllum albidium were collected from an industrial and academic areas at Ado-Odo, Ota, Nigeria; during the dry season of January to March 2018. Biochemical parameters; leaf-pH, relative leaf water content, total chlorophyll content, and ascorbic acid content were analyzed to compute the APTI values. Combined APTI, botanical and socioeconomic indices were graded to evaluate the API of the different plant species. The APTI for the species ranged between 4.79 and 10.7, ideal for sensitive species category (APTI < 11), and the plants are classified as bio indicators of air pollution. The API indicates Mangifera indica and Syzygium malaccense (API = 4) as good performers while Chrysophyllum albidum is a moderate performer (API = 3). The three tree species were identified as suitable green belt plants and thus valuable additions to the green belt development plant list in tropical Africa

Orientational binding modes of reporters in a viral-nanoparticle lateral flow assay

Using microscopy and image analysis, we characterize binding of filamentous viral nanoparticles to a fibrous affinity matrix as models for reporter capture in a lateral flow assay (LFA). M13 bacteriophage (M13) displaying an in vivo-biotinylated peptide (AviTag) genetically fused to the M13 tail protein p3 are functionalized with fluorescent labels. We functionalize glass fiber LFA membranes with antibodies to M13, which primarily capture M13 on the major p8 coat proteins, or with avidin, which captures M13 at the biotin-functionalized tail, and compare orientational modes of reporter capture for the side- versus tip-binding recognition interactions. The number of captured M13 is greater for side-binding than for tip-binding, as expected from the number of recognition groups. Whereas two-thirds of side-bound M13 captured by an anti-M13 antibody bind immediately after colliding with the membrane, tip-bound M13 prominently exhibit three additional orientational modes that require M13 to reorient to enable binding. These results are consistent with the idea that the elongated M13 shape couples with the complex flow field in an open and disordered fibrous LFA membrane to enhance capture