Microrheology of entangled polymer solutions

Microrheology of semidilute polymer solutions is investigated. In this paper we calculate a response function of a probe particle embedded in a semidilute polymer solution by analyzing the two-fluid model. We find that when the size of the probe particle is comparable to the viscoelastic length, the response from the longitudinal compression modes becomes more important than that of the transverse shear modes. As a result, depending on the circumstances, the obtained complex shear modulus cannot be well approximated by that measured in macroscopic rheology experiments. The present results are due to the dynamical asymmetry coupling and the existence of the cooperative dynamics, which are intrinsic to entangled polymer solutions.     

Microrheology of aggregated emulsion droplet networks, studied with AFM-CSLM

We studied the mechanical behavior of densely packed (up to approximately 30% v/v), sedimented layers of (1 microm) water-in-oil W/O emulsion droplets, upon indentation with a (10 microm) large spherical probe. In the presence of attractive forces, the droplets form solid like networks which can resist deformation. Adding a polymer to the oil phase was used to control droplet attraction. The droplet layers were assembled via normal gravity settling. Considering that both the network structure and the droplet interactions play a key role, we used a combination of atomic force microscopy (AFM) and confocal scanning laser microscopy (CSLM) to characterize the mechanical behavior. Here the AFM was used both as indentation tool and as force sensor. Indentation experiments were performed via a protocol consisting of approach, waiting, and retract stages. CSLM was used to observe the network structure at micron resolution in real time. Use of refractive index matched fluorescent droplets allowed the visualization of the entire layer. Upon compression with the probe, a markedly nonhomogeneous deformation occurred, evidenced by the formation of a dense corona (containing practically all of the displaced droplets) in the direct vicinity of the probe, as well as more subtle deformations of force-chains at larger distances. Upon decompression, both the imprint of the indenter and the corona remained, even long after the load was released. The force-distance curves recorded with the AFM correspond well to these observations. For each deformation cycle performed on fresh material, the retract curve was much steeper than the approach curve, thus corroborating the occurrence of irreversible compaction. Contrary to classic linear viscoelastic materials, this hysteresis did not show any dependence on the deformation speed. Our force-indentation approach curves were seen to scale roughly as F approximately delta(3/2). The pre-factor was found to increase with the polymer concentration and with the density of the network. These findings suggest that this new AFM-CSLM method could be used for rheological characterization of small volumes of "granular networks" in liquid. Our hypothesis that the mechanical resistance of the networks originates from interdroplet friction forces, which in turn are set by the interdroplet potential forces, is supported by the predictions from a new mechanical model in which the interdroplet bonds are represented by stick-slip elements.     

Microrheology with optical tweezers: peaks & troughs

Since their first appearance in the 1970s, optical tweezers have been successfully exploited for a variety of applications throughout the natural sciences, revolutionising the field of microsensing. However, when adopted for microrheology studies, there exist some peaks and troughs on their modus operandi and data analysis that I wish to address and possibly iron out, providing a guide to future rheological studies from a microscopic perspective.     

Microrheology of chemically crosslinked polymer gels by diffusing-wave spectroscopy

The Brownian motion of probe particles in aqueous solutions of poly(vinyl alcohol)(PVA) and in chemically crosslinked PVA gels has been studied by diffusing-wave spectroscopy (DWS). At long time scales the measurements allow us to determine the effect of the crosslinking ratio on the macroscopic viscosity of sols and the shear modulus of gels. The local shear modulus of gels as obtained from the characteristic length of the Brownian cage was found to agree with that measured by classical rheometry and dynamic light scattering (DLS). These microrheological techniques were applied to two polymer gel systems. Substrate induced gradient structure of hydrogels was studied from a microrheological point of view using DLS. It is clearly seen that hydrophobic substrate induces weakly crosslinked network formation at the interface region up to a few millimeters as expected from other experimental facts. Magnetic particle motion in gels under external magnetic field was investigated by DWS. The translational motion of the magnetic particles in gels due to the alternating magnetic force can be detected and found to be superimposed on the relaxation due to the thermal motion.     

用探针粒子示踪微流变法测量明胶的动态不均匀性

利用探针粒子示踪微流变法对明胶溶液等温凝胶化过程的动态不均匀性进行了观测.通过向体系中引入探针粒子,利用广义的Stokes-Einstein关系由探针粒子的位移建立了体系结构的空间分布.进而利用van Hove函数和非高斯因子描述了凝胶化转变前后动态不均匀性的变化,结果表明在凝胶化后体系动态在空间和时间尺度上都是不均匀的,与凝胶化前相比,凝胶化后的快动态有更高的贡献.为了进一步探究这种动态不均匀性在空间和时间上相关的变化,本工作在粒子示踪技术的基础上,实现了四点相关函数和极化率的测量.实验结果表明,凝胶化前体系的快松弛可以原位独立完成,而慢松弛则需要周围的结构单元协同完成;凝胶化后体系的快松弛和慢松弛均需要协同完成.     

Microrheology and particle tracking in food gels and emulsions

Particle tracking microrheology, an emerging experimental technique, which utilizes the Brownian motion of embedded particles to probe local dynamics of soft materials, is presented. Particle tracking microrheology is a powerful technique that enables the measurement of viscoelastic responses in small sample volumes, which are inaccessible to macrorheology and to spatially map structural heterogeneities at a microlevel. Therefore, particle tracking microrheology has considerable potential in food emulsions and gels, since these systems are commonly inhomogeneous. Recent advances and achievements are discussed, including the basic principles, operating regimes and limitations of the technique. The application of the technique in the field of food gels and emulsions to study the evolving dynamics of inhomogeneous at microscale length systems and during sol–gel transition is highlighted.     

Microrheology of wormlike micellar fluids from the diffusion of colloidal probes

The microrheology of cationic micellar solutions has been investigated as a function of added organic salts using quasielastic light scattering (QELS). Two organic salts, sodium p-toluene sulfonate and sodium salicylate, were used to induce microstructural changes in cetyl trimethylammonium bromide (CTAB) micelles. The mean-squared displacement (MSD) of polystyrene probe particles embedded in CTAB micellar solutions was monitored by QELS in the single-scattering regime. Through the use of the generalized Stokes-Einstein relationship, the frequency-dependent complex shear moduli of each fluid were estimated from the Laplace transform of the corresponding MSD. The salt-induced transition from nearly spherical to elongated wormlike micelles and consequent changes in fluid response from viscous to viscoelastic are clearly captured by microrheology.     

Microrheology of bacterial biofilms in vitro: Staphylococcus aureus and Pseudomonas aeruginosa

The rheology of bacterial biofilms at the micron scale is an important step to understanding the communal lifecycles of bacteria that adhere to solid surfaces, as it measures how they mutually adhere and desorb. Improvements in particle-tracking software and imaging hardware have allowed us to successfully employ particle-tracking microrheology to measuring single-species bacterial biofilms, based on Staphlococcus aureus and Pseudomonas aeruginosa. By tracking displacements of the cells at a range of timescales, we separate active and thermal contributions to the cell motion. The S. aureus biofilms in particular show power-law rheology, in common with other dense colloidal suspensions. By calculating the mean compliance of S. aureus biofilms, we observe them becoming less compliant during growth, and more compliant during starvation. The biofilms are rheologically inhomogeneous on the micron scale, as a result of the strength of initial adhesion to the flow cell surface, the arrangement of individual bacteria, and larger-scale structures such as flocs of P. aeruginosa. Our S. aureus biofilms became homogeneous as a function of height as they matured: the rheological environment experienced by a bacterium became independent of how far it lived from the flow cell surface. Particle-tracking microrheology provides a quantitative measure of the "strength" of a biofilm. It may therefore prove useful in identifying drug targets and characterizing the effect of specific molecular changes on the micron-scale rheology of biofilms.     

Wire‐Active Microrheology to Differentiate Viscoelastic Liquids from Soft Solids

Viscoelastic liquids are characterized by a finite static viscosity and a yield stress of zero, whereas soft solids have an infinite viscosity and a non‐zero yield stress. The rheological nature of viscoelastic materials has long been a challenge and is still a matter of debate. Here, we provide for the first time the constitutive equations of linear viscoelasticity for magnetic wires in yield‐stress materials, together with experimental measurements by using magnetic rotational spectroscopy (MRS). In MRS, the wires were subjected to a rotational magnetic field as a function of frequency and the motion of the wire was monitored by using time‐lapse microscopy. The studied soft solids were aqueous dispersions of gel‐forming polysaccharide (gellan gum) at concentrations above the gelification point. It was found that soft solids exhibited a clear and distinctive signature compared with viscous and viscoelastic liquids. In particular, the average wire rotation velocity equaled zero over a broad frequency range. We also showed that the MRS technique is quantitative. The equilibrium elastic modulus was retrieved from the wire oscillation amplitudes, and agrees with polymer‐dynamics theory.     

Nanostructure of polysaccharide complexes

The interaction of gum arabic (GA) with chitosan (Ch) of different degree of deacetylation was studied by turbidity measurements, dynamic light scattering and atomic force microscopy. The structure of the complexes was found to be directly related to the charge density of chitosan molecules. Gum arabic and chitosan with a degree of deacetylation of 75% form soluble complexes with a loosely globular structure of about 250 nm, at weight ratios up to 1.2, if the concentrations are kept low (total biopolymer concentration up to 0.06%). If chitosan has a higher charge density (degree of deacetylation of 93%), colloidal particles are formed, independently of the polymer concentration or ratio. At low concentrations and GA/Ch ratios of 1 or 1.2, the particles have diameters of 200-250 nm. The formation of soluble complexes is attributed to a chitosan lower charge density and the presence of non-charged monomers, which prevent the efficient self-assembly of the macromolecules.     

离子液体双水相萃取山楂黄酮和多糖的相行为研究

用分光度法研究了[Bmim]BF4/(NH4)2SO4双水相体系萃取山楂黄酮和多糖的相行为,考察了同时提取这两种组分时[Bmim]BF4的浓度、(NH4)2SO4的浓度、山楂的用量、超声萃取时间等因素对双水相的上下相体积以及分配系数的影响。结果表明:(1)离子液体浓度的增加,双水相的上、下相体积分别明显增大和减小。但黄酮和多糖在双水相中的分配系数仅有波动,均小于5%;(2)硫酸铵浓度的增加,双水相的上下相体积分别明显减小和增大。同时黄酮和多糖的分配系数均有较明显先降低后升高的趋势;(3)山楂质量的增减,不影响双水相的形成,但存在一个实验条件下的溶出饱和值,该值为0.15g,此时黄酮和多糖的分配系数最大;(4)超声萃取时间的延长或缩短,同样不影响双水相的形成,但存在对山楂有效成分达到饱和溶出的阈值,对于黄酮为25min,多糖为20min。     

An acidic polysaccharide having activity on the reticuloendothelial system from the root of Astragalus mongholicus.

An acidic polysaccharide, designated as AMon-S, was isolated from the roots of Astragalus mongholicus BUNGE. It was homogeneous on electrophoresis and gel chromatography, and its molecular mass was estimated to be 7.6 x 10(4). It showed significant reticuloendothelial system-potentiating activity in a carbon clearance test. It is composed of L-arabinose: D-galactose: D-galacturonic acid: D-glucuronic acid in the molar ratio of 18:18:1:1, in addition to small amounts of O-acetyl groups and peptide moiety. A part of the hexuronic acid residues exist as the methyl esters. Methylation analysis, carbon-13 nuclear magnetic resonance and periodate oxidation studies enabled elucidation of its structural features and revealed mainly alpha-arabino-beta-3,6-galactan type structural units.     

A novel neutral polysaccharide having activity on the reticuloendothelial system from the root of Glycyrrhiza uralensis

A neutral polysaccharide, named glycyrrhizan UC, was isolated from the root of Glycyrrhiza uralensis Fischer. It was homogeneous on electrophoresis and gel chromatography, and its molecular mass was estimated to be 69,000. Glycyrrhizan UC is composed of L-arabinose:D-galactose:D-glucose:L-rhamnose in the molar ratio of 10:30 27:1. Methylation analysis, carbon-13 nuclear magnetic resonance and periodate oxidation studies indicated its structural feature as an arabino-3,6-galacto-glucan type polysaccharide.     

Self-organization in the chitosan-clay nanoparticles system regulated through polysaccharide macromolecule charging. 1. Hydrogels

Scanning and transmission electron microscopy and dynamic rheology are used to thoroughly characterize the morphology and mechanical properties of bionanocomposite hydrogel prepared by mixing cationic chitosan with negatively charged particles of a synthetic clay (saponite) followed by gradual increasing of the charge of chitosan macromolecules by decreasing the pH of a medium. Gelation of the system is found to be due to the formation of a fibrillar three-dimensional network structure. It is shown that the size fibrils, the density of the network structure, and its mechanical properties are determined by the concentration ratio between the polysaccharide and saponite particles.     

Self-organization in the chitosan-clay nanoparticles system regulated through polysaccharide macromolecule charging. 2. Films

Formation conditions are studied for bionanocomposite films prepared by mixing cationic chitosan with negatively charged nanoparticles of a synthetic clay (saponite) followed by gradual increasing of the charge of macromolecules by decreasing the pH of a medium. The data on the swelling of the bionanocomposite films in water are used to determine the stoichiometric ratio between the concentrations of macromolecules and nanoparticles that provides the most intense electrostatic interactions stabilizing the films. Their properties and structure are investigated by means of scanning electron microscopy, dynamic thermomechanical analysis, and small-angle X-ray scattering. The films are shown to occur in a glassy state and undergo a number of phase transitions, the temperatures of which depend on the chitosan-to-saponite concentration ratio. In particular, their glass transition temperature increases from 62 to 175°C when passing to the stoichiometric composition. The bionanocomposite films are found to have a layered structure. The layers are, in turn, composed of highly uniform microsized plates 20?C30 nm thick. Small-angle X-ray scattering shows a structural order with a periodicity of 1.78 nm. The structure of the bionanocomposite films is discussed.     

Biological activities of biodegradable polysaccharide

A low toxicity of chitin was demonstrated to be mostly due to biodegradability and the fast metabolization of hydrolysate in animal body. Chitosan, a deacetylated derivative of chitin, is also known to be a biocompatible polymer in spite of a slight immunoadjuvant activity in animal body. Chitosan is easily regenerated to fiber, film, beads and non woven fabrics owing to its high solubility toward diluted aqueous organic acids such as formic acid, acetic acid, glutamic acid and ascorbic acid. The regeneration of chitin was achieved into fibers, gels, porous foams and non woven fabrics following to dissolution of chitin with formic acid or calcium chloride dihydrate saturated methanol. Chitin and its derivatives have been applied as biomedical materials due to remarkable advantages such as antimicrobial activity, acceleration of epidermal cell assembly, low toxicity, and biodegradability.