Direct Observation of Relaxation of Aqueous Shake-Gel Consisting of Silica Nanoparticles and Polyethylene Oxide

Controlling the rheological property of suspensions consisting of colloidal particles and polymers is necessary in industry. Especially, gels induced by shear (shake-gel) are interesting phenomena in rheological field. To gain insight into the shake-gel phenomena of the aqueous suspensions of silica nanoparticles and poly(ethylene oxide) (PEO) and its temporal change, we observed the state transition and measured the viscosity of the silica-PEO suspensions. Our results showed that PEO dose, pH, and molecular weight of PEO influence the state of suspension greatly, and revealed the differences of the suspension states, namely, cloudy, permanent gel, shake-gel, and high viscosity sol. We found that the relaxation time from shake-gel to flowable sol increases to the maximum and decreases again with increasing PEO dose. Shake-gels at pH 8.4 relaxed more slowly than at pH 9.4, and shake-gel did not form at pH above 10 in most of cases, indicating high pH inhibits the formation of shake-gels. PEO of molecular weight of 1000 and 4000 kDa easily bonds more silica nanoparticles by bridging and results in the formation of gels with more stable polymer networks. PEO of molecular weight of 1000 and 4000 kDa also led to longer relaxation time of the silica-PEO suspensions from gel to sol

Removal Characteristics of Water Treatment Systems Applied to Deconstruction Effluents

汚染源を対象とする浄化対策へ水処理システムを適用する場合, 水質変動への安全率や事業コストを考慮した合理的なシステムが求められる。合理的なシステムを構築するためには, 様々な実践データの蓄積が重要である。本論文では, 焼却施設解体工事の除染工程から発生する排水に対し, 現場において凝集沈殿処理, 膜ろ過処理, 促進酸化処理からなる水処理システムが適用された事例について報告する。当該処理システムにおいて, 重金属類は凝集沈殿処理および膜ろ過処理によって環境基準以下まで除去可能であった。ダイオキシン類については, 微粒子の凝集操作を適切におこなうことにより, 膜ろ過および促進酸化処理による除去性能が向上する傾向が見られ, 環境基準以下まで除去可能であった。合理的な水処理システムを適用するためには, 排水の発生過程と性状, 処理システム全体での除去特性を考慮して検討する必要があることを示した

Aggregation and charge reversal of humic substances in the presence of hydrophobic monovalent counter-ions: Effect of hydrophobicity of humic substances

To investigate the effect of hydrophobicity of humic substances (HSs) on their charging and aggregation, we studied the electrophoretic mobility and aggregation-dispersion of HSs in the presence of hydrophobic monovalent cations, namely, tetraphenylphosponium TPP+. The used HSs were standard Suwannee river fulvic acid (SRFA), Suwannee river humic acid (SRHA), and Leonardite humic acid (LHA) with different contents of aromatic carbons. All of the HSs in the presence of TPP+ showed charger reversal. The charge reversal pH or iso-electric point (IEP) of LHA was higher than that of SRFA and SRHA in every concentrations of TPPCl, demonstrating the strong hydrophobic interaction between HSs, especially LHA, and TPP+. We also found that the formation of large visible aggregates of all the HSs at lower pH in the presence of TPP+. Large HS aggregates were markedly formed for LHA, manifesting the existence of stronger hydrophobic attraction among LHAs with TPP+. The appearance of aggregates of all the HSs with TPP+ was confirmed by microscopic observation and the size determination by dynamic light scattering. The HS aggregates showed fractal structure. The values of fractal dimension Df of HS aggregates were 2–2.2 in quiescent conditions, indicating that the HS aggregates with TPP+ were formed via cluster-cluster aggregation with restructuring. The Df increased to 2.8–2.9 in stirring conditions, implying that the compact aggregates were formed through the continuous aggregate breakage and the regrowth between smaller aggregates and larger aggregates

Affirmation of the effect of pH on shake-gel and shear thickening of a mixed suspension of polyethylene oxide and silica nanoparticles

The mixture of silica nanoparticles and polyethylene oxide (PEO) shows unique rheological behaviors, such as reversible gelation and shear thickening, when moderate shear is applied. These behaviors are attributed to the transient bridging network, which is formed via the simultaneous adsorption of a PEO chain onto multiple silica nanoparticles. The adsorption affinity depends on pH and thus, we expect that these behaviors change with pH. Nevertheless, the effect of pH on shake-gel behavior and shear thickening has not yet been systematically examined. In order to improve our understanding, we attempt to study the influence of pH and ionic strength on the relaxation time of the gel and the viscosity of the mixture. Our experimental results demonstrate that shake-gel and shear thickening can be observed in the pH range of 8.0–9.9. Moreover, the relaxation time required for the gel recovery to sol increases and the critical shear rate at which the viscosity begins to rise decreases as the pH decreases. Furthermore, we determined that irreversible shake-gels can be obtained in a narrow range of PEO concentrations at the pH of approximately 8.0. In conclusion, we determined that the relaxation time of gel is longer at low pH, and the critical shear rate decreases with the decrease in interparticle repulsion of silica

Electrophoretic mobility of carboxyl latex particles: effects of hydrophobic monovalent counter-ions

To investigate the effect of hydrophobicity on the charge reversal of colloidal particles, we measured and analyzed the electrophoretic mobility (EPM) of carboxyl latex particles in mixed electrolytes solutions containing potassium chloride KCl and tetraphenylphosphonium chloride TPPCl. Tetraphenylphosphonium (TPP+) ion strongly adsorbs on the particle surface due to its hydrophobicity and thus causes the charge reversal/overcharging. Measurements of EPM were carried out as functions of pH, ionic strength, and the mixed molar ratio of X = [TPP+]/[K+] to unveil the influence of surface charge on hydrophobic interaction. Experimental EPM was analyzed by using 1-pKH Stern Gouy Chapmann model with the Ohshima equation including the relaxation effect or the Smoluchowski equation neglecting the relaxation effect for calculating theoretical EPM values. Our results demonstrate that carboxyl latex particles show charge reversal indicated by positive EPMs at low pH due to the accumulation of TPP+ ions on the surface and the reversed EPM values at low pH are augmented with increasing the mixed molar ratio of X = [TPP+]/[K+]. Also, we observed that charge re-reversal at higher pH as the deprotonation of surface carboxyl groups proceeded. The pH at which the charge re-reversal occurred increased with increasing the mixed molar ratio. From the comparison between our experiments and theoretical analysis, we found that the intrinsic energy of adsorption decreases with increasing the surface charge density to describe the observed charge re-reversal. These results indicate that the intrinsic adsorption energy of TPP+, which is probably due to hydrophobic interaction, decreases with increasing the surface charge density

Syndecan- and integrin-binding peptides synergistically accelerate cell adhesion

AbstractIntegrins and syndecans mediate cell adhesion to extracellular matrix and their synergistic cooperation is implicated in cell adhesion processes. We previously identified two active peptides, AG73 and EF1, from the laminin α1 chain LG4 module, that promote cell attachment through syndecan- and α2β1 integrin-binding, respectively. Here, we examined time-dependent cell attachment on the mixed peptides AG73/EF1. The AG73/EF1 promoted stronger and more rapid cell attachment, spreading, FAK phosphorylation that reached a maximum at 20min than that on AG73 (40min) or EF1 (90min) supplied singly. Thus, the syndecan- and α2β1 integrin-binding peptides synergistically affect cells and accelerate cell adhesion

Eye contrast polarity is critical for face recognition by infants

Just as faces share the same basic arrangement of features, with two eyes above a nose above a mouth, human eyes all share the same basic contrast polarity relations, with a sclera lighter than an iris and a pupil, and this is unique among primates. The current study examined whether this bright-dark relationship of sclera to iris plays a critical role in face recognition from early in development. Specifically, we tested face discrimination in 7- and 8-month-old infants while independently manipulating the contrast polarity of the eye region and of the rest of the face. This gave four face contrast polarity conditions: fully positive condition, fully negative condition, positive face with negated eyes ( negative eyes ) condition, and negated face with positive eyes ( positive eyes ) condition. In a familiarization and novelty preference procedure, we found that 7- and 8-month-olds could discriminate between faces only when the contrast polarity of the eyes was preserved (positive) and that this did not depend on the contrast polarity of the rest of the face. This demonstrates the critical role of eye contrast polarity for face recognition in 7- and 8-month-olds and is consistent with previous findings for adults

Kinetics of turbulent hetero-coagulation of oppositely charged colloidal particles

We investigated the hetero-coagulation rates between oppositely charged particles subjected to a turbulent flow. The turbulent hetero-coagulation was induced by mixing and stirring the aqueous suspensions of positively and negatively charged latex particles. The sign and magnitude of the surface charge densities of the particles were conrmed by the analysis of measured electrophoretic mobilities. The turbulent coagulation experiments were carried out as a function of particle diameter, KCl concentration, and turbulent intensity. The rate of coagulation was obtained by measuring temporal change of suspension turbidity. The experimental results show that the hetero-coagulation rate constant increases with decreasing KCl concentration and with increasing particle size and turbulent intensity. These trends qualitatively agree with the theoretical prediction based on the aggregation kinetics in a turbulent ow combined with a trajectory analysis including the attractive electrostatic interactions. The facilitated coagulation rates in lower KCl concentrations are considered to be due to the increase of the magnitude of attractive electrostatic force accompanied by the development of electrical double layer.第63回理論応用力学講演会(The 63rd National Congress of Theoretical and Applied Mechanics)における英文論文

Charge reversal of sulfate latex induced by hydrophobic counterion: effects of surface charge density

We studied experimentally and theoretically the charge reversal of sulfate latex colloid in the presence of monovalent hydrophobic counterion TPP+ (tetraphenylphosphonium). The intrinsic or chemical energy of adsorption of TPP+ on the latex was evaluated from the concentration at charge reversal. The isoelectric point (IEP) increases with increasing the surface or electrokinetic charge density of sulfate latex spheres. That is, at low surface or electrokinetic charge density, the charge inversion concentration is low, and IEP shifts to higher values with the increase of surface or electrokinetic charge density. The intrinsic energy of adsorption decreases with increasing the surface or electrokinetic charge density. Finally, our experimental and theoretical results suggest that the hydrophobicity is a determining factor for the charge inversion of hydrophobic colloids, and the intrinsic energy of adsorption also varies with the variations of surface or electrokinetic charge density