The Relationship between Gelation Behavior and the Amount of Polymer Dose per Silica Surface Area of "Shake-Gels" Consisting of Silica Nanoparticles and Poly(Ethylene Oxide).

The understanding and control of the rheological behaviors of colloids and polymer mixtures is an important issue for scientific interests and industrial applications. Aqueous mixed suspensions of silica nanoparticles and poly(ethylene oxide) (PEO) under certain conditions are interesting systems called "shake-gels", whose states vary reversibly between sol-like and gel-like under repeated shaking and being left to stand. Previous studies have indicated that the amount of PEO dose per silica surface area (Cp) is a crucial parameter for the formation of shake-gels and the relaxation time from gel-like to sol-like states. However, the relationship between the gelation dynamics and the Cp values has not been fully investigated. To determine how the gelation dynamics are affected by the Cp, we measured the time taken for silica and PEO mixtures to gelate from the sol-like to gel-like states as a function of the Cp under different shear rates and flow types. Our results show that the gelation time decreased with increasing shear rates and depended on the Cp values. Moreover, the minimum gelation time was found around a certain Cp (=0.03 mg/m2) for the first time. The finding suggests that there is an optimum Cp value at which the bridging of silica nanoparticles using PEO is significant, and thus, the shake-gels and stable gel-like states are most likely to form.

Conditions for Shake-Gel Formation: The Relationship between the Size of Poly(Ethylene Oxide) and the Distance between Silica Particles.

Colloidal silica suspensions are widely used in many fields, including environmental restoration, oil drilling, and food and medical industries. To control the rheological property of suspensions, poly(ethylene oxide) (PEO) polymers are often used. Under specific conditions, the silica-PEO suspension can create a phenomenon called a shake-gel. Previous works discussed the conditions necessary to form a shake-gel and suggested that the bridging effect of the polymer is one of the important mechanisms for shake-gel formation. However, we noted that the influence of PEO size compared to the separation distance between silica particles regarding shake-gel formation has not been systematically investigated, while the PEO size should be larger than the particle-particle separation distance for polymer bridging in order to form gels. Thus, we conducted a series of experiments to examine the effects of the radius of gyration of the PEO and the distance between the silica particles by controlling the PEO molecular weight and the silica concentration. Our results elucidated that the radius of gyration of the PEO should be 2.5 times larger than the distance between the silica surfaces in order to promote the formation of a shake-gel. This result supports the hypothesis that the bridging effect is the main cause of shake-gel formation, which can help us to understand the conditions necessary for shake-gel preparation.

Effect of shear flow on the hydrodynamic drag force of a spherical particle near a wall evaluated using optical tweezers and microfluidics.

The hydrodynamic drag force on a spherical particle in shear flow near-wall is investigated using optical tweezers and microfluidics. Simple shear flow is applied using a microfluidic channel at different volumetric flow rates. The hydrodynamic drag force exerted on the particle is detected from the displacement of the trapped particle. The effect of the wall is obtained from the force balance of the trapping and hydrodynamic drag force employing the exact solution of the theoretical model using the lubrication theory for a sphere near the wall. Here, we report the experimentally obtained hydrodynamic drag force coefficient under the influence of shear flow. The drag correction factor increases with decreasing distance from the wall due to the effect of the wall surface. We found that the calculated hydrodynamic drag force coefficient is in quantitative comparison with the theoretical prediction for a shear flow past a sphere near-wall. This study provides a straightforward investigation of the effect of the shear flow on the hydrodynamic drag force coefficient on a particle near the wall. Furthermore, these pieces of information can be used in various applications, particularly in optimizing microfluidic designs for mixing and separations of particles or exploiting the formation of the concentration gradient of particles perpendicular to flow directions caused by the non-linear hydrodynamic interactions.

Charging and Aggregation Behavior of Cellulose Nanofibers in Aqueous Solution.

To understand the charging and aggregation of cellulose nanofibers (CNFs), we performed the following experimental and theoretical studies. The charging behavior of CNFs was characterized by potentiometric acid-base titration measuring the density of deprotonated carboxyl groups at different KCl concentrations. The charging behavior from the titration was quantitatively described by the 1-pK Poisson-Boltzmann (PB) model for a cylinder. The electrophoretic mobility of CNFs was measured as a function of pH by electrophoretic light scattering. The mobility was analyzed with the equation for an infinitely long cylinder considering the relaxation of the electric double layer. Good agreement between experimental mobilities and theoretical calculation was obtained by assuming a reasonable distance from the surface to the slipping plane. The result demonstrated that the negative charge of CNFs originates from the deprotonation of ß(1-4)-d-glucuronan on the surface. The aggregation behavior of CNFs was studied by measuring the hydrodynamic diameter of CNFs at different pH and KCl concentrations. Also, we calculated the capture efficiencies of aggregation, using interaction energies of perpendicularly and parallelly oriented cylinders. The interaction energies between cylinders in both orientations were obtained by the Derjaguin, Landau, Verwey, and Overbeek theory, where the electrostatic repulsion was calculated from the surface potential obtained by the 1-pK PB model. From comparison of the theoretical capture efficiency with the measured hydrodynamic diameter, we suggest that CNFs can be aggregated in perpendicular orientation at low pH and low salt concentration, and the fast aggregation regime of CNFs is realized by the reduction of electric repulsion for both perpendicularly and parallelly interacting CNFs. Meanwhile, the application of Smoluchowski's equation to the mobility of CNFs results in the underestimation of the zeta potential.

Rheological Behavior of an Aqueous Suspension of Oxidized Carbon Nanohorn (CNHox).

Oxidized carbon nanohorn (CNHox) a carbon nanomaterial that has attracted attention due to its unique material properties. It is expected to be applied in various areas like cancer treatment, gene-expression technology, fluids with high thermal conductivity, lubricants, and so on. While the rheological measurements of suspensions provide information on the effective size and interactions of suspended particles, the rheological behaviors of aqueous suspensions of CNHox have never been systematically investigated. To clarify the rheological behaviors of aqueous suspensions of CNHox, their viscosity and dynamic viscoelasticity were measured with changing particle concentration and salt concentration. The viscosity of a CNHox suspension showed yield stress at low shear rates and showed shear-thinning behavior with increasing shear rates. The viscosity of 5 weight % CNHox suspensions was comparable to that of 60 weight % silica suspensions. This high viscosity at a low CNHox concentration is probably due to the porous structure and large effective volume of the CNHox particle. The estimated effective volume of CNHox calculated by the Krieger-Dougherty equation was 18.9 times larger than the actual volume calculated by the mass concentration and density. The dependence of rheological behavior of the CNHox suspension on salt concentration was weak compared to that of the colloidal silica suspension. This weak dependence on salt concentration may be due to the roughness of the particle surface, which would weaken the effect of electric double-layer interactions and/or van der Waals interactions between particles. These rheological behaviors of the aqueous suspension of CNHox shown in this research will be useful in efforts to improve the efficiency of its utilization for the various applications.

Effects of ionic valence on aggregation kinetics of colloidal particles with and without a mixing flow.

HYPOTHESIS: The classical Schulze-Hardy rule states that the critical coagulation concentration (CCC) of colloidal particles is inversely proportional to the counter-ionic valence at powers ranging from 2 to 6. However, the inverse Schulze-Hardy rule has recently been proposed, suggesting that the CCC can also be inversely proportional to the co-ionic valence. Previous studies on these rules did not consider the effect of flow on aggregation kinetics and the CCC. This study aims to investigate the effect of multivalent counter-ions and co-ions on aggregation kinetics and the CCCs in systems with and without a mixing flow. EXPERIMENTS: We measured the aggregation rate coefficients of polystyrene sulfate latex particles as a function of the salt concentration with different ionic species. Furthermore, we analyzed these measurements using theoretical models based on hydrodynamic pair-diffusion in a random flow and trajectory analysis in two steady flows. The analysis was conducted using zeta potentials determined through electrophoretic measurements. FINDINGS: Although the trajectory analysis underestimates the CCCs, the hydrodynamic pair-diffusion model can capture the shift of critical coagulation concentrations in the mixing flow to higher values than those in Brownian aggregation and also shows a better agreement with the experimental results. This result suggests that combining random flow and Brownian diffusion is crucial for developing a consistent framework for predicting both Brownian aggregation and aggregation in a mixing flow.

Adsorption kinetics of polyacrylamide-based polyelectrolyte onto a single silica particle studied using microfluidics and optical tweezers.

HYPOTHESIS: Polyelectrolyte adsorption is considered important in tuning the surface property and the fate of particles; however, often studied on macroscopic surfaces. To gain insights into how polyelectrolytes are adsorbed onto a single particle, it is imperative to utilize techniques capable of elucidating adsorption kinetics on a single-particle level in a controlled flow field. EXPERIMENTS: The polyelectrolyte adsorption kinetics was investigated by electrophoretic mobility measurements combined with the kinetics study onto a single-particle viewpoint using microfluidics and optical tweezers. We directly evaluated the thickness, δH, of adsorbed polyelectrolyte onto a negatively-charged silica particle to deduce the adsorbed polyelectrolyte's conformation. The effect of charge density and salt concentrations were studied. FINDINGS: All polyelectrolytes exhibited dependence of δH on salt concentration. The attractive interactions control the cationic polyelectrolytes adsorption process. The δH depends on charge density indicating more loops and tail confirmation for the weakly-charged polyelectrolytes. The anionic polyelectrolytes showed a dependence of the initial rate and saturation value of δH on salt concentrations, attributed to the repulsion between charged segments and the silica surfaces. Here, we present new insights into the polyelectrolyte adsorption kinetics, particularly the influence of electrostatic interaction from the single-particle perspective, inaccessible to conventional bulk measurements.

Alleviation of ammonia inhibition via nano-bubble water supplementation during anaerobic digestion of ammonia-rich swine manure: Buffering capacity promotion and methane production enhancement.

Anaerobic digestion (AD) of ammonia-rich swine manure (SM) with nano-bubble water (NBW) supplementation was studied in this work with the expectation of ammonia inhibition alleviation, buffering capacity promotion, and methane production enhancement. Results indicated that cumulative methane yield was elevated by 12.3-38.7% in NBW groups. Besides, the reduced methane production rate and elongated lag phase under ammonia inhibition were increased and shortened by NBW supplementation, respectively. The rapid increase of total alkalinity (TA) and partial alkalinity (PA) could be observed with NBW supplementation, as well as the rapid decline of VFA/TA, thus improved buffering capacity and alleviated ammonia inhibition. Moreover, higher level of extracellular hydrolases and coenzyme F420 could be detected in NBW groups. In conclusion, NBW with higher mobility and zeta potential (absolute value) could be a promising strategy for the alleviation of ammonia suppression during the AD of SM.

Novel insight into enhanced recoverability of acidic inhibition to anaerobic digestion with nano-bubble water supplementation.

Nano-bubble water (NBW) has been proven to be effective in promoting organics utilization and CH4 production during anaerobic digestion (AD) process, suggesting its potential in improving the stability of the AD process and thereby alleviating acidic inhibition. In this work, the effect of NBW on digestion stability and CH4 production was investigated to evaluate the ability of NBW on AD recovery from acidic inhibition. Results showed that NBW supplementation increased the total alkalinity (TA) and partial alkalinity (PA), and reduced the ratio of VFA/TA, thus maintained the stability of the AD process. Generation/consumption of VFAs was also enhanced with NBW supplementation under acidic inhibition with pH values of 5.5, 6.0 and 6.5. The cumulative CH4 production was 246-257 mL/g-VS in NBW groups, which was 12.1-17.2% higher than the control. Moreover, with NBW supplementation, the maximum CH4 production rate was raised according to the modeling results.

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.

Strength of Flocs Formed by the Complexation of Lysozyme with Leonardite Humic Acid.

Aggregation and aggregates properties of natural organic and nanosized macromolecules such as humic substances and proteins are crucial to explore so-called colloid-mediated transport and the fate of substances in soil and water environments. Therefore, the aggregation and dispersion, charging, and floc strength of lysozyme (LSZ)-leonardite humic acid (LHA) flocs were experimentally investigated. The experiments were performed in different salt concentrations and LSZ to LHA mass ratios as a function of pH. We obtained the stronger flocs at pH 4.4, where the isoelectric point (IEP) of the complex with the mass ratio 2.5 was confirmed. Thus, the aggregation of LSZ-LHA flocs is mainly caused by charge neutralization. We obtained the floc strength of 4.7 nN around IEP at low salt concentration of 3 mM, which was stronger than 2.8 nN in high salt concentration of 50 mM. The effect of salt concentration can be rationalized by charge-patch attraction at low salt concentration. With increasing mass ratio, the IEP shifted to higher pH. This is due to the increase in positive charge from LSZ in the mixture. The effect of the LSZ to LHA mass ratio on the maximum strength was weak in the range studied.

Effect of nano-bubble water on high solid anaerobic digestion of pig manure: Focus on digestion stability, methanogenesis performance and related mechanisms.

In this study, high solid anaerobic digestion of pig manure (PM) under nano-bubble water (NBW) addition was investigated with focus on digestion stability, methanogenesis performance and related mechanisms. Volatile fatty acids (VFAs) inhibition occurred when total solids (TS) was about 8% without NBW addition, which was alleviated with improved digestion stability under NBW addition, facilitating the process of high solids anaerobic digestion (HSAD). The cumulative CH4 yield, on the other hand, was 201-230 mL/g-VS in the NBW reactors at TS of 3-6%, about 20.3-25.0% higher than the control reactors. At the same time, with higher water mobility and zeta potential, NBW was found to promote the consumption of soluble proteins/carbohydrates during the above AD process.

Strength of Humic Acid Aggregates: Effects of Divalent Cations and Solution pH.

Aggregation-dispersion, charging, and aggregate strength of Leonardite humic acid (LHA) were investigated in CaCl2 and MgCl2 solutions as a function of pH and ionic strength (I). The strength or the withstanding force of aggregates of humic substances (HSs) against breakage is important because this force influences the transport and distribution of pollutants and nutrients along with HSs through the change in the size of HS aggregates as a transport unit. We observed the dominancy of aggregation of LHA at high pH than at low pH in every case of CaCl2 and MgCl2 solutions. This observation suggests the higher binding efficiency of these divalent ions at high pH, though there was no obvious relation with electrophoretic mobility and aggregation of LHA. Further, we first revealed the numerical value of the strength of HS aggregates by using a simple experimental setup of aggregate breakup under laminar converging flow through a capillary tube. The obtained values of the strength of LHA aggregates were higher in the presence of CaCl2 solution than MgCl2 solution, and the strength increased with pH. The highest strengths of LHA aggregates in 30 mM (I) CaCl2 and MgCl2 solutions were around 5.8 and 2.4 nN, respectively, at pH around 9.

An analysis on the electrophoretic mobility of cellulose nanocrystals as thin cylinders: relaxation and end effect.

Cellulose nanocrystals (CNCs) are extracted from cellulosic fibers via sulfuric acid hydrolysis and found to exhibit unique properties due to their nanoscale, ordered structure, and surface morphology. The dispersion stability of a CNC suspension is a significant factor when CNCs are applied for reinforcement of a composite or ink jet printing. Since sulfuric acid hydrolysis introduces sulfate groups on CNC surfaces, we considered that charging conditions needed to be characterized, typically based on electrophoretic mobility. After the electrophoretic mobility was measured, several theoretical equations were applied to fit those values to assume the proper CNC particle shape. While Smoluchowski's equation is often used for this purpose, its applicability to CNCs should be reconsidered due to the thin, rod-like shape of CNCs with a finite length and high charge density. In this sense, we measured the surface charge and electrophoretic mobility of well-characterized CNCs. The obtained experimental data have been analyzed by using various electrokinetic equations. Our analytical results suggested that Smoluchowski's equation and the Ohshima-Henry equation overestimated the magnitude of the mobility of CNCs because it ignores the double layer relaxation and end effect. They also suggested that neither the Ohshima-Overbeek averaged equation nor the Ohshima-Overbeek perpendicular equation described the mobility of CNCs appropriately because those equations consider the double layer relaxation and end effect of a cylinder in a limited manner. Instead, the modified Ohshima-Overbeek equation was presented to be preferred for such a charged cylinder with a small aspect ratio.

Characteristics of ultra-fine bubble water and its trials on enhanced methane production from waste activated sludge.

In this study biogas production efficiency was evaluated by adding ultra-fine bubble water (UFBW) into waste activated sludge (WAS) through anaerobic digestion (AD). Four kinds of gases, i.e. Air, N2, CO2 and H2 were introduced into tap water (TW) to prepare the UFBW with their properties being first investigated. Results show that hundreds of millions of nanoscale bubbles with the negative zeta potential could be stable in the UFBW for longer than two weeks whereas almost no nanometer bubbles could be detected in the raw TW. As for their impact on subsequent AD of WAS, the cumulative methane production with the addition of UFBW was 14-21% higher than that from the raw TW addition group. Interestingly, the Air-UFBW also could promote the biogas production in this study, which is different from the common understanding of AD, an obligate anaerobic process.

Effects of nanobubble water on the growth of Lactobacillus acidophilus 1028 and its lactic acid production.

Nanobubble water (NBW) has been applied in various fields due to the unique properties of nanobubbles (NBs) including long-term stability, negative zeta potential and generation of free radicals. In this study, the performance of four kinds of NBW from different gases (air, N2, H2, and CO2) in addition to deionized water (DW) were investigated and compared in terms of the growth of the probiotic Lactobacillus acidophilus 1028. The NB density, size distribution, zeta potential, pH and dissolved oxygen (DO) of the NBW were firstly investigated. Results indicate that N2-NBW had the highest absolute value of zeta potential and NB density (-25.3 ± 5.43 mV and 5.73 ± 1.0 × 107 particles per mL, respectively), while the lowest was detected in CO2-NBW (-6.96 ± 2.36 mV and 3.39 ± 1.73 × 107 particles per mL, respectively). With the exception of CO2-NBW, all the other types of NBW showed promotion effects on the growth of the strain at the lag and logarithmic phases. Among them, N2-NBW demonstrated the best performance, achieving the highest increase ratio of 51.1% after 6 h cultivation. The kinetic models (Logistic and Gompertz) indicate that the culture with N2-NBW had the shortest lag phase and the maximum specific growth rate when compared to the H2-NBW and DW groups under the same cultivation conditions. Preliminary analysis on the mechanisms suggested that these effects were related to the properties (zeta potential and density) of the NBs, which might affect the transport of substances. This study suggests that NBW has the potential for promoting the production efficiency of probiotics via fermentation.

Enhanced hydrolysis of waste activated sludge for methane production via anaerobic digestion under N2-nanobubble water addition.

Anaerobic digestion (AD) is a relatively safe and economically feasible disposal technique for waste activated sludge (WAS), in which hydrolysis of complex organic matters is the rate-limiting step. The aim of this study is to explore the efficiency of applying nitrogen gas nanobubble water (N2-NBW) to AD of WAS and reveal the possible mechanisms. The possible effects of N2-NBW on different processes during AD of WAS were investigated and N2-NBW was expected to enhance the hydrolysis step. Results showed that after N2-NBW addition, sludge particles possessed more negative charges (indicated by zeta potential) than the control with deionized water (DW) addition. The total methane production of NBW group was 402 mL/g-VSreduced, 29% higher than the control group. Moreover, mechanism investigations revealed that N2-NBW addition not only improved the disintegration of high molecular weight compounds (proteins and polysaccharides), but also enhanced the activities of four extracellular hydrolases by 14-17%. Results from the present work showed that the enhancement of N2-NBW addition on methane production from AD of WAS was mainly through the augmentation of hydrolysis of WAS, as little effect on methanogenesis and VS reduction was discerned. The promotion effect of N2-NBW on hydrolysis suggests that N2-NBW addition is a promising pretreatment strategy for AD of WAS with no chemical addition at low energy consumption, thus, increasing the economic feasibility of WAS disposal.

Enhanced bioconversion of hydrogen and carbon dioxide to methane using a micro-nano sparger system: mass balance and energy consumption.

Simultaneous CO2 removal with renewable biofuel production can be achieved by methanogens through conversion of CO2 and H2 into CH4. However, the low gas-liquid mass transfer (k L a) of H2 limits the commercial application of this bioconversion. This study tested and compared the gas-liquid mass transfer of H2 by using two stirred tank reactors (STRs) equipped with a micro-nano sparger (MNS) and common micro sparger (CMS), respectively. MNS was found to display superiority to CMS in methane production with the maximum methane evolution rate (MER) of 171.40 mmol/LR/d and 136.10 mmol/LR/d, along with a specific biomass growth rate of 0.15 d-1 and 0.09 d-1, respectively. Energy analysis indicated that the energy-productivity ratio for MNS was higher than that for CMS. This work suggests that MNS can be used as an applicable resolution to the limited k L a of H2 and thus enhance the bioconversion of H2 and CO2 to CH4.

Transport of colloidal silica in unsaturated sand: Effect of charging properties of sand and silica particles.

We have studied the transport of colloidal silica in various degrees of a water-saturated Toyoura sand column, because silica particles are widely used as catalyst carriers and abrasive agents, and their toxicity is reported recently. Since water-silica, water-sand, and air-water interfaces have pH-dependent negative charges, the magnitude of surface charge was controlled by changing the solution pH. The results show that, at high pH conditions (pH 7.4), the deposition of colloidal silica to the sand surface is interrupted and the silica concentration at the column outlet immediately reaches the input concentration in saturated conditions. In addition, the relative concentration of silica at the column outlet only slightly decreases to 0.9 with decreasing degrees of water saturation to 38%, because silica particles are trapped in straining regions in the soil pore and air-water interface. On the other hand, at pH 5 conditions (low pH), where sand and colloid have less charge, reduced repulsive forces result in colloidal silica attaching onto the sand in saturated conditions. The deposition amount of silica particles remarkably increases with decreasing degrees of water saturation to 37%, which is explained by more particles being retained in the sand column associated with the air-water interface. In conclusion, at higher pH, the mobility of silica particles is high, and the air-water interface is inactive for the deposition of silica. On the other hand, at low pH, the deposition amount increases with decreasing water saturation, and the particle transport is inhibited.

Strength of natural soil flocs.

To obtain the strength of flocs against breakup is crucial for controlling flocculation in water treatment and predicting transport of colloidal particles in aqueous environments. Recently, the author reported a method to obtain floc strength from a simple experiment of floc breakup subjected to a laminar converging flow. In this study, this method was applied to natural soil flocs. The flocs were formed by coagulation with 0.5 M NaCl (pH 5.4-5.5, pH 6.6) solutions, 0.1M CaCl2 (pH 6.4-6.9) solutions, or acidified distilled water with dilute HCl (pH 5.6). Obtained floc strengths were 0.3, 0.7 and 4 nN for Na-, Ca-, and H-coagulated flocs, respectively. Also, floc strength did not change with floc size. These values of floc strengths were 1-3 orders smaller than those of flocs formed with polymer flocculants and/or precipitated ferric or aluminum coagulants.