A microscale system for in situ investigation of immobilized microalgal cell resistance against liquid flow in the early inoculation stage.

In attached microalgae cultivation systems, cell detachment due to fluid hydrodynamic flow is not a subject matter that is commonly looked into. However, this phenomenon is of great relevance to optimizing the operating parameters of algae cultivation and feasible reactor design. Hence, this current work miniaturizes traditional benchtop assays into a microfluidic platform to study the cell detachment of green microalgae, Chlorella vulgaris, from porous substrates during its early cultivation stage under precisely controlled conditions. As revealed by time lapse microscopy, an increase in bulk flow velocity facilitated nutrient transport but also triggered cell detachment events. At a flow rate of 1000 µL min-1 of growth medium for 120 min, the algal cell coverage was up to 5% lower than those at 5 µL min-1 and 50 µL min-1. In static seeding, the evolution of attached cell resistance toward liquid flows was dependent on hydrodynamic zones. The center zone of the microchannel was shown to be a "comfortable zone" of the attached cells to sequester nutrients effectively at lower medium flow rates but there was a profile transition where outlet zones favored cell attachment the most at higher flow rates (1.13 times higher than the center zone for 1000 µL min-1). Besides, computational fluid dynamics (CFD) simulations illustrated that the focusing band varied between cross-sections and depths, while the streamline was the least concentrated along the side walls and bottom plane of the microfluidic devices. It was intriguing to learn that cell detachment was not primarily happening along the symmetry streamline. Insight gained from this study could be further applied in the optimization of operating conditions of attached cultivation systems whilst preserving laminar flow conditions.

Coalescence of a Ferrofluid Drop at Its Bulk Surface with or without a Magnetic Field.

The coalescence of a ferrofluid drop at its bulk surface, with or without a magnetic field, was investigated experimentally by a high-speed camera. Shape deformations of both the pendant ferrofluid drop and the bulk surface in the axial direction were observed during the approaching process even in the absence of a magnetic field. The angle of the upper pendant peak at the first contact decreases with the magnetic flux density, while the lower ferrofluid peak displays an opposite trend. The coalescing width of the ferrofluid drop follows a power-law relationship. The exponent of 0.64 under medium and high magnetic fields as well as the case without magnetic field confirms the inertial regime of drop coalescence. Under the low magnetic field, the significant exponent increasing from 0.59 to 3.02 at about 4 ms is in coincidence with the sudden change to a smooth coalescing section according to the visualized images. A high-speed microparticle image velocimetry (micro-PIV) technique was employed with a transparent model fluid to reveal the flow fields during the drop coalescence instead of opaque ferrofluids.

Quantitative hydrodynamic characterization of high solid anaerobic digestion: Correlation of "mixing-fluidity-energy" and scale-up effect.

High solid anaerobic digestion (HSAD)'s complex rheological behavior exhibits short-circuiting and dead zone. Mixing optimization is potential to enhance HSAD hydrodynamics. Besides, scale-up effect is quite essential for HSAD's applications, but remains rarely studied yet. Effect of impeller with different width on the correlation of "mixing-fluidity-energy" at different rotating speeds was first investigated at pilot-scale in present work. Then, scale-up effect based on rotating speed and a generalized Reynolds number was revealed from the aspects of fluidity and energy consumption. Results show that impeller width of 100 mm (10 rpm), 200 mm and 300 mm (5 and 10 rpm) are preferred for hydrodynamics and energy economics. Furthermore, Re similarity has better referential significance for the scale-up. In this study, new insight is gained into the correlation of "mixing-fluidity-energy" within a pilot-scale digester. Scale-up effect based Re similarity could potentially offer guidance for HSAD's application in the practical engineering.

Visualization of mass transfer in mixing processes in high solid anaerobic digestion using Laser Induced Fluorescence (LIF) technique.

High solid anaerobic digestion (HSAD) is a promising technology for the treatment of organic waste. Mixing process in HSAD is quite difficult with long mixing time, poor homogenization, significant short-circuiting and stagnant zones. However, the mass transfer in mixing process in HSAD has not been visualized due to the lack of a proper method. In this study, a novel approach for experimentally quantifying the mass transfer in HSAD's mixing process was proposed in a mixing tank equipped with multistage impellers by means of the Laser Induced Fluorescence (LIF) technique. Flow field was investigated for better illustrating the mass transfer, thus Particle Image Velocimetry (PIV) and Computational Fluid Dynamics (CFD) technique were conducted for flow field measurement. The obtained results revealed that the feedstock tended to accumulate around the impeller in the HSAD system, especially near the 1st stage. The tracer diffused rapidly around the 1st impeller in t = 10 s, but the diffusion around the 2nd impeller was quite tough even after 4 h 08 min 23 s. And prolonging mixing time could not significantly improve the flow pattern along with the mixing. In this study, new insight was thrown into the visualization of the mass transfer in mixing process within a HSAD reactor. The visualization of mass transfer in the mixing process in HSAD could offer reference information to the study of the mixing process of HSAD even in full-scale.

Investigation of hydrodynamics in high solid anaerobic digestion by particle image velocimetry and computational fluid dynamics: Role of mixing on flow field and dead zone reduction.

High solid anaerobic digestion (HSAD) was a potential organic waste treatment. Compared with low solid anaerobic digestion, it had the advantages of small footprint, less digestate, and low heating energy. However, HSAD's methane production is poor, mainly due to the complex hydrodynamics. In this study, computational fluid dynamics were utilized for HSAD's hydrodynamics investigation at 14.3% solid content and compared to the particle image velocimetry measurement. Then, effects of mixing on hydrodynamics were investigated. The results indicated that the diameter of impeller was critical for the radial mixing, and the distance between the impellers dictated the axial mixing. Besides, rotating speed affected flow velocities significantly, but displayed less effect on expanding the mixing range. Furthermore, HSAD's treating capacity could be increased at large extent by optimizing mixing. The visualization of the hydrodynamics in this study could potentially offer conceptual basis for HSAD's design in practical engineering.

Mineral pollutants removal through immobilized microalgae-bacterial flocs in a multitrophic microreactor.

Microalgae-bacterial flocs (MaB-flocs) immobilization technique using polyvinyl alcohol (PVA) crosslinked with sodium alginate represent a novel approach for sustainable pollutants removal. The present work was performed to evaluate the performance of a multitrophic batch reactor at microscale for treating two synthetic wastewater solutions prepared with two different initial Chemical Oxygen Demand (COD): 200 mg.L-1 and 450 mg.L-1, respectively. Three MaB-flocs concentrations were entrapped into PVA-alginate beads: C1 (2%, v/v), C2 (5%, v/v) and C3 (10%, v/v), without O2 supply, during three periods 2, 4 and 6 days of batch incubation. PVA-alginate beads containing the highest concentration C3 of MaB-flocs improved the performance of the microreactor to remove significantly NH4+ and PO43- of about 61% and 82%, respectively, from wastewater more than two other concentrations used. This result confirms that C3 of MaB-flocs displays not only a good potential for nutrients removals but also the highest MaB-flocs morphological progression after 6 days of treatment with the highest COD of 450 mg.L-1. The feasibility of the PVA-alginate for cells immobilization, investigated through microscopy analysis, reveals that the evolution of multicellularity in MaB-flocs, for all experiments.

Initial coalescence of a drop at a planar liquid surface.

The initial coalescence of a pendant drop at bulk liquid was jointly investigated by an ultrahigh-speed DC electrical device, a high-speed camera, and a fast micro-Particle Image Velocimetry (micro-PIV). Extended to highly viscous non-Newtonian liquids, the variation of the coalescing width vs time confirms the distinct regimes reported for drop-drop configuration: linear in the inertially limited viscous regime; square root in the inertial regime; possibly a transient viscous regime in between with a logarithmic correction. The measured flow fields during coalescence reveal the transformation of surface energy to kinetic energy, so that the highly located inertia could play a dominant role in relation to the viscous force.

Self-Sustained Coalescence-Breakup Cycles of Ferrodrops under a Magnetic Field.

The self-sustained coalescence-breakup cycles of ferrodrops were investigated for the first time by a high-speed camera under various magnetic fields. Under an axial magnetic field, the upper ferrodrop would deform into a conic shape before coalescing with the bottom ferropeak. Within 0.2 ms after coalescence, the minimum width of the expanding neck obeys a power-law relationship with time, while the exponents increase with the magnetic field and deviate with a decreasing trend in the later coalescence. The cone angle of the upper ferrodrop before coalescence gradually decreases while it increases before breakup with the magnetic field. A critical magnetic field around 35 mT was reported, above which the ferrofluid column undergoes the periodic phenomenon of coalescence and breakup. The frequency for the whole coalescence-breakup cycle increases exponentially with the applied magnetic field. A simplified force balance allows capturing the periodic mechanism involved in this driven harmonic oscillator.

Flow field investigation of high solid anaerobic digestion by Particle Image Velocimetry (PIV).

High solid anaerobic digestion (HSAD) is a promising anaerobic digestion technology. Homogenization and mixing mechanism are essential for HSAD's performance, but relative knowledge still remains poor. In order to investigate HSAD's mixing behavior, a novel flow field measuring approach was proposed as following. Firstly, laponite suspension was selected as the model fluid of HSAD digestate, because the rheological properties and material structure they displayed were highly similar. Then, water and polyacrylamide (PAAm) solution were chosen as basic reference fluid and another non-Newtonian fluid respectively. Flow fields of the three fluids under different rotation speeds were measured via Particle Image Velocimetry (PIV). The evolution of working fluids did induce consecutively the significant flow and mixing behavior of HSAD, because their rheological properties and complexity were getting progressively closer to the real HSAD digestate. Results indicated that the flow field of simulated HSAD fluid was quite different from those of water and PAAm solution, i.e. only the fluid around the impeller could be mixed in HSAD. Besides, increasing rotation speed could not significantly enhance the mixing area of HSAD. Thus, multilayer impellers arranged abreast were recommended for HSAD's mixing. Considering that HSAD's flow field had never been measured before, this study proposed a novel flow field measuring method for such opaque non-Newtonian fluid for the first time. The visualization of HSAD's complex hydrodynamic conditions was also firstly achieved in this study, and thus could further help improve the homogenization of HSAD.

Rheological characterization of digested sludge by solid sphere impact.

An impact method was applied to investigate the rheological characteristics of digested sludge and reveal its transient dynamics. A high-speed camera allowed visualizing the dynamic impact process and observing interaction between impacting sphere and targeted sludge. A damping oscillation was observed after the impact. The crater diameter followed an exponential function, while the crater depth varied as a logarithmic function of both sphere diameter and free fall height. Furthermore, the viscosity and elasticity of digested sludge were evaluated by establishing a simplified impact drag force model. The impact elastic modulus was consistent with the Young's modulus measured by a penetrometer. The impact viscosity was reasonable as the estimated impact shear stress was greater than the yield stress of digested sludge resulting in the formation of crater. The impact method offers an alternative way to reveal the viscoelasticity of digested sludge through a dynamic process.

Dynamics of bubble breakup at a T junction.

The gas-liquid interfacial dynamics of bubble breakup in a T junction was investigated. Four regimes were observed for a bubble passing through the T junction. It was identified by the stop flow that a critical width of the bubble neck existed: if the minimum width of the bubble neck was less than the critical value, the breakup was irreversible and fast; while if the minimum width of the bubble neck was larger than the critical value, the breakup was reversible and slow. The fast breakup was driven by the surface tension and liquid inertia and is independent of the operating conditions. The minimum width of the bubble neck could be scaled with the remaining time as a power law with an exponent of 0.22 in the beginning and of 0.5 approaching the final fast pinch-off. The slow breakup was driven by the continuous phase and the gas-liquid interface was in the equilibrium stage. Before the appearance of the tunnel, the width of the depression region could be scaled with the time as a power law with an exponent of 0.75; while after that, the width of the depression was a logarithmic function with the time.

Self-similar pinch-off mechanism and scaling of ferrofluid drops.

The pinch off of heterogeneous ferrofluid drops at a nozzle in air was experimentally investigated with a magnetic field (downward or upward) and without a magnetic field. Compared to homogeneous drops, the self-similarity and universal scaling law were verified through modifying the initial conditions, such as the nozzle diameter, flow rate, and magnitude and direction of the magnetic fields. Two pinch-off points were observed, and the two consecutive pinch-off dynamics were characterized through scaling laws. Here our scaling exponent remains within the scope of (0.70-0.80) for the primary whereas it remains within the scope of (0.60-0.70) for the secondary pinch off, respectively, comparable to the classic range from 2/3 to 1 for homogeneous drops. The gravity-compensating and gravity-superimposing magnetic fields display a negligible effect on the exponent but determine the sequence of double pinch offs. The universal character of the self-similar pinch off is extended to a heterogeneous fluid.

Effect of the fluid injection configuration on droplet size in a microfluidic T junction.

The effect of confinement on the droplet formation in T junctions was studied for three configurations of fluid injection. The sizes of the main droplets and the satellite droplets were measured in the squeezing and dripping regimes. The evolution of droplet sizes with capillary number in the continuous phase is similar to that in flow-focusing junctions, i.e., the size of the main droplets decreases with an increase of this capillary number, while the size of the satellite droplets increases with an increase of this capillary number. While in the range of flow rates investigated the injection configuration does not exhibit a significant effect on the main droplet sizes, it does have an effect on the size of the satellite droplets. The latter ones are smaller when the neck rupture of the droplet occurs on an angle of the microsystem.

Multiscale hydrodynamic investigation to intensify the biogas production in upflow anaerobic reactors.

Hydrodynamics plays a main role for the performance of an anaerobic reactor involving three phases: wastewater, sludge granules and biogas bubbles. The present work was focused on an original approach to investigate the hydrodynamics at different scales and then to intensify the performance of such complex reactors. The experiments were carried out respectively in a 3D reactor at macroscale, a 2D reactor at mesoscale and a 1D anaerobic reactor at microscale. A Particle Image Velocimetry (PIV), a micro-PIV and a high-speed camera were employed to quantify the liquid flow fields and the relative motion between sludge granules and bubbles. Shear rates exerted on sludge granules were quantified from liquid flow fields. The optimal biogas production is obtained at mean shear rate varying from 28 to 48s(-1), which is controlled by two antagonistic mechanisms. The multiscale approach demonstrates pertinent mechanisms proper to each scale and allows a better understanding of such reactors.

Microscale investigation of anaerobic biogas production under various hydrodynamic conditions.

This work presents an investigation at microscale of various mechanisms affecting anaerobic reactions within the microdevices. In particular, the effect of different hydrodynamic conditions associated with the granular particles' size and density on the biogas production was studied in order to intensify the performance of the anaerobic reactor. The image analysis techniques offer an opportunity to observe and quantify the nucleation and growth of biogas bubble at microscale on a single granule. In addition, the flow conditions were perfectly controlled in the microdevice, and the liquid flow fields were measured by a microparticle image velocimetry (micro-PIV) system. The effect of real hydrodynamic conditions exerted directly on granules onto the maximum biogas production rate was described for the first time. Finally, the role of hydrodynamic conditions on the biogas production at microscale is discussed through a straightforward relationship between the shear rates exerted on the granule and the optimal biogas production rate. The investigation reveals that big granules could have satisfactory biogas production ability under relatively weak hydrodynamic conditions. Thus they would be priority selection for industrial reactors.

Effects of increase modes of shear force on granule disruption in upflow anaerobic reactors.

Sludge washout is listed among the top practical problems of the high rate upflow anaerobic reactors. This study investigated quantitatively two sludge washout processes operated under different hydrodynamic shear increase modes with the intervals of 1 and 10 days respectively. The results reveal that the sludge washout accompanying with large-scale granule disruption could lead to performance failure with heavy sludge loss ratio of about 46.1% at sludge loss rate about 0.35 gVSS L(-1) d(-1) during the process with shear increase interval of 1 day, while the highest sludge loss rate was only 0.12 gVSS L(-1) d(-1) during the process with 10-day interval. The intensified shear conditions could weaken the granules through inhibiting the extracellular polymers production and bioactivity. As consequences, an outbreak of large-scale granule disruption would raise and then significantly accelerate the sludge washout. Since long interval could provide the granules the opportunity to recover from these negative effects to some extent, the shear increase strategy of long interval over 10 days is favorably recommended to operate full-scale reactors during the start-up and shock load periods. The pioneer use of the micro particle image velocimetry in this study offers the possibility to discover the real hydrodynamic conditions around granules at microscale for the first time and reveals that the shear force exerts directly on the granular surface as a mechanical disruption force and big granules undergo high disruption force. The granule disruption is a result of the competition between the granule and the ambient hydrodynamic shear conditions rather than a process with shear force as a sole dominant factor. These could facilitate the understanding of the complicated interactions between the hydrodynamics and reactor performance and favor then a better control of the full-scale reactors.

Bubble formation dynamics in various flow-focusing microdevices.

The aim of this study is to investigate three types of gas-liquid micromixer geometries, including a cross-shape and two converging shape channels for the bubble formation in different liquids. The bubble shape, size, and formation mechanism were investigated under various experimental conditions such as the flow rates of two phases, physical properties of the liquid, and mixer geometries. A micro particle image velocimetry technique and a high-speed camera were used to characterize and quantify gas-liquid flows. It was revealed that the bubble formation, in particular the bubble size, depends on the geometry of the mixing section between two phases. A correlation gathering numerous experimental data was elaborated for the estimation of the bubble size. The influence of different parameters such as the flow rate ratio between two phases, surface tension, and liquid viscosity is well taken into consideration on the basis of the understanding of the bubble formation mechanism at the microscale. This paper marks an original improvement in the domain where no flow field characterizations or correlations were established in flow-focusing devices.

Negative wake behind a sphere rising in viscoelastic fluids: a lattice Boltzmann investigation.

We investigate the complex flow field around a sphere rising in a Maxwell fluid by means of the lattice Boltzmann simulation to provide insights into the strange negative wake experimentally observed behind a bubble or particle in non Newtonian fluids. The influence of the rise velocity, sphere diameter, and fluid's rheology is considered through two dimensionless numbers: the Deborah number De and the Reynolds number Re. Our simulation shows that the negative wake appears behind the sphere when De>2 . On the other hand, the shape of the negative wake described by the opening angle theta of the upward flow cone surrounding the negative wake is mainly determined by the Reynolds number Re. These results reveal that the physical origin of the negative wake stems mainly from the competition between the elastic and viscous stresses in the fluid.

Complex flow around a bubble rising in a non-Newtonian fluid.

Our experimental investigation by both particle image velocimetry and birefringence modulation method shows very complex flow features around a bubble rising in a non-Newtonian fluid. We model this two-phase flow by coupling the free-energy-based lattice Boltzmann scheme and the fluid rheology in the framework of the sixth-order Maxwell model with shear thinning effects. A Newtonian low viscosity drop is used to simulate the rising bubble. Numerical results including noticeably negative wake behind the bubble, stress field, as well as the bubble's teardrop shape are obtained, and compare satisfactorily with our experiments.