Balance Of Hydrodynamic Forces Research Articles

Simulation of fiber-induced melt pressure fluctuations within large scale polymer composite deposition beads

The process-structure-property relationship in Large Area Additive Manufacturing (LAAM) technology is an ongoing area of research as the inherent microstructural properties (chiefly fibers and voids) affect the performance of printed parts. Unfortunately, we currently lack adequate understanding of micro void nucleation and evolution during the LAAM and fused deposition modelling (FDM) processes. Modeling of the polymer melt flow during the extrusion process is important in understanding the underlying microstructural formation and associated properties of the print, that determines the part performance in service conditions. In this paper we compute fiber-induced local pressure fluctuations which may promote void formation in the bead’s microstructure. On a macro-scale, we determine flow fields of a purely viscous, Newtonian planar polymer deposition flow through a LAAM nozzle which are utilized on a micro-scale model where we simulate the evolution of a single ellipsoidal fiber along streamlines obtained from the macro-model. On the micro-scale, we determine instantaneous values of the translational and rotational velocities of the rigid ellipsoidal fiber that satisfies a balance of hydrodynamic forces and couples on the fiber’s surface based on a Newton Raphson algorithm and we track the fiber’s motion along the flow path via an explicit numerical iterative algorithm. Model verification is achieved by benchmarking results with solutions from well-known Jeffery’s equation of motion of a particle in homogeneous simple shear flow. We account for rotary diffusivity due to short-range fiber-fiber interaction in the FEA simulation by determining an effective fluid domain size representative of the interaction coefficient of the melt flow through a correlation analysis that yields an equivalent steady state orientation based on the Advani-Tucker equation. We also consider different possible motions of the fiber along individual LAAM flow paths from a given set of random initial fiber conditions to determine pressure bounds on the fiber surface along each streamline. For improved computational efficiency, calculations are carried out with respect to the fiber’s local coordinate axes to overcome the rigor of adaptive remeshing during the quasi-transient analysis. Results show low pressure extremes near the fiber’s surface which varies across the printed bead as well as through its thickness. Discussion is provided to gain insight into the effect of low-pressure extremes on micro void formation, particularly at the nozzle exit and during die swell/expansion.

Microwave Contactless Current-Sensing for Live/Dead Differentiation of Single Bioparticles on a Microfluidic Platform

The combination of microwave and microfluidic technologies enables the creation of efficient platforms to rapidly quantify live/dead bioparticles in industrial processes, biomedical research and environmental monitoring applications. In this regard, this work demonstrates the differentiation of single live/dead bioparticles with a microwave-microfluidic platform. The system is composed of a contactless current-sensing superheterodyne architecture with broadband frequency-operation bowtie electrodes. Furthermore, the 20mm × 20mm bowtie electrode has been designed and manufactured with a –10 dB frequency range, from 4GHz to 8GHz, and capable of confining the electric field in a micrometric volume. The gap of the bowtie geometry of the electrodes is adjusted to the dimensions of the 50 μm × 50 μm cross-section and 40mm long microfluidic channel. In particular, 10mL sample with 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sup> bacteria/mL flows through the microchannel at 19.8 μL/min, focused with elasto-inertial effect enhanced with 500 ppm of polyethylene oxide. Accordingly, from the signal captured by the electrodes, the energy variation of the applied electric field produced by the bioparticles is sensed by a current-based transimpedance amplifier. Here, the incorporation of the transimpedance amplifier in the measurement system improves the SNR by 4 dB. In addition, the superheterodyne transceiver has been optimized in terms of local oscillator power and bandwidth. Furthermore, the microfluidic system has been adjusted so that the balance of hydrodynamic forces confines bioparticles at the centerline of the microchannel within less than 5 μm. As a result, the system measures the cytoplasmic content of live and dead flowing bacteria with a difference of 6 dB and relative deviation of 3.2%.

Numerical simulations of lift force and drag force on a particle in cross-flow microfiltration of colloidal suspensions to understand limiting flux

The aim of this study was to understand the limiting flux in cross-flow microfiltration of a colloidal suspension in terms of the balance of hydrodynamic forces on a particle. Numerical simulations were carried out to estimate the lift force resulting from the cross-flow, drag force resulting from the convective flux, and net force. Assuming that the limiting flux is the flux when the net force on a particle is zero, we analyzed the simulation data to determine the correlation of two particle Reynolds numbers: Rep_Jlimit, calculated from the limiting flux, and Rep_uδ, calculated from the cross-flow rate. The results indicate that Rep_Jlimit is proportional to the square of Rep_uδ when a particle is at a dimensionless distance (distance divided by diameter) of less than 6.7 from the cake layer, in spite of the experimental fact that Rep_Jlimit is proportional to the 1.5th power of Rep_uδ. In contrast, Rep_Jlimit is proportional to the 1.5th power of Rep_uδ when a particle is at a dimensionless distance of 48–60. This means that the force balance of a particle at such a position, not at the membrane surface, determines the limiting flux in cross-flow microfiltration of a colloidal suspension.

Investigation of Oily Wastewater Filtration Using Polymeric Membranes: Experimental Verification of the Multicontinuum Modeling Approach

The multicontinuum approach has been developed to investigate the filtration of oil-in-water emulsions using membrane technology. Although the fate of individual oil droplets at the membrane surface is determined by the balance of hydrodynamic forces in addition to the applied pressure, such fates cannot be used to determine the overall macroscopic behavior of the membrane. The fact that both the oil and the membrane represent distributions of sizes highlights the idea that such size distributions may be used to construct multiple continua for both the oil and the membrane. Each oil continuum interacts with all membrane continua according to the rules that determine the fate of each individual droplet in each range. The need to validate this modeling approach is essential to build confidence in the myriad results obtained by this approach. For this reason, an experimental set up has been designed and used to provide a verification framework to test the results obtained by the multicontinuum approach and c...

Measurement of Charge Transfer to Aqueous Droplets in High Voltage Electric Fields.

The electric charge acquired by aqueous droplets when they contact an electrode is a crucial parameter in experimental and industrial applications where electric fields are used to manipulate droplet motion and coalescence. For unclear reasons, many investigators have found that aqueous droplets acquire significantly more positive than negative charge. Extant techniques for determining the droplet charge typically rely on a hydrodynamic force balance that depends on accurate characterization of the drag forces acting on the droplet. Here we present an alternative methodology for measuring the droplet charge via direct measurement of the electric current. As the droplet approaches the electrode the current is observed to gradually increase, followed by a large pulse when the droplet makes apparent contact. We interpret the transient current signals as the superposition of the natural response of an RLC circuit and an induced current described by the Shockley-Ramo theory. Nonlinear regression of the observed current to the theoretical model allows for the droplet charge to be extracted, independent of any assumptions about the force balance on the droplet. We demonstrate that regression of the current signal yields charge values that are on average within 4% of charges measured via a force balance. We use the chronocoulometric methodology to investigate how the charge varies with the applied potential, and we demonstrate that deionized water droplets contacting planar electrodes acquire on average 69% more positive charge than negative charge.

Analytical Void Fraction Profile Near the Walls in Low Reynolds Number Bubbly Flows in Pipes: Experimental Comparison and Estimate of the Dispersion Coefficient

In a recent paper, we derived an analytical expression for the void fraction profile in low Reynolds number bubbly pipe flows, based on a balance of hydrodynamic forces on bubbles. The objective of the present work is to perform a comparison of this analytical Bubble Force Balance Formula (BFBF) with an experiment from the literature. We begin by simulating this experiment with the NEPTUNE_CFD code. In particular we show that using an R ij -e model to account for the liquid velocity fluctuations yields reasonable results. In order to compare our analytical profile with experimental measurements, we restrict ourselves to the near-wall region. In this region, the void fraction profile results from a balance between dispersion and wall forces, and the dispersion coefficient can be considered as uniform. The analytical BFBF profile is seen to be in good agreement with the measurements. We are also capable to estimate the dispersion coefficient in this near-wall region.

Effect of ionic strength and permeate flux on membrane fouling: Analysis of forces acting on particle deposit and cake formation

In Cross-Flow Microfiltration (CFMF), suspended particles deposit to form a cake layer on the membrane surface, which provides a resistance to permeate flow. The cake resistance, which plays an important role on the performance of CFMF, is mainly determined by the packing porosity of the cake and, the physical and chemical properties of particles. This study aimed at understanding the porosity and the specific filtration resistance of the cake for a given condition. These properties have been studied using experiments under a constant permeate flux. Factors such as permeate flux and ionic strength were investigated in terms of the particles deposition and cake formation. This study also adopted a force balance model to predict the deposit rate of particles and then compare with the experimental results. Inter-particle forces (electric double layer repulsion force and Van der Waals attraction force) were incorporated into the calculation of cake structure (cake porosity and specific resistance) together with the equilibrium condition of hydrodynamic forces. The experimental results showed that the higher the permeate flux led to the greater amount of particles deposit and the denser structure of cake. The porosity of cake decreased with the increase in ionic strength (0∼0.01M) and then increased sharply afterwards (0.01∼0.1M). The hydrodynamic force balance model estimated well the tendency of variation in cake structure depending on the ionic strength.

Orientation and detachment dynamics of Bacillus spores from stainless steel under controlled shear flow: Modelling of the adhesion force

Shear-flow induced spore detachment was performed under well-controlled laminar flow conditions, in a specially-designed shear stress flow chamber. By comparing detachment profiles of a panel of four strains, belonging to the B. cereus group (B. cereus and B. thuringiensis) and to less related Bacillus species (B. pumilus), it was shown that the spore ability of attaching to stainless steel, probed under dynamic conditions, was mainly affected by the presence (and number) of appendages. Adhesion force between the B. cereus 98/4 strain and stainless steel was quantified at nanoscale. To this aim, detachment results were combined with a theoretical modelling, based on the balance of hydrodynamic forces and torque exerted over a simplified spore model with a spherical form. The wall shear stress, required to remove 50% of the spores initially attached to stainless steel, was determined. On this basis, an adhesion force of 930 ± 390 pN was obtained. Real-time re-orientation of B. cereus 98/4 spores was experimentally established, by using a high-speed camera for tracking the motions of individual spores with high temporal and spatial resolution. Even though tethered to stainless steel without any detachment occurring, spores kept mobile on the substratum, probably due to the existence of discrete bonds or local clusters of anchoring sites, and tended to re-orientate in the flow direction, for minimizing hydrodynamic forces and torque exerted by fluid flow. A significant heterogeneity within the population was also observed, with the co-existence of both moving and immobile spores.

Evaluation of adhesion force between functionalized microbeads and protein-coated stainless steel using shear-flow-induced detachment

Spherical microbeads functionalized with two types of chemical groups (NH 2, OH) were chosen as a simplified bacterial model, in order to elucidate the role of macromolecular interactions between specific biopolymers and 316L stainless steel, in the frame of biofilm formation in the marine environment. NH 2 microbeads were used in their native form or after covalent binding to BSA or different representative poly-amino acids. OH microbeads were used in their native form. Adhesion force between microbeads and bare or BSA-coated stainless steel was quantified at nanoscale. Shear-flow-induced detachment experiments were combined with a simplified version of a theoretical model, based on the balance of hydrodynamic forces and torque exerted on microbeads. A maximal adhesion force of 27.6 ± 8.5 nN was obtained for BSA-coated NH 2 microbeads. The high reactivity of OH functional groups was assessed (adhesion force of 15.6 ± 4.8 nN for large microbeads). When charge-conducting stainless steel was coated with BSA, adhesion force was significantly lower than the one estimated with the bare surface, probably due to an increase in hydrophilic surface properties or suppression of charge transfer. The mechanism for microbead detachment was established (mainly rolling). The flow chamber and the associated theoretical modelling were demonstrated to be a relevant approach to quantify nanoscale forces between interacting surfaces.

Evaluating the Adhesion Force Between Saccharomyces Cerevisiae Yeast Cells and Polystyrene From Shear-Flow Induced Detachment Experiments

The present paper focused on the quantification of the adhesion force between Saccharomyces cerevisiae yeast cells and polystyrene, using laboratory and industrial yeast strains, in a suspending medium of high ionic strength (150 mM NaCl). To this end, shear-flow induced detachment experiments, performed in a flow chamber, were combined with a simplified version of a theoretical model, based on the balance of hydrodynamic forces and torque exerted over yeast cells. It was assumed that yeast adhesion to solid surfaces is mediated by surface proteins and a characteristic thickness of the protein meshwork was thus introduced (25.0 ± 2.5 nm). The experimental determination of the wall shear stress required to remove 50% of the cells initially adherent to polystyrene allowed the estimation of the adhesion force. On this basis, adhesion forces of 6 ± 2 nN and 11 ± 8 nN were obtained for laboratory and industrial strains respectively, reflecting the non-specific interactions, such as Lifshitz-van der Waals and Lewis acid/base interactions, between yeast cells and the polymeric surface. The force magnitude was in the experimental range reported in the literature (between 1 and 50 nN), for a wide variety of solid surfaces and bacterial or fungal species. The mechanism for yeast detachment was established (mainly rolling). The flow chamber and the associated theoretical modelling were thus demonstrated to be a relevant approach to quantify physico-chemical interactions between biological and physical surfaces.

Dynamic and Static Interactions between Bitumen Droplets in Water

In the commercial bitumen extraction operation, dynamic and static interaction forces between bitumen drops in water determine the likelihood of desirable bitumen coalescence at different process stages. These dynamic and static forces were measured using colloidal particle scattering and hydrodynamic force balance techniques, respectively. In the former technique, dynamic interactions are studied through droplet-droplet collision trajectory measurement. In the latter technique, the static attractive forces between droplets are determined when a doublet is separated with a known and adjustable hydrodynamic force. The dynamic force measurement implies the presence of rigid chains on bitumen surfaces. The mean chain lengths for deasphalted bitumen at pH 7, whole bitumen at pH 7, and whole bitumen at pH 8.5 are 50, 78, and 41 nm, respectively. However, the static force measurement indicates much shorter mean chain lengths (<9 nm) in these three bitumen systems. Shorter chain length indicates weaker repulsive force. This finding of a much weaker repulsion between bitumen droplets under static conditions has important implications on the commercial bitumen extraction operation.

Prediction and Measurement of the Interparticle Depletion Interaction Next to a Flat Wall

A theoretical and experimental study was performed to investigate the depletion interaction between two colloidal particles next to a solid wall in a solution of nonadsorbing macromolecules. By calculating the change in free volume available to the macromolecules upon approach of the two particles, a relatively simple expression was developed for the interparticle depletion attraction in hard sphere systems as a function of the particle–particle and particle–plate spacing. Perhaps the most useful result obtained from this analysis was that the wall has no effect whenever the ratio of the particle radius to the macromolecule radius is greater than four. (In charged systems, this ratio would apply to the effective particle and macromolecule sizes.) A series of experiments was then performed in which the hydrodynamic force balance (HFB) apparatus was used to measure the shear force needed to separate a colloidal doublet consisting of two particles trapped in a secondary energy well formed by a repulsive electrostatic force and an attractive depletion force. The macromolecules used here were small, nanometer-sized spheres of either silica or polystyrene. Agreement between the measured separation forces and those predicted using the force balance model of J. Y. Walz and A. Sharma (J. Colloid Interface Sci.168, 485 (1994)) was within a factor of 1.3 using no adjustable parameters and accounting for polydispersity and uncertainty in the macromolecule size. It is shown that this remaining discrepancy could be caused by the Brownian (stochastic) nature of the doublet breakup process.

Applications of colloidal force measurements using a microcollider apparatus to oil sand studies

Two colloidal force measurement techniques have been applied to oil sand studies both using the same apparatus called the microcollider. The techniques are colloidal particle scattering method for repulsion-dominant systems and hydrodynamic force balance method for attraction-dominant systems. The former is based on calculating the colloidal forces from the magnitude of particle–particle collision trajectory deflections. The latter is based on separating a colloidal doublet with increasing shear and calculating the maximum attractive force at the onset of the doublet breakup. Both methods involve the microscopic observation on two individual particles. The methods are also well suited to emulsion systems where the interaction forces as small as 10 −13 N between two droplets can be detected. In oil sand research, two stable emulsions: water-in-diluted bitumen and bitumen-in-water have been receiving considerable attention because of their detrimental effects on the industrial process. The droplet–droplet forces and the emulsion stability mechanism were determined for both emulsions using either one of the techniques. For the w/o emulsion, steric repulsion is the main contributor to the emulsion stability. For the o/w emulsion, all interactions are exclusively DLVO-type forces and the electrostatic force plays a major role in stabilizing the emulsion. Isolated protrusions of tens of nanometers in thickness have also been detected on bitumen surfaces. The protrusions enhance the repulsive force at a long distance but reduce it near the energy barrier, a useful feature that might help in destabilizing the system.

Determining the Colloidal Forces between Bitumen Droplets in Water Using the Hydrodynamic Force Balance Technique

A novel technique using a “hydrodynamic force balance” was introduced to determine the maximum value of attractive forces between two micrometer-sized bitumen droplets in a doublet suspended in water. The technique is based on breaking up a doublet in a gradually increasing wall shear flow and calculating the colloidal force between the two droplets from the breakup shear rate. The measurable force ranges from 10-13 to 10-11 N. The upper limit can be further raised after some modifications to the instrument. The validity of the method has been verified by comparing the determined Hamaker constant of bitumen−water−bitumen with both the experimental data obtained from another force-measurement technique and the literature value. The methed is applicable to both emulsion and suspension systems although only a bitumen-in-water emulsion was investigated in this study. A bitumen droplet surface contains isolated “bumps” of 50−100 nm in horizontal diameter according to previous study. A disk−sphere model assumin...

Near-Boom Oil-Slick Instability Criterion in Viscous Flows and the Influence of Free-Surface Boundary Conditions

Booms are often used to contain oil spills prior to various oil removal techniques. Under certain conditions, the oil droplets can leave the oil slick and enter the water. A simple balance of hydrodynamic forces on a droplet results in an instability criterion which determines whether the droplet will be swept past the boom or not. For viscous flows, it is shown here that the instability criterion consists of a term proportional to the pressure gradient along the boom, as in the potential-flow case, and a term that is inversely proportional to the Reynolds number, although the magnitude of this new term is found to be very small. The solution of viscous flow past an oil boom is obtained using the fractional-step method in a curvilinear coordinate system and the instability criterion is estimated. The influence of the approximate free-surface conditions, such as the rigid-lid no-slip, rigid-lid free-slip, and the exact free-surface condition, the instability criterion is also investigated. The different approximations of free-surface conditions are shown to influence the pressure distributions, thus resulting in different neutral stability curves.