arXiv subjects

James Q. Feng

Publications and source records attributed to James Q. Feng.

17 recordsLinked to original sources

arXiv2025

A Computational Study of Bubble Formation from an Orifice Submerged in Liquid with Constant Gas Flow

The process of bubble formation from an orifice submerged in liquid with constant gas flow is studied by numerical simulations using an OpenFOAM volume-of-fluid solver named interIsoFoam. The computed results show that the detached bubble size tends to increase with the gas flow rate, orifice size, surface tension, liquid contact angle, etc. in qualitative agreement with most previous authors. For a given orifice size and liquid properties, there exists a critical gas flow rate above which detached bubbles will combine via coalescence known as bubble pairing. At low gas flow rates, the volume of detached bubbles in the quasi-static regime is shown to depend linearly on the gas flow rate, consistent with a physical mechanistic analysis but not recognized by previous authors. The detached bubble size seems insensitive to the contact angle when the liquid adequately wets the orifice wall, but can increase substantially if the contact angle is increased beyond a critical value resulting in contact line motion on the horizontal outside wall of orifice. The value of such a critical contact angle is found to increase with the orifice size and decrease with the gas flow rate. Such revelations would logically suggest that reducing orifice size for generating smaller bubbles could be more challenging for sub-millimeter orifices with constant gas flow.

physics.flu-dyn↗

arXiv2023

The Fallacy in the Paradox of Achilles and the Tortoise

Zeno's ancient paradox depicts a race between swift Achilles and a slow tortoise with a head start. Zeno argued that Achilles could never overtake the tortoise, as at each step Achilles arrived at the tortoise's former position, the tortoise had already moved ahead. Though Zeno's premise is valid, his conclusion that Achilles can "never" pass the tortoise relies on equating infinite steps with an infinite amount of time. By modeling the sequence of events in terms of a converging geometric series, this paper shows that such an infinite number of events sum up to a finite distance traversed in finite time. The paradox stems from confusion between an infinite number of events, which can happen in a finite time interval, and an infinite amount of time. The fallacy is clarified by recognizing that the infinite number of events can be crammed into a finite time interval. At a given speed difference after a finite amount of time, Achilles will have completed the infinite series of gaps at the "catch-up time" and passed the tortoise. Hence this paradox of Achilles and the tortoise can be resolved by simply adding "before the catch-up time" to the concluding statement of "Achilles would never overtake the tortoise".

math.GM↗

arXiv2021

Working mechanism and behavior of Collison nebulizer

The Collison nebulizer (as well as its variations) has been widely used for generating fine aerosol droplets of a few microns from liquids of viscosity up to 1000 centipoise. It was originally developed for producing medical aerosols in inhalation therapy, and now has become an important component as pneumatic atomizer in the Aerosol Jet(R) direct-write system for additive manufacturing. Qualitative descriptions of its working mechanism were given in the literature as an expanding high-speed gas jet creates a negative pressure to syphon liquid into the jet stream, where the liquid is subsequently blown into sheets, filaments, and eventually droplets. But quantitative analysis and in-depth understanding have been lacking until rather recently. In this work, we present a logical description of the working mechanism of Collison nebulizer based on OpenFOAM(R) CFD analysis of compressible jet flow in the jet expansion channel. The positive-feedback liquid aspiration mechanism becomes clear by examining the CFD results as the jet expansion channel geometry is varied. As a consequence, the output mist density can be rather insensitive to the liquid viscosity, which is illustrated by a set of experiments with the Collison-type pneumatic atomizer in an Aerosol Jet(R) direct-write system. Thus, an intrinsic self-regulation mechanism is elegantly incorporated in the Collison nebulizer design. As intuitively expected, experimental data also supports the notion of the existence of an upper limit for liquid holdup in the limited space of jet expansion channel; therefore, the output mist density cannot increase indefinitely by increasing the atomization gas flow rate.

physics.app-ph↗

arXiv2019

Water vapor assisted sintering of silver nanoparticle inks for printed electronics

In printed electronics, conductive traces are often produced by printing inks of silver nanoparticles dispersed in solvents. A sintering process is usually needed to make the printed inks conductive by removing the organic dispersants and allowing metal-to-metal contacts among nanoparticles for atomic diffusion and neck formation. It has been challenging to sinter silver nanoparticle inks in a thermal oven at a temperature < 150 C to avoid thermal damage to the plastic substrate while achieving desired conductivity. This work presents a simple yet effective way to sinter a silver nanoparticle ink below 120 C (even at 80 C) by exposing the printed ink to water vapor in the oven. The results consistently show a significant reduction of line resistivity for the samples sintered in a moist oven compared to those sintered in a dry oven. Hence, solvent vapor-assisted sintering of metal nanoparticle inks can become an enabling approach to broaden product range of printed electronics.

physics.app-ph↗

arXiv2018

Mist Flow Visualization for Round Jets in Aerosol Jet Printing

With the microdroplets of water serving as light scattering particles, the mist flow patterns of round micro-jets can be visualized using the Aerosol Jet(R) direct-write system. The visualization images show that the laminar mist jet appears to extend to more than 20 times the diameter of nozzle orifice, D, for jet Reynolds number Re < 600, especially with D = 0.3 mm and less. For smaller jets (e.g., with D = 0.15 mm), laminar collimated mist flow might be retained to 40xD for Re < 600 and for Re ~ 1500 within 20xD from the nozzle. The laminar part of mist flow associated with larger jets (e.g., with D = 1.0 mm for Re < 600) tends to exhibit noticeable gradual widening due to viscous diffusion. For free jets, their breakdown length--the distance from nozzle where transition from laminar to turbulent mist flow takes place as signaled by the inception of a rapid widening of mist stream--is shown to decrease with increasing Re. The presence of impingement wall tends to prevent turbulence development, even when the wall is placed further downstream of the free-jet breakdown length for a given Re . The critical Re for an impinging jet to develop turbulence increases as the standoff S is reduced. The mist flow of impinging jet of D = 1.0 mm seems to remain laminar even for Re > 4000 at S = 12 mm.

physics.flu-dyn↗

arXiv2016

A computational study of particle deposition patterns from a circular laminar jet

Particle deposition patterns on the plate of inertial impactor with circular laminar jet are investigated numerically with a Lagrangian solver implemented within the framework of the OpenFOAM$^{\circledR}$ CFD package. Effects of taper angle of the nozzle channel and jet-to-plate distance are evaluated. The results show that tapered nozzle tends to deposit more particles toward the circular spot edge than straight nozzle. At jet Reynolds number $Re = 1132$, a tapered nozzle deposits particles to form a pattern with a high density ring toward the deposition spot edge, especially for particle Stokes number $St > St_{50}$, which is absent with a straight nozzle. Increasing the jet-to-plate distance tends to reduce the value of particle density peak near deposition spot edge. Reducing $Re$ to $283$ (e.g., for $300$ ccm flow through a $1.5$ mm diameter jet nozzle) yields particle deposition patterns without the high density ring at the deposition spot edge when the same tapered nozzle is used. The particle deposition patterns with the straight nozzle at $Re = 283$ exhibit further reduced particle density around the spot edge such that the particle density profile appears more or less like a Gaussian function. In general, the effect of reducing $Re$ on particle deposition pattern seems to be similar to increasing the jet-to-plate distance. The computed particle deposition efficiency $η$ shows the fact that very fine particles with extremely small values of $St$ near the jet axis always impact the center of plate, indicating that the value of $η$ does not approach zero with a substantial reduction of $St$. Such a "small particle contamination" typically amounts to $\sim 10\%$ of small particles (with $\sqrt{St} < 0.1$) at $Re \sim 1000$ and $\sim 5\%$ at $Re \sim 300$, which may not be negligible in data analysis with inertial impactor measurement.

physics.flu-dyn↗

arXiv2016

A computational study of high-speed microdroplet impact onto a smooth solid surface

Numerical solutions of high-speed microdroplet impact onto a smooth solid surface are computed, using the interFoam VoF solver of the OpenFOAM CFD package. Toward the solid surface, the liquid microdroplet is moving with an impinging gas flow, simulating the situation of ink droplets being deposited onto substrate with a collimated mist jet in the Optomec Aerosol Jet printing process. The computed values of maximum spread factor, for the range of parameters of practical interest to Aerosol Jet printing, were found in very good agreement with some of the correlation formulas proposed by previous authors in the literature. Combining formulas selected from different authors with appropriate modifications yields a maximum spread factor formula that can be used for first-order evaluations of deposited in droplet size during the Aerosol Jet technology development. The computational results also illustrate droplet impact dynamics with lamella shape evolution throughout the spreading, receding-relaxation, and wetting equilibrium phases, consistent with that observed and described by many previous authors. This suggests a scale-invariant nature of the basic droplet impact behavior such that experiments with larger droplets at the same nondimensional parameter values may be considered for studying microdroplet impact dynamics. Significant free surface oscillations can be observed when the droplet viscosity is relatively low. The border line between periodic free surface oscillations and aperiodic creeping to capillary equilibrium free surface shape appears at the value of Ohnesorge number around 0.25. Droplet bouncing after receding is prompted with large contact angles at solid surface (as consistent with findings reported in the literature), but can be suppressed by increasing the droplet viscosity.

physics.flu-dyn↗

arXiv2015

Vapor Transport of a Volatile Solvent for a Multicomponent Aerosol Droplet

This work presents analytical formulas derived for evaluating vapor transport of a volatile solvent for an isolated multicomponent droplet in a quiescent environment, based on quasi-steady-state approximation. Among multiple solvent components, only one component is considered to be much more volatile than the rest such that other components are assumed to be nonvolatile remaining unchanged in the droplet during the process of (single-component) volatile solvent evaporation or condensation. For evaporating droplet, the droplet size often initially decreases following the familiar "d^2 law" at an accelerated rate. But toward the end, the rate of droplet size change diminishes due to the presence of nonvolatile cosolvent. Such an acceleration-deceleration reversal behavior is unique for evaporating multicomponent droplet, while the droplet of pure solvent has an accelerated rate of size change all the way through the end. This reversal behavior is also reflected in the droplet surface temperature evolution as "S-shaped" curves. However, a closer mathematical examination of conditions for acceleration-deceleration reversal indicates that the acceleration phase may disappear when the amount of nonvolatile cosolvent is relatively small and ambient vapor pressure is relatively high. Because the net effect of adding nonvolatile cosolvent is to reduce the mole fraction of the volatile solvent such that the saturation vapor pressure is lowered, vapor condensation onto the multicomponent droplet is predicted to occur when the ambient vapor pressure is subsaturated with respect to that for the pure volatile solvent. In this case, the droplet will grow asymptotically toward a finite size. But when the ambient vapor pressure becomes supersaturated with respect to that for the pure volatile solvent, the condensation growth of droplet can continue indefinitely without bound.

physics.chem-ph↗

arXiv2014

Deficient Reasoning for Dark Matter in Galaxies

Astronomers have been using the measured luminosity to estimate the {\em luminous mass} of stars, based on empirically established mass-to-light ratio which seems to be only applicable to a special class of stars---the main-sequence stars---with still considerable uncertainties. Another basic tool to determine the mass of a system of stars or galaxies comes from the study of their motion, as Newton demonstrated with his law of gravitation, which yields the {\em gravitational mass}. Because the luminous mass can at best only represent a portion of the gravitational mass, finding the luminous mass to be different or less than the gravitational mass should not be surprising. Using such an apparent discrepancy as a compelling evidence for the so-called dark matter, which has been believed to possess mysterious nonbaryonic properties and present a dominant amount in galaxies and the universe, seems to be too far a stretch when seriously examining the facts and uncertainties in the measurement techniques. In our opinion, a galaxy with star type distribution varying from its center to edge may have a mass-to-light ratio varying accordingly. With the thin-disk model computations based on measured rotation curves, we found that most galaxies have a typical mass density profile that peaks at the galactic center and decreases rapidly within $\sim 5%$ of the cut-off radius, and then declines nearly exponentially toward the edge. The predicted mass density in the Galactic disk is reasonably within the reported range of that observed in interstellar medium. This leads us to believe that ordinary baryonic matter can be sufficient for supporting the observed galactic rotation curves; speculation of large amount of non-baryonic matter may be based on an ill-conceived discrepancy between gravitational mass and luminous mass which appears to be unjustified.

astro-ph.GA↗

arXiv2014

Mass distribution in rotating thin-disk galaxies according to Newtonian dynamics

An accurate computational method is presented to determine the mass distribution in a rotating thin-disk galaxy from given rotation curve by applying Newtonian dynamics for an axisymmetrically rotating thin disk of finite size with or without a central spherical bulge. The governing integral equation for mass distribution, resulting from the balance between the Newtonian gravitational force and centrifugal force due to rotation at every point on the disk, is transformed via a boundary-element method into a linear algebra matrix equation that can be solved numerically for rotation curves with a wide range of shapes. To illustrate the effectiveness of this computational method, mass distributions in several mature spiral galaxies are determined from their measured rotation curves. All the surface mass density profiles predicted by our model exhibit approximately a common exponential law of decay, qualitatively consistent with the observed surface brightness distributions. When a central spherical bulge is present, the total galactic mass increases only slightly but the mass distribution in the galaxy is altered in such a way that the periphery mass density is reduced while more mass appears toward the galactic center. By extending the computational domain beyond the galactic edge, we can determine rotation velocity outside the cut-off radius which appears to continuously decrease and gradually approach the Keplerian rotation velocity out over twice the cut-off radius. Am examination of the circular orbit stability suggests that galaxies with flat or increasing rotation velocities with radius are more stable than those with decreasing rotation velocities especially in the region near the galactic edge. Our results demonstrate the fact that Newtonian dynamics can be adequate for describing the observed rotation behavior of mature spiral galaxies.

astro-ph.GA↗

arXiv2012

Diffusion-Controlled Quasi-Stationary Mass Transfer for an Isolated Spherical Particle in an Unbounded Medium

A consolidated mathematical formulation of the spherically symmetric mass-transfer problem is presented, with the quasi-stationary approximating equations derived from a perturbation point of view for the leading-order effect. For the diffusion-controlled quasi-stationary process, a mathematically complete set of the exact analytical solutions is obtained in implicit forms to cover the entire parameter range. Furthermore, accurate explicit formulas for the particle radius as a function of time are also constructed semi-empirically for convenience in engineering practice. Both dissolution of a particle in a solvent and growth of it by precipitation in a supersaturated environment are considered in the present work.

math-ph↗

arXiv2012

Music in Terms of Science

To many people, music is a mystery. It is uniquely human, because no other species produces elaborate, well organized sound for no particular reason. It has been part of every known civilization on earth. It has become a very part of man's need to impose his will upon the universe, to bring order out of chaos and to endow his moments of highest awareness with enduring form and substance. It is a form of art dealing with the organization of tones into patterns. Despite of cultural differences, music from different civilizations seems to consist of some building blocks that are universal: melody, harmony, rhythm, etc. Almost all musical systems are based on scales spanning an octave---the note that sounds the same as the one you started off with, but at a higher or lower pitch. It was discovered by Pythagoras, a Greek philosopher who lived around 500 BC, that the note an octave higher than another has a frequency twice high. The notes that sound harmonious together have simple rational number ratios between their frequencies. It is those implicit structures and relationships in apparently mysterious musical experience that I am interested in exploring here. As a scientist by training with a consistent passion for classical guitar playing, I would like make an attempt to explain the musical experience in terms of science and mathematics, hoping to fill some gaps between the knowledge of scientists and artistic intuition of musicians.

physics.pop-ph↗

arXiv2011

Rotating thin-disk galaxies through the eyes of Newton

By numerically solving the mass distribution in a rotating disk based on Newton's laws of motion and gravitation, we demonstrate that the observed flat rotation curves for most spiral galaxies correspond to exponentially decreasing mass density from galactic center for the most of the part except within the central core and near periphery edge. Hence, we believe the galaxies described with our model are consistent with that seen through the eyes of Newton. Although Newton's laws and Kepler's laws seem to yield the same results when they are applied to the planets in the solar system, they are shown to lead to quite different results when describing the stellar dynamics in disk galaxies. This is because that Keplerian dynamics may be equivalent to Newtonian dynamics for only special circumstances, but not generally for all the cases. Thus, the conclusions drawn from calculations based on Keplerian dynamics are often likely to be erroneous when used to describe rotating disk galaxies.

astro-ph.GA↗

arXiv2011

Modeling the Newtonian Dynamics for Rotation Curve Analysis of Thin-Disk Galaxies

We present an efficient, robust computational method for modeling the Newtonian dynamics for rotation curve analysis of thin-disk galaxies. For a disk galaxy with a typical flat rotation curve, our modeling results show that the surface mass density monotonically decreases from the galactic center toward periphery, according to Newtonian dynamics. In a large portion of the galaxy, the surface mass density follows an approximately exponential law of decay with respect to the galactic radial coordinate. Yet the radial scale length for the surface mass density seems to be generally larger than that of the measured brightness distribution, suggesting an increasing mass-to-light ratio with the radial distance in a disk galaxy. In a nondimensionalized form, our mathematical system contains a dimensionless parameter which we call the "galactic rotation number" that represents the gross ratio of centrifugal force and gravitational force. The value of this galactic rotation number is determined as part of the numerical solution. Through a systematic computational analysis, we have illustrated that this galactic rotation number remains within $\pm 10%$ of 1.70 for a wide variety of rotation curves. This implies that the total mass in a disk galaxy is proportional to $V_0^2\,R_g$, with $V_0$ denoting the characteristic rotation velocity and $R_g$ the radius of the galactic disk. The predicted total galactic mass of the Milky Way is in good agreement with the star-count data.

astro-ph.GA↗

arXiv2008

Galactic Rotation Described with Bulge+Disk Gravitational Models

Observations reveal that mature spiral galaxies consist of stars, gases and plasma approximately distributed in a thin disk of circular shape, usually with a central bulge. The rotation velocities quickly increase from the galactic center and then achieve a constant velocity from the core to the periphery. The basic dynamic behavior of a mature spiral galaxy, such as the Milky Way, is well described by simple models balancing Newtonian gravitational forces against the centrifugal forces associated with a rotating thin axisymmetric disk. In this research, we investigate the effects of adding central bulges to thin disk gravitational models. Even with the addition of substantial central bulges, all the critical essential features of our thin disk gravitational models are preserved. (1) Balancing Newtonian gravitational and centrifugal forces at every point within the disk yields computed radial mass distributions that describe the measured rotation velocity profiles of mature spiral galaxies successfully. (2) There is no need for gravity deviations or ``massive peripheral spherical halos of mysterious Dark Matter''. (3) The calculated total galactic masses are in good agreement with star count data. (4) The addition of central bulges increases the calculated total galactic masses, possibly more consistent with the presence of galactic gases, dust, grains, lumps, planets and plasma in addition to stars. (5) Compared with the light distribution, our mass distributions within the disk are larger out toward the galactic periphery which is cooler with lower opactiy/emissivity (and thus darker). This is apparent from edge-on views of galaxies which display a dark disk-line against a much brighter galactic halo.

astro-ph↗

arXiv2008

Galactic Rotation Described with Various Thin-Disk Gravitational Models

For mature spiral galaxies, the rotation velocities quickly increase from the galactic center and achieve a constant velocity from the core to the periphery. This dynamic behavior is described by models balancing Newtonian gravitational and centrifugal forces in rotating thin axisymmetric disks. Freeman's disk assumes a mass density decreasing exponentially with radius which correctly produces rotational velocities which increase from the galactic center to a maximum near the outer core, but then decreases out to the periphery contrary to measurements. Mestel's disk assumes a mass distribution decreasing more slowly (inversely with radius) that yields a constant rotational velocity across the entire disk, but has an unrealistic central mass singularity and does not describe the core rotation properly. Thus combine the Freeman and Mestel disks to utilize their strengths and eliminate their deficiencies. Utilize the Freeman formula for the central core, and the Mestel formula beyond the core to the galactic rim. This combined model produces rotation curves comparable to the measurements of mature spiral galaxies. For a more general thin-disk model, we develop an alternative computational method to solve for the mass density distributions for various measured rotation curves We compute radial mass densities that balance the Newtonian gravitational and centrifugal forces at every point. The computational solutions show mass densities which decrease approximately exponentially in the central core (similar to Freeman), and transition to a slower inverse radial decrease (similar to Mestel) to the periphery. Thus these diverse approaches yield similar results and are mutaully self-consistent with Newtonian gravity/dynamics.

astro-ph↗

arXiv2008

Galactic Rotation Described with Thin-Disk Gravitational Model

The measured rotation velocity profiles of mature spiral galaxies are successfully described with a gravitational model consisting of a thin axisymmetric disk of finte radius. The disk is assumed uniformly thin but with variable radial mass density. The governing integral equation is based on mechanical balance between Newtonian gravitational and centrifugal forces (due to galaxy rotation) at each and every point in a finite set of concentric rings. The nondimensionalized mathematical system contains a dimensionless parameter we call ``galactic rotation parameter'' which concisely crystallizes perspective. Computational solutions are obtained for the radial mass distributions that satisfy the measured rotational velocity profiles. Together with a constraint equation for mass conservation, the galactic rotational parameter is also determined from which the total galactic mass is calculated from measured galactic radii and maximum rotation velocities. These calculated total galactic masses are in good agreement with data. Our deduced exponentially decreasing mass distributions in the central galactic core are in agreement with almost all others. However our mass distributions differ toward the galactic periphery with more ordinary baryonic mass in these outer disk regions which are cooler with lower opactiy/emissivity (and thus darker).

astro-ph↗