Mass Transfer Events Research Articles

Mechanistic modeling of hydrophobic interaction chromatography for monoclonal antibody purification: process optimization in the quality by design paradigm

AbstractBACKGROUNDA mechanistic model has been developed for hydrophobic interaction chromatography (HIC) based purification of monoclonal antibodies (mAbs). The model has been used to gain thorough process understanding as desired by the quality by design paradigm. The model is based on the general rate model coupled with an exponentially modified Langmuir adsorption model. The latter has been modeled by using the solvophobic theory for correlating protein adsorption equilibrium on the hydrophobic media with the mobile phase salt concentration.RESULTSThe proposed model satisfactorily predicts the elution profile of the monoclonal antibody product under consideration and its aggregates on a HIC column (Phenyl Sepharose 6 Fast FlowTM low sub) operated in linear gradient mode. The model has been used to elucidate the underlying mechanism behind separation of monomer and aggregate species. A comparison of the relative time scales for the various mass transfer events for mAb and aggregate species indicates that the intra‐particle (pore) diffusion transport of these molecules to their respective binding sites within the porous HIC resin is the rate limiting step whereas convection, film diffusion, and binding kinetics are relatively faster (time scales are 10−1 to 10−3 order of magnitude times the time scale of intra‐particle pore diffusion).CONCLUSIONSThe proposed methodology for modeling can be successfully implemented to achieve efficient aggregate removal by HIC using minimal experimentation compared with the traditional experimentally extensive approaches. © 2017 Society of Chemical Industry

The history effect on bubble growth and dissolution. Part 2. Experiments and simulations of a spherical bubble attached to a horizontal flat plate

The accurate description of the growth or dissolution dynamics of a soluble gas bubble in a super- or undersaturated solution requires taking into account a number of physical effects that contribute to the instantaneous mass transfer rate. One of these effects is the so-called history effect. It refers to the contribution of the local concentration boundary layer around the bubble that has developed from past mass transfer events between the bubble and liquid surroundings. In Part 1 of this work (Peñas-Lópezet al.,J. Fluid Mech., vol. 800, 2016b, pp. 180–212), a theoretical treatment of this effect was given for a spherical, isolated bubble. Here, Part 2 provides an experimental and numerical study of the history effect regarding a spherical bubble attached to a horizontal flat plate and in the presence of gravity. The simulation technique developed in this paper is based on a streamfunction–vorticity formulation that may be applied to other flows where bubbles or drops exchange mass in the presence of a gravity field. Using this numerical tool, simulations are performed for the same conditions used in the experiments, in which the bubble is exposed to subsequent growth and dissolution stages, using stepwise variations in the ambient pressure. Besides proving the relevance of the history effect, the simulations highlight the importance that boundary-induced advection has to accurately describe bubble growth and shrinkage, i.e. the bubble radius evolution. In addition, natural convection has a significant influence that shows up in the velocity field even at short times, although given the supersaturation conditions studied here, the bubble evolution is expected to be mainly diffusive.

Does the Wolf-Rayet binary CQ Cephei undergo sporadic mass transfer events?

Context. Stellar wind mass-loss in binary systems carries away angular momentum causing a monotonic increase in the orbital period, Ṗ > 0. Despite possessing a significant stellar wind, the eclipsing Wolf-Rayet (WR) binary system CQ Cep does not show the expected monotonic period increase, in fact, it is sometimes reported to display the opposite behavior.Aims. The objective of this paper is to perform a new analysis of the rate of period change Ṗ and determine the conditions under which Roche-lobe overflow (RLO) mass-transfer combined with wind mass loss can explain the discrepant behavior. Methods. The historic records of times of light curve minima were reviewed and compared with the theoretical values of Ṗ for cases in which both wind mass-loss and RLO occur simultaneously. Results. The observational data indicate that Ṗ alternates between positive and negative values on a timescale of years. The negative values (Ṗ ~ −0.6 to −8.5 s yr-1 ) are significantly larger in absolute value than the positive ones (Ṗ ~ + 0.2 to +1.2 s yr-1 ). We find that a plausible scenario for CQ Cep is one in which the O star undergoes intense but sporadic RLO events that lead to accretion onto the WR star, at which times Ṗ > 0 when the WR wind, and possibly material swept up from the O star, carries angular momentum away from the system. A scenario in which the WR star is the mass donor cannot be excluded, but requires that either the WR wind mass-loss rate undergoes large sporadic enhancements or that an additional process that removes angular momentum from the system be present.

Physics of microstructures enhancement of thin film evaporation heat transfer in microchannels flow boiling

Performance enhancement of the two-phase flow boiling heat transfer process in microchannels through implementation of surface micro- and nanostructures has gained substantial interest in recent years. However, the reported results range widely from a decline to improvements in performance depending on the test conditions and fluid properties, without a consensus on the physical mechanisms responsible for the observed behavior. This gap in knowledge stems from a lack of understanding of the physics of surface structures interactions with microscale heat and mass transfer events involved in the microchannel flow boiling process. Here, using a novel measurement technique, the heat and mass transfer process is analyzed within surface structures with unprecedented detail. The local heat flux and dryout time scale are measured as the liquid wicks through surface structures and evaporates. The physics governing heat transfer enhancement on textured surfaces is explained by a deterministic model that involves three key parameters: the drying time scale of the liquid film wicking into the surface structures (τd), the heating length scale of the liquid film (δH) and the area fraction of the evaporating liquid film (Ar). It is shown that the model accurately predicts the optimum spacing between surface structures (i.e. pillars fabricated on the microchannel wall) in boiling of two fluids FC-72 and water with fundamentally different wicking characteristics.

Determining stellar parameters of asteroseismic targets: going beyond the use of scaling relations

Asteroseismic parameters allow us to measure the basic stellar properties of field giants observed far across the Galaxy. Most of such determinations are, up to now, based on simple scaling relations involving the large frequency separation, \Delta\nu, and the frequency of maximum power, \nu$_{max}$. In this work, we implement \Delta\nu\ and the period spacing, {\Delta}P, computed along detailed grids of stellar evolutionary tracks, into stellar isochrones and hence in a Bayesian method of parameter estimation. Tests with synthetic data reveal that masses and ages can be determined with typical precision of 5 and 19 per cent, respectively, provided precise seismic parameters are available. Adding independent information on the stellar luminosity, these values can decrease down to 3 and 10 per cent respectively. The application of these methods to NGC 6819 giants produces a mean age in agreement with those derived from isochrone fitting, and no evidence of systematic differences between RGB and RC stars. The age dispersion of NGC 6819 stars, however, is larger than expected, with at least part of the spread ascribable to stars that underwent mass-transfer events.

The history effect in bubble growth and dissolution. Part 1. Theory

The term ‘history effect’ refers to the contribution of any past mass transfer events between a gas bubble and its liquid surroundings towards the current diffusion-driven growth or dissolution dynamics of that same bubble. The history effect arises from the (non-instantaneous) development of the dissolved gas concentration boundary layer in the liquid in response to changes in the concentration at the bubble interface caused, for instance, by variations of the ambient pressure in time. Essentially, the history effect amounts to the acknowledgement that at any given time the mass flux across the bubble is conditioned by the preceding time history of the concentration at the bubble boundary. Considering the canonical problem of an isolated spherical bubble at rest, we show that the contribution of the history effect in the current interfacial concentration gradient is fully contained within a memory integral of the interface concentration. Retaining this integral term, we formulate a governing differential equation for the bubble dynamics, analogous to the well-known Epstein–Plesset solution. Our equation does not make use of the quasi-static radius approximation. An analytical solution is presented for the case of multiple step-like jumps in pressure. The nature and relevance of the history effect is then assessed through illustrative examples. Finally, we investigate the role of the history effect in rectified diffusion for a bubble that pulsates under harmonic pressure forcing in the non-inertial, isothermal regime.

Eruption of a Bifurcated Solar Filament

We study the partial eruption of a solar filament observed by the Solar Dynamics Observatory (SDO) and the Solar TErrestrial RElations Observatory-Ahead (STEREO-A) spacecraft on 9 May 2012. This filament was located in Active Region NOAA 11475 and consisted of two distinct branches, separated in height above the active region’s primary polarity-inversion line. For two days prior to the filament eruption, several threads of filament material were observed to connect the lower branch to the upper branch with evidence of a transfer of mass along them. The eruption commenced as a slow rise of the upper branch that began at 9 May 2012 23:40 UT, with the main eruption occurring half an hour later, producing a coronal mass ejection (CME). During the eruption, the upper branch was observed to rotate approximately 120 degrees in a counter-clockwise direction. We suggest that the mass transfer events also comprised a transfer of magnetic flux that led the upper branch of the filament to lose equilibrium as a result of a helical kink instability or torus instability.

On the origin of W UMa type contact binaries – a new method for computation of initial masses

W UMa type binaries have two defining characteristics. These are (i) the effective temperatures of both components are very similar, and (ii) the secondary (currently less massive) component is overluminous for its current mass. We consider the latter to be an indication of its mass before the mass-transfer event. For these stars we define a mass difference ($\delta M$) between the mass determined from its luminosity and the present mass determined from fitting the binary orbit. We compare the observed values of the mass difference to stellar models with mass loss. The range of initial secondary masses that we find for observed W UMa type binaries is 1.3-2.6 M$_{\odot}$. We discover that the A- and the W-subtype contact binaries have different ranges of initial secondary masses. Binary systems with an initial mass higher than $1.8 \pm 0.1$ M$_{\odot}$ become A-subtype while systems with initial masses lower than this become W-subtype. Only 6 per cent of systems violate this behavior. We also obtain the initial masses of the primaries using the following constraint for the reciprocal of initial mass ratio: $0 < 1/q_i < 1$. The range of initial masses we find for the primaries is 0.2-1.5 M$_{\odot}$, except for two systems. Finally in comparing our models to observed systems we find evidence that the mass transfer process is not conservative. We find that only 34 per cent of the mass from the secondary is transferred to the primary. The remainder is lost from the system.

Chemical Composition and Photometry of BE Lyncis

ABSTRACTHigh-resolution spectroscopic observation was carried out to find the chemical composition of BE Lyn. The abundances of 25 chemical elements from carbon to neodymium were found. The deficiency of iron appeared to be equal to Δ log N(Fe) = -0.26 ± 0.08 with respect to the solar metallicity, and the abundances of other elements were mainly undersolar. Only nitrogen, sodium, aluminum, and sulfur showed overabundances near 0.2–0.3 dex. The abundance pattern showed no clear signs of accretion or mass transfer events. It is not possible to exclude the classification of BE Lyn as a SX Phe–type star with slightly undersolar abundances of chemical elements. In addition, new differential time-series observations of BE Lyn were secured using V filters, and seven new times of light maximum were identified. We collected 162 times of light maximum from the literature, unpublished data, and an open database, and we proceeded to investigate the pulsational properties of BE Lyn. All five harmonic frequencies were identified using the Fourier decomposition method.

Effect of cargo properties on in situ forming implant behavior determined by noninvasive ultrasound imaging

Diagnostic ultrasound has been shown to be an effective method for the noninvasive characterization of in situ forming implant behavior both in vivo and in vitro through the evaluation of the echogenic signal that forms as a consequence of the polymer phase transition from liquid to solid. The kinetics of this phase transition have a direct effect on drug release and can be altered through factors that change the mass transfer events of the solvent and aqueous environment, including properties of the entrapped active agent. This study examined the effect of payload properties on implant phase inversion, swelling, drug release, and polymer degradation. Poly(DL-lactide-co-gylcolide) implants were loaded with either: sodium fluorescein, bovine serum albumin (BSA), doxorubicin (Dox), or 1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI). Fluorescein and Dox were released at near equivalent rates throughout the diffusion phase of release but due to differing drug-matrix interactions, Dox-loaded implants released a lower mass of drug during the degradation phase of release. DiI was not readily released, and due to increased depot hydrophobicity, resulted in significantly lower swelling than the other formulations. The initial echogenicity was higher in Dox-loaded implants than those loaded with fluorescein, but after the initial precipitation, phase inversion and drug release occurred at near equivalent rates for both Dox and fluorescein-loaded implants. Nonlinear mathematical fitting was used to correlate drug release and phase inversion, providing a noninvasive method for evaluating implant release (R(2)>0.97 for Dox, BSA, and fluorescein; DiI had a correlation coefficient of 0.56).

ON THE FORMATION OF DOUBLE WHITE DWARFS THROUGH STABLE MASS TRANSFER AND A COMMON ENVELOPE

Although several dozen double white dwarfs (DWDs) have been observed, for many the exact nature of the evolutionary channel(s) by which they form remains uncertain. The canonical explanation calls for the progenitor binary system to undergo two subsequent mass-transfer events, both of which are unstable and lead to a common envelope (CE). However, it has been shown that if both CE events obey the standard alpha-prescription (parametrizing energy loss), it is not possible to reproduce all of the observed systems. The gamma-prescription was proposed as an alternative to this description, instead parametrizing the fraction of angular momentum carried away in dynamical-timescale mass loss. In this paper, we analyze simultaneous energy and angular-momentum conservation, and show that the gamma-prescription cannot adequately describe a CE event for an arbitrary binary, nor can the first phase of mass loss always be understood in general as a dynamical-timescale event. We consider in detail the first episode of mass transfer in binary systems with initially low companion masses, with a primary mass in the range 1.0--1.3 solar masses and an initial mass ratio between the secondary and primary stars of 0.83--0.92. In these systems, the first episode of dramatic mass loss may be stable, non-conservative mass transfer. This strips the donor's envelope and dramatically raises the mass ratio; the considered progenitor binary systems can then evolve into DWDs after passing through a single CE during the second episode of mass loss. We find that such a mechanism reproduces the properties of the observed DWD systems which have an older component with less than approximately 0.46 solar masses and mass ratios between the younger and older WDs greater than 1.

Contribution of the Double Layer to Transient Faradaic Processes: Implications for Hydrodynamic Modulated Voltammetry of Nanostructures

The electrochemistry of Pt nanostructured electrodes is investigated using hydrodynamic modulated voltammetry (HMV). Here a liquid crystal templating process is used to produce platinum-modified electrodes with a range of surface areas (roughness factor 42.4-280.8). The electroreduction of molecular oxygen at these nanostructured platinum surfaces is used to demonstrate the ability of HMV to discriminate between faradaic and nonfaradaic electrode reactions. The HMV approach shows that the reduction of molecular oxygen experiences considerable signal loss within the high pseudocapacitive region of the voltammetry. Evidence for the contribution of the double layer to transient mass transfer events is presented. In addition, a model circuit and appropriate theoretical analysis are used to illustrate the transient responses of a time variant faradaic component. This in conjunction with the experimental evidence shows that, far from being a passive component in this system, the double layer can contribute to HMV faradaic reactions under certain conditions.

Molecular Aspects of Radical Polymerizations—The Propagation Frequency

The product of the propagation rate constant and monomer concentration is a central issue of radical polymerization. Both quantities are strongly interrelated as the determination of a reliable value of the propagation rate constant requires exact knowledge of the monomer concentration. Even for homogeneous polymerization conditions, exact knowledge of the monomer concentration at the reaction site from the molecular perspective is not trivial. For emulsion polymerizations the situation is additionally complicated by mass transfer events and colloid-chemical effects. Molecular modeling appears as an attractive alternative or complement to the deterministic kinetic description of radical polymerizations.

A new-age determination for γ2 Velorum from binary stellar evolution models

Abstract We derive a new age for the γ2 Velorum binary by comparing recent observations to our set of binary models. We find that it is very unlikely that the stars have not interacted, which implies that previous estimates of the age from single-star models of 3.5 ± 0.4 Myr are incorrect. We prefer an older age of 5.5 ± 1 Myr that agrees with the age of other lower mass stars in the Vela OB association. We also find that our favoured binary model shows the components of the binary have interacted in a Case B, post-main-sequence, mass-transfer event. During the mass-transfer event, the envelopes of both components are radiative and therefore the damping of tidal forces is relatively weak. This explains why the binary is still eccentric after mass transfer.

Light elements in massive single and binary stars

AbstractWe highlight the role of the light elements (Li, Be, B) in the evolution of massive single and binary stars, which is largely restricted to a diagnostic value, and foremost so for the element boron. However, we show that the boron surface abundance in massive early type stars contains key information about their foregoing evolution which is not obtainable otherwise. In particular, it allows to constrain internal mixing processes and potential previous mass transfer event for binary stars (even if the companion has disappeared). It may also help solving the mystery of the slowly rotating nitrogen-rich massive main sequence stars.

Transient Heavy Element Absorption Systems in Novae: Episodic Mass Ejection from the Secondary Star

A high-resolution spectroscopic survey of post-outburst novae reveals short-lived heavy element absorption systems in a majority of novae near maximum light, having expansion velocities of 400-1000 km s−1 and velocity dispersions between 35 and 350 km s−1. A majority of systems are accelerated outward, and they all progressively weaken and disappear over timescales of weeks. A few of the systems having narrow, deeper absorption reveal a rich spectrum of singly ionized Sc, Ti, V, Cr, Fe, Sr, Y, Zr, and Ba lines. Analysis of the richest such system, in LMC 2005, shows the excitation temperature to be 104 K and elements lighter than Fe to have abundance enhancements over solar values by up to an order of magnitude. The gas causing the absorption systems must be circumbinary and its origin is most likely mass ejection from the secondary star. The absorbing gas exists before the outburst and may represent episodic mass transfer events from the secondary star that initiate the nova outburst(s). If SNe Ia originate in single degenerate binaries, such absorption systems could be detectable before maximum light.

The Stability of Double White Dwarf Binaries Undergoing Direct‐Impact Accretion

We present numerical simulations of dynamically unstable mass transfer in a double white dwarf binary with initial mass ratio, q = 0.4. The binary components are approximated as polytropes of index n = 3/2 and the initially synchronously rotating, semi-detached equilibrium binary is evolved hydrodynamically with the gravitational potential being computed through the solution of Poisson's equation. Upon initiating deep contact in our baseline simulation, the mass transfer rate grows by more than an order of magnitude over approximately ten orbits, as would be expected for dynamically unstable mass transfer. However, the mass transfer rate then reaches a peak value, the binary expands and the mass transfer event subsides. The binary must therefore have crossed the critical mass ratio for stability against dynamical mass transfer. Despite the initial loss of orbital angular momentum into the spin of the accreting star, we find that the accretor's spin saturates and angular momentum is returned to the orbit more efficiently than has been previously suspected for binaries in the direct impact accretion mode. To explore this surprising result, we directly measure the critical mass ratio for stability by imposing artificial angular momentum loss at various rates to drive the binary to an equilibrium mass transfer rate. For one of these driven evolutions, we attain equilibrium mass transfer and deduce that effectively q_crit has evolved to approximately 2/3. Despite the absence of a fully developed disk, tidal interactions appear effective in returning excess spin angular momentum to the orbit.

Beryllium in Ultra–Lithium‐Deficient Halo Stars: The Blue Straggler Connection

There exists a small group of metal-deficient stars that have Li abundances well below the Li plateau that is defined by over 100 unevolved stars with temperatures above 5800 K and values of [Fe/H] < -1.0. Abundances of Be have been determined for most of these ultra-Li-deficient stars in order to investigate the cause of the Li deficiencies. These Li-deficient stars have implications on the value of primordial Li. High-resolution and high signal-to-noise ratio spectra have been obtained in the Be II spectral region near 3130 Å for six ultra-Li-deficient stars with the Keck I telescope and its new UV-sensitive CCD on the upgraded HIRES. The spectrum synthesis technique has been used to determine Be abundances. All six stars are found to have Be deficiencies also. Two have measurable but reduced Be, and four have only upper limits on Be. These results are consistent with the idea that these Li- and Be-deficient stars are analogous to blue stragglers. The stars have undergone mass transfer events (or mergers) that destroy or dilute both Li and Be. The findings cannot be matched by the models that predict that the deficiencies are due to extramixing in a subset of halo stars that were initially rapid rotators, with the possible exception of one star, G139-8. Because the ultra-Li-deficient stars are also Be-deficient, they appear to be genuine outliers in the population of halo stars used to determine the Li plateau in that they no longer have the Li in their atmospheres that was produced in the big bang.

Numerical Simulations of the Onset and Stability of Dynamical Mass Transfer in Binaries

Hydrodynamical simulations of semidetached, polytropic binary stars are presented in an effort to study the onset and stability of dynamical mass transfer events. Initial, synchronously rotating equilibrium models are constructed using a self-consistent field technique and then evolved with an Eulerian hydrodynamics code in a fully self-consistent manner. We describe code improvements introduced over the past few years that permit us to follow dynamical mass transfer events through more than 30 orbits. Mass transfer evolutions are presented for two different initial configurations: a dynamically unstable binary with initial mass ratio (donor/accretor) q0 = 1.3 that leads to a complete merger in ~10 orbits, and a double-degenerate binary with initial mass ratio q0 = 0.5 that, after some initial unstable growth of mass transfer, tends to separate as the mass transfer rate levels off.

Which massive stars are gamma-ray burst progenitors?

The collapsar model for gamma-ray bursts requires three essential ingredients: a massive core, removal of the hydrogen envelope, and enough angular momentum in the core. We study current massive star evolution models of solar metallicity to determine which massive star physics is capable of producing these ingredients. In particular, we investigate the role of hydrodynamic and magnetic internal angular momentum transport and binary mass and angular momentum transfer. We follow the evolution of rotating single stars and of binary systems that include rotational processes for both stars. Neglecting magnetic fields, we show that the cores of massive single stars can maintain a high specific angular momentum when evolved with the assumption that mean molecular weight gradients suppress rotational mixing processes. In binary systems that undergo mass transfer during core hydrogen burning the mass receiving star accretes large amounts of high angular momentum material, leading to a spin-up of the core. We find, however, that this merely compensates for the tidal angular momentum loss due to spin-orbit coupling, which leads to synchronous rotation before the mass transfer event. Therefore the resulting cores do not rotate faster than in single stars. We also present models that include magnetic fields generated by differential rotation and we consider the internal angular momentum transport by magnetic torques. We investigate the capability of magnetic torques to efficiently pump angular momentum into the cores of accreting stars. Despite our finding that this mechanism works, the magnetic coupling of core and envelope after the accreting star ends core hydrogen burning leads to slower rotation than in the non-magnetic case.