James C Baygents

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• Hunter, J., & Baygents, J. C. (2013). Grand challenges DELI (Discover, Explore, Learn, Imagine) project update. ASEE Annual Conference and Exposition, Conference Proceedings.
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  Abstract: Here we report on data collected for a project in which five new web-based lines of study, referred to as Elective Units, were developed by engineering faculty members with expertise in topics related to the NAE Grand Challenges.1 In a previous communication,2 we described the rationale, development scheme and topical content of the Elective Units. In this work we summarize selected results for the first full-scale offering of the Elective Units to a large cohort of students (400+) enrolled in an undergraduate engineering program. The Units were designed to give freshmen and prospective engineering students, many of whom are still in high school, an opportunity to explore topics of their choice in the engineering foundation course, Introduction to Engineering, offered by the University of Arizona (UA). The selection of topics was based on a Grand Challenges Interest Survey administered to 100+ freshmen engineering students, as part of the groundwork for the project. Students participating in the Interest Survey were asked to investigate the fourteen Grand Challenges for Engineering established by the National Academy of Engineering and indicate which challenges captured their interests. The five new Elective Units were modeled after a pilot Unit that was developed and successfully delivered in Spring 2010 as a result of a Learner-Centered Course Redesign Innovation Grant, funded by the Arizona Board of Regents. The learning in the Units is experiential in that each Unit allows students to address, first-hand, various types of problems that engineers attempt to solve. The activities require students to use a variety of tools to investigate the topics in order to establish a foundation of knowledge. Students are encouraged to further investigate topics and make connections to the societal, global, environmental and economic context that frame the Grand Challenge. The assignments are designed to motivate students to engage in higher-level thinking. Vignettes, i.e. short videos, describing each challenge and emphasizing the important role that engineers play in solving these challenges, were recorded. In addition, detailed written descriptions of the Units were developed. Students use the vignettes and reference materials to decide which Elective Unit(s) they wish to study. They then have the opportunity to devote four weeks of the semester, roughly one Carnegie unit of effort, to the exploration of the topic by making use of the cyberinfrastructure. © American Society for Engineering Education, 2013.
• Hunter, J., & Baygents, J. C. (2012). Grand Challenges Deli (Discover, Explore, Learn, Imagine) project. ASEE Annual Conference and Exposition, Conference Proceedings.
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  Abstract: In Fall 2011, researchers in the College of Engineering at the University of Arizona received an NSF Transforming Undergraduate Education in STEM (TUES) grant to develop learner-centered materials and strategies for an existing engineering course required of entry-level students. The strategy for the project, named the GC DELI (Grand Challenges: Discover, Explore, Learn and Imagine), is to give freshmen engineering and prospective engineering students-some of whom are still in high school-an opportunity to explore interesting and relevant topics of their choice. Five unique web-based lines of study, referred to as Elective Units, are being designed to capture the interests of students with diverse backgrounds and to encourage higher-level thinking. The goals of the project are to increase the commitment of freshman engineering students to the pursuit of engineering as an academic major and a profession, to enhance the interest of high school students in engineering, and to increase the number of women and underrepresented minorities matriculating into engineering and ultimately graduating with a degree in engineering. © 2012 American Society for Engineering Education.
• Oakes, W. C., Dexter, P., Hunter, J., Baygents, J. C., & Thompson, M. G. (2012). Early engineering through service-learning: Adapting a university model to high school. ASEE Annual Conference and Exposition, Conference Proceedings.
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  Abstract: The challenges of this next century require a new generation of engineering talent. In the United States, interest in engineering has remained flat and many groups within remain underrepresented relative to the overall population, specifically women and ethnic minorities. Attracting the next generation of diverse engineers requires a diverse set of pre-college experiences to connect diverse pathways leading to an engineering degree. One exciting approach is the use of service-learning to expose students to design and engineering. Servicelearning has been well established in many disciplines with positive impacts on interest, motivation, student satisfaction, personal success, desire, and retention of students who participated in service-learning projects. Service-learning is pedagogically consistent with literature on the recruitment and retention of women and other underrepresented groups in science and engineering. These benefits have been studied at the higher education level and show promise for pre-college as well. Service-learning connected to engineering also has an enormous potential for capitalizing on the wave of interest in community engagement among teenagers nationally. Connecting service to our community with engineering aligns perfectly with the National Academy's Changing the Conversation. This paper describes the adaptation of a successful university model to high school having been disseminated to more than 50 schools in 10 states. This paper highlights high school programs that have been integrated into the school day and are supported by a large Midwestern university and two large Southwestern universities. Example projects are described as well as the academic structure and teacher training processes. Demographic data shows that the model is attracting more female students and students from groups traditionally underrepresented in engineering. Data also shows that students are becoming more interested in engineering as a result of their experience in the service-learning programs. © 2012 American Society for Engineering Education.
• Safier, P. A., & Baygents, J. C. (2009). Electrohydrodynamic deformation of a miscible fluid stream by a transverse electric field. Langmuir, 25(10), 6000-6004.
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  PMID: 19374452;Abstract: Electrohydrodynamic deformation of a cylindrical fluid stream is analyzed with a quasi-electroneutral model. The stream is miscible with the surrounding liquid, though of different electrical conductivity and permittivity, and is subject to an electric field that acts transverse to the axis of the cylinder. The formulation allows for natural gradients of electrical conductivity and dielectric constant in the transition region between the stream and the surrounding liquid; these property variations are fully coupled to the fluid motion and are assumed to stem from concentration gradients of charge-carrying solutes. Dielectric and Coulombic body forces attendant to the time-dependent, spatial nonuniformities are accounted for. The strength of the electrically driven flows is such that transport of solutes is dominated by advection. As a consequence, the initial conductivity and dielectric constant differences, between the interior of the stream and the surrounding liquid, persist through significant deformation of the stream and characterize the rate at which the stream (continuously) deforms. Calculations for aqueous systems dominated by conductivity effects agree with measurements of stream deformation made by Rhodes et al. [J. Colloid Interface Sci. 1989, 129, 78]. Calculations for systems controlled by dielectric effects show that relative permittivity differences must be at least O(1) if noticeable deformations are to occur in a matter of seconds, which may explain why Trau et al. [Langmuir 1995,11,4665] discerned no deformations controlled by dielectric effects in low permittivity, low conductivity systems. An implication of these latter predictions is that experiments to isolate the role of dielectric constant mismatch may not be practicable. © 2009 American Chemical Society.
• Sounart, T. L., & Baygents, J. C. (2007). Lubrication theory for electro-osmotic flow in a non-uniform electrolyte. Journal of Fluid Mechanics, 576, 139-172.
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  Abstract: A lubrication theory has been developed for the electro-osmotic flow of non-uniform buffers in narrow rectilinear channels. The analysis applies to systems in which the transverse dimensions of the channel are large compared with the Debye screening length of the electrolyte. In contrast with related theories of electrokinetic lubrication, here the streamwise variations of the velocity field stem from, and are nonlinearly coupled to, spatiotemporal variations in the electrolyte composition. Spatially nonuniform buffers are commonly employed in electrophoretic separation and transport schemes, including iso-electric focusing (IEF), isotachophoresis (ITP), field-amplified sample stacking (FASS), and high-ionic-strength electro-osmotic pumping. The fluid dynamics of these systems is controlled by a complex nonlinear coupling to the ion transport, driven by an applied electric field. Electrical conductivity gradients, attendent to the buffer non-uniformities, result in a variable electro-osmotic slip velocity and, in electric fields approaching 1 kV cm-1, Maxwell stresses drive the electrohydrodynamic circulation. Explicit semi-analytic expressions are derived for the fluid velocity, stream function, and electric field. The resulting approximations are found to be in good agreement with full numerical solutions for a prototype buffer, over a range of conditions typical of microfluidic systems. The approximations greatly simplify the computational analysis, reduce computation times by a factor 4-5, and, for the first time, provide general insight on the dominant fluid physics of two-dimensional electrically driven transport. © 2007 Cambridge University Press.
• Short, M. B., Baygents, J. C., & Goldstein, R. E. (2006). A free-boundary theory for the shape of the ideal dripping icicle. Physics of Fluids, 18(8).
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  Abstract: The growth of icicles is considered as a free-boundary problem. A synthesis of atmospheric heat transfer, geometrical considerations, and thin-film fluid dynamics leads to a nonlinear ordinary differential equation for the shape of a uniformly advancing icicle, the solution to which defines a parameter-free shape which compares very favorably with that of natural icicles. Away from the tip, the solution has a power-law form identical to that recently found for the growth of stalactites by precipitation of calcium carbonate. This analysis thereby explains why stalactites and icicles are so similar in form despite the vastly different physics and chemistry of their formation. In addition, a curious link is noted between the shape so calculated and that found through consideration of only the thin coating water layer. © 2006 American Institute of Physics.
• Short, M. B., Baygents, J. C., & Goldstein, R. E. (2005). Stalactite growth as a free-boundary problem. AIChE Annual Meeting, Conference Proceedings, 439-.
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  Abstract: The astonishing variety and beauty of structures found in limestone caves, from stalactites and stalagmites to soda straws, draperies, and helictites, have been the subject of human wonder for hundreds if not thousands of years. There is little debate about the fundamental chemical processes responsible for their development. Water enters the cave from the overlying environment with significant concentrations of dissolved carbon dioxide and calcium. As the partial pressure of carbon dioxide in the cave is lower than that in the overlying rock, carbon dioxide outgases from the water. This raises the pH and leads to supersaturation and then precipitation of calcium carbonate. Yet, this chemical picture is only part of the story, for it does not in any direct way answer the most obvious morphological question: Why are stalactites long and slender, often roughly conical (resembling icicles)? While some studies address the dynamics of speleothem morphology, none quantitatively explains this most basic fact. In the work to be presented, we show that the combination of thin film fluid dynamics, calcium carbonate chemistry, and carbon dioxide diffusion and outgassing leads to a local geometric growth law for the surface evolution that quantitatively explains the shapes of natural stalactites. Here we provide details of this free-boundary calculation, exploiting a strong separation of time scales between that for diffusion within the layer, the time during which a fluid parcel is in contact with the growing surface, and the time scale of growth. When the flow rate, the scale of the stalactite, and the chemistry are in the ranges typically found in nature, the local growth rate is proportional to the local thickness of the fluid layer, itself determined by Stokes flow over the surface. Analytical and numerical studies of this law establish that a broad class of initial conditions is attracted to an ideal universal shape. Found under a set of limiting assumptions, this may be thought of as the Platonic ideal of speleothem growth. While real stalactites have more complex shapes due to instabilities and cave inhomogeneities, statistical analysis of stalactite shapes from Kartchner Caverns (Benson, AZ) yields excellent agreement between the average shape of natural stalactites and the ideal shape. This work serves to emphasize a broad class of problems that demands considerable attention-free-boundary dynamics in precipitative pattern formation. Beyond speleothems, these include structures as diverse as hydrothermal vents, chemical gardens, mollusc shells, and tubes whose growth is templated by bubbles.
• Short, M. B., Baygents, J. C., & Goldstein, R. E. (2005). Stalactite growth as a free-boundary problem. Physics of Fluids, 17(8), 1-12.
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  Abstract: Stalactites, the most familiar structures found hanging from the ceilings of limestone caves, grow by the precipitation of calcium carbonate from within a thin film of fluid flowing down their surfaces. We have recently shown [M. B. Short, J. C. Baygents, J. W. Beck, D. A. Stone, R. S. Toomey III, and R. E. Goldstein, "Stalactite growth as a free-boundary problem: A geometric law and its Platonic ideal," Phys. Rev. Lett. 94, 018501 (2005)] that the combination of thin-film fluid dynamics, calcium carbonate chemistry, and carbon dioxide diffusion and outgassing leads to a local geometric growth law for the surface evolution which quantitatively explains the shapes of natural stalactites. Here we provide details of this free-boundary calculation, exploiting a strong separation of time scales among that for diffusion within the layer, contact of a fluid parcel with the growing surface, and growth. When the flow rate, the scale of the stalactite, and the chemistry are in the ranges typically found in nature, the local growth rate is proportional to the local thickness of the fluid layer, itself determined by Stokes flow over the surface. Numerical studies of this law establish that a broad class of initial conditions is attracted to an ideal universal shape, whose mathematical form is found analytically. Statistical analysis of stalactite shapes from Kartchner Caverns (Benson, AZ) shows excellent agreement between the average shape of natural stalactites and the ideal shape. Generalizations of these results to nonaxisymmetric speleothems are discussed. © 2005 American Institute of Physics.
• Short, M. B., Baygents, J. C., Beck, J. W., Stone, D. A., S., R., & Goldstein, R. E. (2005). Stalactite growth as a free-boundary problem: A geometric law and its platonic ideal. Physical Review Letters, 94(1).
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  Abstract: The chemical mechanisms underlying the growth of cave formations such as stalactites are well known, yet no theory has yet been proposed which successfully accounts for the dynamic evolution of their shapes. Here we consider the interplay of thin-film fluid dynamics, calcium carbonate chemistry, and CO2 transport in the cave to show that stalactites evolve according to a novel local geometric growth law which exhibits extreme amplification at the tip as a consequence of the locally-varying fluid layer thickness. Studies of this model show that a broad class of initial conditions is attracted to an ideal shape which is strikingly close to a statistical average of natural stalactites. © 2005 The American Physical Society.
• Sounart, T. L., Safier, P. A., & Baygents, J. C. (2005). Theory and simulation of isoelectric focusing. Separation Science and Technology, 7(C), 41-68.
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  Abstract: IEF is an electrophoretic process that hinges on the mobility versus pH behavior of ampholytes. The basic theory of IEF tells us that separands focus about their pI in a Gaussian distribution. Peak height and variance depend on the competition between diffusion away from the pI and electromigration towards it. Comprehensive simulations of IEF involve the numerical solution to conservation laws for all the relevant amphoteric and ionogenic species in the separations milieu. The conservation relations are non-linearly coupled to the driving electric field and through the complicated mass-action relations. With the increased speed of microprocessors, one-dimensional simulations of the type shown here are now readily managed. The simulations offer the possibility of gaining insight into the detailed dynamics of the unfolding separations process. To see a more comprehensive exposition of IEF simulation results, particularly with regard to pH gradient development, the reader should refer the monograph by Mosher et al. © 2005 Elsevier Inc.
• Stone, D. A., Lewellyn, B., Baygents, J. C., & Goldstein, R. E. (2005). Precipitative growth templated by a fluid jet. Langmuir, 21(24), 10916-10919.
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  PMID: 16285753;Abstract: Tubular growth by chemical precipitation at the interface between two fluids, a jet and its surroundings, underlies the development of such important structures as chimneys at hydrothermal vents. This growth is associated with strong thermal and/or solute gradients localized at those interfaces, and these gradients, in turn, often produce radial compositional stratification of the resulting tube wall. A fundamental question underlying these processes is how the interplay between diffusion, advection, and precipitation determines the elongation rate of the tubes. Here we report experimental and theoretical results that reveal a regime in which there exists a new scaling law for tube growth. The model system studied consists of a jet of aqueous ammonia injected into a ferrous sulfate solution, precipitating iron hydroxides with varying oxidation states at the jet boundary. Despite the complex chemistry and dynamics underlying the precipitation, the tube growth exhibits a strikingly simple scaling form, with characteristic lengths and times increasing linearly with the mean velocity of the jet. These observations follow from a kinetic model of advection-dominated flows. © 2005 American Chemical Society.
• Sounart, T. L., & Baygents, J. C. (2001). Electrically-driven fluid motion in channels with streamwise gradients of the electrical conductivity. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 195(1-3), 59-75.
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  Abstract: Electroosmotic motion through charged, narrow-bore channels and capillaries is analyzed for the case where there are dominantly-axial gradients in the composition of the flowing electrolyte. The channel width is assumed to be large compared with the Debye screening length, and the electroosmotic slip velocity along the channel wall is taken to vary locally with the ionic strength, pH and electric field. Owing to the wall slip condition, the velocity distribution is nonlinearly coupled to the composition variations within the fluid. The prototype problem studied is one in which buffer ions and other solutes (e.g. analytes) are initially distributed in a sample zone that is sandwiched between uniform running buffer. For the situations considered, the conductivity of the sample zone differs significantly from that of the running buffer; such configurations are common to stacking and electroosmotic pumping protocols. In a frame of reference that moves with the mean velocity of the flow, the velocity field exhibits flow separation in the neighborhood of the conductivity variations and this gives rise to solutal mixing and dispersion in and about the sample zone. Copyright © 2001 Elsevier Science B.V.
• Sounart, T. L., & Baygents, J. C. (2000). Simulation of electrophoretic separations by the flux-corrected transport method. Journal of Chromatography A, 890(2), 321-336.
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  PMID: 11009036;Abstract: Electrophoretic separations at typical experimental electric field strengths have been simulated by applying the flux-corrected transport (FCT) finite difference method to the transient, one-dimensional electrophoresis model. The performance of FCT on simulations of zone electrophoresis (ZE), isotachophoresis (ITP), and isoelectric focusing (IEF) has been evaluated. An FCT algorithm, with a three-point, central spatial discretization, yields numerical solutions without numerical oscillations or spurious peaks, which have plagued previously-published second-order solutions to benchmark ZE and ITP problems. Moreover, the FCT technique captures sharp zone boundaries and IEF peaks more accurately than previously-published, first-order upwind schemes. (C) 2000 Elsevier Science B.V.
• Sounart, T. L., & Baygents, J. C. (2000). Simulation of electrophoretic separations: Effect of numerical and molecular diffusion on pH calculations in poorly buffered systems. Electrophoresis, 21(12), 2287-2295.
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  PMID: 10939437;Abstract: A poorly buffered cationic isotachophoresis separation, first simulated by Reijenga and Kasica, has been revisited to demonstrate that an inconsistent description of solute and charge transport can lead to significant errors in the pH calculation. The separation is first simulated using a second-order finite difference scheme to show that omission of molecular diffusion from the charge balance results in a pH profile with spurious dips in the steady-state zone boundaries. The separation is also simulated using two first order methods that employ numerical diffusion to stabilize solutions against spatiotemporal oscillations. Similar pH dips are generated by these first-order schemes, even when molecular diffusion is included in the charge balance, if numerical diffusion is not considered amongst the charge transport mechanisms. When numerical diffusion, inherent in the discretization of the component balances, is introduced to the charge balance, the spurious pH dips are eliminated. The results indicate that (i) pH dips originally reported by Reijenga and Kasicka are merely artifacts of their numerical model, and (ii) nonoscillatory numerical techniques, such as upwinding and flux limiters, should incorporate artificial transport mechanisms in the charge as well as the solute balances.
• Logan, B. E., Camesano, T. A., DeSantis, A. A., Unice, K. M., Baygents, J. C., Bolster, C. H., Hornberger, G. M., Mills, A. L., & Wilson, J. L. (1999). Comment on 'a method for calculating bacterial deposition coefficients using the fraction of bacteria recovered from laboratory columns' [2] (multiple letters). Environmental Science and Technology, 33(8), 1316-1319.
• Baygents, J. C., & Baldessari, F. (1998). Electrohydrodynamic instability in a thin fluid layer with an electrical conductivity gradient. Physics of Fluids, 10(1), 301-311.
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  Abstract: The onset of electrohydrodynamic motion associated with the imposition of an electric field across a thin layer of liquid has been investigated for the case in which the electrical conductivity varies linearly over the depth of the layer. The variation of the conductivity is due to concentration gradients in the charge-carrying solutes and its spatiotemporal evolution is represented by a convective-diffusion equation. When the viscous relaxation time is long compared to the time for charge relaxation, the analysis reveals that the neutral stability curves for the layer can be characterized by three dimensionless parameters: Rae≡d∈E02Δσ/μK effσ0, an electrical Rayleigh number; Δσ/σ0, the relative conductivity increment: and α, the transverse wave number of the disturbance. Here d is the thickness, ∈ is the dielectric constant, and μ is the viscosity of the layer, E0 is the applied field strength at the lower conductivity boundary, and Keff is an effective diffusivity associated with the Brownian motion of the charge-carrying solutes. With stress-free boundaries, at which the electrical conductivity and current are prescribed, the critical Rae is 1.416×104 at a critical transverse wave number of 1.90 when Δσ/σ0 is 8. As Δσ/σ0 increases, the critical Rae increases and shifts to slightly shorter wavelength disturbances; the critical imposed field strength, however, passes through a minimum because the lower-conductivity boundary exerts a considerable stabilizing influence in the presence of steep conductivity gradients. For Δσ/σ0 ≲8, the critical Rayleigh number increases as Δσ/σ0, decreases and the layer is only sensitive to long wavelength disturbances (α
• Baygents, J. C., Rivette, N. J., & Stone, H. A. (1998). Electrohydrodynamic deformation and interaction of drop pairs. Journal of Fluid Mechanics, 368, 359-375.
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  Abstract: The motion of two drops in a uniform electric field is considered using the leaky dielectric model. The drops are assumed to have no native charge and a dielectrophoretic effect favours translation of the drops toward one another. However circulatory flows that stem from electrohydrodynamic stresses may either act with or against this dielectrophoretic effect. Consequently, both prolate and oblate drop deformations may be generated and significant deformation occurs near drop contact owing to enhancement of the local electric field. For sufficiently widely spaced drops, electrohydrodynamic flows dominate direct electrical interactions so drops may be pushed apart, though closely spaced drops almost always move together as a result of the electrical interaction or deformation.
• Baygents, J. C., Schwarz, B. C., Deshmukh, R. R., & Bier, M. (1997). Recycling electrophoretic separations: Modeling of isotachophoresis and isoelectric focusing. Journal of Chromatography A, 779(1-2), 165-183.
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  Abstract: The conventional, one-dimensional model that is the foundation for the dynamic simulation of electrophoretic separations is modified to describe the essential features of recycling electrophoresis instruments. Simulations are performed for the isoelectric focusing (IEF) and the isotachophoresis (ITP) modes. Results are compared with experimental data from prototype recycling free-flow IEF and ITP devices. Agreement between the experiments and the simulations is favorable, though electrode and electroosmotic effects, which are omitted from the model, are noticeable in the IEF instrument. Three parameters relevant to the design of recycling electrophoresis devices, viz. the number of ports for the recycle manifold, the residence time in the separation chamber per cycle, and the current density, are investigated with the model.
• Belongia, B. M., & Baygents, J. C. (1997). Measurements on the diffusion coefficient of colloidal particles by Taylor-Aris dispersion. Journal of Colloid and Interface Science, 195(1), 19-31.
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  Abstract: Taylor-Aris dispersion in narrow-bore capillaries is used to measure the diffusion coefficient of colloidal particles in aqueous suspension. The method is shown to yield accurate results for particles up to about 0.3 μm in diameter; the measurement time for larger particles is prohibitively long and impractical. For hydrophobic particles, interactions with the capillary walls can introduce error into the interpretation of the data. The measurements also suggest that buoyancy-driven particle motion can introduce error. Consequently, a method similar to capillary hydrodynamic fractionation was developed to establish when these factors were of negligible effect. The results constitute an order-and-a-half improvement in the sensitivity of the technique, which has been recently shown to work for nanometer-sized proteins. The data suggest that, when matched with the appropriate theory, dispersion in capillaries may be a useful probe of colloidal and gravitational interaction potentials.
• Erker, J. A., & Baygents, J. C. (1996). Electrohydrodynamic interaction of a pair of spherical drops. NASA Conference Publication, 731-736.
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  Abstract: The axisymmetric electrohydrodynamic interaction between two spherical emulsion drops has been examined, using the leaky dielectric model to represent the constitutive behavior of the liquid phases. The results follow from the general solutions in bispherical coordinates to the Laplace equation for the electric potential and the Stokes equations for the velocity field. For drops of similar composition, the electrical interactions induced between the drops by the imposition of the electric field are always attractive, meaning they favor coalescence of the drop pair. The hydrodynamic interactions, however, are not always favorable and, indeed, are shown in certain circumstances to drive the drops apart.
• Erker, J. A., & Baygents, J. C. (1996). The equilibrium electric potential and surface charge density of spherical emulsion drops with thin double layers. Journal of Colloid and Interface Science, 179(1), 76-88.
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  Abstract: Analytic approximations are derived for the solution to the Poisson-Boltzmann equation as applied to a spherical emulsion drop containing a binary electrolyte. Particular attention is given to the drop interior and the formulas that result are easily evaluated. The approximations are obtained by two separate asymptotic methods, which are analogous to those used previously by others to describe the electric potential profile on the exterior of a spherical colloidal particle. The analyses apply to emulsion drops with thin double layers, meaning the drop radius a is large compared to κ-1 and κ̄-1, the respective Debye screening lengths for the exterior and interior of the drop. Using δ = (aκ̄)-1 as a perturbation parameter, we obtain a matched-asymptotic solution that adds corrections through O(δ3) to the flat-plate and Debye-Huckel solutions of the Poisson-Boltzmann equation. In the process, we recover expressions for the drop exterior that constitute an O(δ) improvement over the previously published results. Through a nonlinear transformation of the independent variable, we also derive a uniformly valid approximation that iteratively adds a correction to the flat-plate problem. Each technique yields accurate solutions. For example, the maximum relative error over the drop interior is on the order of 1% for aκ as low as 5 with surface potentials as high as 250 mV. Accuracy improves for larger values of aκ̄, with a maximum relative error below 0.1% for aκ̄ > 15. The asymptotic techniques are also used to obtain expressions for the surface charge density, with equally satisfactory results.
• Rivette, N. J., & Baygents, J. C. (1996). A note on the electrostatic force and torque acting on an isolated body in an electric field. Chemical Engineering Science, 51(23), 5205-5211.
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  Abstract: Expression for the electrostatic force and torque on an isolated body immersed in a dielectric medium have been developed for the case where the imposed electric field is either uniform or varies linearly with position. The expression apply to a body of general shape that carries no net native charge; illustrative examples are provided for ellipsoidal and spheroidal bodies. The force on a body in a uniform field is shown to be zero, which rectifies an incorrect result derived previously by others. The expressions obtained can be adapted to dielectric, leaky dielectric and perfectly conducting bodies subject to either dc or ac forcing fields.
• Baygents, J. C. (1994). Electrokinetic effects on the dielectric response of colloidal particles: Dielectrophoresis and electrorotation. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 92(1-2), 67-77.
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  Abstract: Electrokinetic effects on a.c. field dielectrophoretic and electrotational phenomena have been examined. Asymptotic methods have been used to carry out computations on the net electric force and torque acting on a spherical colloidal particle and its attending electric double layer. The dielectrophoretic force and the electrorotational torque are shown to depend on the real and imaginary parts of the (complex) particle dipole coefficient respectively. Moreover, it is demonstrated that the dipole coefficient that appears in the force and torque expressions is equivalent to that manifested by the particle in a uniform a.c. field. Thus the force and torque calculations reduce to a problem that has been solved previously in connection with dielectric dispersions in colloidal suspensions. The results for aqueous KCl solutions indicate that at modest ζ potentials (around 25 mV), the dielectrophoretic and electrorotational spectra are influenced strongly by ε{lunate}Pε{lunate}, the ratio of the particle and solution dielectric constants. At higher ζ potentials (100 mV), the spectra of particles with very thin double layers are affected by Stern layer ion transport below forcing frequencies of 1 MHz. © 1994.
• Rosen, L. A., Baygents, J. C., & Saville, D. A. (1993). The interpretation of dielectric response measurements on colloidal dispersions using the dynamic Stern layer model. The Journal of Chemical Physics, 98(5), 4183-4194.
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  Abstract: The standard description of electrokinetic phenomena deals with a particle whose charge is uniformly smeared over its surface and considers ion transport only within a Gouy-Chapman diffuse layer. Experimental studies with colloidal dispersions have shown that this model is not applicable to many systems. To encompass a wider class of behavior, the standard model was extended to include ion migration within the Stern layer, the region between the shear envelope and the rigid particle. Computations show that Stern layer transport increases the conductivity and dielectric response of suspensions as well as the magnitude of the ζ potential inferred from mobility measurements. Model predictions are compared with experimental measurements on two well-defined systems - colloidal silica and a polymer latex. The inclusion of surface transport processes markedly improves agreement between theory and the experimental data. For example, in situations where the standard theory underpredicts the measured dielectric increments by factors of 2 or 3, the dynamic Stern layer model yields results within 5% to 20% of the experimental data at frequencies in the kHz range. © 1993 American Institute of Physics.
• Baygents, J. C., & Saville, D. A. (1991). Electrophoresis of drops and bubbles. Journal of the Chemical Society, Faraday Transactions, 87(12), 1883-1898.
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  Abstract: We have examined the electrophoresis of drops and bubbles, computing the electrophoretic mobility as a function of the ζ-potential and several other parameters. Our treatment differs from previous work in that we incorporate a more representative picture of the interface. We have found that drops and bubbles are electrophoretically distinct from particles; perhaps the most striking result obtained was that, when the diffuse layers are thin, conducting drops do not always migrate in the direction that would be anticipated from the sign of their surface charge. Thus, the ζ-potential alone is not sufficient to characterize the surface. The analysis shows the sense of the migration is dictated by the net electrochemical stress acting along the interface. For similar reasons, large inviscid spheres tend to remain stationary at modest ζ-potentials and, in contrast to rigid particles, their mobility is actually enhanced by polarization of the double layer. Further, we have uncovered conditions for which the mobility of non-conducting drops is insensitive to the interior viscosity. This 'solidification effect' stems in part from interfacial tension gradients associated with specific adsorption of the ionic solutes, as well as from polarization and, moreover, need not involve the presence of surface-active impurities.
• Baygents, J., & Saville, D. (1991). Electrophoresis of small particles and fluid globules in weak electrolytes. Journal of Colloid And Interface Science, 146(1), 9-37.
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  Abstract: The electrical double layer plays an essential role in the electrokinetic behavior of both rigid and fluid spheres. One of the assumptions woven into the formulation of the classical balance laws of electrokinetics is that the ionogenic solutes are fully ionized. In aqueous media, common inorganic electrolytes such as KCl and NaOH dissociate completely into their constituent ions; the high dielectric constant of water favors dissociation by lowering the energy required to ionize a solute. Not all ionic solutions are aqueous, however, and electrokinetic effects are important in these media, too. Less polar liquids have a much lower dielectric constant, and so they are unable to sustain a high degree of solute ionization; in fact ionogenic solutes may dissociate less than a percent. Here we examine the influence of partial ionization on the electrophoresis of small particles and fluid globules, with a view toward understanding how, and under what conditions, dissociation-association alters the electrokinetics. We find generally that mass-action, consistent with Le Chatelier's principle, works to minimize disturbances to the electrical double layer, resisting polarization of the diffuse ion cloud. Thus, dissociation-association processes are quantitatively important in cases where double layer polarization and relaxation would otherwise prevail. Consequently, the predicted impact on the electrophoretic mobility is greatest for drops and bubbles, since their surfaces are fluid and convection within the interface is a factor. Mass-action can reduce the mobility of a conducting drop by an order of magnitude, and sizeable decreases (50% and more) in drop mobility are even found at ζ-potentials below 50 mV. Rigid particles are affected less dramatically and quantitative effects rarely exceed 10%; particles are markedly insensitive to partial solute ionization unless the ζ-potential is high (above ca. 100 mV) and aκ > 1. The computation scheme employed applies strictly to situations in which the magnitude of the forcing-field is small. Nevertheless, the results imply that for electrokinetic phenomena driven by strong forcing-fields, dissociation-association processes involving ionogenic solutes may be significant in apolar liquids. © 1991.
• Rhodes, P. H., Snyder, R. S., Roberts, G. O., & Baygents, J. C. (1991). Electrohydrodynamic effects in continuous flow electrophoresis.. Applied and theoretical electrophoresis : the official journal of the International Electrophoresis Society, 2(2-3), 87-91.
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  PMID: 1756188;Abstract: We demonstrate experimentally and theoretically the importance of electrohydrodynamic (EHD) flows in continuous-flow electrophoresis (CFE) separations. These flows are associated with variations in the conductivity or dielectric constant, and are quadratic in the field strength. They appear to be the main cause of extraneous and undesired flows in CFE which have degraded separation performance and have until now not been explained. We discuss the importance of EHD flows relative to other effects. We also describe possible techniques for reducing the associated degradation of CFE separations.