Determination of the zeta potential for highly charged colloidal suspensions

We compute the electrostatic potential at the surface, or zeta potential $\zeta$, of a charged particle embedded in a colloidal suspension using a hybrid mesoscopic model. We show that for weakly perturbing electric fields, the value of $\zeta$ obtained at steady state during electrophoresis is statistically indistinguishable from $\zeta$ in thermodynamic equilibrium. We quantify the effect of counterions concentration on $\zeta$. We also evaluate the relevance of the lattice resolution for the calculation of $\zeta$ and discuss how to identify the effective electrostatic radius.Comment: 8 pages, 3 figures with 2 panel

Ion specificity and the theory of stability of colloidal suspensions

A theory is presented which allow us to accurately calculate the critical coagulation concentration (CCC) of hydrophobic colloidal suspensions. For positively charged particles the CCC's follow the Hofmeister (lyotropic) series. For negatively charged particles the series is reversed. We find that strongly polarizable chaotropic anions are driven towards the colloidal surface by electrostatic and hydrophobic forces. Within approximately one ionic radius from the surface, the chaotropic anions loose part of their hydration sheath and become strongly adsorbed. The kosmotropic anions, on the other hand, are repelled from the hydrophobic surface. The theory is quantitatively accurate without any adjustable parameters. We speculate that the same mechanism is responsible for the Hofmeister series that governs stability of protein solutions.Comment: Phys. Rev. Lett. (in press

Aggregation kinetics of stiff polyelectrolytes in the presence of multivalent salt

Using molecular dynamics simulations, the kinetics of bundle formation for stiff polyelectrolytes such as actin is studied in the solution of multivalent salt. The dominant kinetic mode of aggregation is found to be the case of one end of one rod meeting others at right angle due to electrostatic interactions. The kinetic pathway to bundle formation involves a hierarchical structure of small clusters forming initially and then feeding into larger clusters, which is reminiscent of the flocculation dynamics of colloids. For the first few cluster sizes, the Smoluchowski formula for the time evolution of the cluster size gives a reasonable account for the results of our simulation without a single fitting parameter. The description using Smoluchowski formula provides evidence for the aggregation time scale to be controlled by diffusion, with no appreciable energy barrier to overcome.Comment: 6 pages, 5 figures, Phys. Rev. E (Accepted

Effect of Salt Concentration on the Electrophoretic Speed of a Polyelectrolyte through a Nanopore

In a previous paper [S. Ghosal, Phys. Rev. E 74, 041901 (2006)] a hydrodynamic model for determining the electrophoretic speed of a polyelectrolyte through an axially symmetric slowly varying nanopore was presented in the limit of a vanishingly small Debye length. Here the case of a finite Debye layer thickness is considered while restricting the pore geometry to that of a cylinder of length much larger than the diameter. Further, the possibility of a uniform surface charge on the walls of the nanopore is taken into account. It is thereby shown that the calculated transit times are consistent with recent measurements in silicon nanopores.Comment: 4 pages, 2 figure

Dynamics of Labor Demand: Evidence from Plant-level Observations and Aggregate Implications

This paper studies the dynamics of labor demand at the plant and aggregate levels. The correlation of hours and employment growth is negative at the plant level and positive in aggregate time series. Further, hours and employment growth are about equally volatile at the plant level while hours growth is much less volatile than employment growth in the aggregate data. Given these differences, we specify and estimate the parameters of a plant-level dynamic optimization problem using simulated method of moments to match plant-level observations. Our findings indicate that non-convex adjustment costs are critical for explaining plant-level moments on hours and employment. Aggregation generates time series implications which are broadly consistent with observation. Further, we find that a model with quadratic adjustment costs alone can also broadly match the aggregate facts.

CL 615 MS 653 ML 715 Cross Cultural Leadership

Required Texts for Course (CRIB Notes Due on “Read By” Dates Below) Read By – Sep 12 • Book(s):TheMulticulturalLeader:DevelopingaCatholicPersonality.DanSheffield,ClemmentsPublishing,2005. • Article(s)“BodyRitualsoftheNacerima”and“MindlessnessinAmerica” Read By – Oct 10 • Book(s):LiftingtheWhiteVeil:AnExplorationofWhiteAmericanCultureinaMultiracialContext.JeffHitchcock. Crandall Dostie & Douglass, 2003. • Article(s): “HowIBenefitfromWhitePrivilege,”“JustBeneathMySkin,”“APrayerGodCouldNotAnswer” Read By – Oct 24 • Book(s): Riding The Waves of Culture: Understanding Diversity in Global Business. F. Trompenaars. McGraw-Hill, 1998. • Article(s):“EmicsandEticsofCulturallyImplicitLp.Theories”and“CulturalConstraintsinMgmtTheories” Read By – Nov 10 • Book(s):DividedbyFaith:EvangelicalReligionandtheProblemofRaceinAmerica.M.Emerson,C.Smith.Oxford,2001. • Article(s):“HarderthanAnyoneCanImagine”and“God’sNewCommunity:TheChallengesandHopesfor Multi-racial Churches in America” Read By – Nov 17 • Choose 1 of 4 Options (Graduate Participants) 1. United by Faith: The Multiracial Congregation As an Answer to the Problem of Race DeYoung et al. 2. Marginality: Key to Multicultural Theology J. Y. Lee 3. Grace Matters: A Memoir of Faith, Friendship, and Hope in the Heart of the South, Chris P. Rice 4. Invitation to Lead: Guidance for Emerging Asian American Leaders. Paul Tounaga 5. Living in Color: Embracing God\u27s Passion for Ethnic Diversity. Randy Woodley, John Dawson • Choose 1 of 5: (Doctoral Participants) 1. TheGreatTurning:FromEmpiretoEarthCommunityDavidC. Korten. 2. BasicConceptsinInterculturalCommunication:SelectedReadings Milton Bennett. 3. CulturesInfluenceonBehaviorRichardBrislin. 4. ARaceIsaNiceThingtoHave:AGuidetoBeingaWhite Person or Understanding the White Persons in Your Life Janet E. Helms. Content Communications, 1992 5. TheNacirema:ReadingsonAmericanCultureJamesP Spradley; Michael A Rynkiewich. Publisher: Boston : Little, Brown, 1975. Read By – Nov 28 • Book(s): Robert Fuller, All Rise: Somebodies, Nobodies, and the Politics of Dignity. Berrett-Koehler, 2006. • Article(s): “Why Do You Call Me Lord, Lord?” and “Is a Leader\u27s Character Culture-Bound or Culture- Free?”https://place.asburyseminary.edu/syllabi/2351/thumbnail.jp

Separation of suspended particles by arrays of obstacles in microfluidic devices

The stochastic transport of suspended particles through a periodic pattern of obstacles in microfluidic devices is investigated by means of the Fokker-Planck equation. Asymmetric arrays of obstacles have been shown to induce the continuous separation of DNA molecules of different length. The analysis presented here of the asymptotic distribution of particles in a unit cell of these systems shows that separation is only possible in the presence of a driving force with a non-vanishing normal component at the surface of the solid obstacles. In addition, vector separation, in which different species move, in average, in different directions within the device, is driven by differences on the force acting on the various particles and not by differences in the diffusion coefficient. Monte-Carlo simulations performed for different particles and force fields agree with the numerical solutions of the Fokker-Planck equation in the periodic system

Velocity fluctuations and hydrodynamic diffusion in sedimentation

We study non-equilibrium velocity fluctuations in a model for the sedimentation of non-Brownian particles experiencing long-range hydrodynamic interactions. The complex behavior of these fluctuations, the outcome of the collective dynamics of the particles, exhibits many of the features observed in sedimentation experiments. In addition, our model predicts a final relaxation to an anisotropic (hydrodynamic) diffusive state that could be observed in experiments performed over longer time ranges.Comment: 7 pages, 5 EPS figures, EPL styl

The Effects of Inter-particle Attractions on Colloidal Sedimentation

We use a mesoscopic simulation technique to study the effect of short-ranged inter-particle attraction on the steady-state sedimentation of colloidal suspensions. Attractions increase the average sedimentation velocity $v_s$ compared to the pure hard-sphere case, and for strong enough attractions, a non-monotonic dependence on the packing fraction $\phi$ with a maximum velocity at intermediate $\phi$ is observed. Attractions also strongly enhance hydrodynamic velocity fluctuations, which show a pronounced maximum size as a function of $\phi$. These results are linked to a complex interplay between hydrodynamics and the formation and break-up of transient many-particle clusters.Comment: 4 pages 4 figure

Systematic characterization of thermodynamic and dynamical phase behavior in systems with short-ranged attraction

In this paper we demonstrate the feasibility and utility of an augmented version of the Gibbs ensemble Monte Carlo method for computing the phase behavior of systems with strong, extremely short-ranged attractions. For generic potential shapes, this approach allows for the investigation of narrower attractive widths than those previously reported. Direct comparison to previous self-consistent Ornstein-Zernike approximation calculations are made. A preliminary investigation of out-of-equilibrium behavior is also performed. Our results suggest that the recent observations of stable cluster phases in systems without long-ranged repulsions are intimately related to gas-crystal and metastable gas-liquid phase separation.Comment: 10 pages, 8 figure