Partner: Gerhard Nägele 

Recent publications
1.  Abade G.C.^{♦}, Cichocki B.^{♦}, EkielJeżewska M.L., Nägele G.^{♦}, Wajnryb E., Rotational and translational selfdiffusion in concentrated suspensions of permeable particles, JOURNAL OF CHEMICAL PHYSICS, ISSN: 00219606, DOI: 10.1063/1.3604813, Vol.134, pp.24490317, 2011 Abstract: In our recent work on concentrated suspensions of uniformly porous colloidal spheres with excluded volume interactions, a variety of shorttime dynamic properties were calculated, except for the rotational selfdiffusion coefficient. This missing quantity is included in the present paper. Using a precise hydrodynamic force multipole simulation method, the rotational selfdiffusion coefficient is evaluated for concentrated suspensions of permeable particles. Results are presented for particle volume fractions up to 45% and for a wide range of permeability values. From the simulation results and earlier results for the firstorder virial coefficient, we find that the rotational selfdiffusion coefficient of permeable spheres can be scaled to the corresponding coefficient of impermeable particles of the same size. We also show that a similar scaling applies to the translational selfdiffusion coefficient considered earlier. From the scaling relations, accurate analytic approximations for the rotational and translational selfdiffusion coefficients in concentrated systems are obtained, useful to the experimental analysis of permeableparticle diffusion. The simulation results for rotational diffusion of permeable particles are used to show that a generalized StokesEinsteinDebye relation between rotational selfdiffusion coefficient and highfrequency viscosity is not satisfied. Keywords:selfdiffusion, permeable particles, concentrated suspensions Affiliations:
 
2.  Cichocki B.^{♦}, EkielJeżewska M.L., Nägele G.^{♦}, Wajnryb E., Firstorder virial expansion of shorttime diffusion and sedimentation coefficients of permeable particles suspensions, PHYSICS OF FLUIDS, ISSN: 10706631, DOI: 10.1063/1.3626196, Vol.23, pp.08330318, 2011 Abstract: For suspensions of permeable particles, the shorttime translational and rotational selfdiffusion coefficients, and collective diffusion and sedimentation coefficients are evaluated theoretically. An individual particle is modeled as a uniformly permeable sphere of a given permeability, with the internal solvent flow described by the DebyeBuecheBrinkman equation. The particles are assumed to interact nonhydrodynamically by their excluded volumes. The virial expansion of the transport properties in powers of the volume fraction is performed up to the twoparticle level. The firstorder virial coefficients corresponding to twobody hydrodynamic interactions are evaluated with very high accuracy by the series expansion in inverse powers of the interparticle distance. Results are obtained and discussed for a wide range of the ratio, x, of the particle radius to the hydrodynamic screening length inside a permeable sphere. It is shown that for x≥10, the virial coefficients of the transport properties are wellapproximated by the hydrodynamic radius (annulus) model developed by us earlier for the effective viscosity of porousparticle suspensions. Keywords:Stokes equations, hydrodynamic interactions, diffusion, sedimentation, permeable particles, suspesnion, virial expansion Affiliations:
 
3.  Abade G.C.^{♦}, Cichocki B.^{♦}, EkielJeżewska M.L., Nägele G.^{♦}, Wajnryb E., Shorttime dynamics of permeable particles in concentrated suspensions, JOURNAL OF CHEMICAL PHYSICS, ISSN: 00219606, DOI: 10.1063/1.3274663, Vol.132, pp.014503117, 2010 Abstract: We study shorttime diffusion properties of colloidal suspensions of neutral permeable particles. An individual particle is modeled as a solventpermeable sphere of interaction radius a and uniform permeability k, with the fluid flow inside the particle described by the Debye–Bueche–Brinkman equation, and outside by the Stokes equation. Using a precise multipole method and the corresponding numerical code HYDROMULTIPOLE that account for higherorder hydrodynamic multipole moments, numerical results are presented for the hydrodynamic function, H(q), the shorttime selfdiffusion coefficient, Ds, the sedimentation coefficient K, the collective diffusion coefficient, Dc, and the principal peak value H(qm), associated with the shorttime cage diffusion coefficient, as functions of porosity and volume fraction. Our results cover the full fluid phase regime. Generic features of the permeable sphere model are discussed. An approximate method by Pusey to determine Ds is shown to agree well with our accurate results. It is found that for a given volume fraction, the wavenumber dependence of a reduced hydrodynamic function can be estimated by a single master curve, independent of the particle permeability, given by the hardsphere model. The reduced form is obtained by an appropriate shift and rescaling of H(q), parametrized by the selfdiffusion and sedimentation coefficients. To improve precision, another reduced hydrodynamic function, hm(q), is also constructed, now with the selfdiffusion coefficient and the peak value, H(qm), of the hydrodynamic function as the parameters. For wavenumbers qa > 2, this function is permeability independent to an excellent accuracy. The hydrodynamic function of permeable particles is thus well represented in its qdependence by a permeabilityindependent master curve, and three coefficients, Ds, K, and H(qm), that do depend on the permeability. The master curve and its coefficients are evaluated as functions of concentration and permeability. Keywords:Stokes equations, hydrodynamic interactions, selfdiffusion, sedimentation, permeable particles, suspension Affiliations:
 
4.  Abade G.C.^{♦}, Cichocki B.^{♦}, EkielJeżewska M.L., Nägele G.^{♦}, Wajnryb E., Highfrequency viscosity of concentrated porous particles suspensions, JOURNAL OF CHEMICAL PHYSICS, ISSN: 00219606, DOI: 10.1063/1.3474804, Vol.133, pp.08490619, 2010 Abstract: We determine the highfrequency limiting shear viscosity in colloidal suspensions of rigid, uniformly porous spheres of radius a as a function of volume fraction and inverse porosity parameter x. Our study covers the complete fluidstate regime. The flow inside the spheres is modeled by the Debye–Bueche–Brinkman equation using the boundary condition that fluid velocity and stress change continuously across the sphere surfaces. The manysphere hydrodynamic interactions in concentrated systems are fully accounted for by a precise hydrodynamic multipole method encoded in our HYDROMULTIPOLE program extended to porous particles. A truncated virial expansion is used to derive an accurate and easytouse generalized Saitô formula for. The simulation data are used to test the performance of two simplifying effective particle models. The first model describes the effective particle as a nonporous sphere characterized by a single effective radius dependent on x. In the more refined second model, the porous spheres are modeled as spherical annulus particles with an inner hydrodynamic radius as a function of x, defining the nonporous dry core and characterizing hydrodynamic interactions, and an outer excluded volume radius a characterizing the unchanged direct interactions. Only the second model is in a satisfactory agreement with the simulation data. Keywords:Stokes flow, permeable particles, effective viscosity, lubrication, concentrated suspensions Affiliations:
 
5.  Abade G.C.^{♦}, Cichocki B.^{♦}, EkielJeżewska M.L., Nägele G.^{♦}, Wajnryb E., Highfrequency viscosity and generalized Stokes–Einstein relations in dense suspensions of porous particles, JOURNAL OF PHYSICSCONDENSED MATTER, ISSN: 09538984, DOI: 10.1088/09538984/22/32/322101, Vol.22, pp.32210116, 2010 Abstract: We study the highfrequency limiting shear viscosity, η∞, of colloidal suspensions of uncharged porous particles. An individual particle is modeled as a uniformly porous sphere with the internal solvent flow described by the Debye–Bueche–Brinkman equation. A precise hydrodynamic multipole method with a full account of manyparticle hydrodynamic interactions encoded in the HYDROMULTIPOLE program extended to porous particles, is used to calculate η∞ as a function of porosity and concentration. The secondorder virial expansion for η∞ is derived, and its range of applicability assessed. The simulation results are used to test the validity of generalized Stokes–Einstein relations between η∞ and various shorttime diffusion coefficients, and to quantify the accuracy of a simplifying cell model calculation of η∞. An easytouse generalized Saitˆo formula for η∞ is presented which provides a good description of its porosity and concentration dependence. Keywords:Stokes flow, hydrodynamic interactions, permeable particles, dense suspensions, effective viscosity Affiliations:
 
6.  Abade G.C.^{♦}, Cichocki B.^{♦}, EkielJeżewska M.L., Nägele G.^{♦}, Wajnryb E., Dynamics of permeable particles in concentrated suspensions, PHYSICAL REVIEW E, ISSN: 15393755, DOI: 10.1103/PhysRevE.81.020404, Vol.81, pp.02040414, 2010 Abstract: We calculate shorttime diffusion properties of suspensions of porous colloidal particles as a function of their permeability, for the full fluidphase concentration range. The particles are modeled as spheres of uniform permeability with excluded volume interactions. Using a precise multipole method encoded in the HYDROMULTIPOLE program, results are presented for the hydrodynamic function, H(q), sedimentation coefficient, and selfdiffusion coefficients with a full account of manybody hydrodynamic interactions. While selfdiffusion and sedimentation are strongly permeability dependent, the wavenumber dependence of the hydrodynamic function can be reduced by appropriate shifting and scaling, to a single master curve, independent of permeability. Generic features of the permeable sphere model are discussed. Keywords:Stokes equations, hydrodynamic interactions, permeable particles, concentrated suspensions, selfdiffusion, hydrodynamic function, collective diffusion Affiliations:
