Current Activities

We discuss the depletion interaction between a wall and a mesoscopic particle of ellip-soidal shape induced by long, flexible, nonadsorbing polymer chains. Both a force and a torque are exerted on the particle. We concentrate on the case in which the particle size is much smaller than typical polymer lengths, such as the radius of gyration R g ,
where a rigid polymer approximation of the Asakura-Oosawa type cannot be applied. Explicit analytical results are obtained for ideal polymers. For particle-wall distances z large compared to R g an orientation of the ellipsoid perpendicular to the wall is favored. For z small compared to R g (but z still large compared to the particle size),
parallel orientation is favored. The perturbation of the polymer system due to the small particle is represented by a series of point-operators in the corresponding field theory, with next-to-next-to-leading anisotropic derivative-operators characterizing the particle orientation. For the interaction between a spherical particle and a wall the simple ana-lytical results predicted by the proposed small particle expansion beyond leading order display an interesting structure which is confirmed by direct numerical computation.[P10]

(E. Eisenriegler, A. Bringer, and R. Maassen)

One of the basic interactions in colloid physics is the force induced between colloidal particles or a particle and a wall by adding nonadsorbing free polymer chains. For entropic reasons the chains avoid the space between the particle and the wall leading to an unbalanced pressure which pushes the particle towards the wall. We find that the particle-wall interaction crucially depends on the particle-to-polymer size ratio p: While for large p the force decreases monotonically with increasing distance between particle and wall,for small p the force displays a maximum [P5].
(E.Eisenriegler, A. Bringer, A. Hanke, F. Schlesener)

The depletion interaction between colloidal particles and nonadsorbing polymer chains has mainly been studied for large particle size and is less understood in the so-called protein-limit of small particle size. At a first glance this interesting case looks complicated since it is dominated by configurations with the chain coiling around the particle and approximations treating the chain as nondeformable fail completely. However, on relating it via the polymer-magnet analogy to a field theory at the critical point, exact results can be achieved for the monomer-density distribution around one and two small particles [P6].
(E.Eisenriegler)

Understanding polymer chains interacting with a hard wall is a first step toward understanding colloid-polymer mixtures . A hard wall changes the fractal structure of the polymers in a profound way which is reflected in the forms of the various polymer density profiles. We have analytically calculated the center of mass density profile for ideal chains with radius of gyration R_g. Its decay on approaching the wall is much steeper than the power law decays of the end-point or monomer density profiles and is characterized by an essential singularity. The reason is that there are far fewer chain conformations with center of mass close to the wall than with an end-point or an arbitrary monomer close to the wall [P9].
(Eisenriegler, Maassen)