Model Systems

Colloidal beads are grafted with DNA, where the functionality (up to highly packed brushes) as well as the brush thickness (up to about 10 microns) can be varied. The DNA can be fluorescently  backbone-labeled or end-labelled (as in the figure), to enable the investigation of the brush structure as a function of functionality, DNA length, and ionic strength. At high colloid concentrations, the deformation of the brushes due to interactions as well as brush-interpenetration can be studied.

E. Stiakakis

A single stranded DNA fragment connects two stiff, double stranded DNA fragments. The length of both the flexible and stiff parts can be controlled within a single-base accuracy. Depending on the spacer length and the length of the stiff strands, various phases may occur. A novel smectic phase has been found for sufficiently long spacers, where the stiff parts of each duplex reside within the same smectic layer.

E. Stiakakis

Each of the 2700 major coat proteins of fd-virus bear two solvent exposed amine and five carboxyl groups that give the virus a negative charge over a wide range of pH (a TEM picture of a native fd-virus in given in the upper figure). These chemical groups are used to modify the surface of the viruses, and thus tune their mutual interactions. The viruses can be grafted with polymer chains [1], [2], fluorescently labeled, and their charge can be reversed to obtain positively charged viruses particles [3] (as schematically depicted in the lower figure).

J. Buitenhuis

In addition to the chemical modifications, different types of filamentous viruses and mutants of fd-viruses are used. For example, the virus fd-Y21M, which is stiffer than native fd, is used as a model-rod to study the dynamics in smectics [4], and are used in rheological studies. The various bacteriophages that we use have a contour length varying from 0.4 to 2.1 micron, with persistence lengths varying from 2.8 to 9.9 micron.

J. Buitenhuis, P. Lettinga

Concerning spherical colloids, fluorinated latex particles [1], [2] and silica spheres [3], [4] have been of recent interest.
Fluorescent molecules can be included into the particles and several coating procedures can be applied to tune the inter-particle interactions and to achieve comparability with varies materials.
Poly(ethylene glycol)-grafted colloidal silica particles have been synthesized with an improved stability in aqueous solvents [5], This improves on previous synthesis routes that lead to grafting layers that detach from the silica surface and dissolve in water within a few days.

J. Buitenhuis, P. Lang

In addition to the bio-inspired rod-like colloids, we synthesize rods composed of boehmite, and platelets consisting of gibbsite.
Of recent interest are gibbsite platelets [6], [7], for which the flow behavior is systematically studied.

J. Buitenhuis, P. Lettinga