Soft Condensed Matter Group:

Prof. Dr. Alfons van Blaaderen - Dr. Arnout Imhof - Prof. Dr. Ir. Marjolein Dijkstra

Soft condensed matter systems are materials which deform easily under the influence of external fields such as electric fields, shear or gravity. They contain structures with length scales that are much larger than atomic or molecular dimensions, and display dynamics that are much slower. Nevertheless, their structure and dynamics are still governed by thermal fluctuations. We study the physics of soft matter model systems and, using these insights, fabricate new mesostructured materials from them. Our research tools consist of confocal microscopy, scattering methods, colloid synthesis, computer simulations, and theory.
The SCM group is part/member of:

Utrecht University Dept. of Physics and Astronomy	Debye Institute
Transregio SFB Physics of Colloidal Dispersions in External Fields	Network of Excellence SoftComp

News

March 22nd, 2011: PhD Defense of Peter van Oostrum

Using Light Scattering to Track, Characterize and Manipulate Colloids

Colloidal particles are about as large as the wavelength of light. The scattering of light by these particles is a rich and interesting phenomenon. During my PhD I have used the scattering of light by colloids in various ways. First of all I co-developed a new method to analyze in-line holographic micrographs. Our particles are illuminated with a collimated laser beam. Part of the light is scattered and interferes with the remaining, not-scattered, light. This interference pattern is called a hologram. By combining Mie theory and the well known interference, it is possible to calculate holograms. We minimize the difference between an experimental hologram and a calculated hologram. Doing so, we can determine the particle's lateral position to within 1 nm, and, more interestingly to within 10 nm in the axial direction, from a single snapshot. Using a fast CCD camera it is then possible to track colloids in space with unprecedented accuracy and speed. This fitting procedure also gives the radius and the refractive index of the individual particle to about 1 %.

The same scattering behaviour that we use to track and characterize the colloids can be used to exert forces on them using optical tweezers. I performed extensive calculations of the forces exerted by optical tweezers on various colloids, including colloids with a core-shell structure. We used optical traps to measure colloidal forces between PMMA particles in CHC. A more sensitive measurement can be achieved when the particles diffuse freely while they move under influence of interaction forces between them. From the statistics of many holographically measured trajectories one can measure the interaction forces between the colloids with fN accuracy. We demonstrated that this technique allows measuring the screened charge interaction between colloids in various solvents.

January 2011: Synthesis of monodisperse silica rods

Synthesis of monodisperse, rodlike silica colloids with tunable aspect ratio
A. Kuijk, A. van Blaaderen, and A. Imhof, Journal of the American Chemical Society, Article ASAP (2011).

We have succeeded in synthesizing colloidal silica rods that spontaneously form a smectic phase. These particles can be viewed in 3D with a confocal microscope enabling studies of the phase behaviour and dynamics of concentrated suspensions of rodlike particles.

 
(Left) Transmission electron micrograph of the silica rods. (Center) Confocal micrograph of an aligned phase of fluorescent silica rods. (Right) Scanning electron micrograph of a dried smectic of silica rods.

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